The Science of the Disease

The Irish potato, Solanum tubersum, is a very important food crop world wide. If was first found in the Andes Mountains of South America. In 1537, the Spanish conquistadors found the Incas growing papas, or potatoes; they later introduced the potato to Spain, where Italy, Europe, Germany, Russia, and Ireland caught on to the cool new food source. Although the potato can be grown in many different climates and is a main food source to many countries, it is suseptable to over 160 diseases and disorders.

The horrible disease that caused this tragic famine was called the late blight, this is to separate it from the early blight, though both names are misidentifying. It is also referred to as potato blight, downy mildew, and "rust". The late blight is one of the oldest, well known, and serious potato diseases. The first case was found in Europe and the United States in about 1830. Over time, it became worse and worse in Western Europe until 1845 when it caused the Irish Potato Famine. Since Potatoes constituted the main diet of the Irish and the disease was extremely serious in Ireland, thousands of people died of starvation and others immigrated to the Unites States, Canada, and other countries.

Some people thought that if you would eat a potato infected with the late blight you would get infected with spina bifida and anencephaly. Tests on animals proved that this was not true. About forty years later, scientists accidentally discovered the Bordeaux mixture; a mixture made up of copper sulfate, hydrated lime, and water. This mixture helped suppress the late blight. It was used all over until more efficient and less toxic fungicides were discovered.

The late blight can be found in Africa, Asia, Australia, Europe, North America, Central America, and South America; pretty much everywhere. In Western Europe, the eastern United States, and Canada, where cool, humid conditions are present, the late blight is at it's best. These areas take most of the impact of this destructive disease. In 1946, the late blight began to affect tomatoes during an unusually cool, wet growing season.

The late blight is caused by Phytophthora infestans D By. Phytophthora infestans D By is a member of the Phycomcetes. The older name is Botrytis infestans. It is related to the downy mildews which it gets one of it's names. In the decade of 1840 to 1850, questions aroused as to how the late blight was caused. The majority of the people said it was simply the weather. When the fungus was found on the dead foliage it was thought to be saprophytic organisms, meaning organisms that ate dead matter. Rev. M.J. Berkeley was the man of whom invented the fungus hypothesis, Dr. Montagne was the first to describe the disease in 1845, he called it Botrytis infestans. In 1846, the late blight was renamed Phytophthora infestans (Mont.) D By by De Bary. This ended the spontaneous generation theory, and began the germ theory. The spontaneous generation theory basically says that there is really no beginning to disease, it just becomes. The germ theory states that germs are the cause of all disease.

The late blight affects all parts of the plant; the leaves, stem, and tubers, or roots. The first sign of the late blight is light green water soaked spots or lesions, that later turn brown, on the top of the leaves; these would appear during cool, wet weather. These lesions may have a yellowish halo around them. If the weather remains wet or humid, then white conidiophores and conidia, fungus, develops on the under part of the leaves around the halo. The disease spreads rapidly all over, attacking petioles and stems. The disease can destroy whole fields of potatoes, and when it does so, it can put off a decaying odor. The spores are washed down into the soil and attack the roots of the plant causing a thin, reddish brown dry rot. If the plant is stored in cool, dry conditions, the infection will progress incredibly slowly if it progresses at all. Apposing this fact, if they are stored in warm, humid conditions the fungus will continue to kill the plant.

Signs of the disease are fungus around the infected areas and fungus protruding from the stomata of the plant cell. This bacteria is called conidia. Conidia is thin walled and oval shaped. This fungus can penetrate into the plant cell either by going in through the stomata of by forcing itself through the cuticle. The etiology of the disease if fairly simple, it is like a mass take over. If the infected seed of a potato or a potato itself is planted or dumped on the ground, it starts to reproduce a suitable host for the agent, the disease, to grow off of. The fungus starts to reproduce and if the weather is favorable to it, the rain and wind can spread it over whole fields. Each cell of the fungus reproduces 8-12 of itself, this is a pretty fast reproduction rate.

To prevent the late blight from occuring or to help get rid of the late blight, many potato cultivators use fungicides to protect their potatoes from the late blight. Also, keeping formerly infected plants and seeds away from their fields helps considerably. There is a wide variety of fungicides used against the late blight: Bordeaux mixture, Phygon, Maneb, Zineb, and many more. The Bordeaux mixture was the first to be discoved as mentioned earlier. It has been partially replaced by other less toxic fungicides. More research is being conducted as you read on the subject to help absolve the late blight fungus.