infestans is an obligate pathogen therefore, it cannot live in the absence of living host tissue. infestans are limited to members of the nightshade family (tomato, potato, pepper, eggplant, petunia, and some weedy members of the family, such as hairy nightshade). Some isolates from potato are not too aggressive on tomato, but some can be aggressive on both tomato and potato, such as the new strains isolated in the United States in 2009. infestans, and they vary in their aggressiveness on potato and tomato. They are well-adapted for long-distance transport in wind currents to new locations miles away or for splash dispersal from diseased plant tissue onto new tissue or soil where they may release hundreds of infective, swimming zoospores. The sporangia are easily dislodged from the sporangiophores by air currents, mechanical force, and/or splashing water. infestans are hyaline (clear) and branched with characteristic swollen ends where the lemon-shaped sporangia are produced (figure 2). The ability to rapidly produce sporangia under favorable environmental conditions is the basis for the epidemic potential of P. infestans rapidly produces abundant sporangia (lemon-shaped, zoospore-containing structures) on host tissue during wet, cool weather (figure 1). This characteristic has practical implications because P. One important difference between fungi and Phytophthora species is that Phytophthora species produce motile spores called “zoospores.” Zoospores are chemically attracted to plant tissue and able to swim through water (e.g., in soil, drainage ditches, etc.) toward plant tissue where they form an infective cyst. However, as their evolutionary relationships were revealed, they were reclassified within the stramenopiles, a group with many aquatic organisms, such as brown algae. Until the late 20th century, water molds were classified as fungi. Phytophthora infestans belongs to a group of plant pathogens commonly called “water molds” because of their affinity and special adaptations to water. These fungicides were previously very effective against late blight and commonly used by commercial growers for control of late blight. To make matters worse, these strains had developed pesticide resistance to several groups of systemic fungicides. In 2009, new strains identified in the mid-Atlantic region were found to be very aggressive on tomato in comparison to the more endemic strains that prefer potato as a host. When late blight is present in a location (e.g., overwintered) or is introduced (e.g., on transplants or by wind currents carrying inoculum) and environmental conditions are favorable, devastating crop loss can result if preventative control measures have not already been implemented.Ī recent problem that makes late blight more challenging to control is the appearance of new strains of P. However, in major potato-growing regions of Virginia, such as the Eastern Shore, the pathogen may overwinter on potato tubers or on greenhouse tomato plants, making the disease a potential threat each year. In many areas of Virginia, late blight is not typically an annual problem. During wet, cool weather, crop loss due to late blight can be rapid and unstoppable if preventative controls have not been used. Wet, cool environmental conditions, such as those prevalent during the 1840s in Ireland, favor development of late blight. The Irish Potato Famine is a devastating example of the epidemic potential of late blight. This famine resulted in the death of more than 1 million people and the displacement of nearly 2 million more during a short, five-year period. It has historical significance as the cause of the Irish Potato Famine during the 1840s. Late blight, caused by Phytophthora infestans, is an important and destructive disease on potato and tomato.
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