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Missouri Environment & Garden


Zelalem Mersha
Lincoln University Cooperative Extension
Plant Pathology

Cucumber Downy Mildew

Zelalem Mersha
Lincoln University Cooperative Extension

Published: October 14, 2015

Late onset of Cucumber Downy Mildew in Central Missouri in 2015 and its Implications

Cucurbit Downy Mildew (CDM) (Pseudoperonospora cubensis) was confirmed on cucurbits sampled from the Sentinel plots located at Lincoln University’s George Washington Carver Farm in Jefferson City, Missouri. After the Labor Day weekend, angular, yellowish lesions were seen on ‘Straight Eight’ cucumbers, ‘Walthrum’ butternut squash and ‘Hale’s Best’ cantaloupe. Althought the Sentinel plots started in the summer of 2013 and continued annually, this is the first time that CDM was seen in these plots.

Vein bounded angular lesions on the upper side of a cucumber leaf.

Vein bounded angular lesions on the lower side of butternut squash.

In addition to the ‘Straight Eight,’ other cucumbers such as ‘Cobra,’ and ‘Dasher II’, which have shown a relative resistance to powdery mildew, were also infected with P. cubensis. Basil plants on the sentinel plots were free of downy mildew infection. Nonetheless, an organic grower from Holt, Missouri, reported a high level of basil downy mildew disease in the summer and fall of 2015.

Symptoms and Signs: In an attempt to isolate the casual organism, leaves with angular, yellowish lesions were placed in Ziploc® plastic bags and were kept moist overnight for diagnosis. Under a stereoscope, vein-bounded fuzzy and blackish sporulation was clearly visible on the lower side of the leaves. In addition, the lemon shaped sporangia and dichotomously branched sporangiophores (the sporangia-carrying organ) of the oomycete causing CDM (P. cubensis), were seen under a microscope.

Sporulation on the lower leaf side.

Sporangia and Sporangiophore.

Most field production of cucurbits will end soon in Missouri. Even so, growers who produce cucurbits in protected systems such as greenhouses or tunnels may still experience the consequence of this aggressive disease which may cause severe damage to their crop in a short time. The first action, as listed below will be to monitor, detect, and report the disease to prevent and curb its further spread.

The 2015 growing season has been unique compared to the past few years in terms of weather. According to the Missouri Climate Center, the total precipitation in Missouri from May to July was at least seven inches higher than the average. While optimum moisture is critical for plants, too much of it harms them in many ways. Extra leaf wetness usually favors this group of microbes, water molds including P. cubensis. This could partly be the reason for the early detection of CDM in Michigan, Ohio, Indiana and Illinois in 2015.

Epidemiology: Pseudoperonospora cubensis is an obligate pathogen. This means that a host plant needs to be present for it to survive. The pathogen alone will not live through the cold winter unless it is shielded in protected systems such as greenhouses and tunnels. In most years, under normal weather conditions, the cold fall and winter will arrive before the pathogen makes its way to Missouri. It is most likely that the spores of P. cubensis are carried from neighboring states. The spores can travel long distances by strong winds and storms. In some instances, spores of this pathogen are reported to have been blown as far as 600 miles in 48 hours! Alternatively, they might have reached Missouri through transplants from areas with a heavy CDM infection. Very often, the disease is reported in Kentucky, Tennessee, Oklahoma and Illinois. Key weather parameters that favor the disease are a relatively mild temperature (59 - 68°F ), high humidity and long hours of leaf wetness.

Management of Cucurbit Downy Mildew

Early planting and other best management practices. In most years, CDM may not usually migrate and reach to Missouri from the southern states owing to the long distance. Even in exceptional years like 2015, it arrived late and may only affect fall crops. Planting cucurbits as early as possible will greatly avoid downy mildew disease. Optimal distance between plants, trellising, avoiding overhead irrigation, and improved ventilation to reduce relative humidity are some of the best management practices to prevent the disease. 

Monitor and report downy mildew as early as possible. Please contact your county Extension office, University of Missouri’s Plant Diagnostic Clinic or Lincoln University’s Plant Pathology Program if you see a symptom similar to what is pictured above. Once confirmed, the disease can easily be reported on a website (http://cdm.ipmpipe.org/). This will keep other growers informed of the prevalence of the disease. Then they can be alerted and prepare to take actions to prevent this disease and/or cure affected plants.

Fungicide sprays. The Midwest Vegetable Production Guide for Commercial Growers 2015 is a very good resource to find best-performing products against cucurbit downy mildew. In addition, Dr. Mary Hausbeck’s group at Michigan State University has issued a recent report and advice based on an ongoing research project. More information can also be found from an extension publication of the Ohio State University.

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REVISED: October 15, 2015