downy mildew infected cucumber leaf

Tan to dull yellow angular lesions on the top of a cucumber leaf

Updated: May 11, 2021

Symptoms of downy mildew that infects cucumbers, squash, melons, and spinach

Symptoms of downy mildew,  which can affect cucumbers, squash, melons, and spinach include tan to dull yellow angular lesions (spots) that enlarge. These spots first appear on leaves at the center of the plant. Lesions then grow between the small leaf veins and brown areas can appear on leaf undersides. In wet weather, a fine gray to purplish mold can also be observed on leaf undersides.

comparing downy mildew and powdery mildew on the back of a leaf
Comparison of Downy mildew and Powdery mildew on squash. Downy mildew: yellow spot on top and dark mold on bottom of leaf. Lesions can appear angular. Powdery mildew: noticeable white, moldy growth.
Photo: David B. Langston, University of Georgia, Bugwood.org

 

  • Closeup of sporulation on leaf underside

    downy mildew fungal spores on the bottom of an infected leaf
  • Downy mildew can quickly kill leaves and entire plants during warm, wet weather

    advanced symptoms of downy mildew causes plant damage

Management

Many bacterial, fungal, and viral diseases attack vegetable crops in Maryland home gardens. Most of these are not serious and in very few cases is spraying a fungicide recommended. Foliar diseases are frequently weather-dependent and vary in severity from season to season according to rainfall and temperature. Regular plant inspection, especially on lower and inner leaves, will alert gardeners to foliar problems. Foliar diseases are progressive- they begin as small spots on a few leaves. Lesions grow and coalesce and may cause leaves to yellow and die. Identify problems early on to determine the cause of the problem. Monitor affected plants through the season.

12 tips that can help you prevent downy mildew disease problems

  1. Select disease-resistant varieties, particularly for those diseases that appear in your garden each year.
  2. Purchase certified, disease-free potato tubers, garlic bulbs, and asparagus and rhubarb crowns.
  3. Avoid planting on wet, poorly drained sites. Pull soil up into raised beds if drainage is not very good.
  4. Dig or till compost into the soil each year.
  5. Grow healthy plants by providing adequate light, water, and nutrients. Give each plant adequate space to ensure good air circulation.
  6. Keep the bare ground covered with organic mulch. Newspaper covered with straw works very well.
  7. Avoid watering foliage in the evening. It is best to use soaker hoses and drip irrigation, or direct irrigation water around the plant base where it can quickly reach the root zone.
  8. Avoid handling wet foliage.
  9. Harvest your vegetables before they become over-ripe.
  10. Cut off and discard leaves and pull up and discard entire plants that are badly infected by the disease.
  11. Clear your garden at the end of the season of all plant debris. This should be composted or tilled into the soil. Plant parts infected with especially damaging diseases, like late blight of tomato and potato, southern blight, and white rot (garlic and onions), should be bagged and put out with your trash.
  12. Keep weeds to a minimum and control those insect pests like thrips, aphids, flea beetles, and cucumber beetles that are most likely to spread diseases.

When disease symptoms are observed it is often too late to apply a fungicide, although fungicide treatments can help to protect new or un-infected foliage. Fixed copper, sulfur, and horticultural oil are some organic fungicides used by home gardeners. Always, carefully read and follow all pesticide label information and test the spray on a small part of the crop to check for signs of leaf injury (phytotoxicity.)