Cucurbit downy mildew is caused by the fungus-like oomycete pathogen Pseudoperonospora cubensis (P. cubensis).
Cucurbit crops such as cucumber, watermelon, melon, cantaloupe, squash and pumpkin.
Leaves are severely affected, which can result in yield reduction or plant death.
Downy mildew causes angular, yellow to brown lesions on top of the leaves that are restricted by leaf veins (Figure 1). When conditions are favorable, for example early in the morning when moisture is higher, lesions observed on the underside of the leaf may be covered in dark “down” (Figure 2). This downy growth is masses of pathogen spores called sporangia, which can be seen in the field with a 20X hand lens. These spores germinate to produce the pathogen and infect leaves. Identification of downy mildew can be more difficult in watermelon and cantaloupe than in cucumber since lesions do not always have the characteristic angular shape, but spores on the underside of the leaf can provide additional evidence of the pathogen.
Downy mildew is often confused with other foliar diseases such as:
- Alternaria leaf blight, caused by Alternaria cucumerina (Figure 3)
- Angular leaf spot, caused by Pseudomonas psyringae pv. lachrymans (Figure 4)
- Anthracnose, caused by Colletotrichum orbiculare (Figure 5)
- Powdery mildew, caused by Podosphaera xanthii or Erysiphe cichoracearum, note white powdery growth on the leaf surface (Figure 6)
- Phytophthora leaf blight, caused by Phytophthora capsici (Figure 7)
- High-humidity and moisture (6-12 hours of moisture are ideal and usually occur as morning dew, rain or overhead irrigation).
- Cool temperatures (~60°F).
- In North Carolina, the disease typically begins in June and lasts throughout the growing season.
Pathogen spores can be transported from state to state through air currents. The pathogen needs a living host to survive and it can only overwinter on plants in sites with warm weather such as the Southern US and greenhouses. Spores come to North Carolina every year from infected plants in surrounding states or in greenhouses with year-round production.
- Plant early in the season to escape high disease pressure
- Do not allow water to remain on leaves for long periods of time
- Scout plants often and remove infected plants
- Sign up to receive alerts at the Cucurbit Downy Mildew IPM Pipe website and report any outbreaks to prevent others
- Plant tolerant varieties when possible
- Protect the crop with fungicides
Fungicides are required to control cucurbit downy mildew. Before disease appears apply fungicides at 7-day interval for cucumbers and 10-day intervals for other cucurbits. After disease is detected in your area apply fungicides at 5-day interval for cucumbers and 7-day intervals for other cucurbits. Alternate products and tank mix with mancozeb or chlorothalonil to avoid generating fungicide-resistant strains. For the latest fungicide recommendations for cucurbit downy mildew see the Southeastern US Vegetable Crop Handbook. Fungicide labels are legal documents, always read and follow fungicide labels.
|Active ingredient||Example product||Pre-harvest Interval (day)||Fungicide group|
|Oxathiapiprolin||Orondis Opti A||0||49|
|Cyazofamid||Ranman 400 SC||0||21|
|Famoxadone + cymoxanilG||Tanos 50WG||3||11+27|
|Ametoctradin + dimethomorph||Zampro||0||45+40|
|Mancozeb + zoxamideG,B||Gavel 75WG||5||M+22|
|G Can be used in greenhouses, see label for restrictions
R Fungicide resistance reported
B Use only before disease
For example, before disease in cucumber you can use: Tanos + Bravo, alternated with Zampro + Bravo, alternated with Gavel every 7 to 10 days in rotation. After disease you can use: Ranman + Bravo, alternated with Previcur Flex + Bravo, alternated with Orondis Opti A + Bravo every 5 to 7 days in rotation. Using the highest rate labeled for products will result in better control and lower risk of fungicide resistance. Check label for restrictions of number of applications to plan your program.
Organic growers have less chemical options that are effective; the only OMRI labeled active ingredients that have some efficacy against cucurbit downy mildew are fixed copper formulations.
Products containing the active ingredients copper or chlorothalonil (the trade name of one product with chlorothalonil is known as ‘Daconil’) are the best and only effective products available to home gardeners. In addition, home gardeners should grow varieties with tolerance if they are worried about cucurbit downy mildew in future years because most chemicals available to the home gardener are not sufficient to control cucurbit downy mildew once it appears. Once plants are infected in a home garden, there is little that can be done to protect them besides weekly fungicide sprays.
- The NCSU Plant Disease and Insect Clinic provides diagnostics and control recommendations
- The Cucurbit Downy Mildew IPM Pipe provides forecasts and reports of disease outbreaks
- The Extension Plant Pathology Portal provides information on crop disease management
- The Southeastern US Vegetable Crop Handbook provides information on vegetable disease management
- The USDA Fungus-Host Distributions Database provides information about reported hosts for fungal and oomycete pathogens
This factsheet was prepared by the NCSU Vegetable Pathology Lab with funding provided by Pickle Packers International, Inc.
Publication date: Dec. 16, 2013
There is an alternate Spanish language version of this document here: Añublo lanoso en cucurbitáceas
Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your county Cooperative Extension agent.
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