Ashy stem blight, or charcoal rot, is a fungal disease of cucurbits. Your melons, squash, and cucumbers are all susceptible. It can also affect common beans, blackeyed peas, lima beans, chickpeas, corn, fenugreek, soybeans, sorghum, and sunflowers.
The soil-borne fungus (Macrophomina phaseoli) responsible for this disease loves hot days (> 85°F) and cool nights. This pathogen can stick around for up to 12 years. It often infects plants within two weeks after being planted. Symptoms generally do not appear until much later in the growing season, as temperatures rise - after you’ve invested weeks of irrigation, feeding, and weeding. So, learning how to recognize and prevent this disease can help ensure a better harvest.
Symptoms of ashy stem blight
The first signs of ashy stem blight are black, water-soaked lesions or cankers along the stem at the soil line, stunting, and chlorosis (yellowing) of the upper or crown leaves. If you look closely at the lesions, you may see concentric rings. Infected pods may ripen prematurely. As the fungi population grows within the plant, you may see an amber gum oozing from the infected plant. Eventually, the stem turns dry and brown. Lesions may girdle the main stem and kill the plant. If you dig up an infected plant, you will see blackened roots and a lack of feeder roots.
Preventing ashy stem blight
This fungus is a stress pathogen. It preys on stressed plants. A heavy fruit load, high temperatures, drought, and water-stress can make plants more susceptible to infection. Keeping your plants healthy can help them protect themselves.
While furrow-irrigated plants rarely have severe cases of ashy stem blight, you may be surprised to learn that the disease is common with drip-irrigated systems due to increased salt levels near the soil surface, creating salt stress.
Monitor plants regularly for signs of infection. Once infection occurs, remove infected plants and throw them in the trash. A 3-year crop rotation with non-susceptible crops may break the disease cycle. There are no effective chemical treatments for this disease.
Now you know.
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