Potato blight is a term that can refer to one of two potato diseases. Late blight is caused by the Oomycete Phytophthora infestans, an organism that used to be considered a fungus, but is now thought to be related to algae. It is a notorious plant disease, being responsible for the Irish potato famine that led to large-scale death in the 1840s. Early blight is caused by the fungus Alternaria solani. Both of these plant diseases can also affect tomatoes, and other members of the Solanaceae family.
Of all the potato diseases, late blight is the most destructive. It is most devastating in moist, cool areas, such as parts of North America, China, and Western Europe. Whole fields can be destroyed within a week or two, under the appropriate weather conditions. The potatoes can easily become infected and rot later in storage, even if there is only a small amount of disease in the field.
Late blight is such a severe plant disease that several countries have considered using it as a biological weapon to destroy a country’s food supply. The devastation that can be wrought by late blight was shown by the Irish potato famine of the 1840s. Irish farmers had come to depend on potatoes as their primary food source, and when the weather turned cold and moist, an infestation of potato blight totally destroyed the potato crop. It is estimated that 1.5 million Irish people died of hunger, and an equal amount emigrated to escape starvation, primarily to the United States.
The initial symptoms on the plants are spots at the edges of the leaves, which enlarge to become blighted areas. If the weather is moist, white mildew then forms underneath the leaves, and the potato blight spreads to kill entire leaves. With continuing wet weather, all of the parts of the plant above ground rot away. The disease can halt if the weather become dry, only to start again if moist conditions return.
Fungicides should be used when conditions favor late blight infection. Left over potatoes at the end of the season should be destroyed, since they can harbor the pathogen. At the end of the season, any above ground plant parts should be sprayed with herbicide, so they do not act as a source of potato blight. It is ideal to use resistant potato varieties, but many of the popular varieties are susceptible. One should only use seed potatoes that are certified to be free of the late blight pathogen.
There are many different races of Phytophthora infestans, which vary in the types of potatoes they will attack. This situation has become more complicated with the spread of an additional, mating type. Theoretically, this pathogen can reproduce both sexually and asexually. Throughout most of the world, however, the reproduction had been asexual. There had not been genetic recombination to help facilitate the production of new strains.
For there to be sexual reproduction, an additional mating type was needed. This mating type was found only in Mexico until the 1980s. Now, it is spreading throughout the world. Not only does this mean that sexual reproduction can lead to the production of new strains and new forms of aggressive pathogens, but the sexual spore produced can survive in the soil for years. This complicates control measures, since the asexual form requires living tissue and cannot survive in the soil.
Early blight, the other form of potato blight caused by Alternaria solani, is generally a less devastating disease. While it is usually present wherever potatoes are grown, this fungal disease rarely causes losses exceeding 20%, unless it is left uncontrolled. Despite its name, it usually causes disease on mature leaves. In some areas, infection of the leaves is the most serious problem, while infection of the tubers is the more serious problem in other places.
The foliar symptoms of early blight are easy to distinguish, and include a series of dark, concentric circles that alternate with bands of tan leaf tissue. By the end of the growing season, the lesions may join together and cover much of the leaf. If the infection of the leaves and stems is severe, this increases the chances for yield loss, and possible infection of the tubers.
The application of foliar fungicides is the most common and effective means of control. Spraying does not need to be started until right after bloom, or at the first sign of disease. Susceptible cultivars should be avoided in areas where the disease is prevalent. The fungus winters in the soil, so rotating with different crops can help prevent infection, as can plowing under plant refuse in the fall.