When petioles and stems are affected, portions of plants beyond blight lesions may dry up rapidly and collapse. This white growth contains the spores that are easily blown around in the wind. In moist conditions, a downy white growth usually develops near the margins of leaf spots on the undersides of leaves. The spots may enlarge rapidly until entire leaflets are killed. Leaf spot margins often are pale green or water soaked. On leaves, pale green to brown spots appear on the upper surfaces. Late blight symptoms can develop on leaves, stems, branches, and both green and ripe fruit and they are very obvious to the naked eye. In 2009, all three components were present throughout the growing season. If any one or more of these components is absent, then late blight will not develop. Keep in mind that in order for the disease to develop, the pathogen (Phytophthora infestans) must be present in combination with a susceptible host (primarily tomato, potato) and environmental conditions favorable for the pathogen to develop and cause disease. In addition, the cool and wet conditions that favor disease development persisted through the growing season. Never before had such an extensive distribution of late blight-infected plants occurred. Another reason was the distribution of infected transplants through local retail stores from Ohio to Maine. By mid-June, late blight had been confirmed in several states in the Northeast, including Pennsylvania. One is that this was the earliest the disease had been reported over such a broad region of the country (Northeast). The occurrence of late blight in 2009 was different for several reasons. Each year there are several confirmed outbreaks on potato and/or tomato in commercial fields. Late blight is not uncommon in Pennsylvania. Other losses for potato were due to the US8 genotype that is widespread and commonly seen on potato annually. In 2009, the majority of losses for tomato and some potato losses were due to the relatively new US22 genotype. The genotypes most common in Pennsylvania and the Northeast can cause disease on either potato or tomato or both. Similar to the seasonal flu in humans, different isolates (genotypes) of the late blight pathogen exist and infect different hosts so it is highly unlikely that a given isolate of the pathogen would be capable of infecting and causing disease on all known host plants. The pathogen may infect solanaceous weeds such as bittersweet nightshade, as well as other plants in the same botanical (Solanaceae) family, including petunias, Chinese lantern, and tomatillos. In addition to tomato and potato, the late blight pathogen has also been reported to infect eggplant and pepper, although no cases have been reported recently. Heavy losses can take place in transit symptoms can occur on infected but symptom-less tomato fruit within 5 days of harvest. Late blight epidemics occurred in 1878 in England, destroying entire plantings 1906, 1927, and 1928 in California 1940 in Ontario 19 throughout the eastern half of North America when 80 to 90 percent of early seedbeds in Florida were a complete loss, over 50 percent of the crop was lost in eastern states from New York to Florida, and 25 percent of the crop was lost in midwestern states 1960 in Ontario and 1976 in southern Georgia and some northern states where infected transplants were used. Since that time, epidemics have happened periodically when weather and other conditions favored disease development. It was directly responsible for the Irish Potato Famine of 18. Then, from 1844 to 1847, it occurred in epidemic and catastrophic proportions throughout Europe and North America. It occurred in France in 1840, was destructive in Germany in 1841, and appeared in North America in 1843. Late blight is thought to have originated in Central America and to have appeared almost simultaneously in Europe and North America about 1830. The disease occurs worldwide where tomatoes and potatoes are grown. However, it was 1863 before deBary established beyond doubt that this organism, Phytophthora infestans, was the cause of late blight. The causal pathogen of late blight from potatoes was first described in 1845 by Montagne and from tomato in 1847 by Payen in France.
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