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What is it?
Potato ring rot disease is caused by the bacterium corynebacterium spedonicum. This potato plant disease will attack both the tubers as well as the stems of potato plants and all infected portions of the potato plant must be discarded and destroyed which can greatly reduce crop yield.
What does it look like?
Potato ring rot gets its name because the damage occurs in rings. If you were to take a cross section of either a shoot or a tuber you would find a ring, or several concentric rings which are yellowish or light brown. Often the areas of damage which appear in the ring are crumbly and dry with decay. Often with potato ring rot, shoots will be stunted and rosettes will form. Ring rot also causes the areas between potato leaves and veins to turn yellow and brown at which point the potato leaves tend to curl upward and inward. Additionally, you may find that infected potato plants in the vegetable garden will also have stems which tend to wilt or droop over. If potato plant stems are cut or broken at the level of the soil, often there will be a creamy white material which oozes from the stem at ground level. However, even if the entire potato plant is infected with ring rot, only a few stems may show signs of corynebacterium spedonicum damage. Tubers may present as cracked or crumbly in areas of the most severe ring rot infection.
How does it manifest?
Corynebacterium spedonicum species ring rot manifests in potato plants by invading the plants through pre-existing wounds. Often the potato plants will have wounds from seed pieces having been cut off prior to planting, or cultivation wounds. Adiitionally, wounds from garden pests such as wireworms are easily invaded by this bacterium. The good news is that ring rot is not spread from plant to plant in the garden. The bad news is that if a row of your potato plants have pre-existing wounds and corynebacterium spedonicum is present in the soil, they may all become infected. Once a tuber has become infected it is inedible and must be discarded and destroyed. Ring rot bacteria can survive between seasons. They do this in infected tubers. Often a tuber may not show evidence of ring rot at harvest, but ring rot can develop up to several months after harvest in storage containers.
What can you do about it?
When you first notice damage from corynebacterium spedonicum species, ring rot in your potato patch or vegetable garden, weed out infected tubers and stems. Be sure to discard and destroy any potato stem or tuber which appears to have been infected with the ring rot. If you find evidence of ring rot in the tubers you have in storage, you will need to discard and destroy the infected tubers and carefully disinfect the storage containers if you wish to reuse them. Do this with a healthy application of household bleach being sure to wash and dry the containers immediately. Storage bags should be discarded. When you plant potatoes the following growing season, be sure to use on seed potatoes which have been certified. Plant the seed potatoes whole to avoid wounds which would invite corynebacterium spedonicum. If you feel you must cut the seed potatoes, do so but be sure to disinfect the knife between each cut by dipping it into common household rubbing alcohol.
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