NC State Extension Publications

Pathogen

Cucurbit downy mildew is caused by the fungus-like oomycete pathogen Pseudoperonospora cubensis (P. cubensis). There are two types of P. cubensis isolates that can infect cucurbit crops and weeds referred to as clade 1 or clade 2 isolates. Isolates belonging to clade 1 tend to more frequently infect watermelon, pumpkin, and squash while isolates belonging to clade 2 more frequently infect cucumber and cantaloupe. Isolates belonging to clade 2 can quickly become resistant to fungicides, thus, fewer effective fungicides ara available for downy mildew control in cucumber and cantaloupe.

Host Crops and Plants

Cucurbit crops such as cucumber, watermelon, melon, cantaloupe, squash and pumpkin.

Host Parts Affected

Leaves are severely affected, which can result in yield reduction or plant death.

Identification

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.

Cucurbit downy mildew on cucumber

Figure 1. Angular, yellow lesions on topside of the leaves that are bound by leaf veins.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Dark cucurbit downy mildew spores on underside of cucumber leaf

Figure 2. Dark spores on underside of the leaf.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Similar Diseases

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)
Alternaria leaf blight

Figure 3. Alternaria leaf blight.

Dr. Mary Hausbeck, Michigan State University

Angular leaf spot

Figure 4. Angular leaf spot.

Dr. Mary Hausbeck, Michigan State University

Anthracnose

Figure 5. Anthracnose.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Powdery mildew

Figure 6. Powdery mildew.

Dr. Mary Hausbeck, Michigan State University

Phytophthora leaf blight

Figure 7. Phytophthora leaf blight.

Dr. Lina Quesada, NCSU Vegetable Pathology Lab

Favorable Environmental Conditions for the Disease

  • 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.

Disease Transmission

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.

General Disease Management

  • 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

Disease Control for Conventional Growers

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.


Table 1. Example products for cucurbit downy mildew control in cucumber and cantaloupe.
Active ingredient Example product Pre-harvest Interval (day) Fungicide group
Cyazofamid Ranman 3.33 SC 0 21
Fluazinam Omega 500 F 7 29
Oxathiapiprolin Orondis Opti A 0 49
MancozebG Dithane 5 M
Chlorothalonil Bravo/Equus/Daconil 0 M
G Can be used in greenhouses, see label for restrictions

For example, in cucumber you can use: Ranman + Bravo, alternated with Omega + Bravo, alternated with Orondis Opti every 7 days before the disease is reported in the state and every 5 to 7 days depending on the weather after disease is reported in the state. Once the disease is reported in the state, using the highest rate labeled for products will result in better control and lower risk of fungicide resistance. Because rain favors this disease, when possible protect crops before and after a rain event. Check label for restrictions of number of applications to plan your program.

Table 2. Example products for cucurbit downy mildew control in watermelon, squash, and pumpkin.
Active ingredient Example product Pre-harvest Interval (day) Fungicide group
Cyazofamid Ranman 400 SC 0 21
Fluazinam Omega 500 F 7 29
Oxathiapiprolin Orondis Opti A 0 49
PropamocarbG Previcur Flex 2 28
Ametoctradin + dimethomorph Zampro 0 45+40
Ethaboxam Elumin 2 22
Mancozeb + zoxamideG Gavel 75WG 5 M+22
Fluopicolide Presidio 2 43
Famoxadone + cymoxanilG Tanos 50WG 3 11+27
MancozebG Dithane 5 M
Chlorothalonil Bravo/Equus/Daconil 0 M
G Can be used in greenhouses, see label for restrictions

For example, in squash you can use: Ranman + Bravo, alternated with Previcur Flex + Bravo, alternated with Zampro every 10 days before the disease is reported in the state and every 7 days after disease is reported in the state. Once the disease is reported in the state, using the highest rate labeled for products will result in better control and lower risk of fungicide resistance. Because rain favors this disease, when possible protect crops before and after a rain event. Check label for restrictions of number of applications to plan your program.

Disease Control for Organic Growers

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.

Disease Control for Home Gardeners

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.

Useful Resources

Acknowledgements

This factsheet was prepared by the NCSU Vegetable Pathology Lab with funding provided by Pickle Packers International, Inc.

Author

Associate Professor, Plant Pathology (Cucurbits and Sweetpotato)
Entomology & Plant Pathology

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 NC State University or N.C. A&T State University 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 local N.C. Cooperative Extension county center.

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