Development and Application of a Multiplex qPCR Method for the Simultaneous Detection and Quantification of Pratylenchus alleni and P. penetrans in Quebec, Canada.

Root lesion nematodes are very common plant-parasitic nematodes that affect a wide range of plants. More than one species can be found simultaneously in a field, and each has a different impact on crop yield. Unfortunately, identifying them using classical morphometric criteria is very difficult and time consuming. The species Pratylenchus alleni was recently observed for the first time in Canada, associated with severe damage in a soybean field in the province of Quebec. The major species, P. penetrans, is also known to be endemic in Quebec but no data exist on its distribution in field crops. This prompted the development of a multiplex quantitative polymerase chain reaction (PCR) assay for the simultaneous detection and quantification of P. alleni and P. penetrans. The method was found to be specific and sensitive, systematically detecting a single larva in a 100-cm soil sample with no cross-amplification with other species, even when they outnumbered the target species. An exogenous internal positive control was included in the test to avoid false negatives due to the presence of PCR inhibitors. This assay was used to study the distribution of P. alleni and P. penetrans in 185 soybean fields in the major soybean-producing areas of Quebec during a 3-year survey. Overall, P. penetrans was found in 42% of the fields, P. alleni in 8%, and both species in 4%. The population density of P. alleni in positive fields was still very low, with only a few larvae detected. However, densities of P. penetrans were much higher: the provincial mean was 51.7 nematodes per 100 cm of soil (in positive samples), and 8% of the fields (15 of 185) exceeded the theoretical economic threshold. The presence of P. penetrans was also strongly correlated with soil texture, with lighter soil being the most favorable.