AN ANALYSIS OF RESPONSES OF RESISTANT AND OF SUSCEPTIBLE TOMATO PLANTS TO VERTICILLIUM INFECTION

When inoculated with V. albo-atrum through roots, there were striking differences in the responses of Gem tomato plants. In susceptible plants, the foliar symptoms and amount of mycelium in the stem increased rapidly for some time. Then the mycelium started to disappear from the stem; this was accompanied by a check to the normal progress of symptoms, and by the formation of tyloses. In resistant plants, a limited invasion of the root and lower stem was accompanied by rapid and extensive tylosis. The mycelium soon disappeared from the stem and the plant then recovered from the initially mild symptoms. There was an inverse relationship between the amount of mycelium and the extent of tylosis in infected plants. The growth of susceptible plants was markedly reduced by infection. Total leaf area was much smaller because the newly produced leaves did not expand normally. The root system in infected plants was smaller bacause there were fewer tertiary roots. In resistant plants infection stimulated growth. Tomato cuttings inoculated with conidia reacted similarly to root-in-oculated plants. Hyphae grew well in the vascular system of susceptible cuttings, while in resistant cuttings the pathogen started to grow but soon disappeared. Detached leaves of susceptible plants, inoculated through cut ends, wilted more than did leaves from resistant plants. It is suggested that resistance is mainly of the active type that develops after infection.