INDUCED RESISTANCE TO FUSARIUM-WILT OF WATERMELON UNDER SIMULATED FIELD CONDITIONS

Race 2 of Fusarium oxysporum f. sp. niveum overcomes all current wilt-resistant watermelon cultivars. In previous studies, resistance to race 2 was induced in the greenhouse by prior inoculation with avirulent races (races 0 or 1) or with the cucumber wilt pathogen, F. o. f. sp. cucumerinum. In the present study, microplots were used to evaluate induced resistance under simulated field conditions. When F. o. niveum race 2 was at or near normal field levels (750 cfu/g of soil), F. o. niveum race 1 provided good protection of Calhoun Gray watermelons throughout the season. On plants induced by race 1, wilt symptoms were delayed by as much as 2 wk, 35% fewer plants died, and induced plants were healthier overall. When F. o. cucumerinum was used as an inducing agent, wilt symptoms also were delayed 2 wk, but, by the end of the season, disease severity was similar to the noninduced plants. When field populations of F. o. niveum race 2 were increased fivefold to 4,000 cfu/g of soil, F. o. niveum race 1 caused a delay in symptom expression; however, by the end of the season, there was no difference in the number of dead plants or disease severity between the induced and noninduced plants. Thus, it appears induced resistance can be overcome by a high level of challenge inoculum. Race 2 was recovered from the roots and lower stem of plants induced by race 1; therefore, induced resistance did not affect initial penetration of race 2 but did reduce the level of colonization and spread inside the vascular system.