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What is it?
Early blight occurs in potato plants which have been infected with the plant disease alternaria solani, a fungus. Early blight attacks both potato tubers and potato vines and this fungal disease of the alternaria solani species makes infected tubers inedible which can severely hinder the potato crop yield.
What does it look like?
When early blight occurs in potato plants, it first presents itself as spots which are irregular, although roundish. These early blight spots tend to be either a very dark brown or blackish in color. Typically these darkened spots will range in size from 1/8 to 1/2 inch in diameter and they often appear on bottom leaves of the potato plant and continue to affect the potato plant leaves in an upward direction. The fungus known as alternaria solani usually is not seen to affect the top leaves of the potato plant unless the early blight infection is severe. These dark brownish to black spots develop concentric circles within them and these circles may continue to enlarge until such time as the leaves actually die and fall off the potato plant. Often you will not notice damge to the tubers until harvest time at which point they will show signs of infection with dark brown spots which are dry and have a corky texture to them as they crumble.
How does it manifest?
Fungal spores of alternaria solani over winter on plant debris left over from the previous seasons vegetable garden. The fungus then spreads through soil cultivation with this plant debris and infects tubers through preexistent wounds. Tubers may have wounds from planting, insect damage such as wireworm burrow holes, or from cultivation wounds. Any such wound on the tuber allows an opening for alternaria solani to invade and early blight to manifest in the potato plant. Early blight spreads rapidly in potato plants in favorable conditions of temperatures which stay between 75° to 85° F. Early blight tends to be at it’s peak of infection and spread in potato plants toward the end of the growing season. At this time, exhaustive numbers of potato leaves may die off which greatly reduces the potato yield. Potato vines, leaves and tubers may be killed off. Even in extreme cases of alternaria solani infection, the plant rarely dies, even if almost every leave on the potato plant does. However, early blight makes infected tubers inedible and it is likely that if your potato plant is showing that much damage in it’s vines and leaves that the potato tubers are severely infected as well.
What can you do about it?
When leaf spotting begins on potato plants and you notice the tell-tale concentric circles of early blight damage act quickly. By spraying potato plants with a chlorothalonil containing fungicide you can help stave off damage from alternaria solani. Early blight is tenacious and so you may have to re-spray and reapply applications of fungicide at ten day intervals on your potato plants. This should be done until such time as the leaves are dying back naturally. At harvest, destroy all infected plants and tubers as they are inedible. Be sure to clean up any potato plant debris from this process, and any weeds or debris surrounding plants where the fungi could over winter. The following growing season when potato plants reach a height of approximately six inches, begin spraying potato plants with the chlorothalonil fungicide again at intervals of ten days. Repeat applications on potato plants until the tops die back. To avoid spread of alternaria solani, avoid over watering plants and watering them from above. Splashing water can spread early blight spores quickly. Additionally, make sure that any seed potatoes you purchase are certified disease free by your local regulatory agency such as the state department of agriculture.
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