Button mushrooms of Agaricus bisporus (Lange) Imbach are commercially cultivated under environmentally controlled conditions. In California they are the most economically important agricultural crop in Santa Clara and San Mateo counties, and also an important crop in 10 other counties. Trichoderma harzianum Rifai, biotype Th4, can reduce production by preventing formation of fruiting bodies. Biotype Th4 was previously detected and described in Canada (2), Pennsylvania, and Delaware. Unofficial reports suggest its presence in San Mateo County since 1995. Disease incidence and severity on the mushroom farms varied; some mushrooms became severely infected. Green epigeous mycelia and conidia were present on the casing layer resulting in empty patches. On some farms 30% of the production surface was infected during the peak of the epidemic. Initial identification of the species was made by isolating the fungus from the substrate and casing layer. Potato dextrose agar (PDA) cultures coincided with the cultural description of T. harzianum (1,3). Biotype assessments with standard procedures were conducted at Penn State, with polymerase chain reaction (PCR) amplification of total genomic DNA to screen the California isolates of T. harzianum. Random amplified polymorphic DNA (RAPD)-PCR analysis with 14 different primers indicated that they were the same RAPD haplotype as biotype Th4. The Horticultural Research Institute of Ontario relies on morphological observations from cultures grown on 2% MEA (malt extra agar) at 24°C under diffuse daylight to identify biotypes of T. harzianum (2), and microscopic characters of biotype Th4 were also positively confirmed on the California isolates. More than a parasite or pathogen, T. harzianum biotype Th4 is considered a weed mold of mushroom cultivation. The precise interaction is yet unknown. Modified Koch's postulates were confirmed with bags of commercial mushroom substrate (45 kg) inoculated by spraying 100 ml of a spore suspension (3.0 × 106 spores per ml) at filling, to give final concentrations of 103 to 108 spores per kg of compost. Treatments were T. harzianum biotype Th4, strain Th1, an unidentified isolate, control (distilled water only), and noninoculated. Eight replications per treatment were laid out in a randomized block design. Bags were subjected to standard mushroom cultivation practices. Biotype Th4 was reisolated from empty patches on the casing of all Th4 repetitions. Mean percent cover of the mold (therefore mushroom empty patches) was 30% for crops (flushes) 1 and 2, but individual bags varied from 15 to 90%. The mean percent cover in the other two treatments and in the controls was 0% for crops 1 to 4, therefore significantly different. Green mold was covering the total surface on all Th4 repetitions at third crop. No yields were recorded, but serious losses were obvious for the Th4 treatments. Green mold was not observed in the controls.
References: (1) H. M. Grogan et al. Mushroom News 45:29, 1997. (2) D. L. Rinker et al. Mushroom World 8:71, 1997. (3) D. A. Seaby. Plant Pathol. 45:905, 1996.