Pears are the third most important temperate fruit species after grapes and apples (Itai, 2007). There are three major species of pear, Pyrus communis (European pear), P. bretschneideri Rehd. or P. ussuriensis Maxim. (Chinese pear), and P. pyrifolia Nakai (Japanese pear: Nashi), which are commercially cultivated in temperate zones (Bell, 1990). Pears are susceptible to a number of diseases, mostly caused by fungi. In European varieties, resistance against fungal diseases such as scab, powdery mildew, brown spot, and fire blight is an important breeding objective (Bell et al., 1996). In Asian pears on the other hand, resistance against fungal diseases such as scab, rust, and black spot is receiving attention with resistance to black spot disease being a major objective in Japan (Itai, 2007; Kajiura, 1994).
Black spot disease, which is caused by a Japanese pear pathotype of Alternaria alternata (Fr.) Keissler, is one of the most serious diseases in Japanese pear cultivation in Japan (Sanada et al., 1988) and the commercial cultivars Nijisseiki, Shinsui, and Nansui are susceptible (Kajiura, 1994). Successful fruit production is currently maintained by repeated fungicide application and by covering the fruits with paper bags during the growing season. The causal pathogen produces a host-specific toxin named AK-toxin (Nakashima et al., 1982; Tanaka, 1933), which causes necrosis on fruit skin and leaves resulting in decreased yield. AK-toxin is toxic to susceptible cultivars only and is harmless to resistant cultivars and non-host plants. Susceptibility is controlled by a single dominant gene (A), the locus of which is heterozygous in most susceptible cultivars (Kozaki,1973; Terakami et al., 2007).
Ethylene is known to play major roles in regulating plant defense responses against various pathogens, pests, and abiotic stresses (Broekaert et al., 2006). Ethylene is known to increase either susceptibility or resistance in various plants, depending on the plant–pathogen interaction. For example, treatment of tomato with ethylene enhances resistance to the fungus Botrytis cinerea (Diaz et al., 2002), whereas it promotes susceptibility to Xanthomonas campestris pv. vesicatoria (Lund et al., 1998). A similar variety of effects has been observed in Arabidopsis, soybean, and tobacco (reviewed in Van Loon et al., 2006). The role of ethylene in plant defense responses is therefore versatile. Pear farmers have experiential knowledge that fruit infected with black spot disease result in early ripening. Fruit ripening is also related to ethylene production in Asian pears (Itai et al., 1999, 2003a). Thus, there seems to be a relationship between ethylene synthesis and black spot disease. However, despite this knowledge, no report has yet documented the correlation between ethylene production and black spot disease. We report the effects of propylene, an analog of ethylene, and 1-MCP, an inhibitor of ethylene action, on the development of black spot disease concomitant with gene expression of ethylene biosynthetic enzymes.
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