6.4
IDENTIFICATION AND EVALUATION OF PREDISPOSING FACTORS AFFECTING THE OCCURRENCE AND DEVELOPMENT OF TRICHODERMA GREEN MOULD OF AGARICUS BISPORUS

D BEYER, MG ANDERSON, NG CATLIN, J KRESMER and PJ WUEST

Pennsylvania State University, University Park, PA, USA

Background and objectives
Since 1993, Trichoderma green mould has reached epidemic proportions in the mushroom industry in North America. Trichoderma green mould colonizes mushroom compost, competes with Agaricus bisporus mycelium for space and nutrients, and results in large areas of the growing beds that do not produce mushroom fruiting bodies. The epidemiology of this fungal disease is poorly understood and controversial. At first, the impression was that the occurrence of this disease appeared randomly. Earlier research has reported that factors in compost preparation were related to the growth and development of Trichoderma harzianum [1]. Initial observations made at farms in Pennsylvania also suggested that composting (phase I and/or phase II) procedures were often associated with disease development [2]. This research project seeks to evaluate and quantify some predisposing cultural factors for disease occurrence and to learn why severity of disease varies among farms. A goal of this research is to establish a wider base of information regarding compost nutrition, crop management and sanitation correlated with the growth and development of Trichoderma green mould. Preliminary tests were conducted on some predisposing factors identified by the survey.

Materials and methods
Trained technical service personnel experienced in mushroom cultivation were used to develop and conduct a diagnostic survey at mushroom farms. Specific cultural factors in a crop were monitored at individual commercial farms in Pennsylvania as crops progressed from phase I through to harvesting. Cultural factors were quantified and subjectively rated to describe growing characteristics and procedures at critical times in the process. Severity was based on the percentage of growing bed affected, percentage of the room infested and disease onset in relation to the stage of the crop. Standard Mushroom Research Center (MRC) substrate preparation, spawning, casing and mushroom-growing procedures were followed. After spawning the compost was inoculated with 2.0 ml of a spore suspension containing 1x107 spores per ml.

Results and conclusions
Preliminary results of the diagnostic survey indicate that dense, wet, overly mature phase I substrate, problem phase II, spawn infestation and increased fly infestations are the most common contributing factors related to increased incidence of Trichoderma green mould disease. One possible common thread between the compost-related problems is a lack of oxygen during phase I or phase II composting. Experiments have been tested for monitoring oxygen during phases I and II, to investigate the effect of low oxygen on disease development. Tests were conducted to determine the effect of different spawn and spawn-carrying materials on the growth and incidence of Trichoderma green mould. Initial results suggested that fully colonized spawn run compost, used as the source of spawn, and non-grain spawn, resulted in less incidence and severity of Trichoderma growth when compared to grain spawn. Excess compost moisture was found to be a predisposing factor to an increase in severity and incidence of Trichoderma green mould. In other experiments it was determined that Trichoderma spores did not germinate at spore concentrations less than 1x102. This result is of importance to mushroom growers and researchers, in that it suggests that disease development may not occur or may be slower to develop on farms with lower spore loads. It was also shown that spores did not germinate on the surface of the casing using spore inoculum carried in a liquid, either water or nutrient broth. This lack of germination, even with high spore concentrations, is not understood and merits further investigation. This result may suggest that secondary infection after casing is not part of the disease cycle and that fungicides applied after casing have minimal influence in preventing the spread of Trichoderma green mould within a crop.

References
1. Grogan HM, Gaze RH, 1995. Mushroom Science XIV, 653-660.
2. Romaine CPD, Royse J, Wuest PJ, Beyer DM, 1997. Mushroom News 45, 20-28.