The Role of Oospores in the Epidemiology of Potato Late Blight

Abstract

Potato late blight (Phytophthora infestans) is a plant disease feared globally by farmers and the potato industry. P. infestans is a heterothallic oomycete with two mating types. Until recently the pathogen was limited to surviving between seasons as living mycelia in its host plant in most parts of the world. This was due to the fact that populations of P. infestans consisted of only one mating type (A1) in all parts of the world except Mexico, the putative centre of origin of the pathogen. Migration of new genotypes from Mexico, including genotypes of the second mating type (A2) has resulted in that now both mating types can be found worldwide. The formation of oospores is only possible if both mating types coexist. Oospores will give the pathogen the ability of surviving for extended periods of time outside its host, for example in the soil. There are reports of oospore formation under field conditions from many parts of the world. Also, in some places oospores are considered as a new, additional inoculum source and as a consequence the onset of late blight epidemics have become earlier. Oospores are formed through sexual recombination. If they act as a source of inoculum, this will increase the genotypic variation in populations of P. infestans leading to an enhanced adaptability of the pathogen. As a consequence, an earlier start of epidemics caused by oospores in the soil and a more aggressive behaviour of the pathogen due to new sexually formed genotypes could make potato late blight even more difficult to control in the future.

Characterization ofPhytophthora infestanspopulation diversity in Canada reveals increased migration and genotype recombination

“…In Canada and the United States, the potential recombinant genotypes have always made up only a small proportion of the total samples. In contrast, nearly 100% of the isolates from Europe that were tested were recombinant in origin (Drenth et al 1993;Sujkowski et al 1994;Anderson et al 2009). Continued monitoring will be required to determine whether sexually reproducing populations of P. infestans have now become established in Canada and how this development might affect late blight epidemiology in subsequent years.…”

“…In Canada and the United States, the emergence of new P. infestans genotypes suggests that sexual recombination is now contributing to the increased diversity in genetic composition of the pathogen (Goodwin et al 1995a;Kalischuk et al 2012). Sexual recombination produces thick-walled oospores capable of surviving extreme temperatures and extended periods in the absence of host tissue (Anderson et al 2009). However, evidence of sexual reproduction in Canada and the United States remained limited, probably because both mating types were found together only in a small number of sites in this area (Goodwin et al 1995a;Chycoski & Punja 1996).…”

Effect of Fungicides on Lesion Expansion of Late Blight of Potato

“…In the next growing season the germinating oospores can infect newly planted potatoes and cause early epidemics. However, the most important consequence of the occurrence of oospores is an increase in genetic variation within populations of P. infestans and the appearance of new, recombined strains (Andersson et al, 2009). In addition to oospores and air-borne sporangia, the infected seed tubers are another important source of late blight inoculums (Aylor, 2003).…”

Evaluation of PCR markers for Phytophthora infestans mating type determination

“…Recovery of the diploid state makes sexual reproduction possible and in consequence increases adaptability and plasticity of P. infestans strains. Ongoing numerous changes in P. infestans populations in the world have major implications for late blight and make it more difficult to control (Andersson et al 2009;Fry et al 2015;Govers et al 1997;Smart and Fry 2001). …”

“…The oospores of P. infestans are considered an additional inoculum source. They can infect the newly planted tubers causing late blight epidemics earlier in the season than expected (Andersson et al 2009;Fry et al 2013;Mayton et al 2000). The mating of P. infestans and formation of oospores are complicated.…”