Studies in the control of the powdery mildew disease of strawberry and of the biology of the fungal pathogen Sphaerotheca macularis f. sp. fragariae

Strawberry powdery mildew pathogen Sphaerotheca macularis f. sp. fragariae growth was modeled as a response to climatic variables. To confirm optimal conidial germination conditions, temperature was varied while holding relative humidity (RH) at saturation. To describe rate of lesion growth, this obligate fungal parasite-plant host interaction was observed in controlled environments of constant temperatures and constant RH spanning the ranges of these variables observed under natural conditions. Sporulation was observed at all temperatures (5--30 degrees) and RH (32--100 percent) tested, but was delayed and attenuated outside optimal conditions. Temperature and saturation water vapor pressure offered the most predictive value in mildew pathogen growth response, followed closely by water vapor pressure deficit. RH did not correlate with pathogen growth.;Field monitoring of powdery mildew epidemics on strawberry ( Fragaria x annanasa) revealed patterns closely allied with host phenology. Observations of mildew epidemics, their severity, crop yield, and associated weather were recorded in California fruit production fields at 1 site in 1997 and 3 in 1998. These data were compared to growth predicted from controlled environmental conditions (reported elsewhere). Weather in these fields was not a limiting factor to disease development, but rather the changes in epidemics observed were timely and synchronized with factors of host phenology, with regular temporal patterning despite differences in magnitude and onset.;Augmenting populations of the indigenous hyperparasite Verticillium lecanii controlled powdery mildew of strawberry. Trials were placed in three California fruit production fields during the spring and summer of 1998. Disease was reduced compared to the non-treated controls at all sites. Analysis of variance (ANOVA) for each site showed treatments significantly reduced disease severity at one site, as measured by percentage of leaf canopy colonized by the mildew pathogen, when compared to non-treated controls. T-test for all sites collectively showed treatments significantly reduced disease severity. Efficacy of applications is discussed with respect to environmental constraints to deployment of the mycoparasite as a biocontrol. Autoclaved spore suspensions showed similar efficacy in greenhouse trials, suggesting other than direct parasitism as a means of control.