The word ‘charcoal’ has always had happy connotations for me. I love burgers cooked over a charcoal grill. I enjoy drawing with charcoal pencils. But then one fateful day, ‘charcoal’ took on a different meaning when I made a grisly discovery in my garden. My cantaloupes had developed charcoal rot. My fond memories of charcoal were just as tainted as my cantaloupe plants. So, what is charcoal rot disease, you ask? Read on to learn more.
Cucurbit Charcoal Rot
Charcoal rot, or dry-weather wilt, is a disease that affects all cucurbits. Cantaloupe is a cucurbit along with other plants of the gourd family, including watermelons, pumpkins, cucumbers, zucchini and other squash. The soil-borne fungus, Macrophomina phaseolina, is the culprit for cucurbits with charcoal rot.
This fungus can reside in the soil for 3 to 12 years, where it lies in wait to invade plants that are under duress from hot, dry weather. The fungus infiltrates plants from the roots and spreads to the stem, clogging the plant’s vascular tissue with small, dark, round microsclerotia (fungal structures).
Infection typically happens 1-2 weeks after planting; however, the visual indicators of charcoal rot disease will not usually be seen until 1-2 weeks of harvest.
Cucurbit Charcoal Rot Symptoms
What symptoms do cucurbits with charcoal rot exhibit? The lower part of the stem develops water-soaked lesions, causing the stem to become girdled. Amber colored droplets may excrete from these lesions. Eventually, the stem dries out and turns light gray or silver with black charcoal-looking microsclerotia speckled throughout on the surface.
These microsclerotia can also be observed in the pith of the plant if you were to dissect a cross section of the affected stem. As the disease progresses, the plant’s foliage will start yellowing and browning, beginning at the crown. The wilting and collapse of the entire plant could be an eventuality.
The fruit, unfortunately, can also impacted. When I cut open my cantaloupe, I observed a large black sunken area eerily resembling charcoal – hence the name.
Charcoal Rot Treatment
Is there a charcoal rot treatment available? It’s time to impart some bad news. There is no treatment for charcoal rot of cucurbits. Fungicides (seed treatment and foliar) have shown to be ineffective in managing this disease.
It is suggested to rotate to a non-host crop for 3 years; however, the practicality and efficacy of this is questionable for a few reasons. It is just not cucurbits that are susceptible to charcoal rot. It actually affects more than 500 crop and weed species, which limits your options considerably. You also have to consider the longevity factor of microsclerotia in the soil (3-12 years). Soil solarization also isn’t a remedy because charcoal rot of cucurbits is a disease that favors heat.
In this case, your best offense is a good defense. Your best defense is keeping plants healthy. We know that the onset of charcoal rot can be triggered by water stress, so having a good irrigation program in place can be a good preventative measure against this disease. Also – be sure to maximize your plant’s vitality by tending to their nutritional needs (i.e. fertilizer).