Presently The Big Concern Is Disease

Protect your crop, get the latest fungicide information

Published online: Apr 13, 2010 Feature staff report
Fungicides are classified into groups by their mode of action. The Fungicide Resistance Action Committee (FRAC) is a scientific working group who does the classification.

Consider selected chemical groups and registered product names for fungicides used in your state and consult with experts to create a management strategy.

For instance, FRAC group 1 is methyl benximidazole carbamates; Group 2 includes dicarboxamides; Group 3 is demethylation inhibitors (DMI), and Group 4 is phenylamides.

Plant pathologists note that most new fungicides have single-modes of action thereby making the probability of resistance likely. Managing this resistance takes planning.

Retired University of Idaho sugarbeet expert John Gallian explains, "Target pathogens are more likely to develop resistance against fungicides with specific modes of action (qualitative resistance).

Failure to follow resistance management guidelines carefully will likely result in the loss of these new fungicides as effective control measures."


Registered fungicides are described in two types one being protectant fungicides and the other is described as systemic fungicides.

Protectant fungicides act on the leaf surface to prevent infection and it should be known that they do not "cure" established infections. It takes five or more days from the time of infection before leafspots appear.

If a protectant fungicide is applied immediately after a rainy, humid period, infections will already be established and it may be too late to prevent development of leafspots several days later.
Late application may result in claims that "the fungicide didn't work" or "the fungicide wasn't applied correctly."

It is essential that protectant fungicides be on the leaf before rainy or humid weather occurs. The application must be made early enough to allow spray droplets to dry before rains begin. Then the fungicide is not as easily washed off after it dries.

Systemic fungicides are applied and absorbed by the leaf. The class of systemic fungicides known as sterol inhibitors where most sterol inhibitors are at least locally systemic; that is, they are absorbed by the leaf and distributed within the leaf.

Others are more fully systemic, but most do not move into new foliage formed after the fungicide is applied. Some of the sterol inhibitors may have curative properties for Cer-cospora control. Check for current recommendations, and do not use any fungicides before they are registered.