April 2008
Fungicide resistance management grows ever more popular

Most vegetable growers probably have panic attacks just from the thought of a massive fungicide failure. Virtually every crop has its potential plague, and most growers know of times when fungicides failed if not for them, for others.

Like when metalaxyl lost its ability to control late blight on potatoes and tomatoes. Or when benomyl failed to control powdery mildew on cucurbits, and later DMIs failed, too. Or, when DMI fungicides failed against powdery mildew in grapes and then against apple scab, leading to damage in Eastern and Midwestern orchards and vineyards. Or, when strobilurins failed against downy mildew in cucumbers.

The list of examples is quite long, and growing steadily.

When resistance appears, formerly effective fungicides become as rainwater on ducks, turning poisons to potions having reduced effects or even none.

For many years, growers were content to abandon the old and change to new fungicides, and the chemical companies were quite adroit at producing them. But a series of events environmental concerns, health concerns, Food Quality Protection Act constraints, escalating costs, fewer new offerings changed the climate.

The idea now is that fungicides must be carefully protected to avoid losing effectiveness. Some of the newest and best fungicides are so-called single site fungicides that are quite susceptible to resistance. The whole topic is labeled fungicide resistance management.”

Across the country, university plant pathologists are in broad agreement on steps growers should take to protect the disease-killing properties of the fungicides they use, and they carry out educational programs to inform growers.

The chemical industry has stilled competition enough to cooperate in creating the Fungicide Resistance Action Committee (FRAC), and it has organized all the fungicides into groups so growers can know how their fungicides work and how to mix and match them. Rotating fungicides is a key element in preventing resistance. FRAC codes are listed on fungicide labels, and these labels often contain resistance management recommendations.

Tables are available on Web sites. You can get, on the Web site www.frac.info, two FRAC code lists that sort fungicides by mode of action and by code letter or number.

Resistance Defined

Margaret Tuttle McGrath is a plant pathologist at Cornell University’s Long Island Horticultural Research and Extension Center in Riverhead, N.Y. Her specialty area is powdery mildew on cucurbits a powder keg for resistance problems but she offers general guidelines for managing fungicide resistance that apply to virtually any disease or crop.

“Fungicide resistance is a stable, heritable trait that results in a reduction in sensitivity to a fungicide by an individual fungus,” she said. “This ability is obtained through evolutionary processes. Fungicides with single-site mode of action are at relatively high risk for resistance development compared to those with multi-site mode of action.

“Most fungicides being developed today have a single-site mode of action, because this is associated with lower potential for negative impact on the environment, including non-target organisms.”

The “evolutionary processes” involve repeated exposure of disease organisms to a fungicide, which results in the selection of resistant organisms that proliferate and become the “normal” ones in the environment no longer vulnerable to a fungicide or to other fungicides that work in a similar manner.

Not surprisingly, older fungicides, many predating sophisticated organic chemistry and based on inorganic metals, have multi-site activity and are less likely to result in resistant diseases. These are given FRAC code M, meaning multi-site, and include copper and sulfur, zinc- and magnesium-based materials like mancozeb and maneb, oils and others like chlorothalonil and captan. There are nine groups in the M category, with M numbers 1 through 9. While not included in the FRAC codes, other materials of this nature would include lime, chlorine, chlorine dioxide and ozone.
In addition to the Ms, there are 43 FRAC groups listed with the numbers 1 through 43 (omitting 15 and using 16.1 and 16.2). These are rated individually as high, medium or low risk for developing resistance.

In addition, three groups get P labels (P standing for host plant defense induction) and five get U labels (meaning mode of action unknown). There is an NC group (not classified) containing products like oils, potassium bicarbonate and materials of biological origin.

Degrees of Resistance

When fungicide resistance results from modification of a single major gene, pathogen subpopulations can become highly resistant to the pesticide, McGrath said.

“Resistance in this case is seen as complete loss of disease control that cannot be regained by using higher rates or more frequent fungicide applications,” she said. “This type of resistance is commonly referred to as qualitative resistance. When fungicide resistance results from modification of several interacting genes, pathogen isolates exhibit a range in sensitivity to the fungicide, depending on the number of gene changes.

“Resistance in this case is seen as an erosion of disease control that can be regained by using higher rates or more frequent applications. This type of fungicide resistance is commonly referred to as quantitative resistance.”

The FRAC codes are most useful to growers because they identify the relationship among fungicides and thus the potential for cross-resistance.

Simplifying FRAC Codes

Dealing with the FRAC codes is not as complicated as it sounds. A publication from the University of Kentucky tells growers they are most concerned with systemic fungicides that fall into five groups.
FRAC code 1 contains benzimidazoles like thiobendazole. These are used against various rots and molds, like brown rot and botrytis and scab.

FRAC code 2 contains dicarboximides, widely used against gray molds like botrytis.

FRAC code 3 contains the sterol biosynthesis inhibitors (the DMIs). This group is more prone to gradual rather than sudden loss of effectiveness. They are used against gray mold, apple scab and powdery mildew.

FRAC code 4 contains phenylamidides (like metalaxyl) used against blights and downy mildew.

FRAC code 11 includes the strobilurins (QoI). These are used against powdery and downy mildew, early blight and alternaria blotch on apples.

FRAC code 25 contains the antibiotic streptomycin, used against fire blight.

These groups contain most of the important fungicides, and most of them are medium to high risk for resistance. These are the ones that need to be rotated, limited in use and tank mixed across groups.
FRAC code M contains the low-risk materials that should find a place in any tank mixture of fungicides.

The Real Names

On the West Coast, Extension plant pathologists Jay Pscheidt and Cindy Ocamb maintain an online guide to plant disease control at the University of Oregon. Pscheidt works with fruit crops and Ocamb with vegetables in the Pacific Northwest. The Web site is http://plant-disease.ippc.orst.edu.

“Fungicide resistance is a major threat to disease control,” Pscheidt said. “Many new fungicides are at high risk of encouraging the development of resistant fungi. Although tactics for resistance prevention or delay have been developed, product trade names do not make it easy for producers to implement these procedures.”

FRAC codes are valuable because they group fungicides by mode of action. On his Web site, Pscheidt presents a modified FRAC table that includes the most commonly used at-risk fungicides along with their trade names.

Thus, we can see that the group 4 phenylamide, common name metalaxyl, is sold under the names Ridomil, Subdue, Apron and Allegiance. The group 3 DMI (demethylation inhibitors) includes triazoles, of which one is propiconazole sold as Tilt, Orbit, Banner, Alamo, Break, PropriMax, Bumper and ProPensity.

“A grower should use fungicides with different modes of action. Tank mix or rotate materials found in different fungicide families to prevent or delay the development of resistant fungi,” he said.

Extending Life

“Managing fungicide resistance is critically important to extend the period of time that an at-risk fungicide is effective,” McGrath said. “The primary goal of resistance management is to delay its development rather than to manage resistant fungal strains after they have been selected. Therefore, resistance management programs need to be implemented when at-risk fungicides first become available for commercial use.

“The overall strategy to managing resistance is to minimize use of the at-risk fungicide without sacrificing disease control. This is accomplished by using the at-risk fungicide with other fungicides and with non-chemical control measures, such as disease-resistant cultivars, in an integrated disease management program.”

It is critical to use an effective disease management program to delay the build-up of resistant strains, she said. The larger the pathogen population exposed to an at-risk fungicide, the greater the chance a resistant strain will develop. So apply sprays starting very early in disease development or in a preventative manner to avoid large populations of disease-causing organisms.

Specific Guidelines

McGrath offers a list of guidelines growers should follow to effectively manage fungicide resistance. Here are some of them:

-Get information on current occurrence of resistance and on fungicide risk for the target disease, as well as fungicides (from the FRAC code list) to be used.

-Just as some fungicides are more prone to resistance developing, some pathogens are more prone to developing resistance. Fungi that cause powdery mildew diseases are most prone to developing resistance. Cucurbit powdery mildew has developed resistance to almost every chemical class at risk for resistance. Other pathogens attacking vegetable crops that have developed resistance include those causing late blight of potato and tomato, downy mildew of cucurbits, early blight of potato and tomato, phytophthora blight and gummy stem blight of cucurbits.

-Reduce the need for fungicides at-risk for resistance development by using disease-resistant varieties and other cultural management practices.

-Start fungicide applications very early in disease development or before symptoms are seen. It is practically impossible to control the pathogen in an established lesion as opposed to a germinating spore, thus the potential is greater for resistance to develop.

-Use at-risk fungicides only when needed most and alternate among those with different FRAC codes. Alternating among fungicides in the same chemical group is not really an alternation.

-The most critical time to use at-risk fungicides is early in an epidemic, when the pathogen population is small.

-Multi-site contact fungicides should be used alone late in the growing season, where they have been shown to provide sufficient disease control to protect yield.

-It is not known whether a strict alternation (apply once and then switch) is better than a block alternation (apply twice and then switch).

-Don’t cut rates. At-risk fungicides should be used at the manufacturer’s recommended rate and application interval. Using highest label rates is expected to minimize selection of strains with intermediate fungicide sensitivity when resistance involves several genes (quantitative resistance).

-When one crop could serve as a source of inoculum for a subsequent crop, the alternation scheme among at-risk fungicides should be continued between successive crops. The first at-risk fungicide applied to a crop should belong to a different cross-resistance group than the last at-risk fungicide applied to the previous crop.

-Tank mix at-risk fungicides with multi-site fungicides, because these have low resistance risk. Multi-site fungicides (FRAC code M) will control any resistant strains they contact.

-Check the label: Some at-risk fungicides are formulated with other fungicides as premix products to manage resistance.

-Maximize spray coverage. The better the coverage, the greater the contribution of the multi-site fungicides to control and the lower the selection pressure for resistance development.

-Follow resistance management guidelines specified on the label. The label is a legal document. In addition to manufacturer restrictions pertaining to alternations and tank mixtures, there are often limits on the total amount to be applied and the number of allowable applications per season.

-Assess disease control and report any potential loss of efficacy due to resistance to local Extension specialists. This information is important for the entire industry.”

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