STUDIES ON THE POSSIBLE ROLE OF MICROPROPAGATION IN THE DISSEMINATIONOF THE STRAWBERRY CROWN ROT PATHOGEN PHYTOPHTHORA-CACTORUM

Attempts to develop a method for establishing latent infection by the crown rot pathogen (P. cactorum) in strawberry stolen tips or unrooted plantlets were largely unsuccessful. One month after dipping stolen t ips in suspensions of zoospores (from 10 to 10(3) ml(-1)) 6% had died and 6% had become severely necrotic; only 2% of stolons with no obviou s necrosis yielded P. cactorum in isolation on a selective medium. Whe n young, pre-rooted plantlets were sprayed with more concentrated zoos pore suspensions (from 10(3) to 10(5) ml(-1)) and grown on for only on e week, 9% died, 51% became necrotic and 40% remained apparently healt hy; 22% of the ''healthy'' plantlets yielded P. cactorum. The frequenc y of isolation from different parts of the plantlets indicated that mo st infections originated in the base of the crown, probably via root i nitials. When 298 stolen tips and 357 meristems were excised from symp tomless plantlets one week after inoculation with a suspension of 10(4 ) zoospores per ml, P. cactorum grew conspicuously into the culture me dium and killed the plant tissue in 6% of stolen tip cultures and in 0 .8% of meristem cultures. Of 380 micropropagation cultures successfull y established from explants after inoculation not one yielded the path ogen on destructive sampling although other microorganisms were detect ed. It is concluded that the crown rot fungus does not become cryptica lly established in micropropagation cultures of strawberry and that, c onversely, the technique of micropropagation is a reliable way of ensu ring freedom from this pathogen in plant material.