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 was collected from individual potato plants in 10 commercial fields and from tomato plants in greenhouses at two locations. Sporulating stem and leaf tissue were used to inoculate semiselective rye medium and 147 isolates from potato and six from tomato were recovered. The isolates from the 1995, 1998, and 2005 outbreaks were analyzed to determine genotype at the allozyme loci GPI and PEP (3), mitochondrial haplotype (4), mating type, and metalaxyl sensitivity (2). The 1998 and 2005 outbreaks were similar because both were caused by the relatively aggressive US-11 allozyme genotype and had significant economic impact for commercial potato growers. All 153 isolates from potato and tomato in 2005 displayed the same allozyme pattern as the US-11 genotype, possessed the IIB mitochondrial haplotype, and were mating type A1. Of the 16 isolates tested, all were determined to be metalaxyl resistant because isolates grown on 5 and 100 μg/ml metalaxyl exhibited growth greater than 40% of the unamended control. The 1995 outbreak was caused by the relatively rare US-7 genotype and started so late during the season that economic impact was minimal. Similarly, the 2006 outbreak was noted from only one commercial potato field at the time of harvest in September 2006. However, the genotype of the 2006 isolate has not been determined because the patch was destroyed before adequate samples could be collected. Because the disease occurs so sporadically in Alaska, fungicides are not routinely in use, but it is unlikely that the pathogen has persisted locally between outbreaks. The source of P. infestans is unknown for each of the occurrences in Alaska. However, possible routes include seed potatoes for home gardens or commercial farms, tomato transplants, and retail vegetables shipped to Alaska from out of state. While potato is Alaska's main vegetable crop, there are less than 405 ha (1,000 acres) of potatoes planted in the state, with the majority planted in the Matanuska Valley. To our knowledge, this is the first formal report of P. infestans on both tomato and commercial potato in Alaska. References: (1) E. K. Cash. Plant Dis. Rep. 20:121, 1936. (2) D. E. L. Cooke et al. Plant Pathol. 52:181, 2003. (3) S. B. Goodwin et al. Plant Dis. 79:1181, 1995. (4) G. W. Griffith and D. S. Shaw. Appl. Environ. Microbiol. 64:4007, 1998.