Powdery mildew is one of the most significant diseases affecting grape (Vitis vinifera) production around the world. The disease is caused by the hyaline ascomycete, Erysiphe necator, a pathogen capable of rapid proliferation under optimal environmental conditions. Disease onset begins in the spring with the release of ascospores from over-wintering chasmothecia (Gubler and Hirschfelt 1992). Once initial colonies are established, the fungus can asexually propagate via large numbers of conidia that disperse and re-infect additional leaves and developing fruits. Powdery mildew effects on the host include reduction in berry mass, potential cracking of berries, and increased susceptibility to berry rots (Gubler and Hirschfelt 1992, Calonnec et al. 2004, Gadoury et al. 2007). Economically, the disease may be damaging to California’s grape industry because of lost yield, a shortened shelf life for table grapes, and alterations in wine flavor (Gubler and Hirschfelt 1992, Gadoury et al. 2007).

In California, powdery mildew is principally controlled via periodic application of foliar fungicides, including sulfur and synthetic materials such as demethylase inhibitors and strobilurins (California Department of Pesticide Regulation 2004). A wide range of materials show at least some reduction in disease levels under field conditions (Janousek et al. 2007, Adaskaveg et al. 2008). We continued our annual powdery mildew trials during 2008 to evaluate the efficacy of various fungicide products, including registered synthetic materials of varied chemical classes, oils, and biocontrol products. We present the results of five trials conducted in a Chardonnay vineyard at Herzog Ranch in Sacramento County, California.