Resistance induced in herbaceous and hardwood plant species by vascular wilt fungi
Abstract
The objectives of this study were to test for the occurrence of induced resistance in interactions between unrelated vascular wilt fungal pathogens and herbaceous and hardwood plant species. Soil temperature, inoculum densities and incubation time were controlled. Applicability of induced resistance as a tool for the biological control of vascular wilt diseases was considered. Simultaneous root dip inoculation with conidial suspensions of Fusarium oxysporum f. sp. lycopersici (Sacc.) Sn. et Hans. (inducer) and Verticillium dahliae Kleb. (challenger) resulted in the expression of cross protection in Fusarium resistant tomato (Lycopersicon esculentum Mill.) cultivars. The cross protection was gradually lost as the separation in time between the inducer and challenger inoculations was increased, and also as the concentration of the inducer inoculum in relation to the challenger inoculum concentration was decreased. These results suggest a local type of cross protection. A high level of cross protection induced by V. dahliae against F. oxysporum f. sp. lycopersici in Verticillium resistant tomato cultivar could not be demonstrated. Susceptible tomato cultivars inoculated with increasing inoculum concentrations of F. oxysporum f. sp. lycopersici and/or V. dahliae showed higher disease severity ratings as the concentration of the inoculum was increased. An additive effect was observed in mix inoculations. Seedlings of American elm (Ulmus americana L.), green ash (Fraxinus pennsylvanica Marsh.) and red maple (Acer rubrum L.) were tested for induced resistance top V. dahliae under controlled conditions and in the field. Test plants were simultaneously inoculated with the non-pathogen F. oxysporum f. sp. lycopersici (inducer) and pathogenic isolates of V. dahliae (challenger). In the field, seedlings of red maple and green ash exhibited induced resistance to V. dahliae, as measured by increase in fresh weight, and by reductions in symptoms severity and vascular discoloration. However, induced resistance was incomplete in these hardwood species. Similar results in tests with cultivars of tomato suggest that this phenomenon is not effective as a biological control of the vascular wilt pathogen V. dahliae. A semi-selective medium, to which the fungicides pentachloronitrobenzene and iprodione were added, improved the isolation from stems and recognition of colonies of V. dahliae.
Degree
Ph.D.
Advisors
Green, Purdue University.
Subject Area
Plant pathology
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