Imazalil sulfate as an alternative spawn and supplement treatment for green mold disease.

Trichoderma aggressivum f. aggressivum (Ta2) causes the most devastating form of green mold disease on cultivated mushrooms (Agaricus bisporus) in North America. This disease reached epidemic proportions immediately following the introduction of the pathogen in the early 1990s, and since has become endemic throughout much of North and Central America. Green mold continues to be a source of consternation for the mushroom industry in the United States, typically existing at low levels on most farms, but with severe episodes occurring with some regularity.

Green mold is effectively managed through the use of hygienic practices aimed at excluding the pathogen from the mushroom production area. In addition, treatment of the spawn and supplement with Topsin[R] M (thiophanate methyl) has become the standard in the industry, as it safeguards against major crop loss resulting from sources of Ta2 that escape the farm hygiene program.

Previously, we showed that the Ta2 population had undergone an approximate three-fold increase in benomyl (Benlate) resistance between 1994 and 2002. The results of a 2005 survey revealed that the Ta2 population in the southeastern region of Pennsylvania had developed formidable resistance in the three intervening years to the benzimidazoles, thiophanate-methyl and benomyl. The pathogen population of today has greater than a 1,000-fold higher resistance compared to that of the 1996 population. Considering the magnitude of the resistance, the prevalence of resistance forms of the pathogen, and the findings of Topsin[R] M efficacy trials (Romaine and Royse unpublished data), it has become evident that the industry in the major mushroom growing region is fortunately now managing green mold disease largely through hygiene.

The findings of a Penn State fungicide-screening program identified imazalil sulfate as a candidate alternative for thiophanate methyl for the control of green mold. The present study was carried out with the purpose of optimizing the rate of imazalil sulphate for the treatment of spawn and supplement.

MATERIALS & METHODS

A Ta2 isolate (342) that was collected in 2005 at a Pennsylvania mushroom operation in Chester County and shown to have high resistance to thiophanate methyl was used exclusively in this study.

Parameters for composting, spawn run, synthetic CACing, case hold, and harvest followed the practices of the Penn State mushroom research facility.

Commercial spawns of a commercial hybrid off-white strain were treated with Fungaflor 75 SP (Jannsen-Cilag) at rates equivalent to 0, 0.5, 1, 2, and 4 lbs of active ingredient (a.i.) imazalil sulfate in 100 lbs of gypsum per 1,600 units of spawn. As indicated, a 150-g quantity of treated spawn was inoculated with either 0 or 2000 Ta2 spores contained in 1 ml of water with 0.1 percent Tween-20. Fifty pounds of compost containing a commercial time-release supplement (without fungicide treatment) was mixed with the spawn, and then packed into a plastic tub (2.7 [ft.sup.2] surface area).

ProMycel[R] Target[TM] was treated with rates equivalent to 0, 0.5, 1, 2, and 4 lbs of imazalil sulfate in 100 lbs of gypsum per 2000 lbs of supplement. At spawning, 204 g of supplement received either 0, 2000, or 20,000 Ta2 spores. Fifty pounds of compost was mixed with the supplement (= 3 percent rate) and 150 g of spawn (treated as specified for each experiment), and then packed into a plastic tub.

For an experiment, three or four replicate tubs arranged in a randomized complete block design within the growing room represented each treatment. The weight of mushrooms harvested from each tub was recorded daily for three breaks. Yield data were statistically analyzed using Tukey's pairwise comparison test at P>0.05.

RESULTS & DISCUSSION

Cropping Trial I

Under conditions in which unprotected spawn resulted in a 93 percent reduction in mushroom yield due to green mold disease (0.33 lbs/ft2), the yield from spawn treated with either 0.5, 1, 2, or 4 lbs imazalil sulfate was statistically equivalent to the nondisease control (4.61 lbs/ft2) (Table 1). Of the fungicide treatments, however, the 1-lb rate afforded the highest mushroom productivity (4.90 lbs/ft2).

Cropping Trial II

In order to optimize the fungicide rate for the treatment of supplement, spawn was protected with either 1 or 2 lbs imazalil sulfate, while Ta2 spores were applied to the supplement treated with either 0, 0.5, 1, 2, or 4 lbs a.i. of the fungicide. Irrespective of the fungicide rate on the spawn, complete disease control was obtained at all rates of imazalil sulfate tested on the supplement (Table 2). In contrast, unprotected supplement produced a statistically significant 75 percent crop loss (1.35 lbs/ft2). The highest overall yield (5.44 lbs/ft2) was obtained with 1 lb a.i. on each the spawn and supplement.

Cropping Trial III

This cropping trial was identical to trial II with the exception of a 10-fold higher rate of Ta2 spores being applied to the supplement. The higher rate of inoculum was reflected in a 99 percent reduction in yield (0.04 lbs/ft2) (compared to 75 percent in trial II) for the unprotected supplement treatment relative to the noninoculated control treatment (5.34 lbs/ft2) ((Table 3). As in cropping trial II, all rates of imazalil sulfate tested provided complete disease control, which was statistically significant. Once again, the highest mushroom yield (5.46 lbs/ft2) was obtained with 1 lb a.i. applied to each the spawn and supplement. Up to at least 6 lbs imazalil sulfate in the compost substrate (i.e., 2 lbs on the spawn and 4 lbs on the supplement) had no inhibitory effect on mushroom yield.

Cropping Trial IV

This cropping trial was designed to confirm the optimal rates of imazalil sulfate for the spawn and supplement for disease control, and to evaluate possible toxicity to the mushroom crop. In the absence of disease pressure, up to 2 lbs a.i. on each the spawn and supplement was not toxic to A. bisporus (Table 4).

Under green mold pressure causing a 46 percent crop loss (0.3 lbs/ft2) with unprotected spawn and supplement, a statistically significant 100 percent level of disease control was achieved with all fungicide treatments, which involved either 1 or 2 lbs imazalil sulfate applied to each the spawn and supplement. The highest yield at 6.39 lbs/ft2 was observed with the combination of 2 lbs and 1 lb a.i. applied to the spawn and supplement, respectively.

SUMMARY

An application rate of 1 lb imazalil sulfate (1.33 lb Fungaflor 75 SP) applied to both the spawn and supplement provided effective control of green mold disease caused by the highly pathogenic Ta2 genotype without a measurable toxic side effect on the mushroom crop.

Editor's Note:

Imazalil sulfate is NOT currently labeled for use on mushroom spawn or supplement or on the mushroom crop. AMI and Penn State are working with the USDA IR-4 Project to gain EPA registration for imazalil and the tests outlined in this article were conducted as part of that process. Growers should only use chemicals that are clearly labeled for use in mushroom production.

C. Peter Romaine

Daniel J. Royse

Carl Schlagnhaufer

Dept. of Plant Pathology

Penn State

University Park, PA