a2 spread even more slowly, with britain having low levels and ireland (north and republic) having none-to-trace detections through the '90s. many of the strains that appeared outside of mexico since the 1980s have been more aggressive, leading to increased crop losses. in europe since 2013 the populations have been tracked by the euroblight network (see links below). some of the differences between strains may be related to variation in the rxlr effectors that are present. disease management p. infestans is still a difficult disease to control. there are many chemical options in agriculture for the control of damage to the foliage as well as the fruit (for tomatoes) and the tuber (for potatoes). a few of the most common foliar-applied fungicides are ridomil, a gavel/supertin tank mix, and previcur flex. all of the aforementioned fungicides need to be tank mixed with a broad-spectrum fungicide such as mancozeb or chlorothalonil not just for resistance management but also because the potato plants will be attacked by other pathogens at the same time. if adequate field scouting occurs and late blight is found soon after disease development, localized patches of potato plants can be killed with a desiccant (e.g. paraquat) through the use of a backpack sprayer. this management technique can be thought of as a field-scale hypersensitive response similar to what occurs in some plant-viral interactions whereby cells surrounding the initial point of infection are killed in order to prevent proliferation of the pathogen. if infected tubers make it into a storage bin, there is a very high risk to the storage life of the entire bin. once in storage, there isn't much that can be done besides emptying the parts of the bin that contain tubers infected with phytophthora infestans. to increase the probability of successfully storing potatoes from a field where late blight was known to occur during the growing season, some products can be applied just prior to entering storage (e.g. phostrol). around the world the disease causes around $6 billion of damage to crops each year. resistant plants potatoes after exposure. the normal potatoes have blight but the cisgenic potatoes are healthy. genetically modified king edward (right) next to king edward which has not been genetically modified (left). research field, swedish university of agricultural sciences, 2019 breeding for resistance, particularly in potato plants, has had limited success in part due to difficulties in crossing cultivated potato to its wild relatives, which are the source of potential resistance genes. in addition, most resistance genes only work against a subset of p. infestans isolates, since effective plant disease resistance only results when the pathogen expresses a rxlr effector gene that matches the corresponding plant resistance (r) gene; effector-r gene interactions trigger a range of plant defenses, such as the production of compounds toxic to the pathogen. potato and tomato 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 in the northern hemisphere). 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. genetic engineering may also provide options for generating resistance cultivars. a resistance gene effective against most known strains of blight has been identified from a wild relative of the potato, solanum bulbocastanum, and introduced by genetic engineering into cultivated varieties of potato. this is an example of cisgenic genetic engineering. melatonin in the plant/p. infestans co-environment reduces the stress tolerance of the parasite. reducing inoculum blight can be controlled by limiting the source of inoculum. only good-quality seed potatoes and tomatoes obtained from certified suppliers should be planted. often discarded potatoes from the previous season and self-sown tubers can act as sources of inoculum. compost, soil or potting medium can be heat-treated to kill oomycetes such as phytophthora infestans. the recommended sterilisation temperature for oomycetes is 120 °f (49 °c) for 30 minutes. environmental conditions there are several environmental conditions that are conducive to p. infestans. an example of such took place in the united states during the 2009 growing season. as colder than average for the season and with greater than average rainfall, there was a major infestation of tomato plants, specifically in the eastern 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 10 °c (50 °f) and 75%, respectively. a smith period is at least two consecutive days where min temperature is 10 °c (50 °f) or above and on each day at least 11 hours when the relative humidity is greater than 90%. the beaumont and smith periods have traditionally been used by growers in the united kingdom, with different criteria developed by growers in other regions. the smith period has been the preferred system used in the uk since its introduction in the 1970s. based on these conditions and other factors, several tools have been developed to help growers manage the disease and plan fungicide applications. often these are deployed as part of decision support systems accessible through web sites or smart phones. several studies have attempted to develop systems for real-time detection via flow cytometry or microscopy of airborne sporangia collected in air samplers. whilst these methods show potential to allow detection of sporangia in advance of occurrence of detectable disease symptoms on plants, and would thus be useful in enhancing existing decision support systems, none have been commercially deployed to date. use of fungicides spraying potato, nottinghamshire fungicides for the control of potato blight are normally only used in a preventative manner, optionally in conjunction with disease forecasting. in susceptible varieties, sometimes fungicide applications may be needed weekly. an early spray is most effective. the choice of fungicide can depend on the nature of local strains of p. infestans. metalaxyl is a fungicide that was marketed for use against p. infestans, but suffered serious resistance issues when used on its own. in some regions of the world during the 1980s and 1990s, most strains of p. infestans became resistant to metalaxyl, but in subsequent years many populations shifted back to sensitivity. to reduce the occurrence of resistance, it is strongly advised to use single-target fungicides such as metalaxyl along with carbamate compounds. a combination of other compounds are recommended for managing metalaxyl-resistant strains. these include mandipropamid, chlorothalonil, fluazinam, triphenyltin, mancozeb, and others. in the united states, the environmental protection agency has approved oxathiapiprolin for use against late blight. in african smallholder production fungicide application can be necessary up to once every three days. in organic production in the past, copper(ii) sulfate solution (called 'bluestone') was used to combat potato blight. copper pesticides remain in use on organic crops, both in the form of copper hydroxide and copper sulfate. given the dangers of copper toxicity, other organic control options that have been shown to be effective include horticultural oils, phosphorous acids, and rhamnolipid biosurfactants, while sprays containing "beneficial" microbes such as bacillus subtilis or compounds that encourage the plant to produce defensive chemicals (such as knotweed extract) have not performed as well. during the crop year 2008, many of the certified organic potatoes produced in the united kingdom and certified by the soil association as organic were sprayed with a copper pesticide to control potato blight. according to the soil association, the total copper that can be applied to organic land is 6 kilograms per hectare (5.4 lb/acre)/year. 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. another approach is to destroy the canopy around five weeks before harvest, using a contact herbicide or sulfuric acid to burn off the foliage. eliminating infected foliage reduces the likelihood of tuber infection. historical impact suggested paths of migration and diversification of lineages herb-1 and us-1 the effect of phytophthora infestans in ireland in 1845–52 was 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, from which the irish population has still not fully recouped. 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. all of the potato-growing countries in europe were affected, but the potato blight hit ireland the hardest. implicated in ireland's fate was the island's disproportionate dependency on a single variety of potato, the irish lumper. the lack of genetic variability created a susceptible host population for the organism after the blight strains originating in chiloé archipelago replaced earlier potatoes of peruvian origin in europe. during the first world war, all of the copper in germany was used for shell casings and electric wire and therefore none was available for making copper sulfate to spray potatoes. a major late blight outbreak on potato in germany therefore went untreated, and the resulting scarcity of potatoes led to the deaths of 700,000 german civilians from starvation. since 1941, eastern africa has been suffering potato production losses because of strains of p. infestans from europe. france, canada, the united states, and the soviet union researched p. infestans as a biological weapon in the 1940s and 1950s. 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. whether a weapon based on the pathogen would be effective is questionable, due to the difficulties in delivering viable pathogen to an enemy's fields, and the role of uncontrollable environmental factors in spreading the disease. late blight (a2 type) has not yet been detected in australia and strict biosecurity measures are in place. the disease has been seen in china, india and south-east asian countries. in light of the periodic epidemics of p. infestans ever since its first emergence, it may be regarded as a periodically emerging pathogen - or a periodically "re-emerging pathogen".