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Peronospora sparsa on Cultivated Rubus arcticus and Its Detection by PCR Based on ITS Sequences

In 1994 to 1996, large yield losses were reported in cultivated arctic bramble (Rubus arcticus) due to berries drying in the middle of the growing season in the entire cultivation area (62° to 66° N) in Finland. Interveinal, angular, purple-red lesions on leaves are associated with the dryberry disease. Cultivations of arctic bramble were inspected in 1995 and 1996, and 69 plant samples and 137 rootstocks were collected and examined for fungi in the laboratory. Perono-spora sparsa was the species most commonly found in all types of samples tested, but Fusarium avenaceum, Cylindrocarpon destructans, and Botrytis cinerea also were detected. P. sparsa was shown to overwinter in the underground parts of arctic bramble. Laboratory tests showed that the two main cultivars of arctic bramble (Pima and Mespi) were susceptible to P. sparsa. Sequences of the internal transcribed spacer (ITS) regions 1 and 2 of the rDNA genes were determined in four and six isolates, respectively, of P. sparsa, and little sequence variability was detected. The corresponding ITS regions of arctic bramble, the above mentioned fungi, a Phoma sp. previously isolated from arctic bramble, and Phytophthora cactorum isolated from strawberry were also determined and compared with the corresponding sequences of P. sparsa. Subsequently, two pairs of primers were designed to the ITS regions that could be used to detect either P. sparsa and Phytophthora cactorum, or only P. sparsa, respectively, by polymerase chain reaction (PCR) using two different types of thermal cyclers (“heated block” Mini Cycler and “hot air flow” Rapid Cycler).