Tomato (Solanum lycopersicum) and potato (S. tuberosum) crops are grown on over 67,000 acres (27,114 hectares) in the state of Wisconsin each year. Late blight, caused by Phytophthora infestans (Mont.) deBary, is a potentially devastating oomycete pathogen that sporadically affects tomato and potato crops in the state. Prevention of this disease through prophylactic application of oomycete-specific fungicides can cost producers millions of dollars per year in additional chemical, fuel, and labor expenses. In 2009, late blight was observed on tomato and potato in over 25 Wisconsin counties. The epidemic initiated on tomato in southern WI in early August and progressed northward in the state with additional reports on tomato primarily from home gardens and small farms. Potato late blight was also identified but with limited incidence in central WI, likely due to routine fungicide programs in commercial production. Clonal lineages of P. infestans documented in Wisconsin in previous epidemics included US-1 in the 1970s and US-8 in the mid-1990s. Populations of P. infestans in the U.S. have recently undergone significant genetic changes, resulting in isolates with unique clonal lineages and epidemiological characteristics (1). Symptoms of late blight observed on tomato and potato included water-soaked to dark brown circular lesions with pale green haloes accompanied by signs of pathogen sporulation typically on leaf undersides during periods of high humidity. Isolates of P. infestans were generated from field infected tomato and potato foliar tissues. Axenic, single zoospore derived cultures were generated and maintained on Rye A agar for further characterization. Mycelium was coenocytic with hyphal diameter of 5 to 8 μm (n = 50). Sporangia were limoniform or ovoid, semi- to fully papillate, caducous, had short pedicels, and were 29.6 (h) × 16.8 μm (w) (n = 50). The average length/width ratio was 1.76. Allozyme banding patterns at the glucose-6-phosphate isomerase (Gpi) locus indicated a 100/122 profile, consistent with the US-22 clonal lineage (3). Mating type assays confirmed the isolates to be A2 and in vitro mefenoxam sensitivity was observed (4). Restriction fragment length polymorphic analysis of a representative isolate from Wisconsin with the multilocus RG57 sequence and EcoRI produced the DNA pattern indicative of US-22 (2). The P. infestans clonal lineage US-22 was predominant in U.S. epidemics on tomato in 2009. To our knowledge, this is the first report of P. infestans clonal lineage US-22 causing late blight on tomato and potato in Wisconsin, USA.
References: (1) K. Deahl. (Abstr.) Phytopathology 100(suppl.):S161, 2010. (2) S. B. Goodwin et al. Curr. Genet. 22:107, 1992. (3) C. H. Hu et al. Plant Dis. 96:1323, 2012. (4) A. C. Seidl et al. Phytopathology 101(suppl.):S246, 2011.