One of the benefits of our cold Canadian winters is sometimes getting a little extra help from Mother Nature with reducing overwintering pressure of diseases such as powdery mildew. Unfortunately, the relatively mild winter we just had may not have been enough to kill overwintering mycelium in buds that were infected with powdery mildew last season. Conidia will grow out of the buds in the spring as infected tissue and spread to other leaves causing secondary infection.
Dormant buds infected with powdery mildew are typically feathered, pointed or shriveled (Figure 1) and usually break dormancy later than healthy buds. This means susceptible, green tissue may already be present when the first conidia are produced. If conditions are ideal, even a small powdery mildew population can quickly explode if not managed properly.
Protectant fungicides used for early season scab management do not have activity on powdery mildew. Tank-mixing a low rate of sulphur (3-5 kg/ha) with captan and/or an EBDC, beginning at ½” green, will provide good activity on scab while also suppressing powdery mildew. This early season program will help free up other chemistries, such as the sterol inhibitors (Nova, Fullback), SDHIs (Aprovia, Fontelis, Sercadis, Luna Tranquility) and strobilurins (Pristine, Flint, Sovran) for later use when plenty of new growth is present.
Some key points for effective powdery mildew control this year include:
- Maintain a tight spray schedule with high rates during primary scab infection period. Powdery mildew does not invade mature leaf tissue, so spread of mildew ceases when trees stop producing new terminal leaves.
- Rain deters powdery mildew development by washing off spores. Instead, mildew thrives in dry weather and high relative humidity. So, protectant sprays may still be required during dry periods when there is little risk from apple scab.
- Getting good mildew control following an outbreak will take several seasons. Mildew infected white shoots from last year’s failure will persist through the season, but does not indicate current fungicides are failing. The current season mildew program is designed to prevent spread that would lead to primary infection for next year.
- Include a mildewcide, such as sulphur in all sprays beginning at ½” green until temperatures are greater than 25°C or when applying oil. Sulphur lacks post-infection activity, so must be applied early season. A tank-mix that includes captan, EBDC and sulphur provides excellent protection against scab, rust and mildew.
- If pressure was low last year, oil applied for mites may provide suppression of powdery mildew. Use a 2% solution (20 L/1,000 L water) for dormant sprays or a 1% (10 L/1,000 L water) solution for summer sprays. Do not use captan- or sulphur-containing products within 14 days of an oil application.
- Where they are working, include fungicides from other groups, including sterol inhibitors, strobilurins or SDHIs during the critical infection period, generally at pink and petal fall. Tank mix these products with captan or an EBDC.