Downey mildew of curcurbis (cucumbers, watermelon, pumpkin etc)

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Downy Mildew of Cucurbits

(Cucumbers, cantaloupe, watermelon, pumpkins, etc.)

(Pseudoperonospora cubensis

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Downey mildew is the most common fungal disease of cucurbit crops, which causes withering and premature death of leaves. Fruits formed during an attack will be small and poorly flavored. The pathogen develops best on the lower surface (underside) of leaves, thus a successful management program necessitates controlling the pathogen at the initial stages of disease development on lower as well as the upper surface of the leaf. Wet conditions are favouring rapid development and spread of downy mildew.

Major Challenges in Management of Downy Mildew

Downy mildew can begin to develop at any time during crop development, in contrast to powdery mildew, which typically begins to develop in field-grown cucurbit crops around the time of initial fruit formation. Therefore, applying fungicides preventively throughout crop development for a disease that could begin to develop at any time, including late in the season when yield will not be impacted, is undesirable and not economical.

Knowing how to identify the first symptoms of downy mildew in cucurbit crops is a critical component of an effective management program

Timing of fungicide applications is very important for disease management. Level of control achievable with fungicides can decline greatly when applications are delayed after downy mildew onset.

Systemic fungicides are most effective for control of disease, but unfortunately they have narrow spectrum activity, prone to resistance development and not effective for the most common cucurbit foliar disease. Therefore, fungicide application for the control of downy mildew are only recommended when downy mildew is present.

Learn to Identify Downy Mildew Symptoms

  • Initial symptoms of downy mildew include small yellow spots (Figures 1 & 3 ) and small irregular black spots with yellow-green border ( figures 4 & 5 ). First symptoms also can be a water-soaked spot ( Figure 2).
  • Infection does not expand beyond veins thus spots develop an angular appearance as the spots expand (figures 3 & 8 ). Sometimes several spots occur together forming a yellow patch 2 that can have an orange tint, especially in pumpkin (Figure 6).
  • Spores are lemon-shaped, dark gray with a purplish tint (F igure 7). as spots develop, further spore production may occur resulting in the underside of spots becoming visibly fuzzy due to the dark spores ( Figure 8 ).

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Figure 1: Yellow spots on the upper surface of these pumpkin leaves are early symptoms of downy mildew. These are not diagnostic as similar spots can occur with other diseases, notably powdery mildew.

Photos Courtesy of M.T. McGrath, Long Island Horticultural Research and Extension Center, Cornell University

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Figure 2: Early lesions of downy mildew in melon usually appear water-soaked on the underside of leaves.

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Figure 3: Yellow spots on cucumber leaves caused by downy mildew.

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Figure 4: These small irregular black spots with yellow borders are early symptoms of downy mildew in pumpkin             

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Figure 5 : Early symptoms of downy mildew on the upper (left) and lower (right) surfaces of leaf

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Figure 7: Sporulating Downy mildew fungus  Pseudeoperonospora cubensis . Spores are lemon-shaped, dark gray with a purplish tint.

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Figure 8: Purplish dark gray spores of the downy mildew fungus only develop on lower surfaces of leaves (left). Note that downy mildew spots often have an angular appearance because they do not enlarge beyond major veins (right). This is most evident on the lower surface. Yellow spots are on the upper surface of this leaf opposite where spores have developed on the lower surface. One section of this leaf has died because of downy mildew.

It is difficult to deliver fungicide directly to the lower leaf surface. Consequently, an important component of fungicide programs has been fungicides able to move to the lower leaf surface. Most of these fungicides are systemic or have translaminar activity (e.g. Amistar ® ).

  • Application of systemic, translaminar fungicides such as Ridomil Gold ® and Amstar ® should start at the time when symptoms have just started to develop. For the prevention and/or delay of resistance development, they should be applied in alteration and /or in the cocktail mixture of protectant (contact) fungicides.
  • Apply every 5-7 days depending on disease severity.
  • The first application needs to be made as soon as possible after reaching the threshold of 1 leaf with symptoms out of 50 older leaves examined.
  • It is especially important to examine the lower leaf surface when scouting. Inspect your field twice per week,
  • Systemic fungicides prone to resistance development. Therefore, assessing mildew severity on the lower surface of the leaves is very important component of resistance management. When control is poor on lower surfaces but good on upper surfaces, stop using fungicides at risk!
  • Mistblower (motorblower) or knapsack sprayer with hollow cone nozzle should be used to ensure good spray coverage. Apply 33 to 45 Gallons of spray mixture per acre to ensure good leaf coverage.
  • A sticker should be added and the spray mixture applied. Commercially available sticker is added at the dose rate of 5 cc (5 ml) per one gallon of water.
  • If sprinkle type irrigation is used, do not irrigate immediately after fungicide application. Avoid using of overhead irrigation on cucurbits (this will favour disease development).
  • Cultural measures such as weed control, crop rotation and use of tolerant /resistant varieties are important factors for successful disease management.