Detection of different bacterial and fungal pathogens in apparently healthy strawberry plants

The pathogens Phytophthora fragariae, Colletotrichum spp. and Xanthomonas fragariae may be present in the roots or in the system of strawberry plants showing none or atypical symptoms. All three pathogens are known to cause serious losses in some varieties under certain climatic conditions in Switzerland. For the sanitary control of imported young strawberry plants as green plants, cold-store plants or unrooted stolen-tip plantlets, random samples are drawn at the border and are sent to our lab. In April and March strawberry fields used for the propagation of young plants are monitored for red stele root rot (Phytophthora fragariae) and other root diseases, and fields planted in August for fruit production in the following year are monitored for latent infections with X. fragariae, Colletotrichum or Gnomonia in late fall and spring. It is required that pathogens are detected and identified in the shortest time possible, specially in sanitary control samples. Methods had also to be adapted to the fact that, depending on the kind of plants to be tested, different plant parts are available for the diagnosis. In our diagnostic laboratory serological methods are used for the detection and identification of X. fragariae and P. fragariae. Serological methods may be not very sensitive and not very specific, but they are fast and great numbers of samples can be easily handled. It is assumed that by including control samples and by practicing a carefull interpretation of the results, ELISA-tests can be used to qualify plant material if more accurate methods are not suited for routine control of large numbers of samples, or if they are too expensive. The Paraquat test or isolation on PDA-dishes are used to detect Colletotrichum spp and other pathogens, like Gnomonia comari or Hainesia lythri, present in the system of strawberry plants. However, it was found that latent infections by these pathogens are difficult to detect under certain conditions.