Ashy stem blight, also known as charcoal rot, is a fungal disease of cucurbits. This means that your melons, squash, and cucumbers are susceptible. It can also affect common beans, blackeyed peas, lima beans, chickpeas, corn, fenugreek, soybeans, sorghum, and sunflowers.
Ashy stem blight is a soil borne fungus (Macrophomina phaseoli) that loves hot days (> 85°F) and cool nights. This pathogen can stick around for up to 12 years. It is common in California and often infects plants within 2 weeks after being planted, but symptoms generally do not appear until much later in the growing season, as temperatures begin to 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 sign 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 be able to 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. If lesions girdle the plant, it will die. If you dig up an infected plant, you will see blackened roots and a lack of feeder roots.
Preventing ashy stem blight
Ashy stem blight is known as a “stress pathogen”. This means it preys on stressed plants. Stresses, such as 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. It is believed that this particular set-up increases salt levels near the soil surface, creating salt stress.
Monitor plants regularly for signs of infection. Once infection occurs, affected plants should be removed and thrown in the trash, and a 3-year crop rotation with non-susceptible crops should be put into place. There are no effective chemical treatments for this disease.
Now you know.
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