The Mold Complex of Napa Valley Grapes

¹ St. Helena, California

Over a five-year period, nearly 10% of all Napa Valley grapes were inspected. The tonnage of all mold-infected grapes represented less than 2% of the total crush. Over 98% of the fruit was clean and free of defects. Of the 2% that was infected, over 90% was infected with Botrytis cinerea Pers., approximately 8% with mildew (Uncinula necator), and about 2% with other molds identified as Aspergillus sp., Cladosporium sp., and Penicillius sp. There was no significant relationship between the total mold of grapes and the climatic region of the valley where the grapes were grown. Nor did the grape grade, based on sugar-acid ratio, show any relationship to the total mold content. Although possibily influenced by grape variety and time of harvest, the more mature grapes did show lower levels of mold. A considerable increase in mold during the last week of harvest was noted in all years that had over one inch of rain during the harvest season. Seasons with little or no rain maintained a fairly uniform level of mold growth throughout the vintage. The amount of mold varied considerably with the grape variety. Mildew was a factor in only one year, 1963, and its effect upon the total mold complex was appreciable.