Cherry Powdery Mildew

1. Cherry Powdery Mildew Panel Discussion Duane Holder Dave Stirling Dr. Peter Sholberg

2. Caused by a fungus that will only grow on living cells Produces an infection peg to absorb nutrients from inside the plant cell During the growing season P. leucotricha attacks green foliage and fruit High humidity is conducive to the disease Cultivars vary greatly in susceptibility Cherry Powdery Mildew(Podosphaera clandestina)

3. Young leaves are more susceptible than mature leaves Leaf infections occur 4-6 weeks after bud break Infection causes puckering and distortion White powdery growth forms over leaves dispersing spores in the wind Fruiting bodies (cleistothecia) form on older leaves and allow the fungus to overwinter Leaf Symptoms

6. Slightly depressed circular areas form on the fruit surface Obvious signs of powdery mildew spores are absent As the fruit ripens a white powdery bloom appears on the fruit Fruit infection ceases when brix soluble solids reach 15-16% Fruit Symptoms

9. Tools for Disease Forecasting • Weather recording devices such as Spectrum Watch dogs for recording temperature, RH, rain, and leaf wetness • Disease forecasting software for a) predicting primary infection and b) degree of risk for secondary infection. • Spore traps for collecting powdery mildew spores that are released from bud break until harvest • Microscopic or molecular analysis to positively identify pm spores collected weekly from the orchard air

10. Disease Forecasting

11. Weather Data vs Spore Count Cherry Powdery mildew

12. Powdery Mildew Monitoring • Positive correlation between periods of wetness & release of spores Base 9˚C

13. A modified grape powdery mildew model could be used to predict primary cherry powdery mildew • Groves & Spotts have developed a model with three components • Ascospore degradation • Viability declines from 58% (bud burst) to 0% (after pit hardening • Primary infection • Occurs at 10 to 28C and interacts with relative humidity • Secondary Infection risk index (100 points) • Four consecutive days with ≥ 6h at 15-28.5C initiation • Each day ≥ 6 h between 15-28.5C add 20 index points • Index decrease 10 points < 6 h or temp > 28.5C • Low (0-40); moderate (40-50); high (60-100) disease pressure

14. Spore trap samples a specific volume of air by running 10 min/hour I-rods are collected from the spore trap and brought back to the laboratory I-rods are washed and DNA is extracted Real time (Q)PCR with species specific primers for pm provides the number of pm spores on the rods If zero spores there is no chance of pm Spore numbers could indicate disease risk Molecular techniques