Dan Egel
Southwest Purdue Agriculture Center

The best way to combat any plant disease is through resistant varieties. Muskmelon (cantaloupe) growers have been able to use cultivars resistant to powdery mildew for the last several years. However, that resistance may not serve growers much longer in the future. This article describes powdery mildew of muskmelon, the host resistance situation in the United States and possible management options.

Powdery mildew of muskmelon appears as a talc-like powder on the leaves of muskmelon plants Upper and lower surfaces of leaves may be affected. The disease is caused by a fungus (Podosphaera xanthii) that is readily windborne; the fungus may be blown many miles from field to field. Unlike most fungal plant diseases, this fungus does not require leaf wetness, only high humidity—a commodity in great supply during most Midwest summers. Severe infections may reduce the vigor and yield of plants, and in some cases, reduce fruit quality. Fortunately, the fungus does not survive well in soil; a 2- to 3-year rotation will suffice to manage the disease, but remember, the fungus may blow in from long distances.

Many plants are susceptible to powdery mildew. However, there are several different fungi which cause powdery mildew. The fungus which causes powdery mildew of muskmelon also affects pumpkins, squash, gourds, zucchinis and other plants in the cucurbit plant family. The powdery mildew on your lilacs, for example, will not affect your muskmelon.

Several years ago, muskmelon varieties were introduced that had good resistance to powdery mildew. Those of us associated with muskmelon production in the Midwest began to forget about scouting for the disease or the management options. Last year, it was reported out of the southeast United States that powdery mildew was observed on cultivars previously known to be resistant to the disease. Similar reports have surfaced this year already. It appears that at least in the southeast, the fungus that causes powdery mildew has mutated into a new race or form that overcomes commercially available resistance of powdery mildew in muskmelons.

Although so far the reports of resistance breakdown have not reached the Midwest, muskmelon growers should scout fields for the presence of the disease. Organic growers should follow these management recommendations.

• Continue to purchase cultivars that have good resistance to powdery mildew.
• Follow at least a 2-year crop rotation.
• Some muskmelon growers may already be applying fungicides for Alternaria leaf blight and gummy stem blight on a regular basis, if cultural practices do not provide adequate control. These applications, starting at about vine touch within a row, should also help to protect vines from powdery mildew. Fungicides that may help to protect vines from powdery mildew include:
  • Formulations with the microbial ingredient Bacillus subtillus strain QST 713 (including Serenade).
  • Formulations of copper such as copper hydroxide.
  • Formulations with sulfur.
  • Fungicides with an active ingredient of potassium or sodium bicarbonate.
  • Fungicides with the active ingredient of hydrogen dioxide.

• It is best not to apply microbial-based fungicides in combination with the other fungicides since the copper/peroxide/bicarbonate may limit the effectiveness of the microbial pesticide.
• Scout fields for symptoms of powdery mildew and report possible problems to the Extension representative in your state or area.

Organic growers should always check with their certifying agencies to be certain that all products and management practices are allowed.

Powdery mildew has a talc-like appearance on
muskmelon leaves.