Washington State University viticulture extension specialist Dr. Michelle Moyer suggests growers consider the following when developing a ­disease management program:

•    Reproductive rate of the pathogen. How fast can the disease reproduce in your vineyard? Is weather conducive to reproduction?
•    Speed of your reaction time. Do you have enough sprayers to cover your acreage in a timely manner?
•    Spray coverage. How good is your spray penetration? During years of high disease pressure, such as 2011, poor spray coverage in some vineyards resulted in significant disease problems.
•    Implications for next season. Are you allowing build-up of disease within the canopy that will add to your spore population and disease pressure for the next year?

Understanding fungicide labels

Dr. Gary Grove, WSU plant pathologist, says fungicide labels can be overwhelming because they contain a lot of information, but understanding them can lead to better choices.

Fungicides are classified on labels in six ways:

1 Mobility in the plant. Plant uptake is by contact or systemic. Systemic fungicides are absorbed into plant tissue and may offer some postinfection activity.

2 Role in protection.  Preventive activity provides protective barrier before the pathogen arrives or begins to develop; early infection activity (curative) means the fungicide can penetrate plant tissue to stop the pathogen; antisporulant activity (eradicant) is the ability to prevent spores from being produced.

3 Breadth of metabolic activity. Single-site fungicides target only one point or function in the metabolic pathways or a single critical enzyme or protein needed by the fungus; multisite fungicides affect a number of different metabolic sites and are less prone to build resistance.

4 Mode of action. This is the specific biochemical process by which the fungicide kills or suppresses the target fungus.

5 Chemical group or class. The name given to a group of chemicals that share common biochemical mode of action. They may or may not have a ­similar chemical structure.

6 FRAC Code. The Fungicide Resistance Action Committee has developed a code of numbers and letters to distinguish the different fungicide groups based on mode of action to help growers rotate chemicals for resistance management and choose products with different modes of action.

Grove has developed a color-coded fungicide table for grape growers to help guide their chemical choices. Chemicals with low risk of resistance development are green, those with moderate risk are ­yellow, and high risk are red (see “Fungicides for Grape Powdery Mildew”).