Glucosinolates contained in members of the Brassicaceae release isothiocyanates potentially useful in controlling Fusarium oxysporum pathogens in conifer seedling nursery soils. Our objective was to determine the toxicity of individual isothiocyanates to different growth stages of the fungus. Bioassays with four F. oxysporum isolates were conducted using sealed containers in which 0.3 μl of 2-propenyl, ethyl, buty, phenylethyl, benzyl, or phenyl isothiocyanate was allowed to volatilize. Propenyl and ethyl isothiocyanates were the most fungistatic of those compounds tested. The same concentrations of propenyl and ethyl isothiocyanates that inhibited mycelial growth completely suppressed conidial and chlamydospore germination of all isolates. Other isothiocyanates including ethyl, benzyl, and phenethyl were also fungitoxic to F. oxysporum conidia and chlamydospores. Reduction in pathogen populations resulting from a green-manure crop are likely achievable since chlamydospores are sensitive to isothiocyanate. Pathogenic F. oxysporum isolates infesting nursery soils would likely be most suppressed by species of plants such as Brassica carinata, B. nigra, and B. juncea, which contain glucosi-nolates that release high concentrations of propenyl isothiocyanate.

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Trichoderma harzianum was cotransformed with genes encoding green fluorescent protein (GFP), ␤-glucuronidase (GUS), and hygromycin B (hygB) resistance, using polyethylene glycol-mediated transformation. One cotransformant (ThzID1-M3) was mitotically stable for 6 months despite successive subculturing without selection pressure. ThzID1-M3 morphology was similar to that of the wild type; however, the mycelial growth rate on agar was reduced. ThzID1-M3 was formed into calcium alginate pellets and placed onto buried glass slides in a nonsterile soil, and its ability to grow, sporulate, and colonize sclerotia of Sclerotinia sclerotiorum was compared with that of the wild-type strain. Wild-type and transformant strains both colonized sclerotia at levels above those of indigenous Trichoderma spp. in untreated controls. There were no significant differences in colonization levels between wild-type and cotransformant strains; however, the presence of the GFP and GUS marker genes permitted differentiation of introduced Trichoderma from indigenous strains. GFP activity was a useful tool for nondestructive monitoring of the hyphal growth of the transformant in a natural soil. The green color of cotransformant hyphae was clearly visible with a UV epifluorescence microscope, while indigenous fungi in the same samples were barely visible. Green-fluorescing conidiophores and conidia were observed within the first 3 days of incubation in soil, and this was followed by the formation of terminal and intercalary chlamydospores and subsequent disintegration of older hyphal segments. Addition of 5-bromo-4-chloro-3-indolyl-␤-D-glucuronic acid (X-Gluc) substrate to recovered glass slides confirmed the activity of GUS as well as GFP in soil. Our results suggest that cotransformation with GFP and GUS can provide a valuable tool for the detection and monitoring of specific strains of T. harzianum released into the soil.

Brassica tissues are potentially useful in the control of Aphanomyces root rot of peas (Pisum sativum), but identity of the responsible compounds and specific impacts of those compounds on the pathogen's infection potential remain uncertain. Brassica napus seed meals and water extracts from these meals were used to determine the effect of glucosinolate hydrolysis products on Aphanomyces euteiches f. sp. pisi. B. napus meal ('Dwarf Essex') containing glucosinolates and intact myrosinase, the enzyme responsible for glucosinolate hydrolysis, completely inhibited infection by A. euteiches f. sp. pisi oospores. Water extracts from this meal, likewise, severely inhibited infection by oospores, as well as mycelial growth. Extracts from autoclaved 'Dwarf Essex' meal, in which myrosinase was denatured, and a low glucosinolate B. napus variety ('Stonewall') produced little disease reduction and had less impact on mycelial growth. Gas chromatographic analysis of Brassica tissues and water extracts confirmed that glucosinolates remained in autoclaved 'Dwarf Essex' meal and that 'Stonewall' meal contained low glucosinolate concentrations. 5-Vinyloxazolidine-2-thione was identified by mass spectrometry as a dominant glucosinolate hydrolysis product in aqueous extracts of the inhibitory meal. Bioassays conducted with aqueous solutions of this compound reduced mycelial growth, but not to the extent of those from intact 'Dwarf Essex' meal. Water-soluble compounds produced from the hydrolysis of glucosinolates in B. napus tissues reduced A. euteiches oospore infection and inhibited mycelial growth, thus, demonstrating potential utility of Brassica species in the control of A. euteiches.

Knudsen, G. R., and Bin, Li. 1990. Effects of temperature, soil moisture, and wheat bran on growth of Trichoderma harzianum from alginate pellets. Phytopathology 80:724-727.