The first incidence of Cucurbit Downy Mildew was detected in New Jersey, United States on June 13th, 2023. Previously (in 2021 and 2022) Clade II has been detected in Nova Scotia, in the Annapolis Valley region, which predominantly infects cucumbers and cantaloupes. Clade I, which affects watermelon, pumpkin, squash, and zucchini has not yet been detected, but monitoring should still be undertaken in these crops on the chance that it does make an appearance.
It is important that growers remain vigilant and scout their fields regularly for this disease, as spores may be blown into the province by storm systems in the United States. We are not recommending protectant fungicides at this point in time, but strongly recommend weekly scouting for disease. Downy mildew may be identified initially as water-soaked lesions that appear on the top side of the leaf, which may first appear on any region of the canopy. The most ideal time to identify these lesions are during or right after a heavy dew. The centre of the lesion initially appears chlorotic or yellow before eventually the tissue dies, becoming brown or necrotic (Fig 1 and 2). This disease is very fast spreading, the lesions will continue to spread across the canopy, and quickly kill the entire plant if left unchecked. When conditions are humid, a “downy growth” may be observed on the underside of the initial water-soaked lesions. These symptoms may first appear before lesions on the upper leaf. This growth is particularly apparent in the morning, after a period of wet weather or dew formation.
Growers must stay vigilant, scout often, and report any positive findings they may see.
Figure 1: Chlorotic lesions associated with downy mildew infection on cucurbits.
Figure 2: Necrotic lesions associated with downy mildew infection on cucurbits.
If you suspect downy mildew in your field, please contact Dustin MacLean, the Field Plant Pathologist at dustinmaclean@perennia.ca or 902 324-9623.
Posted by Dustin Maclean, Horticulturalist and Field Plant Pathologist with Perennia
