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Chitosan is a linear heteropolymer consisting of ß 1,4-linked N-acetyl-D-glucosamine (GlcNAc) and D-glucosamine (GlcN). We have compared the antifungal activity of chitosan with DPn (average degree of polymerization) 206 and FA (fraction of acetylation) 0.15 and of enzymatically produced chito-oligosaccharides (CHOS) of different DPn alone and in combination with commercially available synthetic fungicides, against Botrytis cinerea, the causative agent of gray mold in numerous fruit and vegetable crops. CHOS with DPn in the range of 15-40 had the greatest anti-fungal activity. The combination of CHOS and low dosages of synthetic fungicides showed synergistic effects on antifungal activity in both in vitro and in vivo assays. Our study shows that CHOS enhance the activity of commercially available fungicides. Thus, addition of CHOS, available as a nontoxic byproduct of the shellfish industry, may reduce the amounts of fungicides that are needed to control plant diseases.

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The aim of the present study was to characterize sixteen isolates of Trichoderma originating from a field of sugar beet where disease patches caused by Rhizoctonia solani were observed. Use of both molecular and morphological characteristics gave consistent identification of the isolates. Production of water-soluble and volatile inhibitors, mycoparasitism and induced systemic resistance in plant host were investigated using in vitro and in vivo tests in both sterilized and natural soils. This functional approach revealed the intra-specific diversity as well as biocontrol potential of the different isolates. Different antagonistic mechanisms were evident for different strains. The most antagonistic strain, T30 was identified as Trichoderma gamsii. This is the first report of an efficient antagonistic strain of T. gamsii being able to reduce the disease in different conditions. The ability to produce water-soluble inhibitors or coil around the hyphae of the pathogen in vitro was not related to the disease reduction in vivo. Additionally, the strains collected from the high disease areas in the field were better antagonists. The antagonistic activity was not characteristic of a species but that of a population.

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To identify the most important sources of inoculum of Botrytis cinerea (causal agent of gray mold) in commercial strawberry (Fragaria × ananassa) fields in Norway, soil and overwintered plant material were collected from planting beds and alleys at five locations in 2000 to 2002 (13 samples altogether). Plant material was sorted by category (e.g., leaves, stems, mulch, and weeds). After subsamples of each material were incubated for 5 days at 20°C at high humidity, conidiophores of B. cinerea growing from mycelia and sclerotia were counted. Overwintered plant debris within planting beds yielded more than 96% of total conidiophores counted, the remainder originating from plant debris collected from alleys or soil. Overwintered strawberry plant debris produced 98% of the conidiophores within planting beds and 80% of the conidiophores in the alleys, while the remaining was produced by weeds. Senescing and dead leaf laminae produced 45% of the conidiophores while stem residues (i.e., petioles, stolons, inflorescences, and unidentifiable stem parts) produced 50% and mummified fruit produced 5% within planting beds. The contribution of sclerotia, compared with mycelia, to conidiophore production varied greatly between fields and years. Overall, 47% of the total number of conidiophores produced in plant material within planting beds originated from sclerotia. More than 90% of the conidiophores from sclerotia were found in dead stem residues.

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ABSTRACT Trichoderma biocontrol isolates are most effective as highly concentrated inocula. Their antagonism to other fungi may be a result of pregermination respiration. In a nutrient-rich medium, almost all Trichoderma atroviride P1 (P1) conidia initiated germination processes and increased respiration, even in dense suspensions. When 1 x 10(7) P1 conidia/ml were coinoculated with 1 x 10(5) Botrytis cinerea conidia/ml, dissolved oxygen fell to <1% within 2 h and the pathogen failed to germinate. More dilute P1 suspensions consumed oxygen slowly enough to allow coinoculated B. cinerea to germinate. On nutrient-poor media, fewer P1 conidia initiated germination. Oxygen consumption by the inoculum and inhibition of B. cinerea were enhanced when P1 conidia were nutrient activated before inoculation. Pregermination respiration also affected competitive capacity of the antagonist on solid substrates, where respiratory CO(2) stimulated germination rate and initial colony growth. These parameters were directly correlated with inoculum concentration (R(2) >/= 0.97, P < 0.01). After initiating germination, Trichoderma conidia became more sensitive to desiccation and were killed by drying after only 2 h of incubation on a nutrient-rich substrate at 23 degrees C. These results indicate that nutrient-induced changes preceding germination in Trichoderma conidia can either enhance or decrease their biological control potential, depending on environmental conditions in the microhabitat.