Powdery mildew has also arrived in Ontario.
Unlike many fungal pathogens, powdery mildew does not require prolonged leaf-wetness for infection to occur; in fact spores may geminate at humidity levels as low as 20%! The development of this disease does slow down at temperatures above 30 C. However, slightly cooler night-time temperatures may result in an unexpected infection period.
Fungicide efficacy is optimized when they are used preventatively. Scout fields regularly and initiate a spray program no later than the first sign of infection. Look for small, white powdery lesions on the underside of the leaf surface (Figure 1). Symptoms typically first appear on the leaves in the mid-portion of the plant or on the stems.
It is important to keep all new growth adequately protected. The leaves are most susceptible to infection 16-23 days after unfolding. Pumpkins and squash are particularly susceptible to powdery mildew. Uncontrolled infections result in premature defoliation, smaller fruit sizes and lower sugar levels.