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The photosensitization of the cereal pathogen Fusarium culmorum by phenylheptatriyne (PHT), a naturally occurring acetylene of Bidens pilosa L. (Asteraceae), was investigated. PHT strongly inhibited germination of macroconidia (mean effective concentration (EC50) = 1.7 ppm) and growth of mycelia (EC50 = 5.6 ppm) of F. culmorum in the presence of near-ultraviolet radiation. Photosensitization of macroconidia was found to be fungicidal and was not reversed after repeated washings in PHT-free media. Photosensitization was less apparent in germ tube elongation in liquid media. However, nonphotosensitizing growth inhibition was observed in these trials. Endogenous levels of PHT in Bidens pilosa were highest (up to 450 μg/g fresh weight) in all plant parts during the seedling stage and declined during vegetative growth. The concept that PHT acts as a preinfectional inhibitor to F. culmorum is discussed.

Cell‐free 30‐day‐old culture filtrates of 24 isolates of Fusarium oxysporum f.sp. radicis‐lycopersici (FORL) differed considerably in their capacity to induce wilting in 28‐day‐old tomato seedings and to inhibiting the germination of tomato seeds. The wilt effects ranged from mild on leaves and lateral stems, to total collapse of the seedlings in 24 h. Wilt, leaf curl and leaf chlorosis, appearing in this sequence, were the three symptoms clicited by the culture filtrates. Boiled and non‐boiled filtrates elicited similar sympotoms. The high wilt capacity filtrates were pH 7.2; the others were generally below pH 6. The high wilt capacity filtrates showed polyphenoloxidase activity but the overall pattern of this activity did not correlate consistently with wilt capacity. The majority of the lower wilt capacity filtrates showed a net inhibition of dihydroxy‐phenylalanine (DOPA) oxidation. The study suggests that the symptoms in the tomato seedlings were elicited by toxins in the culture filtrates. Further, it appears that the differences obtained in the wilt capacity of filtrates from the isolates were due, at least in part, to inherent differences in the concentration of the toxic factors. The rapidity of the onset of wilt, the total, collaps of filtrate‐treated seedlings and the absence of fungi in wilted seedlings suggest further that the operative mechanisms are physiological and biochemical and not impairment of the seedlings’ translocation system by physical blockage with mycelia.