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What is it?
Late blight occurs in potato plants which have been infected with the plant disease phytophthora infestans, a fungus. Late blight attacks both potato plants and tomato plants and this fungal disease of the alternaria solani species makes infected tubers and fruits inedible which can severely hinder the potato crop yield, as well as the tomato crop yield in your vegetable garden. One way to prevent spread between vegetable plants is to plant potatoes and tomatoes at opposite ends of your vegetable garden. Late blight was the potato plant disease which was responsible for the great Irish famine from 1845 to 1850.
What does it look like?
When late blight occurs in potato plants, it first presents itself as spots which are irregular, although circular. These early blight spots begin rapidly and continue growing in size. The spots from phytophthora infestans will also look water-soaked. Typically these darkened spots will often appear on bottom leaves of the potato plant and continue to affect the potato plant leaves in an upward direction. Due to the quick emergence and spread of this fungal infection, the spots quickly turn black and kill off leaves. The fungus known as phytophthora infestans then quickly begins to kill off leaf stems as well as the main stems of the potato plants. Additionally, when the weather becomes increasingly wet or moist, not only does late blight spread more quickly but you may also notice that a greyish mold begins to grow on the undersides of infected leaves. When tubers are infected, these parts of the potato plant will show brownish or purple spots. In storage these spots on tubers tend to do one of two things, they will either become wet and sunken or they will dry rot. Often tuber skins become infected just prior to the harvest when they are not showing much damage and therefore it may seem like the damage occured during storage, however most likely the tubers were infected prior to the harvest.
How does it manifest?
Fungal spores of phytophthora infestans over winter on plant debris left over from the previous seasons vegetable garden. The spores of late blight can also over winter in tubers in storage, as well as in the garden compost pile. 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 phytophthora infestans to invade and late blight to manifest in the potato plant. Late blight spreads rapidly and can kill off an entire crop within a matter of a few days. Therefore signs of late blight should not be ignored. Late blight tends to be at it’s peak of infection during moist humid weather and the fungus spread rampantly when the days are warm with cool nights.
What can you do about it?
Late blight is often an annual problem in many regions of the country. If you are aware that late blight is an annual trouble in your area, you are one step ahead, so don’t be afraid to ask around if it’s your first time planting potatoes or you move into a new region. When potato plants get approximately six inches in height you can spray them with a chlorothalonil containing fungicide. You should continue repeat applications of the fungicide at seven day intervals until the plants turn yellow and die naturally. After potato plants have died off, you will need to dig up the tubers approximately ten days later, allowing time for the fungal spores to die off. Be sure to avoid any sort of overhead watering system in potato plants as this is sure to spread late blight. It is preferable if you can use either a furrow irrigation or drip irrigation when watering potato plants. Be sure you are diligent in cleaning up and destroying and plant debris post-harvest so there is nothing for phytophthora infestans to over winter on.
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