| Phytophthora infestans (Mont.) deBary causes late blight, one of the most important diseases of potato and tomato worldwide. Multiple aspects of the biology and epidemiology of three recently identified clonal lineages of P. infestans, US-22, US-23, and US-24 were investigated to further an understanding of changing P. infestans populations and enhance management of late blight. Collection and characterization of P. infestans isolates from Wisconsin from 2009 to 2012 revealed three new clonal lineages, US-22 (A2 mating type, mefenoxam sensitive), US-23 (A1, intermediately sensitive), and US-24 (A1, intermediately sensitive). In an investigation of host range, it was discovered that the US-22, US-23, and US-24 lineages all produced lesions and sporulated well on potato and also on hairy nightshade, a common solanaceous weed. Sporulation on tomato was limited for US-24, but not for US-22 or US-23. Through testing of low temperature survival on tomato seeds, survival by all three lineages was seen for 14 days at -3°C and for 3 months at 0°C, indicating the potential for overwintering of P. infestans in tomatoes buried in fields or in protected areas such as compost piles. Resistance in tomato cultivars was explored and cultivars with Ph late blight resistance genes had variable efficacy against US-22, US-23, and US-24. Surprisingly, effective broad-spectrum resistance was found in some heirloom cultivars, including `Wapsipinicon Peach' and `Matt's Wild Cherry'. These cultivars can be immediately used by growers and further explored by breeders. Several organic-approved and biorational fungicides, particularly EF400 and Zonix, showed efficacy against the new lineages, indicating the potential for more environmentally friendly management of tomato late blight through non-conventional fungicides. The pathogen characteristics of lesion growth and sporulation rate were experimentally determined and simulated epidemics generated for each lineage using the LATEBLIGHT simulation model, resulting in conclusions that the US-23 lineage may have the potential to initiate the largest, fastest epidemics and be the most likely to persist in the pathogen population. Together, these results will further an understanding of the biology and epidemiology of P. infestans and lead to enhanced control of the devastating disease of late blight. |
