5.5.8
DECREASED SENSITIVITY TO DMI FUNGICIDES OF STRAWBERRY POWDERY MILDEW FUNGUS AND HEAT TREATMENT AS A COUNTER MEASURE

K OKAYAMA, M NISHIZAKI and T SUGIMURA

Nara Agricultural Experiment Station, 88 Shijo-cho, Kashihara,Nara 634-0813, Japan

Background and objectives
Since a new cultivar Toyonoka of strawberry was brought into an extensive cultivation, powdery mildew became serious problem in Japan. Although sterol demethylation inhibitors(DMI) such as bitertanol, fenarimol, triflumizole and myclobutanil have been mainly sprayed in nurseries, their effects were not enough to control the disease. Poor fungicide efficacy has been attributed to the development of fungicide resistance in the fungus. This study determined the sensitivity of the fungus to DMI fungicides and established an efficient control measure of the disease.

Materials and methods
The strawberry cultivar, Toyonoka, was used throughout the experiments. DMI sensitivity of the powdery mildew isolates was determined by leaf disc assay [1] and the control effect of the fungicides was evaluated with runner-tip plantlets. Conidia of Sphaerotheca maculariswere collected from diseased strawberry leaves and fruits. The EC50 values and the minimum inhibitory concentration (MIC) were correlated with the control effect of the fungicides on runner-tip plantlets. For establishing a control measure of the disease, the effect of temperature on the disease development was determined by using runner-tip plantlets. The control effect of heat treatment and application of fungicides were examined with runner plants in a plastic house.

Results and conclusions
In the leaf disc tests EC50 values and MIC of myclobutanil, triflumizole and fenarimol gradually increased from 1994 to 1996. Sensitivity to DMI fungicides in leaf disc were reflected on control efficacy estimated with runner-tip plantlets tests. When MIC value determined by leaf disc tests exceeded 25mg/l for bitertanol, 5mg/l for myclobutanil, 10mg/l for fenarimol and 20mg/l for triflumizole, control efficacy became impractical even if the respective fungicide was sprayed at recommended concentration.
Runner-tip plantlets were attacked severely by the fungus at 20-25 C, but no plantlets were infected at 30-35 C. The mycelia and conidia on diseased leaves shrunk and no conidia germinated when incubated at 30-35 C,3,000 lux, for a week. When potted diseased plants were put into sealed vinyl bags and incubated at 30-32 C for five days, mycerial colonies disappeared, indicating that the disease could be effectively controlled at this temperature regime. Plastic tunnel and additional capillary watering in a plastic house were found to keep the temperature and humidity high enough to control the disease. When potted diseased plants were kept in the tunnel, the disease was suppressed effectively even after the plants were transplanted to the bed for fruits production in the plastic house. The control effect was enhanced by prior spraying with some fungicides.

Reference
1. Schepers HTAM, 1984. Netherland Journal of Plant Pathology 90, 165-171.