Charcoal Rot Showing Up in Cantaloupe

From Clemson Extension Agent Rob Last and Plant Pathologist Tony Keinath.

Charcoal rot was identified in a cantaloupe crop in South Carolina this week. The fungus responsible for the disease is Macrophomina phaseolina, a soil-borne fungus that survives as microsclerotia.

The fungus affects more than 800 host plants. These range from corn and soybean to potato and strawberry. Watermelon is also a good host, while cucumber and squash are less susceptible than cantaloupe.

Typically, the disease is more prevalent in hot, dry conditions, as we have been experiencing across the state this season.

In cantaloupes, the disease is a post-harvest disease, often with no visible external symptoms to the fruit.

Symptoms

The vines may wilt under high temperatures and a fruit load on an infected plant. When inspecting the plant’s crown (at the soil line), cankers can be visible, similar to a gummy stem blight canker, but unlike gummy stem blight, fruiting bodies will not be present.

The example found this week showed no visible symptoms on the fruit’s exterior. However, twelve hours after harvest, the fruit began to collapse.

The picture above illustrates the fruit at the point of harvest as no visible effects of the disease can be seen.
The fruit exterior twelve hours after harvest. Note the developed blackening on the skin.
When the fruit was cut open, the flesh was blackening with a dry rot and a characteristic red-wine-colored halo around the rot.

Under the microscope, the black lesion consists of microsclerotia.

Treatment and Management

There are no effective conventional or organic fungicides for minimizing the rot, either preventative or curative.

Rotation is crucial; avoiding susceptible hosts for 2 to 3 years can reduce the severity of infection. Wheat and rye are the best crops for rotation. Do not use corn, sorghum, sunflower, or any legumes, including soybean, southern pea, green bean, or peanut, in the rotation.

Fumigants may be effective in reducing the viable microsclerotia in the soil.

When cultivating the field after the season, cultivate the affected area of the field last, and sanitize equipment to prevent spreading the sclerotia to other “clean fields.”

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