Cucurbit Downy Mildew Observed in Arkansas
by Aaron Cato - August 30, 2023
Downy mildew was observed in Arkansas on August 30th on cucumbers near Hamburg
in Ashley County. Cucurbit growers across the state need to scout now and consider
making targeted fungicide applications. See below for more info.
Downy mildew was observed in Arkansas on August 30th on cucumbers near Hamburg in Ashley County (Picture 1 and 3) . Due to the aggressive nature of this disease and its propensity to move through
weather systems, cucurbit growers across the state need to scout now and consider
making targeted fungicide applications. This is important considering the humid conditions
we have had in the last couple of weeks, which favors downy mildew, although the lack
of rainfall and storm systems should help to slow down this disease to some degree.
See below for more information on scouting and management of Cucurbit downy mildew.
Downy Mildew 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.
Cucurbit downy mildew presents as angular, yellow/light-green spots, which turn into
necrotic yellow/brown lesions on the top of leaves (Picture 1 and 2) . It can be further identified by gray/purplish spores that have a water-soaked appearance
on the underside of leaves (Picture 3 and 4) . Growers should keep an eye out for these symptoms and think of sending a sample
to the Plant Health Clinic through their county agent if you have any questions on
what disease you are seeing.
Management Growers should scout at least once a week for downy mildew in all cucurbits. These
scouting efforts should be intensified if we have confirmed cases in the state. Fungicide
products that provide good control of downy mildew are Elumin (ethaboxam, FRAC 22), Orondis Ultra (oxathiapoprolin FRAC 49 + mandipropamid FRAC 40), Orondis Opti (oxathiapoprolin FRAC 49 + chlorothalonil FRAC M05), and Ranman (cyazofamid, FRAC 21). See the 2023 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. Normal fungicide programs that utilize chlorothalonil
containing products (Bravo, etc.) will help to suppress this disease but one of the
downy mildew specific products above should be added alongside when the disease is
first observed.
Picture 1 – Cucurbit downy mildew on cucumbers in Ashley County, AR 8/30/2023. Chlorotic and
necrotic leaf lesions that stay within leaf vein boundaries are good indicators of
this disease. Photo by Aaron Cato.
Picture 2 . 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 3 . The underside of cucumber leaves where cucurbit downy mildew was observed in Ashley
County, Arkansas. Water-soaked growth on the underside of lesions can be found within
the boundaries of leaf veins. Photo by Aaron Cato.
Picture 4 . 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.
