Detection of Phomopsis sclerotioides in Commercial Cucurbit Field Soil by Nested Time-Release PCR

A polymerase chain reaction (PCR)-based molecular method to detect Phomopsis sclerotioides in soil was developed using a species-specific primer pair. To improve sensitivity of the detection, three PCR techniques were used; namely, nested PCR using the primer pair internal transcribed spacer (ITS)1 and ITS4, time-release PCR using two different DNA polymerases (recombinant Taq and AmpliTaq Gold), and fluorescent PCR to obtain fluorescent-labeled PCR products that can be analyzed by capillary electrophoresis. The latter two techniques were combined and termed nested time-release fluorescent (NTRF)-PCR. The minimum concentration of DNA required to obtain species-specific PCR products successfully was 50 fg/μg. Using the NTRF-PCR method, the fungus could be detected in sandy soil that was artificially infested at a density of 10 CFU/g. The pathogen was detected in most soil samples collected from commercial cucumber fields in which visual disease symptoms had appeared, and even in samples collected from fields where visual disease symptoms had not appeared. To prevent the invasion and establishment of root-inhabiting pathogens such as P. sclerotioides, it is critical to detect the fungus in soil as soon as possible after its introduction into a cucumber-growing region.