Development of a species-specific and sensitive detection assay for Phytophthora infestans and its application for monitoring of inoculum in tubers and soil

A specific and sensitive PCR assay for the detection of Phytophthora infestans, the cause of late blight of potato, in soil and plant tissues was developed. A P. infestans-specific primer pair (INF FW2 and INF REV) was designed by comparing the aligned sequences of rDNA internal transcribed spacer regions of most of the known Phytophthora species. PCR amplification of P. infestans DNA with primers INF FW2 and INF REV generated a 613 bp product, and species specificity was demonstrated against DNA from nine other Phytophthora species and seven potato-blemish pathogens. In a single-round PCR assay, 0.5 pg pure P. infestans DNA was detectable. Sensitivity was increased to 5 fg DNA in a nested PCR assay using Peronsporales-specific-primers in the first round. As few as two sporangia or four zoospores of P. infestans could be detected using the nested assay. Procedures are described for detection of P. infestans in leaves, stem and seed potato tubers before expression of symptoms. A soil assay in which 10 oospores per 0.5 g soil were detectable was developed and validated using samples of field soil. The PCR assay was used to examine the long-term survival of sexual (oospores) and asexual (sporangia and mycelium) inoculum of P. infestans in leaf material buried in a replicated experiment under natural field conditions. Oospores were consistently detected using the PCR assay up to 24 months (total length of the study) after burial in soil, whereas the sporangial inoculum was detected for only 12 months after burial. Sporangial inoculum was shown to be nonviable using a baiting assay, whereas leaf material containing oospores remained viable up to 24 months after burial.