Using Resistant Cultivars to Manage Strawberry Diseases

From the Midwest Small Fruit Pest Management Handbook
http://www2.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/MwSmFruitPMHandbook.pdf


In any IPM program, the use of resistant cultivars is an important consideration.  Many commercial cultivars have resistance and/ore tolerance to leaf spot, leaf scorch, red stele, and powdery mildew.  The more resistance within a program, the better.  Table 21 (page 27) lists ratings for disease resistance in several of the more commonly grown strawberry cultivars.

 

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. 

What's this spot on my rose? Downy mildew showing up on rose this spring.

There’s a new disease in town.  Downy mildew of rose is showing up across the state.  It has been several years since downy mildew has been diagnosed on rose in KY, but we have already seen multiple cases this spring.  During the past weeks, there have been reports of an “incurable black spot” that is “unresponsive to fungicides.”

The downy mildew pathogen is a water mold, not a true fungus.  Thus, humid greenhouse conditions are ideal for disease spread.  So far, all reports originate from greenhouse-grown roses.  Additionally, retail centers who purchased finished roses indicated that roses arrived with early stages of the disease. 

Symptoms of downy mildew occur primarily on young apical leaves.  Initial symptoms begin as purplish-red irregular spots (photo below).  Rapid spread occurs with high moisture and reduced air circulation.  Within days, lesions coalesce, resulting in leaf yellowing and/or leaf drop.  The fuzzy sporulation of rose downy mildew occurs on the lower leaf surface, but it is typically very sparse and difficult to see.  This season, infection has been diagnosed on hybrid tea roses and some varieties of Knockout rose. 

Downy mildew on rose begins with irregularly-shaped lesions that are purple to reddish in color.

Roses are susceptible to a variety of leaf spots (photo below).  Black spot, the most common foliar disease of rose, begins as circular spots with feathery margins.  While hybrid tea roses are extremely susceptible to black spot, Knockout roses are mostly resistant.  Cercospora leaf spot, common on many shrub roses, produces circular lesions with purplish red edges and light gray or tan centers.  Both of these leaf spots can be confused with early symptoms of downy mildew.

Leaf spots of rose can appear similar.  Black spot lesions are circular with feathery edges (left, photo by J. Hartman).  Cercospora spots have dark halos with light centers (right, photo by A. Windham).  Downy mildew infections cause blotches, not spots (center, photo by D. Hull).

Management of downy mildew requires a combination of cultural and fungicidal controls.  This first step requires reduction of humidity.  Greenhouses should be well ventilated.  Nursery and greenhouse plants should be spaced for sufficient air circulation to promote leaf drying.  Drip irrigation can greatly reduce leaf wetness, as well.  Growers should practice good sanitation, removing fallen leaves and pruning away diseased plant parts, as the pathogen can survive for several weeks on debris.  Fungicides registered for use in commercial greenhouses and nurseries include Aliette, Banol, Segway, Stature, and Subdue MAXX.  Homeowners may apply copper + mancozeb.  Refer to labels for rates and application intervals.