Sweet Potato Black Rot – How To Manage Sweet Potatoes With Black Rot

Image by Scot Nelson

By Bonnie L. Grant, Certified Urban Agriculturist

Sweet potatoes are one of the major cultivated root crops in the world. They need 90 to 150 frost-free days to harvest. Sweet potato black rot is a potentially damaging disease caused by a fungus. The disease is easily transmitted from equipment, insects, contaminated soil or plant material. Black rot on sweet potato can be easily prevented in most cases, but chemical control of already infected plants is not available.

Signs of Black Rot on Sweet Potato

Dark, dry, bruise-like lesions on sweet potato may be a symptom of a common disease of Ipomoea. The disease can also affect such plants as cacao, taro, cassava, coffee, and mango. The fungus essentially breaks down the outer vascular layer of the root, rarely infecting the interior of the tuber. Sweet potatoes with black rot are essentially animal fodder or garbage once infected.

Small round spots that appear to be slightly sunken are the initial symptoms of the disease. Sweet potatoes with black rot will develop larger spots that darken and have tiny black fungal structures with stalks. These cause a sweet, sickly fruit smell and may invite insects to transmit the disease.

The rot can occasionally spread to the cortex of the sweet potato. The dark areas have a bitter taste and are not palatable. Sometimes, the entire root rots. The disease may be noticeable at harvest or well into storage time or even market.

Preventing Sweet Potato Black Rot

Black rot of sweet potatoes comes most often from infected roots or splits. The fungus can also live in soil for several years and enter through wounds in the tubers. Additionally, it overwinters in sweet potato plant debris or certain host plants, such as wild morning glories. The fungus produces prolific spores, which contaminate machinery, washing bins, gloves, and crates. Often, one infected potato can spread the disease through an entire cured and packed lot.

Insects are also vectors of the disease, such as sweet potato weevils, common pests of the plants. Temperatures above 50 to 60 degrees Fahrenheit (10 to 16 C.) encourage the formation of spores and enhance the spread of the disease.

Black rot cannot be controlled with fungicides or any other listed chemical. The best cure is prevention. Purchase disease free roots and slips. Do not plant sweet potatoes in the same place but once every 3 to 4 years. Remove host plants. Wash and cure the harvest immediately and do not store potatoes until completely dry. Cull diseased or suspicious roots at harvest.

Decontaminate any equipment and avoid damaging slips or roots. Slips or roots can be treated with a pre-planting dip of fungicide. Exercise good care of plants and sanitation practices and most of the sweet potatoes should escape significant damage.

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