Relationship ofVerticillium dahliaeandErwinia carotovorapv.carotovorain the Early Dying Disease of Potato

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Abstract
The effect of simultaneous and sequential inoculations of potato plants with the wilt-causing fungus, V. dahliae (Vd), and the soft-rotting bacterium, E. carotovora pv. carotovora (Ecc), was studied under controlled environmental conditions. All plants were inoculated with Vd by the root dip method. Plants were inoculated with Ecc either through wounded roots, through the cut stem base, or by injection with a micropipette at a leaf axil. Disease severity was assessed by measuring inhibition of stem growth, extent of leaf chlorosis and wilting (disease index), green leaf area remaining at the end of the experiment, and the number of stems in which soft rot developed. Ecc successfully colonized stems inoculated either by the cut stem base or the leaf axil injection method, but not when inoculum was introduced through wounded roots. With the cut stem base inoculation, plants of potato cultivars Norgold Russet and Russet Burbank developed more severe symptoms of early dying in the presence of both pathogens than with either pathogen alone. The effect of concurrent infection with Vd and Ecc on both ''Norgold Russet'' and ''Russet Burbank'' was synergistic; plant growth was reduced, chlorosis and wilting were increased, and development of stem soft rot was enhanced. Ecc alone did not cause stem soft rot following inoculation by this method, but it resulted in reduced green leaf area in ''Norgold Russet''. When the Vd-infected plants were injected with Ecc at a leaf axil, only ''Norgold Russet'' developed symptoms that were more severe than with either pathogen alone. Furthermore, soft rot developed in stems of more plants inoculated with both pathogens than when Ecc was present alone.