As we move into late July and August, I typically see an increase in sample submissions for vascular wilt testing.  Vascular wilts are diseases where the pathogen (typically fungal or bacterial) invades the water-conducting tissue (called the xylem) inside a plant and leads to blockage of this tissue.  The blockage prevents water from moving from the roots to above-ground parts of the plant.  Without water, the plants wilt and typically eventually die.

In late July of 2017, I completed what I affectionately refer to as the “Triple Crown” of woody ornamental vascular wilts, confirming diagnoses of Dutch elm disease, oak wilt and Verticillium wilt all on the same day.

Dutch elm disease

Remember the days when streets were shaded by cathedral canopies of American elms?  Nope, I don’t either.  At 55, I arrived at the tail end of the period when virtually every street in the US was lined with American elms and when Dutch elm disease (DED) was in its heyday of killing pretty much every American elm in sight.  DED is a prime example of what can happen when a particular type of plant is grown in monoculture (i.e., in large numbers in close proximity to the exclusion of other plants) and a non-native (i.e., invasive) pathogen is introduced.

Two fungal pathogens (Ophiostoma ulmi and Ophiostoma novo-ulmi) cause Dutch elm disease and are thought to be of Asian origin.  They appear to have been introduced into US through Europe via imported wood during the late 1920’s to early 1930’s ( ulmi) and the 1940’s (O. novo-ulmi).  These fungi were subsequently spread by elm bark beetles (both imported European and native North American species) which introduced the fungi into (very susceptible) American elms as they tunneled into the trees to lay eggs.  To make matters worse, elms along American streets were root grafted (i.e., their roots were fused together), so the DED fungi, once introduced into an area by bark beetles, were able to rapidly move from tree to tree underground via these grafts.  Thus the disease decimated street after street of American elms across the US.

Elms (including true American elms) still exist in urban landscapes.  Some are “escapes” (American elms that are susceptible to DED, but in some way have avoided infection), some are true American elms that have been bred for resistance, and others are hybrids (usually American elms crossed with Asiatic elm species) again bred for DED resistance.  For large, susceptible American elms, routine (about every other year) fungicide injections can be used to manage DED.  Keep in mind however, that no management strategy is perfect and even resistant and treated elms can succumb to DED.

Oak wilt

I often think of oak wilt as the “Dutch elm disease of oaks” because there are many similarities between the two diseases.  The oak wilt pathogen (the fungus Ceratocystis fagacearum) is often initially introduced into an area via insect (several types of sap beetles can be involved).  These insects are attracted to wounded trees and, if they carry the oak wilt pathogen, can drop spores of the fungus off as they fed on sap oozing from wounds.  Because of sap beetle transmission, I typically recommend pruning oaks only when they are dormant.  In particular, the “January thaw” period in mid-winter is a good time to prune because it is warm enough so that you will not freeze to death while pruning, but not warm enough that the oak wilt fungus and sap beetles will be active.  If for some reason you need to prune an oak during the growing season, you should IMMEDIATELY paint over pruning wounds to make them unattractive to sap beetles (which can visit wounds within 10 minutes of their formation).  You can use a commercial pruning paint if you like, but latex paint (left over from painting the rooms of your house) will work as well.

Once the oak wilt fungus colonizes a tree, branches will begin to wilt and the tree will eventually die.  In wooded areas, the oak wilt fungus can move from oak tree to oak tree via root grafts, causing major tree loss.  Management of oak wilt in woodlots and forested areas typically involves establishing a perimeter around infected trees, trenching around this perimeter to sever root grafts and then removing oak trees within the trenched area.  In landscape settings, single, healthy specimen oaks can be treated with fungicide injections (typically made every two years) to help prevent infection.

Making oak wilt management a challenge is the fact that some people diagnose oak wilt based on visual symptoms.  I do not agree with this method of assessment, and personally will only diagnose oak wilt if I can grow the oak wilt fungus from symptomatic oak branches or trunk tissue, or detect Ceratocystis fagacearum DNA in this tissue.  The danger of visual diagnosis is that there are other diseases and insect problems that can cause branch dieback symptoms that somewhat mimic those of oak wilt.  In particular, I worry about misdiagnosis of Armillaria root disease as oak wilt, because trenching (advocated for oak wilt management) wounds roots and wounded roots are a primary entry point for Armillaria (the fungus that causes Armillaria root disease).  Another problem that mimics oak wilt is damage due to two-lined chestnut borer, an insect pest that tends to attack oak trees that are under stress.  Fungicide injections for oak wilt management are a waste of time, effort and money if the real problem is an insect pest such as two-lined chestnut borer.  Proper diagnosis is the first step in developing a successful disease and insect management strategy for oak wilt (or any disease for that matter).

Verticillium wilt

Verticillium wilt is disease that can affect a wide range of woody ornamentals including, but not limited to, maples (particularly Japanese and Korean maple), ash, redbud, magnolia, and smokebush/smoketree.  The disease can also cause problems in vegetables (e.g., potato, tomato, pepper, vine crops and especially eggplant) as well as herbaceous ornamentals (I diagnosed Verticillium wilt in purple coneflower just recently).  Verticillium (typically Verticillium dahliae) is soilborne and can be introduced into a location via contaminated soil, mulch (be cautious of using mulch composed of chipped up street trees that might have died from Verticillium wilt) or even leaves that have fallen from infected trees and been blown into an area.  The fungus infects through roots, colonizing and blocking the xylem, resulting in branch dieback.  In particularly susceptible trees (e.g., Japanese maples) and vegetables (e.g., eggplant), death can follow very rapidly.  Proper diagnosis of Verticillium wilt is important because if Verticillium is present at a location, use of Verticillium immune or resistant plants is the best method to prevent problems in the future.  That said, over the past three years, the PDDC has documented previously unreported hosts for Verticillium including seven-son flower, wafer ash, buttonbush and Eastern leatherwood.

For More Information:

Check out these Fact Sheets:

• Dutch elm disease
• Oak wilt
• Verticillium wilt:
  • of trees and shrubs
  • of vegetables

To receive updates on emerging diseases and their management,  follow the PDDC on Facebook and Twitter (@UWPDDC).

On July 26, late blight (caused by the water mold Phytophthora infestans) was formally diagnosed in Wisconsin for the first time in 2017.  The late blight sample was of infected tomato fruits from Waukesha County.  Late blight attacks both potatoes and tomatoes, and unchecked the disease can rapidly kill plants.  Late blight is the disease that caused the Irish potato famine in the 1840’s, resulting in the starvation of approximately 1 million Irish and the mass emigration of approximately another 1 million Irish, many to the US.

Nowadays, late blight can wipe out home garden tomatoes and potatoes, and can have a huge impact on fresh market tomato production.  Most importantly in Wisconsin however is late blight’s potential impact on commercial potato production.  Wisconsin is a leading potato producer in the US with greater than 60,000 acres in production in 2016.

Symptoms of late blight on potato and tomato leaves and stems typically appear as somewhat large, dark, oily areas, sometimes with a lighter border.

On tomato fruits, the disease often appears as large, leathery areas with somewhat wavy margins and sometimes visible concentric rings.  The underside of infected leaves will typically have a fuzzy white-gray appearance, an indication that pathogen is sporulating.  Sporulation also occurs on infected fruits, but can be more difficult to see.  The disease progresses rapidly and kills plants.  The pathogen can eventually also infect potato tubers where it can survive over the winter.

Because of the importance of late blight to Wisconsin agriculture, the PDDC offers free diagnosis of suspect late blight potato and tomato samples.  If you see anything that you think is late blight on potato or tomato (or even if you don’t have what you think is late blight on potato or tomato, but want to know what your potato or tomato problem is), send in a sample, invoke the words “late blight” and the diagnosis is free.  To submit a sample, place symptomatic leaves, stems, fruits, and/or tubers in sealable plastic bags.  DO NOT wrap the sample in wet paper toweling as this can accelerate the deterioration of the tissue and make diagnosis more difficult.  Place the bagged sample(s) in a sturdy box with lots of padding and mail the sample to the:

PDDC
Department of Plant Pathology
University of Wisconsin-Madison
1630 Linden Drive
Madison, WI  53706-1598

Be sure to include complete contact information (i.e., complete mailing address, phone number and email address).  If the sample is positive for late blight, PDDC staff will contact you via phone to provide guidance on how to manage the problem.  All submitters will receive a written report outlining any disease problems in their samples.  Samples positive for late blight will be forwarded to the lab of Dr. Amanda Gevens (the UW-Madison/Extension vegetable pathologist) for genotype testing.  There are numerous variants (genotypes) of Phytophthora infestans and knowing which variant(s) [or genotype(s)] are present in Wisconsin can provide critical control information for commercial potato and tomato producers.  Certain variants of Phytopthora infestans are resistant to certain fungicides; others are not.

To learn more about late blight and its management in home garden, check out the “Late Blight” University of Wisconsin Garden Facts in the fact sheets section of the PDDC website:

• Late Blight Fact Sheet – English
• Late Blight Fact Sheet – Spanish

Also, be sure to follow the PDDC on Facebook and Twitter @UWPDDC for updates on plant diseases such as late blight, as well as to learn about PDDC educational events.

For additional resources on late blight (particularly for commercial growers) see the following links:

• Late Blight Symptoms and Prevention – Poster
• Potato Late Blight Fungicides Registered for Wisconsin – Table
• Managing Late Blight in the Organic Tomato Crop – Research Paper
• UW Vegetable Pathology Late Blight Information – Links

In my June web article, I mentioned cedar-apple rust and other Gymnosporangium rusts as diseases that I expected to see a lot of this year.  This has certainly been the case over the last month.  What has surprised me (and I probably shouldn’t be surprised given our continuing wet weather) is the plethora of other rust diseases that I have seen in the clinic this season.  Here’s a rundown some of the more interesting examples of rust diseases that I’ve seen this year.

White pine blister rust

This is an alternating rust where the pathogen requires two different plants to complete its life cycle.  In this case the hosts are white pine and Ribes species (in particular gooseberries and currants).  The white pine phase of this disease reared its head in May in the Madison area (see my PDDC Facebook page post from May 20) and the Ribes phase of the disease should be in full force right now.  Watch for orange, powdery masses of spores of the undersides of gooseberry and currant leaves.  Management of this disease relies on not growing white pines and gooseberries/currants in close proximity.

Cedar-apple rust (and other Gymnosporangium rusts)

Gymnosporangium rusts have definitely had a big year in 2017 and like white pine blister rust are alternating rusts.  Check out apples, crabapples and hawthorns right now for characteristic yellow-orange leaf spots.  I posted photos of the juniper stage of this disease on the PDDC Facebook page back on May 20.  If our weather continues to be wet, I expect a banner year for the juniper stage of the disease next spring.  Management of Gymnosporangium rusts is also most successful when the two hosts are not grown near one another.

Crown rust

This alternating rust has been particularly dramatic on buckthorn this year (see photos posted on the PDDC Facebook page post on June 22).  There are several variants of the crown rust fungus and each variant has a specific grass alternate host.  The most common alternate hosts that I encounter are oats (in agricultural settings) and turfgrass (in urban settings).  If you need another reason to eradicate buckthorn (in addition to the fact that this plant is incredibly invasive), control of crown rust is that reason.

Orange rust

This rust requires just a single host (raspberry) to complete its life cycle, and leads to stunting of raspberry plants, yellowing of raspberry leaves, and eventual formation of masses of orange powdery spores on the undersides of leaves (see the PDDC Facebook page post on this disease from June 3 for photos).  Unfortunately the orange rust fungus becomes systemic in plants (surviving as fungal hyphae/threads) and management requires plant removal and destruction.

Mayapple rust

This is another single host rust, with the fungus completing its life cycle on mayapple.  Watch for angular (i.e., vein-bordered) yellow leaf spots with spores of the fungus produced directly beneath these spots on the undersides of the leaves.  The resting spores produced by this fungus are two-celled and spiny (see the PDDC Facebook page post from June 30 for photos).  This rust can be managed by a combination of removal of infected plants (as they go dormant for the season) and timely fungicide applications the following growing season as new plants emerge.

Bean rust

This is another single host (snap/pole beans) rust that I have seen in the past, but I have only ever seen the later stages of the disease where the fungus produces orange and brown-colored spores (called urediniospores and teliospores respectively).  This year I got to see (for the first time ever) a third type of spore produced by the fungus (called aeciospores).  These spores are white and are produced in rings of fruiting bodies (i.e., reproductive structures) on the underside of bean leaves (see the PDDC Facebook page post from June 30 for photos).  This disease is relatively uncommon in home gardens, most likely because many snap bean varieties have at least some resistance to the disease.

To learn more about plant diseases and their management, explore the Plant Disease Diagnostics Clinic (PDDC) website (https://pddc.wisc.edu/) or follow the PDDC on Facebook and Twitter @UWPDDC.

May has been a fairly wet month in many parts of Wisconsin.  When spring rains overlap with leaf emergence on broad-leafed trees and shrubs, expect leaf diseases to run rampant over the summer.  Some of diseases that I have already seen or I am expecting to see this year include:

Anthracnose

Anthracnose refers to a large group of fungal leaf diseases.  There are many different types of anthracnose fungi and they are somewhat host specific.  However, all of these fungi tend to cause irregular, blotchy necrotic (i.e., dead) areas on leaves.  If anthracnose occurs early (on leaves that are not fully expanded), leaves can become cupped and curled.  On some trees (white oaks come to mind), anthracnose can be so severe that it will cause defoliation, but typically these trees will releaf and by July, you would never know the trees had anthracnose earlier in the year.

Tar spot

This is a disease I most commonly see on maples.  The distinctive symptoms (either large, solid tar-like spots, or circular, diffuse clusters of smaller tarry spots) typically are not visible until later in the season.  However, check maple leaves (particularly on Norway maples) right now for small yellow spots that are a clue that infections have already occurred.  If you have a good (10X or 20X) hand lens, you may be able to see very small, tarry spots in the middle of the yellow areas.  Continue to watch for more spectacular symptoms to develop as the summer progresses.

Peach leaf curl

If you have a peach tree with curled, cupped and bubbly looking leaves, you have this disease.  The distorted leaves often have a pinkish and/or yellowish color.  There is nothing else that will cause these sorts of symptom on peach leaves.

Apple scab

I expect a banner year for this disease on apple and crabapple trees.  The causal fungus survives in apple and crabapple leaf litter and releases spores during wet periods as leaves are beginning to emerge.  Initial infections lead to dark, roughly circular leaf spots with somewhat feathery edges.  Spores are produced in the infected areas leading to additional infections.  Eventually entire leaves can look gray/black and sooty.  Highly susceptible apple and crabapple varieties usually totally defoliate due to the disease by mid-season.  Unfortunately the affected trees do not releaf.

Cedar-apple rust (and other Gymnosporangium rusts)

I received a number of photos of the juniper stage of this fungal disease earlier in the spring, and I posted several of these photos to my clinic Facebook page.  The orange, marmalade-like masses that form on junipers produce spores that infect leaves of apples, crabapples and hawthorns.  The leaf spots that eventually form (they are at their most vibrant in July and August) are often roughly nickel-sized and bright yellow, orange or maroon (on red-foliaged varieties of crabapples).  Spores produced in these spots eventually reinfect junipers, thus completing the pathogen’s life cycle.  Also watch for spiny, salmon-colored fruit on hawthorns, a variation of the disease called cedar-quince rust.

To learn more about these diseases (as well as other plant diseases) and their management, explore the Plant Disease Diagnostics Clinic (PDDC) website (https://pddc.wisc.edu/) or follow the PDDC on Facebook and Twitter @UWPDDC.

If you are like many vegetable gardeners, you have transplants growing in your basement under artificial lights and are chomping at the bit to get those plants out into your garden.

Before you do that, here are a few things to think about to make your summer vegetable garden more successful.

Finish any last minute garden clean up.

If you have leftover vegetable plant debris in your garden, remove it now.  These leftovers are where disease-causing fungi and bacteria overwinter and they can serve as a source of pathogens that can infect your new garden plants.  Burn (where allowed), deep bury or hot compost these materials.

Clean other gardening items.

Disease-causing organisms can survive the winter on gardening tools, stakes and cages.  These items should be decontaminated before using them again:

• Remove any clinging soil or plant debris.
• Use 70% alcohol (e.g., rubbing alcohol, spray disinfectants) or 10% bleach to complete the process:
  • Treat metal items with alcohol, either dipping them for 30 seconds or spraying them until they drip and allowing them to air dry.
  • For non-metal items, soak in bleach for 20-30 minutes, then rinse thoroughly to remove bleach residues.
    Tip:  Be sure to wear old clothes, rubber gloves and eye protection when working with bleach.

Invest in a soaker or drip hose.

If you use a sprinkler to water, you are getting leaves wet and this provides an environment that is perfect to get diseases started.  Soaker and drip hoses keep water off of leaves and apply it into the soil where it is most useful.

Map out your garden.

One way to reduce disease problems is to make sure you move vegetables around in your garden each year.  This is called rotation and helps prevent buildup of disease-causing organisms in the soil.

For details on how to use rotation most effectively, check out the University of Wisconsin Garden Facts fact sheet “Using Crop Rotation in the Home Vegetable Garden.”  Each year make a map of where you have specific vegetables and keep these maps so that you know where to rotate your vegetables each year.

Keep a journal.

Write down observations of what goes on in your garden and when.

• When did you plant?
• When did seedlings emerge?
• When did plants bloom?
• When did they set fruit?  When did you harvest?
• Did you see particular diseases or insect pests?
• When did they start?

All of this sort of information can be helpful in planning your garden in the future.  After several years, you will also get a sense of what disease and insect problems are common and when they typically arrive.  Armed with this information, you can more efficiently and effectively develop  management strategies.

Enjoy growing the old standards that you love, but also do not be afraid to try new (and what may seem like exotic) vegetables.  Trying new things keeps gardening fresh and exciting, and exposes you to new flavors and cuisines.

Have fun!

That’s what gardening should be all about.

Additional Resources

To learn more about plant diseases and their management, explore the Plant Disease Diagnostics Clinic (PDDC) website (https://pddc.wisc.edu/) or follow the PDDC on Facebook and Twitter @UWPDDC.