There is nothing more frustrating than to see the fruits of your garden labor lost to diseases and pests.

Though the past few months have been very pleasant, the warm days and cool nights have contributed greatly to the development and spread of many diseases that can attack home garden vegetables. Some pathogens attack a wide variety of plants; others attack only specific plants. Some pathogens attack all parts of the plant; others attack only certain tissues.

If you have vegetables growing in the garden that are part of the cucurbitacea family, such as cucumbers, squash, zucchini and melons, you might have noticed some discoloration or fuzzy white areas starting to form on the leaves.

This is likely mildew, and vegetables are affected by two types — downy and powdery.

Powdery mildews predominate during dry and warm-to-hot temperatures, and downy mildews thrive during cool and wet periods.

Powdery mildew will develop white powdery spots on leaf surfaces and expand as the infection grows. The leaves will eventually turn yellow or brown and fall off, exposing the plant or fruit to sunburn. In some cases, powdery mildew will cause the leaves or shoots to become twisted or distorted.

Downy mildew causes light green to yellow angular spots on leaf surfaces. A white fluffy growth will begin to appear on the lower sides of these angular spots. Over time, these lesions turn brown and dry up. Downy mildew progresses from the older leaves, moving up the plant to the younger leaves. Severely infected leaves may die. On rare occasions the pathogen can become systemic, causing dark discoloration of stem tissue.

Plants can be protected with synthetic chemicals or biopesticides.

There are also practices that can help to prevent these diseases from the beginning:

  • Plant in full sun, which helps dry leaf surfaces quickly so as not to spread the disease through water droplets sitting on the foliage.
  • Keep the garden well mulched.
  • Provide proper spacing between plants to allow sunlight and air flow through the crop to help dry the foliage.
  • Water early in the morning. 
  • Don't overfertilize. Use a slow-release product, which comes in many forms, such a compost, manures and resin-coated fertilizer, such as Osmocote.

If you are unable to determine whether you have downy mildew or powdery mildew, consider submitting a sample of the foliage to the LSU AgCenter Plant diagnostic center. Information on how to submit and sample, the forms needed and diagnostic fees are located at lsuagcenter.com.

Got a question?

Email gardennews@agcenter.lsu.edu.