A simple model system was constructed to evaluate the microbistatic and microbicidal properties of gaseous allyl isothiocyanate (AIT) against bacterial cells and fungal conidia deposited on agar surfaces. Salmonella typhimurium, Listeria monocytogenes Scott A, and Escherichia coli O157:H7 were inhibited when exposed to 1,000 μg AIT per liter. Pseudomonas corrugata, a Cytophaga species, and a fluorescent pseudomonad failed to grow in the presence of 500 μg AIT per liter. Germination and growth of Penicillium expansum, Aspergillus flavus, and Botrytis cinerea conidia was inhibited in the presence of 100 μg AIT per liter. Bactericidal and sporicidal activities varied with strain and increased with time of exposure, AIT concentration, and temperature. E. coli O157:H7 was the most resistant bacterial species tested.

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Pseudomonas fluorescens isolate 1100-6 was evaluated as a potential biological control agent for apple blue mold caused by Penicillium expansum or Penicillium solitum. Both the wild-type isolate 1100-6 and a genetically modified derivative labeled with the gene encoding the green fluorescent protein (GFP) were compared. The P. fluorescens isolates with or without GFP equally reduced the growth of Penicillium spp. and produced large zones of inhibition in dual culture plate assays. Cell-free metabolites produced by the bacterial antagonists reduced the colony area of Penicillium isolates by 17.3% to 78.5%. The effect of iron chelate on the antagonistic potential of P. fluorescens was also studied. The use of iron chelate did not have a major effect on the antagonistic activity of P. fluorescens. With or without GFP, P. fluorescens significantly reduced the severity and incidence of apple decay by 2 P. expansum isolates after 11 d at 20 degrees C and by P. expansum and P. solitum after 25 d at 5 degrees C when the biocontrol agents were applied in wounds 24 or 48 h before challenging with Penicillium spp. Populations of P. fluorescens labeled with the GFP were determined 1, 9, 14, and 20 d after inoculation at 5 degrees C. The log CFU/mL per wound increased from 6.95 at the time of inoculation to 9.12 CFU/mL (P < 0.05) 25 d after inoculation at 5 degrees C. The GFP strain did not appear to penetrate deeply into wounds based on digital photographs taken with an inverted fluorescence microscope. These results indicate that P. fluorescens isolate 1100-6 could be an important new biological control for apple blue mold.

Twenty-one isolates of the bacterium Bacillus subtilis and one of Enterobacter aerogenes were tested on agar for antagonism to Alternaria alternata, Armillariella mellea, Botrytis allii, Botrytis cinerea, Colletotrichum lindemuthianum, Monilinia fructicola, Penicillium expansum, Phytophthora cactorum, Pythium ultimum, Rhizoctonia solani, Sclerotinia sclerotiorum, Sclerotium cepivorum, Verticillium dahliae, and Venturia inequalis, causal organisms of many plant diseases. Enterobacter aerogenes was antagonisic to all of the pathogenic fungi tested except Verticillium dahliae and Armillariella mellea. Similarly, Bacillus subtilis was antagonistic to all of the pathogenic fungi tested except Pythium ultimum. When Enterobacter aerogenes and Bacillus subtilis were tested in vivo on cherry fruit for control of postharvest brown rot and alternaria rot, Enterobacter aerogenes was ineffective. Eleven isolates of Bacillus subtilis provided effective alternaria rot control and 15 isolates provided brown rot control which ranked with the best fungicide control.

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