Ohio cucurbit growers have been battling Phytophthora blight in squash for at least the past 2 weeks, and yesterday we diagnosed an unusual outbreak in cucumber seedlings. Phytophthora can attack any part of cucurbit plants, but we don’t usually see seedling disease in cucumbers in Ohio. This may be due in part to an “escape” scenario, since many cucumber crops are planted early before Phytophthora blight pressure builds. In addition, cucumber plants usually hold up well to Phytophthora blight, although fruits are highly susceptible. It looks like this may be another season of high Phytophthora blight pressure.
Management of Phytophthora blight in cucumbers and other cucurbits is complicated by the fact that downy mildew may also be a problem. Currently we are only seeing downy mildew on cucumbers in northern Ohio, although we expect to find it in melons soon – see my posts on June 28 and July 8. Phytophthora blight is different from downy mildew in a number of ways, and similar in others. One big difference is that Phytophthora movement through the air is limited, unlike downy mildew. Another is that the pathogen survives over the winter in Ohio. Third, Phytophthora has to be introduced into a field, usually by contaminated water or movement of soil. Phytophthora tends to thrive in hot weather and downy mildew in cooler weather, but there is considerable overlap. Phytophthora and the downy mildew pathogen are related; some of the fungicides that are effective against downy mildew are effective to a degree against Phytophthora. All cucurbits are susceptible to Phytophthora blight, as are peppers and some other vegetable crops. No varieties of cucurbits are resistant to Phytophthora blight, so cultural practices (raised beds, good drainage, clean irrigation water; no cull piles) and fungicides are needed to manage this disease. The pathogen is a “water mold” and moves around readily in wet, and especially flooded fields. The sporangia that contain the zoospores that swim to and infect plants can also be splashed onto stems, leaves and fruit.
Keeping ahead of both diseases is important, but any efforts to manage them can be undone by long periods of wet weather, when it is particularly important to keep a tight schedule of fungicide applications. The Midwest Vegetable Production Guide for Commercial Growers lists the products labeled for these diseases on cucurbits – see page 117 in the 2017 Guide for a chart on relative efficacy of fungicides against different diseases of cucurbits. Dr Mary Hausbeck, Michigan State University, recommends applying fungicides when pickles are 1”, 3” and 5” long to suppress Phytophthora blight. Fungicides with 0-days pre-harvest interval (PHI) – Orondis Ultra, Ranman, Zampro and Zing! – will be useful when fruits are close to harvest. Make sure to note re-entry intervals. While copper in itself is not very effective against Phytophthora blight, tank mixing with a copper fungicide (e.g. Kocide 3000 – also 0 days PHI) improves activity against Phytophthora. Dr. Mohammad Babadoost’s research at the University of Illinois indicates that Revus tank mixed with a copper fungicide and alternated with Ranman + copper, Tanos + copper, or Zampro + copper (7-day intervals) is effective in suppressing Phytophthora blight.
Be sure to read and follow label directions for application of fungicides and other pesticides. Alternating fungicides with different modes of action is critical to reduce the risk of resistance development in pathogen populations.
Click on table to enlarge.