Paclobutrazol and other plant growth-retarding chemicals increase resistance of melon seedlings to fusarium wilt

Studies were made of plant growth retardation and effects on resistance of melon to fusarium wilt by soil application of paclobutrazol, other ergosterol biosynthesis inhibitors, the fungicide benomyl and the herbicide dinitramine. Paclobutrazol and ancymidol delayed the onset of wilting and were the most effective in reducing wilt incidence. A relationship between effectiveness in retarding seedling elongation and increasing resistance to fusarium wilt was observed. No chemical had a significant effect on pathogen population level in the plant, as determined by stem colonization and direct assessment tests in seedlings grown in soil treated with the chemicals. Excluding benomyl, and to a lesser extent dinitramine, leaf and stem extracts had no inhibitory effect on conidial germination. Gibberellins GA₄₊₇, when applied to seedlings inoculated after germination in paclobutrazol-treated soil, nullified growth retardation and increased disease incidence. We suggest that disease incidence reduction by paclobutrazol is due to an effect on plant metabolic processes and not to direct fungitoxicity of the compound.