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 famine s . The organism can also infect tomatoes and some other members of the Solanaceae . Biology . The spore s 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 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 transposon s and many gene families encoding for effector protein s 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 enzyme s such as protease s , lipase s and glycosylase s that act to degrade plant tissue , enzyme inhibitor s 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 . 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 pesticide s 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 stem s 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 weapon s 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 .