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Real versus fake fungicide resistance

Steve Koike is not a man of the cloth per se, yet he pounds the pulpit on the importance of laboratory verification to prove whether difficult pathogen control in crops is the fault of the fungicide or another culprit.

“Verification is the best way to nail down whether unsuccessful control is the result of fungicide resistance or fake resistance,” said Koike, who operates the University of California Cooperative Extension (UCCE) plant pathology diagnostic lab in Salinas, Calif.

In most cases, Koike says fungicide resistance, also called tolerance and insensitivity, is seldom the real culprit.

“Growers and crop advisers are critical in recognizing the signs of uncontrolled pathogens in the field,” Koike said. “Pathogen testing should follow under clinical or greenhouse conditions to scientifically determine if the issue is fungicide resistance or another problem.”

Koike addressed fungicide resistance with crop industry members at the 2010 Desert Ag Conference in Casa Grande, Ariz.

Koike defines fungicide resistance as a decreased response of a pathogen to a fungicide due to fungicide applications. This differs from genetic resistance bred into plants to resist infection and disease.

A good example on the importance of verification occurred about a decade ago. Salinas Valley lettuce growers suspected reduced crop protection for downy mildew from the fungicide Aliette. Until then, the product had delivered good, reliable control for years without a hint of resistance.

Koike, UCCE Monterey County plant pathologist, cautiously suspected a reason other than resistance; perhaps improper product usage, late spraying, or an incorrect application rate or fungicide rotation schedule.

Koike, practicing what he preached, took samples to the lab where he and his colleagues tested the mildew against an established baseline isolate. They challenged different mildew isolates (strains) with several fungicide amounts. Tested isolates were from the Salinas Valley; plus an isolate from England where Aliette was never used.

Aliette provided good control of the English isolate, yet poor results in the Salinas isolates even with product rates applied above the recommended label amounts. The growers were correct; the mildew showed signs of fungicide resistance.

In the short term, impacted growers relied more on other fungicide products. However, for reasons never understood, the Aliette resistance problem was short lived. In a few years the resistance problem went away. Aliette is used successfully today.

Fungicide resistance can lead to lost disease control, reduced yields, and unnecessary expense by applying products which no longer work.

There are few new instances of fungicide resistance in the world, Koike says, yet resistance remains a front-burner issue for growers and crop advisers.

Some groups or species of fungal pathogens are historically more prone to fungicide resistance. These include downy and powdery mildews, Botrytis cinerea, Cercospora species, Penicillium species, Phytophthora infestans, Pyricularia, Septoria species, and Venturia. Bacterial pathogens often linked to resistance include Pseudomonas species and Xanthomonas species (example - bacterial spot on tomato).

If fungicide resistance is suspected, Koike says online searches and research reports are good information resources.

“If you see a possible breakdown on fungicide performance, then look around and see what other people are reporting on this issue,” Koike suggests. “There could be other cases that mirror your situation: the same product, crop or disease.”

If tests prove the problem is not resistance related, the culprit could be fake resistance; symptoms which simulate fungicide resistance. The causes of fake resistance can include improper product application timing, incorrect product rates and poor fungicide distribution.

Another faux resistance cause is applying the product during the latency period; the time between the actual infection and when symptoms appear in the field.

The latency period for downy mildew in lettuce is two to four days.

“Growers may say the product was used according to the label and then several days later a major outbreak of mildew occurs,” Koike said. “They may have sprayed late, after the infection event had already taken place. It looks like the product didn’t work. It’s actually late application timing.”

What causes fungicide resistance? Pathogens can adapt to fungicides through mutation and selection pressure.

“Pathogens are subject to change and mutate all the time,” Koike said. “Mutation allows a few pathogens to withstand a chemical. They are rare, but do exist.”

Koike compared plant pathogen mutation to human health scenarios where antibiotic-resistant bacteria strains evolve due to the overuse or misuse of antibiotics.

Resistance management is critical to maintain the fungicide effectiveness over the long term. Koike recommends judicious fungicide use: Apply products strictly according to the label, apply only when needed, avoid overuse and apply products in a timely manner at the appropriate rates.

“If we fall down on any of these aspects then the application is not as effective,” Koike said. “This could be a deterrent in long-term resistance management.”

Another management method is to tank mix or alternate products with different modes of action. He recommends combining single- and multi-site fungicides, and scheduling sprays with different modes of action group numbers.

The numbers are compiled by the Fungicide Resistance Action Committee, referred to as the FRAC list, available online at

Appropriate resistance management also requires using integrated disease or pest management strategies, including good site selection, resistant plant varieties, field sanitation, cultural practices, plus weed and vector management.


TAGS: Management
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