Life Cycle of Late Blight on Potatoes

Symptoms

Late blight infestation causes dark brown to black colored patches to appear on the leaves of potato plants. The leaves often have a yellow outline on the edges. The reverse side, or downward-facing part of the leaves, develop whitish spores, especially in moist conditions. The infection can quickly spread to other parts of the plant, including the stems, in a matter of days, turning the entire foliage into an infected rotting mass. The potato tubers themselves also show symptoms of infection. Parts of the potato may show signs of rot, giving off a foul smell.

The Phytophthora infestans fungus normally produces sexual spores that survive in the soil or inside potato tubers that may have been saved for seeding purposes. These spores are known as sporangia and their function is to facilitate the reproduction of the fungus. Sporangia need ideal conditions to germinate. For instance; the temperature has to be between 44 and 55 degrees Fahrenheit. They also need moisture, so germinate after it rains. They can also germinate when there is fog or heavy dew, together with the right temperature. During the germination process, zoospores are released from the sporangia. These spores are spread by winds to other potato plants. The zoospores penetrate the surface of new or uninfected potato leaves where they repeat the same destructive actions on the plant.

One method of controlling the spread of potato late blight is to remove infected potatoes from the pile of seed tubers. Infected potato plants, which survived the winter, should also be destroyed. Another means of controlling the disease is through the use of fungicides. The type and strength of fungicide depends on the level of infection. For instance, a moderate infection with only a few scattered infected plants requires a combination of culling infected plants and fungicide application. An advanced outbreak that has affected between 70 to 90 percent of the plants may require the destruction of the entire crop and treatment of the ground to kill the sporangia there.

One method for preventing potato late blight is for the farmer to only obtains potato seed tubers from reliable sources. The use of excessive nitrogen rich fertilizers and manure encourage the growth of the disease. Also, selecting resistant potato varieties and remaining vigilant as to any symptoms helps prevent an outbreak of the disease.