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Downy Mildew of Cucurbits
(Cucumbers, cantaloupe, watermelon, pumpkins, etc.)
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
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
Figure 2: Early lesions of downy mildew in melon usually appear water-soaked on the underside of leaves.
Figure 3: Yellow spots on cucumber leaves caused by downy mildew.
Figure 4: These small irregular black spots with yellow borders are early symptoms of downy mildew in pumpkin
Figure 5 : Early symptoms of downy mildew on the upper (left) and lower (right) surfaces of leaf
Figure 7: Sporulating Downy mildew fungus Pseudeoperonospora cubensis . Spores are lemon-shaped, dark gray with a purplish tint.
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 ® ).