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by Aaron Cato - September 2, 2020
This week the plant health clinic confirmed two cases of downy mildew on Jack-O-Lantern
pumpkins, one in White County and another in Clay County. Due to the aggressive nature
of this disease and its propensity to move through weather systems, pumpkin growers
need to scout now and consider making targeted fungicide applications. This is even
more important considering the incredibly humid/wet conditions we have had over the
last week, and how many storms have moved across the state. See below for more information
about this disease and recommendations for forecasting, scouting, and management.
Downy mildew is one of the most devastating diseases of cucurbits. In Arkansas downy
mildew of cucurbits is only occasionally observed because the fungus that confers
this disease, Pseudoperonospora cubensis, can only survive in living cucurbit plants. These plants do not survive the winter
in Arkansas and therefore this disease does not overwinter in the state. However,
this disease does overwinter in states further South and it can work its way across
the Southeast in the summer as spores are moved in the wind. This makes scouting very
important for disease management, especially considering the limited number of products
that provide good suppression.
Identification and Monitoring
Cucurbit downy mildew presents as angular, faint yellow-light green spots early, which
turn in to necrotic yellow-brown lesions on the tops of leaves (Picture 1). It can
be further identified by gray/purplish spores on the underside of leaves (Picture
2). Growers should keep an eye out for these symptoms and check the forecasting tool
that has been created to determine areas at the highest risk of infection. This tool
works via reports from extension specialists of confirmed cases of downy mildew, which
are then combined with weather data to determine the risk of surrounding areas. Please
consult the CDM IPM Pipe Website for downy mildew forecasting to get out ahead of
any potential issues. Provided below is the Current forecast for downy mildew (Picture 3). Expect this to change with reports that we sent in on 9/2/2020.
Growers should scout at least once a week for diseases in pumpkins. These scouting
efforts should be intensified when forecasts indicate a risk for our area, or when
we have confirmed cases in the state. Fungicide products that provide good control
of downy mildew are Ranman (cyazofamid, FRAC 21), Orondis Ultra (oxathiapoprolin FRAC 49 + mandipropamid FRAC 40), and Orondis Opti (oxathiapoprolin FRAC 49 + chlorothalonil FRAC M05). See the 2020 Southeastern US Vegetable Crop Handbook for further fungicide recommendations and products that provide fair control/prevention.
None of these products will provide good control as a salvage application, making
it necessary to scout often and watch the IPM Pipe forecasting.
Picture 1. Downy mildew symptomology observed in Clay county. Leaves toward the bottom of the
picture show signs of early symptomology with yellow angular lesions. Many leaves
of this picture show advanced symptomology with leaves rolling inward. Photo by Allison
Howell, Clay County UAEX.
Picture 2. Angular lesions exhibiting grey/purple spore growth on the underside of the leaf.
This is a key characteristic of potential cucurbit downy mildew. Photo by Allison
Howell, Clay County UAEX.Picture 3. Forecast for September 1st that indicates a high risk of cucurbit downy mildew for areas close to Arkansas.
This forecast is prior to samples from Arkansas being sent in on 9/2/2020.