Revising the Gubler-Thomas Model for Powdery Mildew – Adding Real Time PCR and a Revised High Temperature Threshold
Use of the Gubler-Thomas Model for powdery mildew risk assessment by California grape growers has already achieved goals of better disease control by targeting fungicide applications to high risk conditions over many of the temperature ranges seen in California grape growing areas. Use of the model in some years has reduced fungicide applications significantly. We report here our results to extend the predictive power of this model to higher temperature regimes. Better control of powdery mildew of grape has the potential to improve crop yield and quality as well as sustainability. Extending the high temperature range of the Gubler-Thomas Model would potentially allow for even fewer fungicide applications per season. With global climate change, the need for better understanding of the role of high temperatures on disease may increase in importance. We have conducted 2.years of controlled environment studies which have enabled us to understand the influence of high temperatures on grape powdery mildew (Backup 2009). Under controlled laboratory conditions, temperature and duration were carefully tested and their effects characterized on germination, growth and sporulation of the fungus. Our work shows that E. necator continues to germinate, infect, grow, and sporulate in the lab at higher temperatures than previously thought. In 2009, after controlled work with single heat exposures, we tested how multiple heat spikes affect fungal growth and reproduction. We found that temperature is increasingly lethal to the pathogen, and slows or reduces colony survival, delays spore production, and reduces the number of spores produced. The higher temperatures, such as 36 and 38° C for 4 and 2 hours, respectively, had a more pronounced effect than 34º C or the room temperature control, as did the higher number of consecutive heat treatments (1, 2 or 3), although to a lesser degree than temperature increases alone. However, repeated exposures to 4 hours of 36 and 38 C up to 3 times, which would total 12 hours, did not result in the same colony death and lack of spore production as one longer exposure of 12 hours straight had resulted. It appears that the fungus can and does recover with these shorter high temperature intervals. We have completed one year of testing the results of our integrating the lab work with our field studies. We are focusing on 36 °C and 38° C as important high temperatures and at 2 to 4 hours duration for the new high temperature threshold for the GT model. We are also adjusting how the index accounts for observed delays in fungal growth and reproduction due to the sublethal conditions experienced in the vineyard including not adding points to the index for several days after a high temperature spike to mimic the delays in growth observed in the lab. We are using the results from this work to assist both public and private end users of weather data and the model. With funding from the CDFA Specialty Crops Block grant only, we are developing vineyard spore trapping and molecular diagnostic techniques for the pathogen.