Phytophthora infestans

Phytophthora infestans is an oomycete or water mold 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"). Late blight was a major culprit in the 1840s European, the 1845 Irish and 1846 Highland potato famines. The organism can also infect tomatoes and some other members of the Solanaceae.

Biology
The spores of this water mold over-winter on infected tubers, particularly those that are left in the ground after the previous year's harvest, in cull piles, soil or infected volunteer plants and are spread rapidly in warm and wet conditions. This can have devastating effects by destroying entire crops.

Spores develop on the leaves, spreading through the crop when temperatures are above 10 °C and humidity is over 75% for 2 days or more. Rain can wash spores into the soil where they infect young tubers. Spores can also travel long distances on 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.

Strains
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 molds of different mating-types and is required for the production of resistant spores.

Genetics
In 2009 scientists completed the sequencing of the genome of P. infestans. It was found that the genome is considerably larger (240 Mb) compared to other Phytophthora species whose genomes have been sequenced; Phytophthora sojae has a 95 Mb genome and Phytophthora ramorum had a 65 Mb genome. It also contained a diverse variety of transposons and many gene families encoding for effector proteins that are involved in causing pathogenicity. These proteins are split into two main groups depending on whether they are produced by the water mold in the symplast (inside plant cells) or in the apoplast (between plant cells). Proteins produced in the symplast included RXLR proteins, which contain an arginine-X-leucine-arginine (where X can be any amino acid) sequence at the amino terminus of the protein. RXLR proteins are avirulence proteins, meaning that they can be detected by the plant and lead to a hypersensitive response, killing the oomycete. P. infestans was found to contain around 60% more of these proteins than other Phytophthora species and this may allow it to overcome host defences more quickly. Those found in the apoplast include hydrolytic enzymes such as proteases, lipases and glycosylases that act to degrade plant tissue, enzyme inhibitors to protect against host defence enzymes and necrotizing toxins. Overall the genome was found to have an extremely high repeat content (around 74%) and to have an unusual gene distribution in that some areas contain many genes whereas others contain very few.

Management
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 grow throughout the season, but it is estimated the tubers stop growing when 75% of the canopy has been destroyed. Around the world the disease causes around $6 billion of damage to crops each year.

Genetic engineering
In recent years, a resistance gene effective against all known strains of blight has been identified and successfully copied from a wild relative of the potato, Solanum bulbocastanum, and introduced into the genome of cultivated varieties of the potato. This is an example of cisgenic genetic engineering.

Sources of inoculum
Blight can be controlled by limiting the source of inoculum. Only good quality seed potatoes obtained from certified suppliers should be planted. Often discarded potatoes from the previous season and self-sown tubers can act as sources of inoculum.

Environmental conditions
There are several environmental conditions that are conducive to P. infestans. During the 2009 growing season, which was colder than average, and with greater than average rainfall, there was a major infestation of tomato plants in the eastern United States. 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.
 * A Beaumont Period is a period of 48 consecutive hours, in at least 46 of which the hourly readings of temperature and relative humidity at a given place have not been less than 20 °C and 75%, respectively.
 * A Smith Period is at least two consecutive days where min temperature is 10 °C or above and on each day at least 11 hours when the relative humidity is greater than 90%.

Potato varieties
Potato varieties vary in their susceptibility to blight. Most early varieties are very vulnerable; they should be planted early so that the crop matures before blight starts (usually in July). Many old crop varieties, such as King Edward potato are also very susceptible 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 blight and not others, so their performance may vary depending on which are around. These crops 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. Defender is an American cultivar whose parentage includes Ranger Russet and Polish potatoes resistant to late blight. It is a long white-skinned cultivar with both foliar and tuber resistance to late blight. Defender was released in 2004. 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.

Use of fungicides
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 is a fungicide that was marketed for use against P. infestans, but suffered serious resistance issues when used on its own. It is strongly advised to use metalaxyl along with carbamate compounds, or the especially effective synergistic Cymoxanil and Mancozeb combination as it is effective at managing metalaxyl resistant strains.

In the past, copper sulfate solution (called 'bluestone') was used to combat potato blight. Copper pesticides remain in use in rare instances on organic crops.

Control of tuber blight
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 canopy can also be destroyed around 2 weeks before harvest. This can be done via a contact herbicide or using sulfuric acid to burn off the foliage.

Historical impact
The effects of Phytophthora infestans in Ireland in 1845-57 were one of the factors which caused over one million to starve to death and forced another two million to emigrate from affected countries. Most commonly referenced is the Great Irish Famine, during the late 1840s.

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. It crossed the Atlantic Ocean with a shipment of seed potatoes for Belgian farmers in 1845.

Use as a biological weapon
Potato blight was one of more than 17 agents that the United States researched as potential biological weapons before the nation suspended its biological weapons program. France, Canada, USA and the Soviet Union also researched P. infestans as a biological weapon in the 1940s and 50s.