University of Maryland Extension

Downy Mildew on cucumbers, squash, melons, spinach - Vegetables

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foliage of muskmelon infected with downy mildew

Symptoms of downy mildew, (see above photo on muskmelon), 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.

downy mildew symptoms on squash leaf
Downy mildew symptoms on squash


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.

downy mildew on spinach
In wet weather, a fine gray to purplish mold can also be observed on leaf undersides

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 bare ground covered with an 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 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.)

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