Fungicide-Resistant Downy Mildew Detected in Kentucky Vineyard

This summer, a grape grower in central Kentucky reported persistent downy mildew in his vineyard.  He noted that regular applications of Abound and Pristine fungicides failed to manage the disease.  After laboratory analysis, the pathogen was deemed completely resistant to the two fungicides at the lowest recommended rates and 85% resistant at the highest recommended rates. 

 

What is fungicide resistance?

In the simplest terms, pathogens become resistant to fungicides when the chemical no longer manages disease symptoms.  However, even the most effective fungicides fail to completely eradicate a pathogen population.  There are always a few fungal spores or other fungal inoculum that survive the pesticide application.  Those survivors may be the result of ineffective spray coverage, but individual pathogens may have a trait that provides some type of resistance to the fungicide.  Think back to high school biology when we learned the theory of “survival of the fittest.”  Unfortunately, a single survivor can multiply into thousands of individuals while passing that resistance gene onto its offspring, much the way our parents passed on eye color to us. 

 

How did resistance develop?

Consider that it is highly unlikely that a fungal population will incur resistance to more than one chemical type, at least over the short term.  As a fungal population can become resistant to a single chemical, growers should rotate sprays with a different chemical group.  These chemical rotations can become confusing, and many growers do not fully understand the concept of chemical groups. 

 Chemical groups are classified by biochemical mode of action, not necessarily by active ingredient.  For example, within the strobilurin group of fungicides, active ingredients include azoxystrobin, pyraclostrobin, trifloxystrobin, and kresoxim-methyl, all of which are quinone-outside inhibitors.  Because information on biochemical modes of action can be confusing for growers, the Fungicide Resistance Action Committee (FRAC) developed numeric codes that represent these chemical groups.  Strobilurins are classified as FRAC group 11.  These codes appear on the top right side of all pesticide labels.  Thus, growers may simply refer to the coded chemical group number on labels as opposed to depending upon complex information such as mode of action.

Considering that all fungicides within the same group have the same mode of action, it is clear that if a grower fails to properly rotate fungicide groups, fungicide resistance risk is high.  Additionally, fungicide labels indicate the maximum number of applications allowed per growing season.  A maximum of four applications of strobilurins are allowed per growing season.  The grower mentioned above used Abound and Pristine fungicides consistently over a two-year period, exceeding the maximum number of applications and failing to rotate with a different chemical group.  This rapidly induced the development of a resistant population of the downy mildew pathogen.

 

Abound fungicide is classified as a FRAC Group 11 fungicide.  The chemical group code appears on the top right corner of fungicide labels.

 

How does a grower know if a resistant population developed?

Pathogen populations do not begin as 100% resistant.  In fact, resistance develops gradually.  Thus, growers should be aware of efficacy and disease control.  If a product(s) begins to become less effective over time, he should contact his local Extension agent immediately. 

 

What next?

If resistant pathogen populations develop within a vineyard, growers should immediately stop using the fungicide in question and all others in the same FRAC group.  With the assistance with an Extension agent or specialist, growers should identify other fungicides that will effectively manage disease.  In the aforementioned case, the grower stopped using strobilurin fungicides and substituted a phosphorous acid fungicide (ProPhyt, Rampart, etc.) for management of downy mildew.  If strobilurins are used for management of other diseases, tank-mix with another product (within a different FRAC group) that provides downy mildew control.

 

More Information

Fungicide resistance can appear complicated, so growers should not hesitate to seek assistance in development of a spray program.  Contact University of Kentucky Cooperative Extension agents or specialists for assistance. 

April showers bring May… Disease?

An Introduction to Water Molds

Spring rains can create growing conditions that are devastating to most landscape plants.  For example, excess water is responsible for a disorder called “wet feet,” which results from the suffocation of plant roots as waterlogged soil loses oxygen.  However, suffocation is not the only injury to nursery or landscape plants during rainy spring weather.  Wet soils are favored by a group of pathogens called water molds, or oomycetes, which cause a range of root and stem diseases. 

Water molds are found in most soils, but plant stress and high pathogen numbers can lead to severe disease.  Common water molds such as Phytophthora and Pythium cause roots rots, stem rots, collar rots, and damping off diseases in both woody and herbaceous plants.  They are also responsible for downy mildews and some foliar blights in upper plant parts.

 

Symptoms

Symptoms differ according to plant type and infection site.  Root rot symptoms begin, not surprisingly, at the roots.  However, because roots are concealed, disease often goes undetected until plants begin to decline or upper plant parts wilt as a result root reduction.  Disease often begins during rainy spring weather, but it is typically not noticed until hot dry weather initiates wilting. 

Water molds can also cause above-ground infections.  These symptoms can range from yellow mottling of leaves to water-soaked lesions on leaves and succulent stems.  Woody stems and trunks may develop cankers just above the soil line, often at a wound site.  Cankers are usually dark-colored and may exude sap or “bleed.”

 

Uniqueness of water molds

Water mold pathogens are very different from true fungi; they require free water to complete their life cycles.  Initial infections often occur during rainy spring weather as temperatures begin to warm.  After infection, pathogens release large numbers of “swimming” spores that move on films of water.  This is the repeating stage that leads to disease epidemics if wet conditions continue.  Spores are spread by splashing water and movement of contaminated soil particles.

Once established, water molds can produce survival structures that allow them to lie dormant during hot dry seasons.  Available water can reinvigorate these structures and the disease cycle can begin again.  Many water molds occur naturally in soils, and proliferation under wet conditions can be devastating to plants.

Disease Prevention Using Cultural Practices

Most water mold diseases can be prevented or managed using cultural practices.  Consider the management tips below to prevent infections or to help manage infected nursery or landscape plants.

·         Improve drainage

o   Manage surface water

o   Plant in raised beds

o   Divert downspouts

o   Use organic matter to improve internal drainage

o   Limit irrigation

o   Manage nursery runoff from infested areas

·         Disinfest tools, containers, and greenhouses  to eliminate spread

o   Commercial sanitizers are available

o   10% Lysol® concentrate and 10% bleach are also effective.  Bleach is corrosive on metals, so rinse tools well before storage.

·         Dispose of infested potting media

o   Do not reuse contaminated soils

·         Destroy infected nursery and greenhouse plants as soon as possible

o   Do not compost infected plants

·         Remove plant debris and other sources of inoculum before spring

o   Rake and destroy leaves and debris

·         Reduce splash

o   Use drip irrigation

o   Mulch exposed soils

·         Use resistant cultivars whenever possible

Management using fungicides

Water molds are not true fungi, so not all fungicides will be effective against these pathogens.  Fungicides must be specifically labeled for oomycetes.  Select fungicides that contain one of the active ingredients listed below.  Efficacy of these fungicides is dependent on plant and pathogen type; read labels carefully. 

Fungicide active ingredients effective against water molds:

·         Azoxystrobin (Heritage)

·         Cyazofamid (Segway)

·         Etridiazole (Terrazole, Banrot)

·         Mefenoxam (Subdue)

·         Propamocarb (Banol*)

·         Phosphorus acid (Alude, Agri-Fos**)

* Not for use in residential landscapes, for commercial use only.

** Available to homeowners.

See our fungicide guides PPFS-OR-W-14 and PPFS-GH-3 at www.ca.uky.edu/agcollege/plantpathology/extension for more information concerning fungicide use or contact your local UK Extension agent for assistance.

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