VIEW ARTICLE
Research Induced Resistance to Fusarium Wilt of Watermelon Under Simulated Field
Conditions. R. D. Martyn, Associate Professor, C. L. Biles, Graduate
Research Assistant, and E. A. Dillard III, Technician II, Department of Plant Pathology
and Microbiology, Texas A&M University, College Station 77843. Plant Dis. 75:874-877. Accepted for publication 20 March 1991. Copyright 1991
The American Phytopathological Society. DOI: 10.1094/PD-75-0874. 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. Additional keywords:
Citrullus lanatus, cross-protection |