Mildew Diseases of Cucurbits

Downy mildew and powdery mildew are diseases that may look similar at first glance, but are actually very different. Both usually affect only the leaves. Downy mildew can be identified from the fungal layer on the underside of leaf, in moist weather, accompanied by leaf spots on the top of the leaf. Powdery mildew causes white, powdery, fungal growth in the absence of any leaf spotting. This growth is on both sides of the leaves, and is present at all times.

Downy mildew is a fungal disease caused by Pseudoperonospora cubensis. It affects cucurbits (cucumber, melon, squash, pumpkin) and a few weeds. The disease is found worldwide in temperate and tropical areas and is one of the most important foliar diseases of cucurbits. It occurs sporadically in the northeast. It causes small fruit, due to lost leaf area, and the fruit may have a poor flavor. After the leaves die, fruit is exposed to the sun and may become sunscalded. This disease is favored by cool (61o-72o F), moist weather.

Symptoms of Downy Mildew.

  • Pale green areas that change to yellow angular spots bounded by leaf veins in cucumber, irregular edges for other cucurbits.
  • The spot is brighter on the upper leaf surface than on the lower leaf surface.
  • The spots may turn brown or may remain yellow.
  • During moist weather, the undersides of the spots are covered with a layer of fungus that can be white to purplish to almost black.
  • The entire leaf dies quickly.
  • Usually older leaves are affected first, then younger leaves. Petioles remain green after the leaf blade dies.
  • Fruit are usually not affected, but muskmelons can be covered with the fungal growth.

Prevention of Downy Mildew.

  • Promote good air circulation and leaf drying; use wide spacing between plants.
  • Select sites with good drainage and air movement.
  • Avoid overhead irrigation.
  • Water early in the day to allow leaves to dry thoroughly.
  • In the greenhouse, the use of blue polyethylene sheets to filter the light has been shown to decrease spore production by the fungus. This did not increase the yield.
  • Resistant cucumber varieties are available.
  • See current recommendations for chemical control measures.

Powdery Mildew is caused by the fungi Erysiphe cichoracearum and Sphaerotheca fuliginea. This disease is a major production problem. It has been observed since the 1800s. Although there are powdery mildew fungi on many plants, these two infect only muskmelon, cucumber, squash, pumpkin, gourd, and watermelon. This disease can cause fruit to be smaller in size, fewer in number, less able to be successfully stored, sunscalded, incompletely ripe, and have a poor flavor. It may also predispose plants to black rot (link to black rot of cucurbits factsheet). It is favored by moderate temperatures. These fungi are unusual in that they do not require the leaves to be wet for them to infect the leaf, but rather the humidity must be high. They produce spores when the humidity is low.

Symptoms of Powdery Mildew.

  • This fungus occurs mainly on leaves, which are most susceptible 2 to 3 weeks after unfolding.
  • A whitish, powdery, fungal growth is present on the leaves.
  • Fungal growth often starts on the shaded undersurface of the crown leaves.
  • Fungal growth can be found covering both leaf surfaces.
  • Severely affected leaves can become dry and brittle, or can wither and die.
  • Exposed vines and fruit are more prone to sunscald and the vines may shrivel.
  • Rarely, fruit infection occurs in watermelon and cucumber.

Prevention of Powdery Mildew.

  • Choose a site with good air circulation and low humidity.
  • Separate new plantings from old plantings.
  • Use a crop rotation of at least 1 year.
  • Control cucurbit weeds and other weeds.
  • Resistance is available in muskmelon, cucumber, summer squash, and pumpkin.
  • See current recommendations for chemical control measures.


  • McGrath, M. T. 1997. Proceedings of the New England Vegetable and Berry Conference. Cooperative Extension System.
  • McGrath, M. T. and C. E. Thomas. 1996. Powdery Mildew in Compendium of Cucurbit Diseases. T.A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 28-30.
  • Reuveni, R., and M. Raviv. 1997. Control of Downy Mildew in greenhouse-grown cucumbers using blue photoselective polyethylene sheets. Plant Disease. 81:999-1004.
  • Sherf, A.F. and A. A. MacNab. 1986. Vegetable Diseases and Their Control. John Wiley and Sons, New York.
  • Thomas, C.E. 1996. Downy Mildew in Compendium of Cucurbit Diseases. T.A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. APS Press, St. Paul, MN. pp. 25-27

By: Pamela S. Mercure, IPM Program Assistant, University of Connecticut, 1998.

Reviewed by: T. Jude Boucher, IPM, University of Connecticut. 2012

This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

The information in this document is for educational purposes only.  The recommendations contained are based on the best available knowledge at the time of publication.  Any reference to commercial products, trade or brand names is for information only, and no endorsement or approval is intended. The Cooperative Extension System does not guarantee or warrant the standard of any product referenced or imply approval of the product to the exclusion of others which also may be available.  The University of Connecticut, Cooperative Extension System, College of Agriculture, Health and Natural Resources is an equal opportunity program provider and employer.