EVIDENCE OF A TOXIN IN CULTURE FILTRATE OF PHYTOPHTHORA FRAGARIAE

We have investigated the potential use of Phytophthora fragariae Hickman culture filtrate (CF) for in vitro selection of strawberry (Fragaria X ananassa Duch.) variants resistant to red stele (red core). CF first was tested for its ability to induce wilting of tomato and strawberry seedlings. CF induced wilting of tomato seedlings but not strawberry seedlings. Second, we observed the effect of CF on ion leakage of strawberry leaf and root tissue. A slight increase in conductance was observed in roots of susceptible and resistant varieties tested. Third, the effect of CF on shoot or callus regeneration from leaf sections of 4 strawberry varieties was investigated. Use of 10% or more CF resulted in browning of all leaf sections of susceptible varieties 'Honeoye' and 'Marlate'. However, in the presence of 10% CF, approximately 20% of leaf sections of resistant varieties 'Allstar' and 'Guardian' remained green and/or produced callus. And, fourth, the effect of CF on growth of crown explants was examined. CF had an adverse effect on growth of all varieties tested. Thus, our results provide preliminary evidence that a CF-based method of screening for resistant strawberry variants may be feasible and worth further study.