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

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


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. 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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Preventing Mold on a Sweet Potato Plant

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Sweet potatoes (Ipomoea batatas) provide ornamental beauty and edible root tubers, depending on the variety. These vining plants grow as perennials in U.S. Department of Agriculture plant hardiness zones 9 to 11, thriving in moist soil and full sun. Mold problems can attach the roots or the leaves, and they range from a deadly disease to nothing more than a minor nuisance, depending on the type. Early identification and prevention helps ensure your sweet potatoes remain in the best health.


Preparing Your Sweet Potato Slips for Planting

When the slips are six to eight inches long, they are ready for transplanting into the containers. But first, you have to separate them into individual slips.

  1. Carefully twist them off the slips to separate them from the tuber. The slips may have already developed some rudimentary root structures. Be careful not to damage them.
  2. Place the sweet potato slips in a bowl with the bottoms submerged in water. This will encourage further root development and should take about a week.
  3. When the roots grow to about an inch long, you can transplant the slips to the containers.

Note: Like with any other seedlings, you need to harden off your sweet potato slips. You can do this by gradually introducing the slips to outdoor conditions. Do this one to two weeks before breaking them off the mother sweet potato.


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