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Pseudomonas syringae
(saprophytic strain)
and "Fruit Yeasts"

by Wojciech Janisiewicz, USDA, Appalachian Fruit Research Station, Kearneysville, WV

The antagonist bacterium, Pseudomonas syringae, is a saprophytic strain which was originally isolated from an apple leaf and can be frequently isolated from apple fruit. "Fruit yeasts" consist of many pink and white yeasts belonging to Deuteromycota, Basidiomycota and Ascomycota, isolated from a variety of fruits including apple, pear, and citrus. Both P. syringae and fruit yeasts grow exponentially on wounded apple and pear tissue and prevent necrotrophic pathogens such as Penicillium expansum or Botrytis cinerea from entering fruit through wounds and initiating infection and subsequent decay.

Habitat

P. syringae is a nutritionally versatile organism. There are 41 pathovars of P. syringae which are able to cause diseases on various plants. The antagonistic strain is their non-pathogenic counterpart and is antagonistic to pathogens on many plants. It can grow well on wounded plant tissue and can control a variety of diseases on different fruits, including pome fruits, banana, and citrus fruits as well as vegetables. The fruit’s surface is a natural habitat for many yeasts which dominate fruit microflora, especially close to harvest. The yeasts grow rapidly on this substrate, which has a high concentration of readily available carbon sources, such as those contained in juices leaking from wounded fruits. Mummified fruit and soil are important over-wintering sites for yeasts and serve as an inoculum source for developing fruit. The composition of the yeast microflora on the fruit changes as the fruit matures. Humans have been taking advantage of yeasts naturally occurring on fruit from ancient times by allowing them to ferment fruit to make wine or cider. Although yeasts are common inhabitants on many fruits, most of our knowledge concerns yeasts on grape and apple because interest in vinification and cider making has stimulated research in these areas.

Diseases controlled

The discovery of the potential of P. syringae and yeasts that naturally occur on apple and pear as biocontrol agents against fungi that cause postharvest decay of the fruits (2) was followed by the isolation of microorganisms from various fruits and the testing of their antagonistic potential in many systems in laboratories world wide. This resulted in the discovery of many bacterial and yeast antagonists which are effective against postharvest diseases on a variety of fruits. Some of these appear to have commercial potential. In developing this page, we are focusing initially on antagonists developed in our program. These include P. syringae, which has been commercialized, and "fruit yeasts" in general, because many of them are effective in controlling postharvest decays of apple and pear.

P. syringae (strain L-59-66 renamed as strain ESC- 11) can control blue mold caused by P. expansum, gray mold caused by B. cinerea, and Mucor rot caused by Mucor spp. on apple and pear (4,5). It can also control blue mold caused by Penicillium italicum, and green mold caused by P. digitatum on citrus fruit. Another strain (ESC-10) of this bacterium, superior in controlling decays of citrus fruit, has been isolated by EcoScience, Corp. Strain ESC-11 has been shown to reduce crown rot of banana, which is caused by a complex of fungi, including Fusarium semitectum and F. moniliforme (10), and it also reduced Fusarium dry rot on potato caused by F. sambucinum (9). This strain also prevented growth of the foodborne pathogen, Escherichia coli O157:H7, in apple wounds (3). This pathogen can grow quickly on damaged apple tissue and consumption of unpasteurized apple cider contaminated with this bacterium has caused outbreaks of illnesses in recent years.

"Fruit yeasts" can control postharvest decays on many different fruits. We have isolated a yeast, Sporobolomyces roseus, and many other pink and white yeasts (most of them identified by isolate numbers) that can control blue mold and gray mold of pome fruits (2,6,7). In addition, other investigators have reported a number of "fruit yeasts" that were effective antagonists against postharvest fruit decay. A white yeast, Candida oleophila, has been commercialized for the control of blue mold and green mold on citrus fruit and blue mold and gray mold on apple (1). In general, many "fruit yeasts" can reduce fruit decays to varying degrees but only a few can reduce decay below the threshold acceptable to the fruit industry and at concentrations realistic for commercial development.

Relative effectiveness

Postharvest biological control of blue mold and gray mold on Golden Delicious apples.
Left: wounded apples were sprayed with conidia of P. expansum and B. cinerea only (control);
Right: wounded apples were sprayed with the conidia of the pathogens and BioSaveTM 110. The fruit was stored for 3 months at 1°C before evaluation.

Biological control in the postharvest environment has significant advantages over that under field conditions because the two most important factors effecting biocontrol, temperature and relative humidity are constant and under strict control. In addition, the targeted area (fruit) is easily accessible. These factors greatly reduce the variability of biocontrol and also make this system more amenable to manipulation. The effectiveness of biocontrol may be influenced by postharvest treatments, such as diphenylamine and ethoxyquin, which are used on pome fruits against superficial scald, a physiological disorder, and additives, such as flotation salts and waxes. In addition, fruit maturity and cultivar may effect biocontrol. However, if these factors are considered in developing a biocontrol strategy, highly consistent and commercially acceptable control of fruit decay developing from infections at wound sites can be achieved. Postharvest biocontrol can also be integrated with other non-fungicidal methods such as calcium infiltration or heat treatment, which by themselves cannot provide adequate control but in combination with biocontrol may increase the performance margin of the latter (8).

Commercial availability

P. syringae ESC-11 is sold under the name BioSaveTM 110 and is recommended for the control of postharvest decays of pear and apple (4). P. syringae ESC-10 is commercially available under the name BioSaveTM 100 and is recommended for the control of postharvest decays of citrus fruits.

The white yeast, Candida oleophila, has been commercialized for the control of blue mold and green mold on citrus fruit and blue mold and gray mold on apple and is sold by Ecogen, Inc. under the name Aspire (1).

References

1. Droby, S., Cohen, L. Daus, A., Weiss, B., Horev, B., Chalutz, E., Katz, H., Keren-Tzun, M., and Shachnai, A. 1998.Commercial testing of Aspire: a yeast preparation for the biological control of postharvest decays of citrus. Biological Control, 12: 97-101.

2. Janisiewicz, W. J. 1987. Postharvest biological control of blue-mold on apples. Phytopathology, 77: 481-485.

3. Janisiewicz, W. J., Conway , W. S., and Leverentz, B. 1999. Biological control of postharvest diseases of apple can prevent growth of Escherichia coli O157:H7 in apple wounds. J. Food Protection, 62: 1372-1375.

4. Janisiewicz, W. J., Jeffers, S. N. Efficacy of commercial formulation of two biofungicides for control of blue mold and gray mold of apples in cold storage. Crop Protection,16: 629-633. 1997.

5. Janisiewicz, W. J. and Marchi, A. 1992. Control of storage rots on various pear cultivars with saprophytic strain of Pseudomonas syringae. Plant Disease, 76: 555-560.

6. Janisiewicz, W. J., Peterson, D. L. and Bors, R. H. 1994. Control of storage decay of apples with Sporobolomyces roseus. Plant Disease, 78: 466-470.

7. Janisiewicz, W. J. 1996. Ecological diversity, niche overlap and coexistence of antagonists used in developing mixtures for biocontrol of postharvest diseases of apples. Phytopatholog,y 86: 473-479.

8. Janisiewicz, W. J. 1998. Biological control of postharvest diseases of temperate fruits: challenges and opportunities. In: Plant-Microbe Interaction and Biological Control, G. J. Boland and L. D. Kuykendall eds., Marcel Dekker, Inc., New York, pp. 171-198.

9. Kenwick, S., and Jacobsen, B. J. 1998. Biological control of Fusarium dry rot on potato with antagonistic bacteria in commercial formulation. Phytopathology, 88: S47.

10. Williamson, S. M., Guzman, M., Anas, O., Marin, D. H., Jin, X., and Sutton, T. B. 1999. Evaluation of potential biocontrol agents for crown rot of banana. Phytopathology, 89: S85.

No endorsement of named or illustrated products is intended, nor is criticism implied of similar products that are not mentioned or illustrated.

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