<P> Around the world the disease causes around $6 billion of damage to crops each year . </P> <P> Breeding for resistance, particularly in potato, 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 . </P> <P> 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 . </P> <P> 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 . </P>

The botanist who proved that phytophthora infestans was the cause of the late potato blight was