Specific detection of Phytophthora nicotianae using the polymerase chain reaction and primers based on the DNA sequence of its elictin gene ParA1

Six primers based on the sequence of the flanking and coding regions of the elicitin gene ParA1 of Phytophthora nicotianae were tested for specific detection of the fungus by the polymerase chain reaction (PCR). One combination, IL7/IL8, with IL7 in a flanking region and IL8 in a coding region of the gene, gave an intense 378 bp signal with a diverse collection of isolates of P. nicotianae, that included some from black shank disease of tobacco and others from a variety of hosts. The sequence of the amplification product obtained with an isolate that produces elicitin and one that does not, was homologous with the known sequence of the ParA1 gene. The same primer combination gave no signal with sixteen other Phytophthora species tested except for two isolates P. palmivora with which it gave a weak 800 bp signal. It gave no signal with DNA from healthy tobacco and tomato plants but P. nicotianae was detected in inoculated tobacco and tomato plants. Small numbers of zoospores (>100) trapped onto a nitrocellulose membrane after filtration from suspension were also detected after two successive rounds of PCR.