Research progress of pear black spot and breeding for disease resistance

Abstract: Pear is one of the most important fruit crops in china, pear plantation areas were 1 124 040 m², and production was 18 698 560 t in 2015 according to Ministry of Agriculture statistics. Areas andproduction of the pear were the first in the world. However, pear diseases have become to a major constraint to the pear industry worldwide. Pear black spot disease, caused by Alternaria sp. is one of the mostserious diseases in pears. Pear black spot is a widespread disease in world, which was serious occurrencein Japan, Korea and South Chinese pear producing areas. Pear black spot was first reported in Japan in1933, and spread to the rest of the world. Pear black spot disease could cause necrosis on pear leaves,twigs, fruits and early falling leaves, reduce the productivity and quality of fruit. In high temperatures andhigh humidity areas, pear black spot disease could cause pear yield loss up. This disease is primarily controlled by application of synthetic fungicides. However, large-scale use of fungicides has potential toxic effects on humans and wildlife, and lead to environment pollution. Moreover, it is becoming increasingly dif⁃ficult and costly due to heavy use of fungicides to ensure good quality fruit. Pear black spot pathogen wasreported by many research scholars. A. gaisen Nagano was considered to be the pathogen of pear blackspot in 1920 by Nagano. However, A. kikuchiana Tanaka was considered to be the pathogen of pear blackspot in 1933, and many scholars used A. kikuchiana as pear black spot pathogen in a long time. Simmonused A. gaisen Nagano as pear black spot pathogen according to the nomenclature, and the A. kikuchianaas a junior synonym. A. alternata (Fr.) keissl could also cause pear black spot was reported in1937, Nishimura suggested that A. kikuchiana might be pathotype of A. alternata according to its morpho⁃logical studies. In 1993, Simmons thought that A. kikuchiana and A. alternata were two distinct species according to the morphological and toxin evidence of hundreds of strains of Japanese pear. At present, thereare 9 species of Alternaria isolated from pear fruit, and 6 species were reported in China, they are A. alternata (Fr.) Keissler, A. gaisen K. Nagan, A. tenuissima, A. yaliinficiens R. G. Roberts, A. infectoria and A.ventricosa R. G. Roberts. Spores and mycelium of disease shoots, leaves and fruit were used by pear blackspot pathogen to live through the winter. Overwintering spores were spread by rain in the spring of secondyear, and fall into the leaves of pear, germinated germ tube, grown along the pear leaf epidermis with suitable temperature and humidity conditions. The top of the germ tube expands to form an attachment to invade the pear leaf by stoma or wound. Under the stoma, the infected hypha was released into the nearbycells to absorb nutrients and water from the leaves of pear, and completed the primary infection. Host specific toxin AK-toxin was isolated from A. kikuchiana by Nakashima in 1982, and analyzed the composition and structure of AK toxin. AK toxin can cause physiological damage to the plasma membrane of hostcells, and AK toxin is one of the main reasons pear black spot to infect the pear. In recent years, the pearblack spot disease resistance of pear cultivars has been evaluated by the many domestic scholars. Different pear cultivars resistance to pear black spot disease were significantly different, however, the resistanceto black spot disease of sand pear is relatively stable by different investigation methods. So we could findthe resistance gene from sand pear cultivar. Plants often were invaded by various kinds of pathogens ingrowth and development, plants could recognize pathogen, and release substances, established defensesystem against external disease, gene encoding plant disease resistance protein is called resistance genes.Because of the long period of the pear tree, the genetic background is complex, and most of the varietiesare self incompatible, the traditional breeding for disease resistance is of high cost and low efficiency, andthe use of molecular breeding can shorten the breeding period and improve the breeding efficiency. In recent years, the research on the identification of disease resistance genes in pear has been carried out byusing map based cloning, homologous cloning, and transcriptome sequencing. 5 213 differently expressedgenes related to pear black spot resistance were obtained, 34 microsatellites were detected in these genes,28 genes were found to be closely related to pear black spot resistance by transcriptome sequencing,which were reported in 2015. Pear black spot is one of the most important diseases of pear, many researches on pathogen and pear resistance to pear black spot disease were reported, but the overall research related to pear black spot was still lagging behind. The researches of pear resistant mechanism to the blackspot disease, drug resistance mechanism of pathogen and pear black spot disease mechanism were still notthorough. Practical molecular marker assisted breeding of pear disease resistant and comprehensive prevention and control measures to pear black spot were still relatively few studied. This paper expoundspear black spot disease symptoms, pathogen, identification and evaluation of resistance to black spot ofpear germplasm resources, molecular biology of pear resistance and others. This review provides a refer⁃ence for the further research and integrated control of pear black spot.