Genetics of Fungicide Resistance to Bayleton, Rally, and Rubigan

Objective: To determine the genetics of fungicide resistance in U^. necator to the triazole and pyrimidine DMI fungicides currently registered on grapes in California. Twenty isolates were collected from Lone Oak in Monterey County and twenty isolates were collected from the Renaissance Vineyards in Lassen County in 1990. Some Lone Oak isolates have previously exhibited DMI resistance to a high degree. Renaissance isolates have never been exposed to DMI’s and do not demonstrate resistance. Thirty single conidia were transferred to separate isolated plants from the most resistant Lone Oak isolate and thirty single conidia from the most sensitive Renaissance isolate. The resulting colonies were allowed to proliferate on young Carignane plants to test for sexual mating type. Fifteen isolates from each vineyard (Renaissance and Lone Oak) were tested for sexual mating type by rubbing conidia from a colony of New York type ‘F’ or New York type ‘G’ directly onto a leaf infected with the colony to be tested and adjacent to the California test colony. The plants containing both colonies were grown in isolation tubes for 30 to 60 days and then observed by microscope for cleistothecia formation. If an isolate produced cleistothecia when crossed with New York ‘F’ it was determined to be type ‘G’ and if it produced cleistothecia when crossed with New York ‘G’ it was determined to be type ‘F’. At this time, two California type ‘G’ sensitive isolates, one California ‘F’ sensitive isolate, and one California ‘F’ resistant isolate has been identified. However, viability of the ascospores has not yet been verified and is currently being tested. Because of the difficulty in obtaining successful crosses this report is preliminary. During the next year, we will screen the crosses and their progeny for DMI sensitivity. The results of these screenings will be reported as they occur.