Category Archives: Monthly Column

August: Late Blight Arrives in Wisconsin

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.

On tomato leaves (left), late blight leads to brown-black, water-soaked, oily areas that may have a white-gray fuzzy look. On tomato fruits (right), late blight leads to large, often sunken, golden- to chocolate-brown, firm spots with distinct rings.
On tomato leaves (left), late blight leads to brown-black, water-soaked, oily areas that may have a white-gray fuzzy look. On tomato fruits (right), late blight leads to large, often sunken, golden- to chocolate-brown, firm spots with distinct rings.

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:

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:

July: Rust – It’s Not Just for Iron Anymore

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 cankers on branches and stems often have a cracked or blistered appearance and may bear yellow to orange spores in fruiting bodies of Cronartium ribicola.

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

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 RustMayapple 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

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.

June: Foliar Diseases of Trees and Shrubs

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 on White Oak
    Anthracnose on White Oak

    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.

  • Symptoms of tar spot on silver maple leaves.
    Symptoms of tar spot on silver maple leaves.

    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.

  • Leaf distortions and discoloration typical of peach leaf curl.
    Leaf distortions and discoloration typical of peach leaf curl.

    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.

  • Leaf spots typical of apple scab on apple. Similar lesions occur on fruits.
    Leaf spots typical of apple scab on apple. Similar lesions occur on fruits.

    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.

  • Circular, yellow-orange diseased areas typical of cedar-apple rust on apple.
    Circular, yellow-orange diseased areas typical of cedar-apple rust on apple.

    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.

May: Seven Tips for a Successful Vegetable Garden

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:

1.   Remove any clinging soil or plant debris.

2.   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.

April: Winter Injury

Temp changeThis past winter’s weather was a roller coaster ride of seasonably cold temperatures in December and January, followed by record warm stretches in February, followed by cold temperatures again for much of March. These extreme temperature fluctuations can injury many woody landscape plants.

The Most Typical Forms of Winter Weather-related Plant Injury

  • Cold injury: Cold injury is a physical damage to plants due to cold temperatures.  Fruit trees (e.g., apples, pears, cherries, plums and especially peach and apricot) are particularly prone to this problem.  Plants can suffer from cold injury in the fall if temperatures stay relatively warm and then precipitously plummet, not allowing plants enough time to properly harden off for the winter.  Cold injury can also occur in the middle of the winter if there are extended warm periods where plants break dormancy, followed by cold temperatures that kill actively growing tissue.  Finally cold injury can occur in the spring if plants leaf out and then are exposed to an unexpected frost.  Trees and shrubs with cold injury typically have scattered branches that never leaf out, or scattered branches where leaves emerge and then shrivel, dry up and die.  In extreme cases, cold injury can kill an entire tree or shrub.  Cold injury in apples and pears is often misdiagnosed as fire blight.
  • Winter burn: Winter burn occurs when trees or shrubs (e.g., yew, Alberta spruce and boxwood) run out of internally stored water and dry out.  Winter burn can occur during the winter, but most often is noticeable as trees and shrubs break dormancy.  Browning of needles (e.g., on yew) or bleaching of leaves and stems (e.g., on boxwood) are characteristic symptoms.  Winter burn can affect just a few branches (which in some cases eventually recover) or an entire plant (which in some cases may die).

How to Minimize the Effects of Winter Injury

  • Choose hardy plants. Consult the USDA plant hardiness zone map to determine your hardiness zone and select plants that are rated to survive in your climate.
  • Plant marginally hardy plants in protected areas. You can still try to grow your favorite marginally hardy trees and shrubs, but plant these near your house, particularly in sheltered areas that may be slightly warmer than the rest of your yard.
  • Plant at the right time of year. Plant in the spring or late summer (August or September) to allow for root growth before the onset of winter and to avoid hot mid-summer temperatures that can be stressful on new transplants.
  • Mulch properly. Mulch around trees and shrubs out to their driplines, keeping mulch at least three inches from the bases of plants.  Use one to two inches of a high quality mulch (e.g., shredded oak bark mulch) on heavier, clay soils and three to four inches on lighter, sandier soils.  Proper mulching can help insulate roots over the winter.
  • Water properly. It is particularly important to water evergreens into the fall up until the ground freezes or there is a significant snowfall.  This will help prevent winter burn.  Established trees and shrubs require approximately one inch of water per week; newly transplanted trees and shrubs requite up to two inches of water per week.  If supplemental watering is needed, use a soaker or drip hose; DO NOT use a sprinkler.
  • Fertilize properly. DO NOT fertilize trees and shrubs in late summer and fall.  Fertilizing late stimulates growth that is more likely to be injured as temperatures drop in the fall.
  • Prune properly. Avoid pruning in later summer and fall.  Late pruning on some trees and shrubs can stimulate growth that is more likely to be injured by cold fall temperatures.
  • Protect plant for the winter. Build barriers (e.g., of burlap or canvas) around sensitive plants to deflect and reduce the drying effects of winter winds.

For more information on winter injury/winter burn and additional specifics on how to prevent winter injury/winter burn from being a problem, see the University of Wisconsin Garden Facts “Winter Burn” available at https://pddc.wisc.edu/fact-sheet-listing-all/.

March: Seed Starting and Damping-Off

Preparing for Spring

March is the month when many gardeners begin to think about starting vegetables and ornamental annuals from seed indoors.  While seed-starting can be an economical method for producing plants for home gardening, damping-off can put a damper (no pun intended) on this fun winter gardening activity.

Forms of Damping-Off

Damping-off can take on many forms:

  • rot of seeds prior to germination,
  • rot of seedlings before they reach the soil surface,
  • collapse of seedling stems after emergence followed by plants toppling onto the soil surface (the most classic form of the disease).

Gardeners often blame damping-off on poor seed quality, but disease-causing fungi (and other similar organisms) are the real cause of the problem.  Luckily, only seeds and young plants are susceptible to damping off; older plants are immune to the disease.

Lower stem collapse of Zinnia seedlings due to damping-off.
Lower stem collapse of Zinnia seedlings due to damping-off.

Prevention of Damping-Off

  • Use new or decontaminated growing containers, working surfaces and tools. Damping-off pathogens can survive on inert surfaces.  Rinse items like pots and potting stakes thoroughly to remove soil and then soak them for 20 to 30 minutes in 10% bleach.  Rinse items well after treatment to remove bleach residues.  This treatment works well to decontaminate clay and ceramic containers, but is less reliable for plastic items.  If you have had damping-off problems in the past, discard plastic containers and labels and start with new items.  Also use 10% bleach to decontaminate surfaces where you work with seeds and plants.  For metal tools, use a 30 second dip in 70% alcohol (e.g., rubbing alcohol) to decontaminate or alternatively use a spray disinfectant (spray until tools drip and then allow them to air dry).
  • Use pasteurized soil to start seeds. Pasteurized soils are steam treated which helps kill disease-causing organisms.  Never use garden soils as these typically contain low levels of damping-off organisms.
  • Use high quality, vigorous seed. High quality seed germinates rapidly and resulting plants quickly mature and become immune to damping off.
  • Plant seeds at the proper depth. Check seed packets for information on proper panting depth.  When seeds are planted too deeply, plants mature less quickly and remain susceptible to damping-off for a longer period of time.
  • Germinate seeds at high temperatures. High temperatures stimulate rapid germination and growth, resulting in a shorter period of time when plants can become infected.  Check out heated seed germination mats, which promote rapid plant growth.  They can help prevent damping-off.
  • DO NOT overwater. Damping-off pathogens are more active in wet soils.  Keep soils drier to reduce pathogen activity and limit infections.

For More Information

See the University of Wisconsin Garden Facts “Damping-Off” available at https://pddc.wisc.edu/fact-sheet-listing-all/.

February: the PDDC Brings Education to You

Did you know that the PDDC offers educational programs throughout the state of Wisconsin?

Take a look at our stats from last year:

This year is shaping up to be even busier.

Talks and Workshops

Brian Hudelson, the PDDC Director, designs and delivers custom, in-person presentations to groups throughout the state of Wisconsin.  Target audiences include home gardeners, professionals in the green industry (e.g., arborists and greenhouse producers), and Master Garden Volunteers.

Talks and Workshops

Topics

Brian’s presentations can cover virtually any topic related to plant diseases including, but not limited to, diseases of fruits and vegetables, diseases of trees and shrubs, and diseases of herbaceous ornamentals.

Schedule

You can find Brian’s 2017 presentation schedule here:
2017 Outreach Events Calendar

Check the calendar for the topics, locations, and what educational materials he will provide for each presentation.

How to Book a Talk

If you’d like Brian to give a presentation to your group, contact the PDDC at pddc@plantpath.wisc.edu.

Be aware that Brian’s schedule fills quickly and he is currently booking presentations well into 2017.

Informational Booths

Want to speak with PDDC staff in person, one-on-one?

The PDDC provides informational booths at select events during the year.  Brian, other clinic staff and graduate students from the UW-Madison Department of Plant Pathology can answer your questions on plant diseases and how to manage them.

Informational Booths

The PDDC’s next informational booth will be at Garden Expo February 10-12 at the Alliant Energy Center in Madison, WI.  Garden Expo is one of the largest garden shows in the Midwest and serves as fundraiser for Wisconsin Public Television.

Radio and Television

Brian Hudelson (aka Dr. Death) is a regular guest on Garden Talk with Larry Meiller on Wisconsin Public Radio (WPR), and has appeared regularly on The Wisconsin Gardener on Wisconsin Public Television (WPT).

Radio and TV

Brian will next appear on Garden Talk on February 3, 2017 at 11 am.

Past Appearances

WPR – listen to Brian’s past appearances on Garden Talk on the WPR website.
WPT – watch Brian’s past appearances on The Wisconsin Gardener on WPT website.

Need More Information?

To learn more about PDDC outreach activities, check out the PDDC Clinic Services and Clinic Outreach brochure.

To book Brian for a talk or event, contact the PDDC at pddc@plantpath.wisc.edu.

January: Pruning in the New Year

January is the time of year to start thinking about maintenance pruning for the trees and shrubs in your yard.

While there are exceptions, for many trees and shrubs, pruning during colder weather (when disease-causing organisms and insects are not active) can help minimize infections through pruning wounds.

The “January Thaw”

If you decide to prune, watch for the “January thaw,” when temperatures are warm enough that you will not freeze to death outside, but cold enough for plant pathogens and insects to have limited activity.

How To’s

For details on pruning, see:

Preventing Infections

As an added precaution to prevent infections, consider routine disinfestation (i.e., decontamination) of pruning tools as you prune.  In the best of all possible worlds, you should decontaminate between every pruning cut.  However, due to time constraints, this may not be feasible, but do consider disinfesting tools between every tree or shrub.

  1. Dip them in alcohol.
    The best and probably easiest way to disinfest pruning tools is to dip them for at least 30 seconds in 70% alcohol (e.g., rubbing alcohol).
  2. Use spray disinfectants.
    You can also use spray disinfectants that contain roughly 70% alcohol, in which case you spray your pruning tools until they drip and then allow them to air dry.
  3. Dip them in a bleach solution.
    As an alternative to alcohol, you can dip tools for at least 30 seconds in a 10% bleach solution.

CAUTION: Bleach is corrosive to metal tools and will induce rusting when used repeatedly.

For More Information:

For addition details on proper pruning, contact your county UW-Extension office or contact the PDDC at pddc@plantpath.wisc.edu.