Determining the Factors that Influence Disease Onset from Overwintering Powdery


  1. Determine how temperatures and chilling affect disease onset from grape buds harboring mycelium of Uncinula necator.
  2. Establish effective control strategies for bud perennation.
  3. Using the data from the first and second objectives, construct a model and subsequent risk assessment program for the control of the infected buds as primary inoculum sources.

Objective 1: From the first collection in December 1996, no bud infections were observed.

These buds were kept at 4°C for 8 months prior to planting in growth chambers due to lack of growth chamber space at the time. It is postulated that the extended length of constant cold temps may have had an affect on the buds. Surplus wood collected from the UC Davis Carignane vineyard is now being stored in a cold chamber maintained at 2-4° C. Buds will be randomly selected and pushed at 20°C to assess what the incidence of infection with different rates of chilling. The second collection was made in January 1998. Buds were observed daily for infection, sporulation, bud-swell, bud-break, flat leaf stage, and infection. Each shoot was measured and any symptoms of stunting were noted. Infected buds were observed in all temperatures except 30°C. The incidence of infected buds was highest at 15 and 20° C (Table 1). Preliminary data show a positive correlation between degree days and infection as well as between days to infection and temperature. Rates of sporulation of Uncinula necator do change with temperature but it is doubtful that is the only reason for the decrease in infected shoots at the higher temperatures. In fact, 25°C is close to optimum temperature for the growth of the pathogen. Further research will be needed to understand the dynamic association between accumulated chilling units and the response of the pathogen to the host growth rate. Material from the March collection is currently being assessed.

Objective 2: In year one of the study (1997), infected buds on each vine in each 1/3 acre replication were mapped (Fig. 1).

In April 1998 we will re-visit these blocks and assess disease incidence. We hypothesize that the vines in the treatment blocks will have a lower incidence of infected shoots in 1998 relative to those in the control blocks simply because early season infection in treated blocks did not occur on adjacent shoots. Therefore, bud infection on adjacent shoots should be reduced. After recording this information we will repeat the protocol as in 1997 and follow the treatment and control blocks into a third season (1999). Prior to treatment application in 1997, there were 734 infected buds in the blocks designated to be treated with Rally 4 oz/A and 762 infected buds in the non-treated control blocks.

Objective 3. Nothing as of yet has been constructed for a model.

Upon completion of mis years second growth chamber study and second field study, data will be analyzed. Model construction will be done in 1999.