Revising the Gubler-Thomas Model for Powdery Mildew

Laboratory studies were developed to measure the effects of high temperature (above known optimal temperatures for the fungus) on spore germination, colony expansion and spore production on 4 isolates of E. necator from California vineyards. Spore germination, colony size, and spore production decreased as a result of either increasing high temperature or duration. Regression analysis showed increasingly negative slopes with shorter (in hours) x-axis intercepts as temperature increased from 30° to 42° C. High temperature-dependent reductions in spore viability, colony growth, and rate of sporulation can reduce the rate of disease development in the field.

At 30° and 32° C, colonies continued to grow, sporulate, and germinate after 24-hr heat treatment. At these temperatures, decreases in pathogen fecundity compared to 22.5º C appeared to be negligible or inconsistent, and not biologically relevant for most growing areas in California. Treatment at 34º C decreased biological activity of E. necator, but at 36º C between 12 and 24 hours was required to kill the pathogen. For all 3 assays, the duration required to extinguish biological activity reduced as the temperature increased in 2º increments. At 44º C, only ½ to 1 hr was required to extinguish all or most activity. In addition to locating the lethal conditions from 30 to 44º C, and from ¼ hr to 24 hr, we also detected sub-lethal conditions that delayed colony growth by up to 6 days. Using multiple regression analysis, we derived equations for each assay to predict cessation of biological function in the pathogen as a function of temperature and duration. The three-factor ANOVA resulted in highly significant main effects and interactions for temperature and its duration, but isolate was not significant, nor were interactions of isolate with any other factor. This information then is potentially relevant to a number of California’s grape growing regions, because under field conditions, vines may be repeatedly exposed to sub-lethal temperatures.

Although our controlled environment results are compelling and internally consistent among the 3 assays, they differ from previous field data used to establish the high temperature threshold of the Gubler-Thomas powdery mildew risk index, and with some published studies. In general, our results indicate pathogen survival at higher temperatures and durations than previously reported. Some of the discrepancy is because our laboratory experiments do not account for effects of uV radiation, sub-optimal humidity, ontogenetic resistance, or resistance to infection induced by high temperatures. But our data strongly suggest that the powdery mildew risk index should be revised for higher temperatures.

We established the second year of a field trail in unsprayed plots in a Sacramento County Chardonnay vineyard to study variations in canopy microclimate and powdery mildew. We installed a Metos weather station with real time satellite data collection and web based access to hourly temperature data and the powdery mildew risk index. The weather station had a sheltered ambient air temperature sensor at 2.2m height above the canopy, and 16 specially designed thin wire thermocouples attached to the underside of leaves for intensive characterization of the vine canopy microclimate. We found that fully exposed external leaves had higher temperatures on average about 2° F than internal shaded leaves, with maximum differences at times of 10° F. We found that internal shaded leaves were closer in temperature to above the canopy sheltered ambient air temperatures. Exposed, external leaves experienced 3 times more hours above the current high temperature threshold for the Gubler Thomas model than internal leaves or the sheltered ambient air temperatures. We found greater disease earlier in the season on the shaded leaves, although eventually a similar incidence but lower severity was observed on the exposed leaves inoculated mid June. Internal leaves inoculated mid July had consistently greater disease than the external leaves through mid August. A handheld IR leaf surface temperature sensor measured about 10° F lower temperatures than the thermocouple sensors attached to the bottom of the same leaves. We found the IR temperature sensor to be quite variable and did not allow for a consistent differentiation of leaf surface temperatures by canopy location.

We will continue with our analysis of both the laboratory and field data collected in 2008, 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. Our goal will be to adjust the lethal and sub-lethal ranges predicted in the lab to conditions found in the canopy. By correlating disease outbreaks with canopy microclimate conditions recorded by environmental sensors, we will be able to identify which temperatures at what durations stop, reduce, or promote powdery mildew epidemics.