Occurrence of late blight caused by Phytophthora infestans on potato and tomato in Alaska.

Phytophthora infestans, causal agent of late blight, was included in a list of plant pathogens found in Alaska in 1934 (1). No notes of symptoms, extent of disease, or dates were recorded. The only reference to the location was given as Wrangell, a town in southeast Alaska with subsistence gardening. Neither P. infestans nor late blight was noted again in the state for another 59 years. Late blight first appeared in Alaska's major potato-growing region in south-central Alaska's Matanuska Valley in 1995. Subsequent outbreaks have been sporadic, occurring only in 1998, 2005, and 2006. Each of these outbreaks was identified from rapidly enlarging brown foliar lesions with branched sporangiophores and lemon-shaped sporangia (~25 × 30 μm). The 1995 and 1998 potato late blight outbreaks in Alaska were not sampled extensively nor have they previously been formally reported. We recovered single isolates of P. infestans from symptomatic potato foliage in the 1995 and 1998 outbreaks. In 2005, symptomatic foliage wa...