The fate of carboxin, benomyl and thiabendazole in seed- and soil-treated plants, as shown by in vitro and in vivo bioassays on some epiphytic yeasts

The yeast, Sporobolomyces roseus, when added to leaves of barley and pea seedlings seed- and soil-treated with carboxin, benomyl or thiabendazole (TBZ), sporulated little or not at all from both sides of the first leaves. Second leaves of barley and third of pea gave similar results. Tests in vivo showed that all three chemicals reduced sporulation of S. roseus at lower concentrations than they did growth, and also delayed sporulation. Leaves from benomyl- or TBZ-treated plants inhibited the growth of sensitive test organisms (S. roseus, Leucosporidium scottii and Aureobasidium pullulans), on agar: it is considered that fungitoxic substances are excreted from both faces of leaves of barley and pea treated with these chemicals. Leaves from carboxin-treated plants, however, did not inhibit sensitive test organisms (S. roseus, L. scottii and Ustilago hordei), so their inhibitory action in vivo is deemed to result from hormonal imbalance in treated plants: carboxin-treated barley plants were greener than control plants, no guttation occurred on leaves, and stomata tended to remain open; treated barley and pea were more sensitive to drought than were controls. Carboxin-treated barley and pea plants sprayed with the cytokinin antagonist, abscisic acid, behaved as did plants without carboxin. It is considered likely that carboxin affects the epiphytes by creating a leaf surface incapable of sustaining them, possibly through dryness.