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Project description

Modeling of Pear Scab, Venturia pirina, Epidemiology and Disease Risk Using an Onsite Weather Monitoring System. (97DS030)
Program UC IPM competitive research grants program
W.D. Gubler, Plant Pathology, UC Davis
C. Feliciano, Plant Pathology, UC Davis
K. Dell, Plant Pathology, UC Davis
G.T. McGourty, Cooperative Extension, Mendocino County
Host/habitat Pears; Tree Crops
Pest Pear Scab Venturia pirina
Discipline Plant Pathology
Decision Support
Start year (duration)  1997 (Three Years)
Objectives Monitor pear scab pseudothecial development, ascospore maturation, ascospore release, and infection of plant parts from primary inoculum at two pear orchard field sites in Mendocino County, each with an onsite weather station with sensors for rain, leaf wetness, temperature, and relative humidity.

Refine the Spotts-Cevantes model for predicting pear scab ascospore release as well as conditions favorable for disease development, and incorporate these into an electronic format.

Test a treatment regime at one field site based on applying fungicides only when needed; i.e., when inoculum is viable and present in sufficient numbers and conditions are favorable for infection.

Results from 1998 demonstrated that fall treatments with lime-sulfur or urea radically reduce the production of ascospores from pear scab pseudothecia in over-wintered leaf samples. In 1999, we made full scale applications of fall treatments in an attempt to reduce ascospore production and fruit disease incidence in the orchard. Our results were not statistically significant because the untreated blocks were not consistently heavily diseased. We did find an indirect measure of the benefits of fall treatments, by relating ascospore production with disease incidence. This relationship, as shown by the equatioin Y=3.73X+0.75, where Y = fruit lesion incidence, and X = ascospore production (x10-3) , showed that the lower the number of ascospores, the lower the incidence of disease. Asci maturation in leaf samples and weather variables were monitored from 4 sites in 1998 and 5 sites in 1999 and compared to the degree-day model developed for Oregon conditions. Our data did not fit the model, indicating the need to substitute appropriate parameters to the model. However, the data may better fit a non-linear model or a model with additional variables, such as leaf-wetness, which we are currently investigating. The model was found to be a useful predictor of the date at which all asci contain mature ascospores. Spore traps from 3 sample sites ceased to catch spores within 7 -- 18 days after the model predicted 99.5% of asci are mature, adding confidence to the prediction.

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