The effect of growth substances on Verticillium wilt of tomato plants

This paper describes the effects of a number of growth-regulating substances on the development of disease in tomato plants caused by Verticillium albo-atrum. Indole-acetic acid usually reduced disease and also reduced the number of hyphae in the stem but it increased tylosis; low concentrations slightly stimulated disease development. Some control of disease was obtained by removing apical buds, and axillary shoots as they developed. When the apical bud alone was removed, developing axillary shoots sometimes wilted; this did not occur in plants with intact buds. Gibberellic acid increased disease in susceptible plants and also induced symptoms in resistant plants. Maleic hydrazide greatly reduced growth of plants, made them more diseased and stimulated growth of the parasite in the vessels. Of other growth-regulating substances tested, 2,4,6-trichlorophenoxy-acetic acid, 2,3,5-triiodobenzoic acid and 2,4-dichlorophenoxyacetic acid increased disease at some concentrations and reduced it at others. Cycocel (2-chloroethyl) trimethyl ammonium chloride and naphthaleneacetamide, gave good control of disease over a range of concentrations when applied to the soil in which plants were growing. Treatments which reduced disease also reduced the growth of the parasite in the shoot and stimulated the formation of tyloses. Indole and 2,4-dichloroanisole had some effect on disease development but this was much less than that induced by the other substances.