Revising the Gubler-Thomas Model for Powdery Mildew Control

Laboratory studies were developed to measure the effects of temperature on spore germination, colony expansion and spore production on 4 isolates of E. necator from California vineyards. Spore germination was decreased as a result of either increasing temperature or duration. Higher temperatures were eventually lethal to conidia. Preliminary regression analysis showed relationships between temperature and spore germination had increasingly negative slopes with shorter (in hours) x-axis intercepts as temperature increased from 30° to 42° C. A majority of the temperatures studied prevented spore germination at shorter durations than for the other 2 assays. Reductions in spore viability due to high temperatures can reduce the rate of new infections in the field and are important in managing the pathogen, even if already established colonies are still viable.

At 30° and 32° C, colonies continued to grow, but at a slower rate than at the optimum growth temperature of 24° C. Higher temperature treatments produced increasingly negative regression slopes. Exposure to 40°, 42°, and 44° C stopped colony growth and was capable of eradicating mature colonies at sufficient durations (3 hr, 1.5 hr, and 1 hr, respectively). Growth rates increased from 9 to 19 mm2/day and sporulation increased from 13 to 41 conidia/ mm2 at 24° C as colonies matured over the durations studied. Colony expansion and spore production rates increased at the lowest temperature studied (30° C), but were dramatically reduced at higher temperatures studied.

Results of the first year laboratory studies are important in developing and refining the 3 assays used to assess high temperature effects on E. necator sporulation, growth, and infection. We established a field trail in unsprayed plots in a Sacramento County Chardonnay vineyard to study variations in canopy microclimate and powdery mildew. A research weather station (Campbell Scientific CR10) with 4 thin-wire thermocouples and light intensity (PAR) sensors was installed. Higher south-side leaf surface temperatures were observed (21.3° C) compared to north (20.7° C) (averaged for 1.5 months), with greatest differences at highest temperatures. Maximum south-side temperatures reached 36.7°C and north-side 35° C, with a maximum difference of 3.5° C. Ambient sheltered temperatures at 2 M were higher than canopy leaf surface temperatures, with greatest difference 10° C higher than north-side of the vines. First detected on May 23, 2007, powdery mildew disease incidence (DI) and severity (DS) were rated weekly. DS north-side was not significantly different from DS south-side. Interestingly there was slightly more disease early season south-side as compared to north-side (60%vs. 51%DI) and (4%vs. 2%DS). Subsequently, DI and DS north-side were greater (although not significantly). The Gubler-Thomas PM risk index showed that environmental conditions were conducive to disease beginning about May 12, 2007 and continued into July.

We intend to conduct further analysis of both the laboratory and field data collected in 2007, and to use the results and additional data gathered in the second year to complete this project. Results from the controlled environment studies and the field studies will be combined to refine the Gubler-Thomas model at high temperatures.