Recent Genotypes of Phytophthora infestans in the Eastern United States Reveal Clonal Populations and Reappearance of Mefenoxam Sensitivity

Abstract

Isolates of Phytophthora infestans (n = 178) were collected in 2002 to 2009 from the eastern United States, Midwestern United States, and eastern Canada. Multilocus genotypes were defined using allozyme genotyping, and DNA fingerprinting with the RG-57 probe. Several previously described and three new mulitilocus genotypes were detected. The US-8 genotype was found commonly on commercial potato crops but not on tomato. US-20 was found on tomato in North Carolina from 2002 through 2007 and in Florida in 2005. US-21 was found on tomato in North Carolina in 2005 and Florida in 2006 and 2007. US-22 was detected on tomato in 2007 in Tennessee and New York and became widespread in 2009. US-22 was found in 12 states on tomato and potato and was spread on tomato transplants. This genotype accounted for about 60% of all the isolates genotyped. The US-23 genotype was found in Maryland, Virginia, Pennsylvania, and Delaware on both tomato and potato in 2009. The US-24 genotype was found only in North Dakota in 2009. A1 and A2 mating types were found in close proximity on potato and tomato crops in Pennsylvania and Virginia; therefore, the possibility of sexual reproduction should be monitored. Whereas most individuals of US-8 and US-20 were resistant to mefenoxam, US-21 appeared to be intermediately sensitive, and isolates of US-22, US-23, and US-24 were largely sensitive to mefenoxam. On the basis of sequence analysis of the ras gene, these latter three genotypes appear to have been derived from a common ancestor. Further field and laboratory studies are underway using simple sequence repeat genotyping to monitor current changes in the population structure of P. infestans causing late blight in North America.

Tomato I2 Immune Receptor Can Be Engineered to Confer Partial Resistance to the Oomycete Phytophthora infestans in Addition to the Fungus Fusarium oxysporum

“…However, the level of HR observed between I2 and AVR3a KI was relatively weak therefore we cannot rule out that other effectors present in this P. infestans determine the observed I2-mediate resistance. Most late blight lineages infect both potato and tomato (Goodwin, Smart et al 1998), although some lineages primarily infect tomato and are not likely to cause disease on potato plants in the field (Hu, Perez et al 2012 identified in this study gives partial resistance to P. infestans strains carrying either of the AVR3a variants in the transient assays used (Fig. 7).…”

Evolution and Management of the Irish Potato Famine Pathogen Phytophthora Infestans in Canada and the United States

“…Because widely used site-specific fungicides, such as mefenoxam (the active isomer in metalaxyl), inhibit sporulation and mycelial growth inside host tissues by specifically inhibiting RNA polymearase-1, a mutation that changes the affinity of target sites could easily lead to fungicide resistance (Davidse et al 1983). Significant variations for mefenoxam sensitivity were identified among P. infestans isolates from the different genotypes (Hu et al 2012;Danies et al 2013;Fry et al 2013;Kalischuk et al 2012). Isolates of the US-8 and US-11 genotypes were resistant to mefenoxam, while isolates of the genotypes US-22, US-23 and US-24 were initially highly sensitive to sensitive to this fungicide.…”

“…For example, from 2002 to 2007, the P. infestans genotypes US-20 and US-21 were the predominant genotypes affecting tomatoes in North Carolina and Florida (Hu et al 2012). However, inoculum distribution enabled the US-22 genotype (mating type A2) to rapidly displace the other genotypes and cause significant damage to tomato and potato growing areas in the northeastern United States and central Canada (Hu et al 2012;Kalischuk et al 2012).…”

“…However, inoculum distribution enabled the US-22 genotype (mating type A2) to rapidly displace the other genotypes and cause significant damage to tomato and potato growing areas in the northeastern United States and central Canada (Hu et al 2012;Kalischuk et al 2012). In addition, the new A1 mating type P. infestans genotypes US-23 and US-24 were found in tomato and potato-growing areas, respectively, in the northwestern United States and Canada (Kawchuk et al 2011;Hu et al 2012;Kalischuk et al 2012). These genotypes showed evidence of host specialization.…”

“…The pathogen is capable of penetrating a healthy tuber periderm which reduces yield and quality and diminishes storability by facilitating secondary infections . The pathogen reproduces predominantly by asexual means in the United States and Canada via sporangia formation on infected host tissues which are readily dispersed by wind and rain Hu et al 2012;Kalischuk et al 2012). These sporangia then directly initiate infection or form motile zoospores.…”