Local and Systemic Resistance Induced in Watermelons by Formae Speciales ofFusarium oxysporum

Abstract

Watermelon cultivars differentially resistant to Fusarium wilt were preinoculated (induced) with Fusarium oxysporum f. sp. cucumerinum or avirulent races of F. o. niveum 24 or 72 hr prior to challenge with a virulent race of F. o. niveum. All of the inducer treatments significantly reduced with symptoms (P.ltoreq.0.05). Avirulent races of F. o. niveum induced a higher level of resistance than did F. o. cucmerinum. An interval of 24 hr between induction and challenge provided significant protection; a 72-hr interval further enhanced resistance. When roots of the wilt-susceptible watermelon cultivar Black Diamond were induced with F. o. cucumerinum and the leaves inoculated with Colletotrichum lagenarium 24 or 72 hr later, 50% fewer lesions developed on leaves of induced plants than on noninduced inoculated controls. This suggests that induced resistance to F. o. niveum is both local and systemic, as well as nonspecific.