The structure of mating type, metalaxyl resistance and virulence of Phytophthora infestans isolates collected from Latvia

Abstract

Potato late blight, caused by the oomycete Phytophthora infestans (Mont.) de Bary, is one of the most serious potato (Solanum tuberosum L.) diseases, causing considerable yield loss in potato production worldwide, including Latvia. At present, the data on the population characteristics of P. infestans in Latvia is sparse. Therefore, the main aim of this study was to collect new data on Latvian isolates of P. infestans, to determine the main characteristics of the pathogen, particularly mating types, metalaxyl resistance and virulence with Black's differential set of potato genotypes containing resistance (R) genes R1-R11. During 2010-2012, 181 isolates of P. infestans were collected from 23 potato fields, from 13 locations in Latvia. Out of 181 isolates tested, 52.5% were A1, 43.1% -A2 and 4.4% -self-fertile mating type. Of 116 isolates screened for resistance to metalaxyl, 25.9% were resistant, 19.8% -intermediate and 54.3% -sensitive. More than 80% of isolates were virulent to R1, R3, R4, R7, R10 and R11, while 33% or fewer isolates were virulent to R5, R8 and R9. The least frequent was virulence against R9 in 24% of isolates. Our study revealed that the Latvian population of P. infestans is diverse. The proportion of mating types and the occurrence of both A1 and A2 in the same field indicate the possibility of sexual recombination in Latvian fields. Thus, it is very important to keep the crop rotation system, to prevent soil contamination with long-living oospores. The Latvian population of P. infestans shares many similarities with other European populations, which suggests gene flow between populations.

Diversity of populations of Phytophthora infestans in relation to patterns of potato crop management in Latvia and Lithuania

“…Possible sexual reproduction and oospore formation in the Latvian and Lithuanian P. infestans populations are supported by the fact that A1 and A2 mating type isolates coexist in the potato fields and the mating type ratio is close to 1:1 in the population (Aav et al ., ; Runno‐Paurson et al ., ). Also, oospores have been found in Latvian potato fields with a frequency of over 80% from collected leaflets, which makes disease management more difficult as the inoculum source is constantly present in the fields (Bimšteine, ).…”

“…13_A2, 37_A2; http://www.euroblight.net). However, the low fungicide application rates in Lithuania and Latvia reduce the likelihood of such selection pressure and may therefore act to maintain the diversity that comes from the oospore population in the soil (Aav et al ., ; Runno‐Paurson et al ., ). This may explain why more unique MLGs, rather than local clones, were detected from small conventional fields where chemical late blight control is usually inadequate, varying from no sprays to up to four sprays per season.…”

“…Furthermore, they do not grow varieties with better resistance to the disease or practise adequate crop rotation. Therefore, in favourable conditions, the pathogen can reproduce and spread freely, causing substantial damage to the crop (Ronis et al ., ; Aav et al ., ; Runno‐Paurson et al ., ).…”

“…Effective late blight management is aided by up‐to‐date knowledge of the pathogen population. However, information on Latvian and Lithuanian populations of P. infestans is scarce and limited to only a few studies of phenotypic characterization (Valskyte et al ., ; Bebre et al ., ; Aav et al ., ; Runno‐Paurson et al ., ). The mating type ratio of nearly 1:1, with both mating types occurring in the same fields, indicates a high potential for sexual reproduction and oospore formation (Aav et al ., ; Runno‐Paurson et al ., ).…”

“…However, information on Latvian and Lithuanian populations of P. infestans is scarce and limited to only a few studies of phenotypic characterization (Valskyte et al ., ; Bebre et al ., ; Aav et al ., ; Runno‐Paurson et al ., ). The mating type ratio of nearly 1:1, with both mating types occurring in the same fields, indicates a high potential for sexual reproduction and oospore formation (Aav et al ., ; Runno‐Paurson et al ., ). In addition, oospores and late blight symptoms characteristic of oospore infections have previously been recorded in potato fields in Latvia (Bimšteine, ).…”