All posts by ddlang

Sudden Oak Death

What is sudden oak death?  Sudden oak death (also called Ramorum leaf blight or Ramorum dieback) is an oftentimes lethal disease that has caused widespread death of tanoak (Lithocarpus densiflorus), coast live oak (Quercus agrifolia), California black oak (Quercus kelloggii), and Shreve oak (Quercus parvula var. shrevei) in California.

Rapid wilting and die back of branch tips can be a symptom of ramorum dieback.
Rapid wilting and die back of branch tips can be a symptom of ramorum dieback.

The disease can affect or has been reported in association with a wide range of woody and herbaceous plants including, but not limited to bigleaf maple (Acer macrophyllum), Bodnant viburnum (Viburnum X bodnantense), ‘Brouwer’s Beauty’ pieris (Pieris floribunda X japonica), California bay laurel (Umbellularia californica), California buckeye (Aesculus californica), California coffeeberry (Rhamnus californica), California honeysuckle (Lonicera hispidula), canyon live oak (Quercus chrysolepis), coast redwood (Sequoia sempervirens), doublefile viburnum (Viburnum plicatum var. tomentosum), douglas-fir (Pseudotsuga menziesii var. menziesii), evergreen huckleberry (Vaccinium ovatum), Formosa firethorn (Pyracantha koidsumii), ‘Forest Flame’ pieris (Pieris formosa X japonica), Himalaya pieris (Pieris formosa), Japanese camellia (Camellia japonica), Japanese pieris (Pieris japonica), laurustinus (Viburnum tinus), madrone (Arbutus menziesii), manzanita (Arctostaphylos manzanita), rhododendron (Rhododendron spp.), Sasanqua camellia (Camellia sasanqua), toyon (Heteromeles arbutifolia), western starflower (Trientalis latifolia), and witch hazel (Hamamelis virginiana), Burkwood viburnum (Viburnum X burkwoodii), California hazelnut (Corylus cornuta), Camellia X williamsii, cascara (Rhamnus purshiana), Chinese pieris (Pieris formosa var. forrestii), common lilac (Syringa vulgaris), David viburnum (Viburnum davidii), drooping leucothoe (Leucothoe fontanesiana), European beech (Fagus sylvatica), European cranberrybush viburnum (Viburnum opulus), European turkey oak (Quercus cerris), European yew (Taxus baccata), fragrant viburnum (Viburnum farreri), grand fir (Abies grandis), Holm oak (Quercus ilex), horse-chestnut (Aesculus hippocastanum), lingonberry (Vaccinium vitis-ideae), mountain laurel (Kalmia latifolia), Northern red oak (Quercus rubra), Pieris formosa var. forrestii X Pieris japonica, poison oak (Toxicodendron diversiloba), Prague viburnum (Viburnum X pragense), reticulate camellia (Camellia reticulata), salmonberry (Rubus spectabilis), Southern red oak (Quercus falcata), strawberry tree (Arbutus unedo), sweet chestnut (Castanea sativa), Viburnum X carlcephalum X Viburnum utile, Victorian box (Pittosporum undulatum), wayfaringtree viburnum (Viburnum lantana), and wood rose (Rosa gymnocarpa).

Sudden oak death was first reported in the US in California and has subsequently been found in other US states, including in Wisconsin in 2019.  Sudden oak death has also been reported in Europe.

What does sudden oak death look like?  Symptoms of sudden oak death vary depending upon the plant species infected.  On some hosts, infections occur primarily on leaves leading to light brown leaf spots and blotches.  These leaf symptoms may be indistinguishable from other, more common, leaf spots and blights, or may mimic sunburn or leaf scorch symptoms.  Twigs and branches that become infected often wilt, forming a “shepherd’s-crook”, and subsequently die back.  Infection of tree trunks leads to cankers (i.e., sore-like areas) that produce large amounts of an amber to black colored ooze.  This ooze can dry to form a stained area on the bark.  Removing the bark over the affected area will reveal discolored wood beneath that sometimes (but not always) has a black border.  Cankers can eventually expand to girdle trunks, thus resulting in the death of the tree or shrub.  Trunk infections appear not to extend into the root system of the plant.  Once sudden oak death cankers develop, other pathogens may invade the infected areas, accelerating tree or shrub death and complicating the diagnosis of the disease.

Where does sudden oak death come from?  Sudden oak death is caused by the fungus-like water mold Phytophthora ramorum, which was first recognized as a pathogen in 1995.  Phytophthora ramorum can be spread over long distances through movement of infected plants or infested plant parts.  The organism can also be moved with contaminated soil (e.g., on vehicle tires, tools, or shoes), or in contaminated water.  Once established on plants in a given location, the fungus produces reproductive structures (called sporangia) that can be moved from plant to plant by rain splash, or wind.  Phytophthora ramorum was introducing into Wisconsin in 2019 on nursery stock grown in the state of Washington.

Ramorum leaf blight symptoms can mimic those of other leaf spots and blights.
Ramorum leaf blight symptoms can mimic those of other leaf spots and blights.

How do I save a plant with sudden oak death?  If you believe you have seen a plant that has sudden oak death, please IMMEDIATELY submit a sample to the Plant Disease Diagnostics Clinic (PDDC), care of the address in the box at the bottom of this page.  Double bag suspect plant tissue in sealable plastic bags and place the bagged specimen in a box or envelope for shipping.  Include contact information (complete address, phone number, email address) in a separate sealable plastic bag with the sample.  Tape over all of the edges of boxes and envelopes used for shipping to keep everything sealed inside.  Write on the box or envelope that the box or envelope contains a suspect SOD sample.  If you have questions about collecting or submitting a sample, contact PDDC staff at (608) 262-2863 or at pddc@wisc.edu.

Because Phytophthora ramorum is a regulated, quarantined pathogen, DO NOT remove the affected plant (or parts thereof) or take the plant from the site where it is located, other than to collect a specimen for submission for a diagnosis.  Be sure to decontaminate any tools or other items that come into contact with the plant (including those used to collect a diagnostic sample) by dipping them for at least 30 seconds in 10% bleach.  If a plant tests positive for Phytophthora ramorum, it will be removed and destroyed to help prevent further spread of the pathogen.

How do I avoid problems with sudden oak death in the future?  Carefully inspect any new nursery stock upon delivery (or prior to purchase, if possible) for symptoms of sudden oak death.  Keep new stock isolated from older stock as long as possible, to minimize possible movement of the pathogen should the disease develop after plants have arrived.  If you see any suspect symptoms, alert the PDDC so that arrangements can be made for proper testing for Phytophthora ramorum.

For more information or help in diagnosing sudden oak death:  Contact Brian Hudelson, UW-Madison Plant Disease Diagnostic Clinic, Department of Plant Pathology, University of Wisconsin-Madison, 1630 Linden Drive, Madison, WI  53706-1598, phone: (608) 262-2863, email: pddc@wisc.edu, see the USDA APHIS sudden oak death website https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/pests-and-diseases/phytophthora-ramorum/sod, or contact your County Extension agent.

July: Cool Fungal Friends for a Hot and Muggy July

MushroomOver the past two weeks, I have received a number of photos of incredibly cool microorganisms that have popped up in people’s gardens or around their homes.  These organisms have all turned out to be non-pathogens (i.e., they don’t cause plant diseases), but they are some of the more extraordinary organisms that I encounter in my job.

Bird’s Nest FungiIf you’ve recently spread new mulch, you may eventually notice small (maybe ¼ to ½ inch in diameter) cup-like structures forming in groups on the mulch surface.  Closer examination of the cups will reveal egg-shaped structures inside.  Looking at these tiny structures as a whole, you’d swear that some sort of miniature bird has built a series of nests in your flowerbed.  But these mini-“nests” are actually reproductive structures of one of the bird’s nest fungi.  The “eggs” are packets of fungal spores attached to the interior of the cup by a cord-like structure.  When a raindrop hits the nest, the momentum of the falling water catapults an “egg” into the air.  The attached “cord” stretches and eventually breaks.  The broken end is sticky and eventually adheres to some object (e.g., a branch or leaf) as the “egg” flies through the air.  The attached “egg” swings around on the cord (think of a bola) and eventually gloms onto something as well.  As the walls of the catapulted “egg” degrades, the spores inside are released.  Being elevated, the spores are more likely to be picked up in air currents.  What an amazing adaptation that increases the likelihood that the spores are spread over a large distance!

Bird's Nest Fungi on Mulch in a Flowerbed
Bird’s Nest Fungi on Mulch in a Flowerbed

StinkhornsSometimes when people dig in their gardens they encounter large (an inch or larger in diameter), soft, egg-like structures in the soil.  Folks often think these are actual bird or reptile eggs.  However, if you cut the “eggs” through the center along the long axis, you discover inside a mushroom-like structure with a stalk and an oftentimes honeycomb-like cap.  These mushrooms enlarge (eventually causing the “eggs” to “hatch”) and poke up above the soil surface.  These emerging mushrooms often resemble a certain, uh-hem, part of the male anatomy.  I have had a number of amusing conversations over the years as clients have attempted to describe stinkhorns without naming the offending body part.  The name stinkhorn comes from the fact that as they produce spores, these fungi tend to smell pretty rank.  The foul small (akin to rotten meat) attracts certain insects (e.g., flies) that pick up the spores and move them long distances.  Again, this is another amazing dispersal mechanism.

Stinkhorn "Eggs"
Stinkhorn “Eggs”
Inside the Stinkhorn "Eggs"
Inside the Stinkhorn “Eggs”

Slime MoldsSlime molds are not true fungi, but members of a unique group of organisms called the myxomycetes.  These organisms spend part of the lives in an amoeba-like stage (so in a sense are animal like), but when they reproduce, form spores the way true fungi do.

The most common slime mold I encounter is one I affectionately call dog vomit slime mold, but is more commonly called scrambled egg slime mold.  This slime mold really looks like someone’s dog upchucked in your flowerbed.  Watch for this slime mold (usually in the genus Fuligo) after you’ve spread a new layer of mulch on your flowerbeds.

Dog Vomit Slime Mold on Mulch in a Flowerbed

Another slime mold you may encounter is Physarum which is often found on turfgrass.  I remember seeing this slime mold in a lawn walking home from work one day.  I thought that someone had been out on their lawn spray painting some furniture and left gray paint residue on their grass.  On closer inspection however, the gray turned out to be thousands of small, spherical fruiting bodies (i.e., reproductive structures) of the slime mold.  I have also seen this slime mold crawl over and fruit on other plants such as ginseng and tomato.

Physarum Slime Mold on a Tomato Leaf
Physarum Slime Mold on a Tomato Leaf

Perhaps the most dramatic slime mold I see is the chocolate tube slime mold (Stemonitis), which often fruits on the walls of houses.  It can form HUGE masses of what appear to be clusters of miniature cattails.  One of my friends saw this slime mold and claimed it was straight out of Stranger Things (the horror television series I’ve been binge-watching of late).  There’s no accounting for taste!  Sigh.  Slime molds will ever remain cool in my book.

Chocolate Tube Slime Mold on a House Soffit
Chocolate Tube Slime Mold on a House Soffit

For addition information on the PDDC and its activities and the cool disease samples I get to see, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.

June: June is Bustin’ Out All Over (With Plant Health Problems)

SunWith the arrival of June, the plant disease floodgates have burst and the PDDC has been deluged with plant specimens with a wide range of plant health issues.

Probably the most striking samples showing up at the clinic right now are those that are showing symptoms of winter/cold injury.  I continue see samples from trees and shrubs where the entire plant has died over the winter with no signs of life this spring.  In particular, I have seen (and have also heard about) burning bushes that did not survive the winter.  While my clients have been distraught over this death and destruction, I have been trying to offer a silver lining (shocking, I know for Dr. Death!).  I point out that burning bush is classified as invasive plant in Wisconsin and that the death of these plants provides an opportunity to replace the shrubs with something more exciting and environmentally friendly.

When entire trees and shrubs have not died over the winter, in many cases parts of them have.  In particular, I have seen boxwoods, arborvitaes and other evergreens where branch tips have died and bleached over the winter.  This is a fairly classic symptom of winter burn/winter injury that I see every year.  This symptom might be a consequence of direct cold injury to tissue (a distinct possibility due to the extreme cold temperatures that we experienced in late January) or due to loss of moisture (from lack of sufficient water in the fall and/or exposure to dehydrating winds over the winter).  Slightly more subtle winter injury comes in the form of deciduous trees that leaf out, typically producing small leaves that then collapse and dry up.  I have been seeing this a lot on fruit trees (e.g., apples, pears, plums, cherries).  In these situations, there was likely sufficient internally stored water in the trees to initiate bud break and start leaf expansion, but also enough cold injury to the vascular (i.e., water-conducting) tissue to limit subsequent water movement into the leaves to further expand them and keep them alive.  I expect to see this collapse of leaves continue throughout the summer.  Many folks see this dieback on apples and pears and assume the problem is fire blight, but I have yet to diagnose this disease so far this year (and I’ve been trying very hard to find it).  Management for this type of dieback is simple pruning.  I suggest pruning four to six inches below obviously dead areas on branches.  The best time to prune for most trees and shrubs tends to be in the winter, but if you need to prune during the summer, be sure to prune only when it is dry.  Even though I don’t believe the dieback that I have described above is disease related, just to be safe, I still recommend that you disinfest pruning tools between cuts by dipping them in 10% bleach or (even better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants can be used as a source of alcohol as well.  I do not recommend using pruning paints except if you are forced to prune an oak tree during the growing season (to prevent transmission of the oak wilt fungus via sap beetles).  You can dispose of branches by taking them to your municipal yard waste recycling center (if you have one available), burning them (where allowed by local ordinance) or burying them.

Another variation on leafing out that I have seen this year that I am attributing to cold injury is where trees leaf out completely, but have smaller than normal leaf size.  In these situation, I am suspicious that there is minor damage to the vascular tissue the trees, but not sufficient to totally prevent water from reaching branch tips.  I have a redbud tree in front of my home showing this symptomology.  The tree typically has large, lush foliage that provides a privacy screen for my front door.  This year the leaves are quite small and I can easily see through the thin canopy.  This tree also had much reduced flowering this year compared to previous years.  I have also seen smaller leaves on a lot of maples (particularly silver maples) this year, but these trees have shown excessive seed production compared to other years.  I call this overflowering/seeding phenomenon the “Oh my gosh, I’m going to die, I’d better reproduce” syndrome.  Over-flowering/seeding is typical for stressed trees.  However, because this year’s flower buds were formed last summer, the underlying stress that led to over-flowering/seeding was not our winter weather, but other stresses that occurred during the growing season of 2018.  For trees with smaller leaves, I suggest making sure they are adequately watered.  I typically recommend that trees and shrubs receive approximately one inch of water per week.  If Mother Nature doesn’t cooperate, then I suggest setting up a soaker or drip hose at the drip line of the tree (i.e., the edge of where the branches extend) and applying whatever additional water is needed.  Affected trees should continue to receive water until they start to turn their normal fall color in the autumn.

And if all of the environmental stress-related issues aren’t enough, I’ve been getting inquiries about (and finally receiving samples of) what appears to be anthracnose on maples.  I expect to eventually see this disease on other trees as well.  Typical symptoms of the disease are brown to black, necrotic (i.e., dead), blotchy areas on leaves.  Our wet spring weather has been very favorable for this disease to develop.  Luckily, anthracnose tends to be a very cosmetic disease and typically causes little long-term damage to trees.  Cultivating the ability to ignore anthracnose symptoms and doing good fall clean-up of the leaves and removing them from your property typically are the best ways to handle this disease.

Phew, what a June!  Now, onward to July.  I have a feeling the deluge is going to continue!

For addition information on the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.

P.S.:  Brownie points for those of you who recognize the origin of the title of this month’s article.

May: Heinous Hitchhikers – Purchased Plants as Pathogen Providers

As we get into May, many gardeners begin thinking about buying perennials to replace plants that have died over the winter, or annuals to fill decorative pots and hanging baskets.  Being the optimist that I am (NOT), whenever I’m visiting my local nursery or garden center, I’m always evaluating plants as potential sources of plant pathogens.  For me, having diseased plants can be good (demo plants for classes and workshops, anyone?).  For sane, rationale gardeners however, avoiding potentially diseased plants is a must.  Here on some pointers on what you can do to try to minimize the likelihood that you will bring home unwanted guests as you garden.

  • Buy plants from a reputable business. Most businesses want to sell a good product.  The livelihood of a nursery or greenhouse depends on the quality of the products that it sells.  If a business sells poor plant materials, this reputation will get around and the business will likely not last long.  One way to decide on where to shop is to check with friends or colleagues on where they have purchased high quality plants in the past.  Word of mouth is often the best recommendation for a business.  That said, keep in mind that sometimes even the best, most conscientious plant producers/sellers can have disease problems.  In the past, diseases like Ralstonia wilt and impatiens downy mildew have been serious, and economically devastating diseases, for producers/sellers and no one has been immune to these disease issues.
  • Buy locally, when possible. Locally produced plants are often better adapted to the local climate, which can translate into better survival long term for perennial plants.  For annuals, plants grown in southern regions are often more likely to be exposed to pathogens earlier during production, with more opportunities for infections to occur.  These pathogens can travel north with plants as they are shipped into Wisconsin for sale.  Introductions of the Southern blight fungus and late blight pathogen have occurred in this manner in the past.
  • Avoid plants showing disease symptoms. Look carefully for any abnormalities in plant size, growth, or color that might indicate disease issues.  Common disease symptoms can include necrotic (i.e., dead) areas on leaves that might indicate a fungal, bacterial, nematode, or even viral  Also watch for lightning bolt-like line patterns, ringspots, or just blotchy light and dark patches on leaf tissue.  These symptoms are typical of viral diseases such as tobacco rattle, cucumber mosaic and hosta virus X.  Be sure to pop plants out of their pots to inspect the roots.  Roots should be plentiful and white.  If the roots are few and far between, or even worse, brown, then root rots could be a problem.  If you see abnormalities of any kind, DO NOT buy the symptomatic plants.
  • Avoid plants showing signs of disease-causing organisms. Some types of pathogens, particularly fungi and water molds, can produce spores on plant surfaces that will be visible to the naked eye.  Typical diseases where pathogens might be visible include powdery mildews and downy mildews, including the infamous impatiens downy mildew and basil downy mildew.  If you see any indication of this sort of growth, again DO NOT buy the plants.
  • Avoid plants with insects. Insects can cause damage to plants on their own through their feeding activities, so it is important not to bring home these pests with your plant purchases.  Insect pests can spread to other plants in your garden and cause substantial damage on their own.  From a disease standpoint, insects are important because they are plant pathogen vectors, moving disease-causing organisms from plant to plant as they feed.  There are insects that are known to move fungal, and bacterial pathogens in the environment, but where insects tend to have their biggest impact is through movement of viral and phytoplasma  In particular, aphids and thrips are important in moving viruses such as Cucumber mosaic virus and Impatiens necrotic spot virus from plant to plant.

With a little bit of effort and by using good observational skills, you can minimize the risk of bringing diseased plants into your garden.  However, even if you follow the advice outlined above, purchasing plants is not totally risk-free.  Sometimes plants harbor disease-causing organisms with nary a symptom nor sign in sight.  These pathogens may rear their ugly heads and start to cause problems once you’ve begun growing the plants in your garden.  Even if you dodge the bullet and successfully avoid purchasing infected plants, know that Mother Nature has tricks up her sleeve to bring plant pathogens to you.  So expect at least a little bit of disease, no matter how careful you are.  In the end though, plant diseases tend to be the exception and not the rule, so remember that most of time when you look at your garden, what you will see will be healthy and happy plants.  KEEP ON GARDENING AND LOVE EVERY MINUTE OF IT!

For addition information on the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.

 

Bagging Apples for Insect and Disease Control

Producing high quality apples in home gardens can be challenging due to damage caused by insects (e.g., apple maggot, codling moth, plum curculio, stinkbugs) and fungal diseases (e.g., apple scab, cedar-apple rust, flyspeck, sooty blotch).  Many insects damage apples when they lay their eggs in developing fruit.  Insect larvae can cause

Bagging apples can reduce fruit damage due to insects and diseases. (Photo courtesy of Janet van Zoeren)
Bagging apples can reduce fruit damage due to insects and diseases. (Photo courtesy of Janet van Zoeren)

additional damage as they tunnel into the fruit.  Wind-borne fungal spores can land on fruit leading to infections that damage fruit, reduce fruit aesthetics or affect long-term storage.

Although insecticide and fungicide sprays can help control insect pests and diseases, regular spraying can be inconvenient and costly for homeowners.  A non-pesticide alternative for protecting fruit is to encase apples in bags that provide protective barriers against insects and fungal pathogens.

What do I need to bag my apples?  First, you will need some sort of bagging material.  This could be a household plastic or paper bag, or a commercially-produced bag designed specifically for apple bagging.  Perhaps the most convenient choice is a common plastic sandwich or quart-size zip-type bag.  Zip-type bags are effective, weather proof, economical, and readily available.  You will also need something (e.g., a twist tie, tape, string, staples) to secure the bags to your apples and a pair of scissors to cut a drainage hole in each bag.

When do I bag my apples?  Start bagging when fruits are approximately ½ to 3/4 inch in diameter.  Apples typically reach this size approximately two weeks after petal fall.  Thin each cluster of apples to a single fruit, keeping the largest, best-shaped fruit in the cluster.  Make sure that the apples you select have not already been damaged.  In particular, plum curculio can damage fruit and codling moth can lay eggs before fruit are large enough to bag.  You may want to consider using an insecticide spray between petal fall and bagging to prevent this early damage.

How do I bag my apples?  Place an apple in the bag of your choice with the top of the bag around the stem.  Carefully secure the bag with a twist tie, tape, string or staple without damaging the stem.  If using a zip-type bag, place the stem in the middle and close the seal to within one inch of the stem on each side.  Staple the bag on each side of the stem to ensure that the bag will remain secure all summer.

Use scissors to cut approximately one-half inch from one of the bottom corners of the bag. This will allow condensation that may form inside the bag to drain.

Leave plastic bags on all summer.  Paper bags may deteriorate and need replacement if there is excessive rain during the growing season.  In addition, paper bags should be removed a few weeks prior to harvest to allow proper fruit color to develop.

Bagging apples can help prevent damage due to insect pests such as codling moth (left) and diseases such as apple scab (right). (Photos courtesy of Christelle Guédot and the UW-Plant Disease Diagnostics Clinic).
Bagging apples can help prevent damage due to insect pests such as codling moth (left) and diseases such as apple scab (right). (Photos courtesy of Christelle Guédot and the UW-Plant Disease Diagnostics Clinic).

What should my apples look like after they have been bagged?  Because bags prevent insects and fungal spores from reaching fruit, most of your bagged apples should be in near perfect condition.  Fruit that you do not bag will likely be blemished from insects and disease.  Blemished fruits should be removed (whether they fall to the ground or remain attached to your tree) and destroyed by burning (where allowed), deep burying or hot composting.  Diseased leaves from your apple tree should be treated similarly once they fall from the tree in the autumn.  Properly disposing of blemished fruits and diseased leaves will help limit overwintering of insect pests and disease-causing organisms, thus reducing insect and disease problems the following growing season.

For more information on bagging apples for insect and disease control:  Contact your county Extension agent.

African Fig Fly

The African fig fly (AFF), Zaprionus indianus, is an invasive vinegar fly closely related to flies in the genus Drosophila [which includes the common vinegar fly (also sometimes called the common fruit fly) and spotted-wing drosophila (SWD)].  AFF is native to Africa, Eurasia and the Middle East.  It was first detected in the United States in Florida in 2005, where it is now well established.  In 2012, AFF (a single adult) was first detected in Wisconsin.  In 2017, larger numbers of AFF were found in the state in traps used to monitor for SWD.

An African fig fly adult. (Photo courtesy of Benjamin Davidson)
An African fig fly adult. (Photo courtesy of Benjamin Davidson)

Outside of Wisconsin, AFF has had its biggest economic impact in commercial fig production, and has not been found to have a significant economic impact on other agricultural crops.  In Wisconsin, the potential economic impact of AFF on fruit and other crops is not yet known.  Because AFF reproduces readily and can rapidly expand its range, careful monitoring for this insect in the state is warranted.

Appearance:  The adult AFF is slightly larger than the common vinegar fly or SWD, and has a light-colored body and bright red eyes.  An adult AFF has four black-bordered, white stripes that run along each side of its head and thorax (but not its abdomen).  These stripes can easily be seen using a hand lens and are not found on any other vinegar flies in Wisconsin.  An AFF larva (maggot) is legless and has a single hook-like tooth at its mouth-end.  The maggot uses this tooth to feed on rotted berries and other soft fruit tissues.  AFF maggots are nearly indistinguishable from maggots of common vinegar flies and SWD.

Host Range:  AFF can feed on a large number of fruit crops grown in Wisconsin, including blackberries, blueberries, grapes, peaches, raspberries and strawberries, with grape being a preferred host.

Symptoms and Effects:  Given that AFF is a newcomer to Wisconsin and its temperate climate, predicting which fruits the insect will affect, and to what extent, is difficult.  What is known about AFF is that it cannot initiate damage to intact, ripe fruit; it can only feed on and lay eggs inside of overripe or already damaged fruit.  The insect contributes to additional breakdown and decay of these fruit.  In Wisconsin, AFF and SWD (which can damage intact, ripe fruit) will likely occur together at a given location, and will likely interact.  AFF may end up laying eggs in fruit damaged by SWD, possibly leading to an increase in AFF populations.  The effects of AFF on SWD populations are not clear at this time.  However, some research conducted in grapes suggests that where AFF and SWD occur together, they may compete, leading to increased numbers of AFF and lower numbers of SWD.

Life Cycle:  As an insect native to the tropics, AFF will likely not be able to survive even a mild winter in Wisconsin; in fact, AFF does not appear to overwinter even as far north as Virginia.  However, AFF can travel long distances on air currents, and could potentially travel from the southern US to Wisconsin in the spring and summer, leading to fruit infestations in the state in the summer and fall.  AFF has only been detected in Wisconsin in 2012 and 2017, suggesting that if the insect does cause problems in Wisconsin, these problems will not likely be a yearly occurrence.

Exactly how many generations of AFF might develop in Wisconsin in a given year is not known.  In the laboratory (at 75°F), AFF develops from egg to adult in 20 days and females lay, on average, 60 eggs.  Based on its short generation time and ability to produce numerous eggs, AFF populations could potentially build quickly within a given year, once the insect arrives in Wisconsin.

Monitoring:  Monitor for AFF using the same traps that you would use to monitor for SWD.  You can buy commercially available traps, or alternatively, you can make simple, inexpensive traps using a 32-ounce clear plastic container with a lid.  To allow adult flies to enter the trap, drill or melt ten 3/16 inch diameter holes around the top of the container.  Bait traps with one tablespoon active dry yeast and four tablespoons sugar in twelve ounces of water.  Alternatively, use a two-inch layer of apple cider vinegar in the bottom of the container.

Add one to two drops of unscented dish soap to the trap to break the surface tension of the liquid so that flies are more likely to

A homemade vinegar fly trap made from a 32-ounce clear plastic container with a lid. (Photo courtesy of Emma Pelton)
A homemade vinegar fly trap made from a 32-ounce clear plastic container with a lid. (Photo courtesy of Emma Pelton)

drown.  Hang traps in a shaded plant canopy where fruit is present.  For strawberries, place traps on the ground.  Check traps and replace liquid bait weekly, using a hand lens to identify trapped insects.  Be careful not to pour bait on the ground near the traps as this will confuse flies and reduce the effectiveness of the traps.  DO NOT bother testing fruit directly to determine infestation rates for AFF, as AFF larvae will be indistinguishable from those of other vinegar flies (e.g., common vinegar fly and SWD).

Control:  Because AFF cannot cause damage to sound fruit, and because recorded populations of AFF in Wisconsin have been low, AFF control may not be needed unless/until severe outbreaks occur.  Control methods recommended for SWD will likely also help control AFF.  In particular, cultural practices such as removing all damaged or rotting fruit and then solarizing (heating) or freezing this material to kill fly larvae may slow buildup of AFF at a given location in a given growing season.  However, because AFF is able to move over long distances from areas in the south where it can easily overwinter, reintroduction of the insect will be possible even if you follow strict sanitation practices.

Exclusion netting, which has been used with some success to manage SWD, will likely also help manage AFF.  When using netting, use a mesh size that is 1/32 inch or smaller.  Start using netting after early fruit set (so as not to interfere with pollination), but before fruit begins to ripen.  Make sure the mesh completely covers fruit with no gaps between sheets of netting or at the soil surface.  Any opening can provide an entry point for AFF.

Chemical control for AFF is not currently recommended in Wisconsin.  Observed AFF populations have not been high enough to warrant such treatments.  If spraying for AFF does become necessary, choose insecticides (e.g., spinosyns, carbamates and organophosphates) that also provide control of SWD.  As always, rotate active ingredients that are in different IRAC chemical classes (i.e., have different modes of action) to delay development of insecticide resistance.  For details on IRAC chemical classes, see https://www.irac-online.org/modes-of-action/.  Also, be sure to consider the effects of the products that you select, on non-target insects, particularly beneficial insects such as bees.  Please check the most recent Midwest Fruit Pest Management Guide (see https://learningstore.uwex.edu) for full product recommendations.

If you suspect that you have found an AFF, please contact the University of Wisconsin-Madison Insect Diagnostic Lab at (608) 262-6510, idl@entomology.wisc.edu or http://labs.russell.wisc.edu/insectlab/contact-us/.

For more information on African fig fly, as well as other vinegar flies:  See University of Wisconsin Garden Facts XHT1102 (Fruit Flies in the Home), University of Wisconsin Garden Facts XHT1237 (Spotted Wing Drosophila), or contact your county Extension agent.

April: To Prune, or Not to Prune, That is the Question.

ShearsAs warmer weather has spread through much of Wisconsin, I have talked with more and more gardeners who are chomping at the bit to get into their yards and start the 2019 gardening season.  One of the activities these gardeners are contemplating is the pruning of their trees and shrubs.  They often ask me whether spring is a good time to prune from a plant disease perspective.

The answer is:  It depends.

Overall, I am a proponent of winter pruning.  Typically there is a slight warming at the end of January or at the beginning of February where it’s warm enough to prune and not freeze to death, but not warm enough that disease-causing organisms are likely to be active.  In my mind, pruning during this window reduces the risk of pathogens infecting through pruning wounds.  There can be exceptions to this rule of thumb however.  There is some research that indicates that pruning honey-locust trees in the summer (during hotter, drier periods) can reduce the risk of Nectria canker compared to pruning in the winter.

When pruning in the spring, the trees that I have the most concern about are oak trees.  In particular, I worry about transmission of the oak wilt fungus, Bretziella fagacearum.  This fungus can be moved from tree to tree by sap beetles that become active as temperatures warm.  These insects are attracted to wounds, including those caused by pruning.  Some municipalities restrict pruning of oaks after April 1 in anticipation that temperatures will shortly be warm enough that sap beetles will be active.  Use of a calendar date as a cut off for pruning oaks can be problematic however if spring arrives early.  For that reason, I really ONLY recommend winter pruning for oaks.  If for some reason, you really need to prune an oak at another time of the year, paint over any pruning wounds.  You can use a commercial pruning paint if you like, but left over latex paint (from painting a room in your home) will work just as well.  The paint provides a physical barrier that makes a wound less attractive to sap beetles.  Be sure to paint wounds on oak trees IMMEDIATELY.  There is research that indicates that sap beetles can visit wounds in as little as 10 minutes.

In general, when pruning tress other than oaks in the spring (or any time other than the winter), always be sure to prune when it’s dry and when there is dry weather predicted for several days post pruning.  Dry weather is less favorable for fungal spores (which might land on a pruning cut) to germinate and infect.  Also be sure to prune properly based on the type of tree or shrub.  Laura Jull of the UW-Madison Department of Horticulture has authored several excellent fact sheets on how to prune evergreens, deciduous trees and deciduous shrubs.  Check these out!!  When pruning out diseased branches, prune four to six inches below obviously diseased areas if you suspect a fungal disease and 12 inches below obviously diseased areas if you suspect a bacterial disease.  In the best of all possible worlds, you should decontaminate your pruning tools between every cut to limit possible movement of pathogens via your tools from branch to branch or from tree to tree.  You can use a 30 second dip in 70% alcohol (e.g., rubbing alcohol) or in a commercial disinfectant for this.  Alternatively, you can use a spray disinfectant, spraying your tools until they drip and then allowing them to air dry.  Except in situations where oaks are pruned during the growing season, I do not recommend using paint on pruning cuts.  Paints tend to slow down the formation of callus tissue, the tissue plants produce to naturally cover over wounds.

Finally, avoid what I tend to try to do when I prune, which is to prune off my fingers.  OUCH!!

For addition information on plant diseases, the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.

Silver Leaf

What is silver leaf?  Silver leaf is a fungal disease that affects a wide range of deciduous trees.  The disease has its biggest impact in fruit trees such as apple, pear and cherry, but can also affect ornamental trees such as willow, poplar, maple, oak, and elm.  Silver leaf has traditionally been considered a disease of older trees that have been physically damaged or are in decline due to other diseases.  However, beginning in 2017, severe cases of silver leaf have been observed on young, healthy apple trees in commercial orchards in Wisconsin.

Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).
Young, vigorous high-density apple trees, with trees showing symptoms of silver leaf (on the right) adjacent to those that do not (on the left).

What does silver leaf look like?  The first symptom of silver leaf is a silver sheen that appears on leaves of affected trees. The number of leaves affected can vary dramatically from tree to tree.  The silver sheen develops when the epidermis of a leaf (i.e., the surface layer of cells) separates from the rest of the leaf, altering the way that the leaf reflects light.  The silvery leaves may also have brown, dead patches.  Leaf symptoms may appear one year, but may be less severe or even nonexistent in subsequent years.

Note that other tree stresses (particularly environmental stresses) can cause leaf symptoms similar to those of silver leaf.  An additional symptom that can help in identifying silver leaf is dark staining just under the bark of branches with symptomatic leaves. This staining can extend several inches down a branch.  Eventually, white edged, purple-brown, shelf-like conks (reproductive structures of the fungus that causes the disease) will appear on branches and/or trunks of the diseased trees.

Where does silver leaf come from?  Silver leaf is caused by the fungus Chondrostereum purpureum.  Spores of the fungus are released from conks during wet periods in the autumn and spring and infect trees at pruning scars or other open wounds (e.g., wounds from branches breaking during severe storms or due to heavy, wet spring snow). The fungus lives in the xylem (i.e., the water-conducting tissue) of infected branches, and its presence in the xylem leads to the dark staining as described above.  A toxin released by the fungus moves up into the leaves causing the epidermis separation that leads to the silver sheen of the leaves. Eventually, wood in infected branches begins to decay, at which point the fungus starts producing conks.

How do I save a tree with silver leaf?  On trees with limited damage, prune out branches showing leaf symptoms.  Also watch for any conks, and immediately remove branches where these are present.  Removing conks limits production of spores that can lead to infections in other trees.  When pruning, cut branches at least four inches below where you can see staining under the bark or where conks are visible.  Decontaminate pruning tools after each cut by dipping them for at least 30 seconds in 70% alcohol, a commercial disinfectant or 10% bleach.  You can also use spray disinfectants that contain approximately 70% alcohol.  Apply sprays to tools until they drip and then allow the tools to air dry.

The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).
The silver sheen of leaves typical of silver leaf (left) and conks (i.e., reproductive structures) of the silver leaf fungus, Chondrostereum purpureum (right).

In plantings where silver leaf symptoms are widespread, pruning out all symptomatic branches may not be practical, and the loss of that many branches might cause more harm than good. Also, trees sometimes show symptoms one year but then appear to recover in subsequent years. Therefore, instead of pruning symptomatic branches, consider marking diseased trees.  Carefully watch the marked trees each year to see if symptoms reoccur, or if the trees lose vigor. If trees lose vigor and/or conks become visible, then the trees should be removed.  Because the silver leaf fungus limits water movement in infected branches, make sure that affected trees receive adequate water.  In general trees should receive approximately one inch of water per week during the growing season from natural rain and/or irrigation.  Eventually infected trees will likely decline to the point where they should be removed.  In some instances, monitoring trees may not be feasible.  In such situations, removing trees the first year that they show silver leaf symptoms may be the best management option.

Any branches or trunk sections removed from trees with silver leaf should be disposed of by burning (where allowed) or burying.

How do I avoid problems with silver leaf in the future?  Whenever possible, prune trees during the winter during dry periods when temperatures are below 32°F.  If you must prune during the growing season, only prune during dry periods.  Pruning at these times will decrease the risk of infection by the silver leaf fungus through pruning wounds. DO NOT use pruning paints or sealants when pruning. At this time, there are no fungicides for silver leaf control.

March: Viral Villains – Gruesome Guests for Indoor Gardeners and Greenhouse Growers

Recently, I have seen an increase in conservatory and greenhouse-grown plants arrive at the PDDC.  It’s certainly the time of year that greenhouses gear up their plant production in anticipation of spring sales (assuming that spring is going to arrive this year – I have my doubts).  There are several viral diseases that I routinely see in home and greenhouse-grown plants that, if undetected, can spread easily and pose challenges for both indoor and outdoor gardeners.

CymMV Orchid
CymMV on an Orchid

I recently received several orchid samples from a local conservatory.  As it turned out, several of the plants were infected with Cymbidium mosaic virus (CymMV), an orchid specific virus.  In some orchid species, CymMV causes few, if any symptoms.  In other orchid species, a typical symptom is the appearance of necrotic (i.e., dead) leaf spots, symptoms that in most other plants I would attribute to fungal or bacterial pathogens.  Over the years, I’ve learned that with orchids, testing right away for viral pathogens like CymMV, particularly when there is leaf spotting (and oftentimes even when there isn’t), is a good idea.  Luckily, I have a quick, easy-to-use serological dip stick test (the plant virus equivalent to a home pregnancy test) to test for CymMV. When plants test positive, my recommendation is to throw out the infected plants.  It’s too easy to accidentally move viruses around in plant sap that you get on tools or even your hands when you are trimming leaves or deadheading flowers.  Once infected plants are removed, it’s important to decontaminate items that may have come into contact with the plants.  For details on what to use, check out the recipes in the University of Wisconsin Garden Facts on Hosta virus X (HVX).  HVX is another common plant virus, albeit on hostas rather than orchids.

INSV Begonia
INSV on Begonia

I also recently received a Lysimachia sample with a viral problem that turned out to be a bit more of a challenge to diagnose.  The plants came from a commercial greenhouse.  I noted that the edges of the leaves were dead and also noted damaged areas elsewhere on the leaves.  Some of the damage seemed to be consistent with that due to thrips feeding.  This sent up a red flag, as thrips can carry plant viruses like Impatiens necrotic spot virus (INSV) and Tomato spotted wilt virus (TSWV).  These viruses can infect a wide range of plants, can spread quickly (it doesn’t take a lot of thrips) and can cause significant economic loss.  I used dip stick tests for INSV and TSWV, as well as for two other common plant viruses [Cucumber mosaic virus (CMV) and Tobacco mosaic virus (TMV)], but all of these tests were negative. To double-check that my thoughts about possible thrips damage were reasonable, I showed this sample to PJ Liesch, the UW-Madison/Extension insect diagnostician.  PJ verified the presence of a small number of thrips in the sample, but indicated that the brown leaf edges were not a typical symptom of thrips feeding.  PJ was on vacation when this sample arrived and he wasn’t able to look at the sample for about a week after submission.  By that time, I had noticed that the plants were developing additional symptoms including growth distortions and botchy color.  Everything was pointing to a viral problem.  Given PJ’s verification of thrips, I again tested the sample for INSV and TSWV, and lo and behold, this time the sample tested positive for INSV.  At that point, everything fell into place and I reported back to my client that I thought INSV was the primary issue with the plants, and recommended plant removal and decontamination.  The conflicting results that I got with this sample point out a difficulty in confirming viral pathogens.  Dip stick tests require a certain amount of a virus to be present in a sample to get a positive reaction and the amount of a virus in a plant can vary both in terms of the age of the plant part being tested (old vs. young leaves), as well as how long the plant has been infected.  Testing symptomatic tissue of different ages, as well testing more than once over a period of several days, can be critical in making an accurate diagnosis.

TMV on Tobacco

Finally, the virus that I haven’t yet seen this year (and that I don’t really want to see) is TMV.  This virus has a particularly wide host range and is particularly nasty given how easily it can be moved around.  You can pick up TMV on your fingers as you handle infected plants and transmit the virus by touch as you handle health plants.  TMV is a very stable virus.  It not only can be found in live plants, but can also be found in dead/dried plant tissue, including dried and processed tobacco.  If you smoke or use chewing tobacco, you are at increased risk of picking up this virus and spreading it around.  TMV can also hang around on inert items (e.g., clothing, boxes, work surfaces, tools) and can eventually be moved from these items to plants.  TMV, given its stability and easy transmission, is one of the most problematic and destructive viruses that I know of.  The growth distortions and blotchy color (i.e., mosaic) caused by TMV make infected plants unmarketable.  Destroying infected plants, and carefully and systematically decontaminating anything that has come into contact with infected plants is a must to get this virus under control.  And if you are thinking of quitting smoking or chewing tobacco, and need another reason, this virus is it.

For addition information on the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.

February: Wacky Wisconsin Winter Weather

After last week’s sobering, subzero weather, I began to think about all of the ways winter in Wisconsin is designed to make the life of plants, particularly woody ornamentals, difficult.

Snow IconSnow:  Snow can be a mixed blessing.  I like to see a few inches of snow on the ground, because snow actually has an insulating effect.  Without a snow layer, soil temperatures can become quite cold far down into the soil profile and this can lead to cold injury to plant roots.  Such injury can outright kill plants, or alternatively lead to a slow, painful death where plants leaf out, but leaves rapidly dry up and die because there is a lack of a functional root system to take up water to supply to the emerging leaves.

Excessive snow can also be problematic.  In particular, I see situations where white cedars (arborvitaes) become so laden with snow that a variety of problems can arise.  In extreme instances, the weight of the snow may be so heavy that branches will snap.  In other instances, the snow simply causes the branches to bend downward.  This may seem innocuous, but if the snow doesn’t melt away and the plants are weighted for an extended period, branches may not spring back into their normal position and the shrubs end up deformed.

Cold IconCold Temperatures:  Late January’s deep freeze here in Wisconsin likely led to significant plant damage.  Plants have particular ranges of temperatures that they can tolerate (usually described in terms of their USDA Hardiness Zone).  If temperatures drop outside of this optimal range, physical injury to branches and trunks, and even plant death can result.  As I mentioned above, lack of snow cover can make cold injury worse by allowing for additional cold injury to roots.  Another contributing factor can be that many gardeners want to “push the envelope” and grow plants (often exceptionally beautiful trees and shrubs) that are not rated for their hardiness zones, but for warmer environments.  Often these marginally hardy plants will do well for many years, until they experience an extreme winter.  We’ll have to wait to find out how severe this winter’s damage has been until spring arrives.  At that point, we’ll be able to see how many trees and shrubs don’t leaf out or are stone dead.  I’m expecting plants like Japanese maple, magnolia, redbud and many types of fruit trees (particularly peach and apricot) to be hardest hit.

Ice IconIce:  Now that temperatures have warmed a bit, I’m seeing some areas of Wisconsin experiencing “wintry mixes” of precipitation.  Often this means freezing rain, which can coat branches and, depending on the duration of the rain and the specific temperature, lead to thick layers of ice that can be so heavy that they cause branches to break.  Some gardeners, in an effort to prevent this breakage, attempt to knock ice from branches, but this technique runs the risk of damaging overwintering buds.  I have fond memories (read EXTREME sarcasm here) of an ice storm in the mid-1970’s that caused substantial tree damage, paralyzed Madison and left my family without power for five days.  We spent a lot at the mall.

Wind IconWind:  High winds can cause extensive plant dehydration.  I most commonly see this as a problem on evergreen trees and shrubs.  I think of windy conditions, alone or in concert with cold temperatures, as being particularly damaging on Alberta spruce, boxwood and white cedar (arborvitae).  Watch for brown or bleached needles and branch dieback on these plants, particularly as they come out of dormancy in the spring.  This damage in often referred to as winter burn.  In extreme situations, high winds can physically damage and break off branches.

Now that I’ve totally depressed you by considering all of the possible adverse effects of winter weather, let’s try to put a sunny spin on things and think of the upside:  All of the winter death and destruction provides ample opportunities to plant new and exciting trees and shrubs (and even herbaceous plants), and watch these new plants grow and mature.  🙂

For addition information on the PDDC and its activities, check out the PDDC website, follow the clinic on Twitter or Facebook (@UWPDDC) or contact the clinic at pddc@wisc.edu.