Studies on the possible role of micropropagation in the dissemination of the strawberry crown rot pathogen Phytophthora cactorum

Summary

Attempts to develop a method for establishing latent infection by the crown rot pathogen (P. cactorum) in strawberry stolon tips or unrooted plantlets were largely unsuccessful. One month after dipping stolon tips in suspensions of zoospores (from 10 to 10³ ml^?1) 6% had died and 6% had become severely necrotic; only 2% of stolons with no obvious necrosis yielded P. cactorum in isolation on a selective medium. When young, pre-rooted plantlets were sprayed with more concentrated zoospore suspensions (from 10³ to 10⁵ ml^?1) and grown on for only one week, 9% died, 51% became necrotic and 40% remained apparently healthy; 22% of the “healthy” plantlets yielded P. cactorum. The frequency of isolation from different parts of the plantlets indicated that most infections originated in the base of the crown, probably via root initials. When 298 stolon tips and 357 meristems were excised from symptomless plantlets one week after inoculation with a suspension of 10⁴ zoospores per ml, P. cactorum grew conspicuously into the culture medium and killed the plant tissue in 6% of stolon tip cultures and in 0.8% of meristem cultures. Of 380 micropropagation cultures successfully established from explants after inoculation not one yielded the pathogen on destructive sampling although other microorganisms were detected. It is concluded that the crown rot fungus does not become cryptically established in micropropagation cultures of strawberry and that, conversely, the technique of micropropagation is a reliable way of ensuring freedom from this pathogen in plant material.