Phytophthora infestans is an oomycete that causes the serious potato disease known as late blight or potato blight. (Early blight, caused by Alternaria solani, is also often called 'potato blight'). It was a major culprit in the 1845 Irish and 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae. It is currently being remedied by genetic engineering, taking a resistance gene from the plant Solanum bulbocastanum and introducing it into the genome of cultivated varieties of the potato.
Spores develop on the leaves, spreading through the crop when temperatures are above 10C (50°F) and humidity is over 75% for 2 days or more. Rain can wash spores into the soil where they infect young tubers, or else spores can be blown in from miles away by the wind.
The early stages of blight are easily missed, and not all plants are affected at once. Symptoms include the appearance of dark blotches on leaf tips and plant stems. White mould will appear under the leaves in humid conditions and the whole plant may quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in store.
Up until the 1970s, there was only one type of blight (A1) in the UK, and this was unable to produce resistant spores that could survive the winter. There are now two types (A1 and A2) which can mate and after that produce resistant spores, although the indications so far are that this rarely, if ever, happens in the UK. Mating can occur only between moulds of different mating-types and is required for the production of resistant spores.
P. infestans is still a difficult disease to control today by ordinary methods. There are many options in agriculture for the control of both damage to the foliage and infections of the tuber. Potatoes fill throughout the season, but it is estimated the assimilates stop going to the tubers (they stop growing) when 75% of the canopy is destroyed. This must also be taken into account when growing potatoes, as it means that plants grown do not have to be 100% resistant to blight.
Blight can be controlled by limiting the source of inoculum. Only good quality seeds obtained from certified suppliers should be planted. Often discarded potatoes from the previous season and self-sown tubers can act as sources of inoculum.
There are several environmental conditions that are conducive to P. infestans. By using weather forecasting systems, such as BLITECAST, if the following conditions occur as the canopy of the crop closes, then the use of fungicides is recommended to prevent an epidemic.
Potato varieties vary in their susceptibility to blight. Most early varieties are very vulnerable; so that the crop matures before blight starts (usually in July) plant them early. Many old crop varieties, such as King Edward potato are also very suceptible but are grown because they are wanted commercially. Maincrop varieties which are very slow to develop blight include Cara, Stirling, Teena, Torridon, Remarka and Romano. Some so-called resistant varieties can resist some strains of the blight and not others, so their performance may vary depending on which are around. These crops tend to have had polygenic resistance bred into them, and are known as field resistant. New varieties such as Sarpo Mira and Sarpo Axona show great resistance to blight even in areas of heavy infestation. These varieties are likely to gain great popularity as consumers increasingly embrace organically produced crops and reject food items that have been grown using fungicides and other chemicals.
Fungicides for the control of potato blight are normally only used in a preventative manner, perhaps in conjunction with disease forecasting. In susceptible varieties, sometimes fungicide applications may be needed weekly. An early spray is most effective. Metalaxyl was a fungicide that was marketed for use against P. infestans, but suffered serious resistance issues when used on its own.
Ridging is often used to reduce tuber contamination by blight. This normally involves piling soil or mulch around the stems of the potato blight meaning the pathogen has farther to travel to get to the tuber.
The origin of Phytophthora infestans can be traced to a valley in the highlands of central Mexico. The first recorded instances of the disease were in the United States, in Philadelphia and New York City in early 1843. Winds then spread the spores, and in 1845 it was found from Illinois to Nova Scotia, and from Virginia to Ontario. The fungus crossed the Atlantic Ocean with a shipment of seed potatoes destined for Belgian farmers in 1845.