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There have been several significant advances in the understanding of grape­vine powdery mildew in ­recent years, some of them local and some not. Perhaps the most useful local tidbit has been the determination of the pathogen’s life cycle in eastern Washington. In other viticulture regions, Erysiphe necator (formerly known as Uncinula) can survive winter as mycelium in dormant buds or as cleistothecial, or both. The mode of survival profoundly influences our approach to disease and fungicide resistance management. For years the mode of overwinter survival of E. necator in eastern Washington was unknown and assumed to occur in the infected dormant buds. In their landmark 1987 study, Pearson and Gadoury documented the survival of E. necator cleistothecial in exfoliating bark on New York grapevines.

Since then, the epidemiological significance of the cleistothecial stage has been documented in parts of California, Italy, France, and Australia. We observed profuse production of cleistothecial on infected foliage during the latter stages of the 1995 and 1996 epidemics, and early in the subsequent growing seasons, primary disease ­ dist­ribution patterns consistent with ­ascosporic infections was observed. Effective management is based on an understanding of the epidemiology of the disease, including the mode of survival. The purpose of our four-year study was to determine the source of primary inoculum for grape powdery mildew epidemics in the region.

During our study, flag shoots and other evidences of point inoculum sources were not observed during vineyard surveys. Viable cleistothecia retrieved from exfoliating bark and senesced leaves (on the vineyard floor) persisted through the prebloom or immediate postbloom periods. Ascospores were trapped from the vineyard air during all years of the study, and their occurrence preceded the appearance of vineyard symptoms and detection of conidia by volumetric spore traps. The first powdery mildew colonies were randomly distributed throughout the vineyard and located on the lower surfaces of leaves in close proximity to bark. This comprehensive body of evidence indicates that cleistothecia are the only known source of primary inoculum for grape powdery mildew epidemics in eastern Washington. These findings are important for several reasons.

The presence of cleistothecial in the life cycle of grapevine mildew indicates that: 1) spring wetting events are necessary and required to initiate epidemics; 2) the presence of this stage increases the risk of fungicide resistance. Furthermore, by tracking the deterioration of the overwintered inoculum supply over a period of several years, we now know that the primary inoculum supply is exhausted by the end of the bloom period. Cleistothecia require at least 0.1 inch of precipitation (or over-the-canopy irrigation) at 50°F or greater in order to ­release ascospores. If these conditions are not met by the end of bloom, primary infection will probably not occur in the vineyard.

Any mildew that does become established later in the season will do so after the period of highest fruit susceptibility and will enter the vineyard from outside. Another very significant contribution to our knowledge of grapevine powdery mildew came from the grape powdery mildew research group located at the New York Agriculture Experiment Station, Geneva. Researchers there did an extensive study on the susceptibility of Chardonnay berries to infection by E. necator.

For years, it was thought that grape berries were susceptible to powdery mildew from bloom through veraison, and disease management factors were built around this phenomenon. In an extensive series of inoculation studies, our colleagues in New York demonstrated that berries were susceptible over a period of time much shorter than previously thought. They demonstrated that the resistance of grape berries to infection increased with berry age. This age-related resistance is known as “ontogenic” resistance. Their studies indicated that fruit were susceptible from early bloom until three to four weeks after bloom and found that two fungicide applications made during this window provided control of fruit mildew equal to full-season programs comprised of up to seven fungicide applications.

Their findings profoundly affect how we should approach disease management in our region: our most effective fungicides should be aimed at this critical period of fruit susceptibility. Since my last Good Fruit Grower article in 2000, there have been several new additions to our fungicide arsenal. We have received registrations for the DMI fungicide tebuconazole (Elite), strobilurin fungicides Pristine and Cabrio, and the quinoline fungicide Quintec.

These new compounds are highly efficacious, excellent materials to be used during the critical bloom period, and safe to beneficial insects. However, all are at moderate-to-high risk for the development of fungicide resistance. Our colleagues at Geneva have documented another important development. They recently reported performance problems with the strobilurin fungicides (e.g., Abound, Sovran, Flint) in selected New York vineyards during the 2003 season.

Unfortunately for some New York growers, they have localized resistance to both DMI and strobilurin fungicides, leaving some growers very few synthetic options for powdery mildew management in a region known for its mildew pressure. The disturbing aspect of strobilurin resistance was that it developed rapidly in a fashion different from DMI fungicides. Whereas the manifestation of resistance to DMI compounds has developed gradually, resistance to the strobilurins occurred as a sudden and almost complete control failure. Resistance developed in vineyards where 15 to 20 strobilurin sprays (since the introduction of the compounds) had been made.

It is important to note that when strobilurin resistance (and DMI resistance for that matter) occurred in other regions, it was dependent on the fungicide use history and pattern in particular vineyards and not regionwide. In other words, in some New York vineyards the strobilurins still perform superbly. Fortunately, strobilurin resistance does not appear to have developed in Washington State yet.

To avoid it, growers should consider the vineyard spray history and rotate out of the strobilurin class if the total number of applications is approaching 15. If the number is less, the grower should limit the use of strobilurin fungicides by using other fungicide modes of action in the spray rotation.In terms of the number of fungicide tools available, at no time have there been so many choices available to the grape grower, but growers should be cognizant of the fact that many of our new—and older—efficacious fungicides are at high risk for the development of resistance.