At this time of year, most Ohio cucumber crops are harvested, and downy mildew is just an unpleasant memory. However, downy mildew is not done yet – today I received a call from Ron Becker, retired from OSU Extension, who reported downy mildew on high tunnel cucumbers in Wayne County. This is a good time to talk about managing cucumber downy mildew in “protected culture”, meaning greenhouses and high tunnels. High tunnels allow a significant extension of the cucumber growing season; greenhouses can permit cucumber production during the winter months.
Several fungicides routinely used to manage downy mildew in cucurbits in open fields may not be used in greenhouses or high tunnels. The labels of these fungicides explicitly prohibit their use in greenhouses. According to the Ohio EPA, high tunnels are equivalent to greenhouses for the purpose of pesticide applications. Labels of fungicides that are allowed in greenhouses and high tunnels either acknowledge this or are silent on this use. A summary of fungicide labeling for greenhouse vegetables is provided in Table 17 of the Midwest Vegetable Production Guide for Commercial Growers.
Amongst the prohibited fungicides are chlorothalanil (Bravo, Equus), Presidio, Zampro and Zing!. Ranman and Previcur Flex are permitted for use on greenhouse cucurbits, and the labels include specific instructions. Manzate and Gavel labels are silent on greenhouse use.
Small dark “dots” are spores of the downy mildew pathogen on the underside of a cucumber leaf.
It is very important to manage downy mildew in greenhouses over the winter and early spring months. The downy mildew pathogen requires living cucurbit plants to reproduce; it does not survive the winter outdoors in northern climates. However, greenhouses can serve as “green bridges” for downy mildew, providing living hosts from fall to spring. Greenhouse cucumbers with downy mildew provide inoculum for field cucumbers in May and June. The earlier downy mildew arrives in Ohio, the more difficult and expensive it is to manage. An effective fungicide program in the greenhouse that alternates downy mildew fungicides tank mixed with a protectant fungicide such as Manzate is a must for greenhouse cucumber production.