Studies on improvement of resistance to loquat canker in loquat breeding

Loquat canker, caused by Pseudomonas syringae pv. eriobotryae, is a serious disease of loquat [Eriobotrya japonica (Thunb.) Lindl.] in Japan and no commercial cultivar is resistant to all groups of the disease. Therefore, it is an important breeding objective to improve canker resistance for loquat breeding. First, we examined period of evaluation after inoculation and relationship between resistance and age or fruit quality, and developed the screening method of resistant seedlings at juvenile age to incorporate the selection of canker-resistant plants into loquat breeding program. Second, to produce canker-resistant seedlings effectively, the inheritance of resistance to loquat canker Groups A and C was investigated. Furthermore, for prevention of inbreeding depression and promotion of canker-resistant breeding, new resistant resources were selected among many cultivars which released in Japan and introduced from foreign countries in recent years. 1. Efficient screening method of resistant seedlings to loquat canker at juvenile age was examined to incorporate the selection of canker-resistant plants into loquat breeding program. Resistant young seedlings to loquat canker could be selected by needle-inoculating at the midribs on the abaxial surface of 30-40% expanded leaves and evaluating 50-60 days after inoculation. Evaluations of canker resistance at two different ages (three or six years old) were consistent in 81 seedlings out of 82 seedlings, which showed canker-resistant seedlings could be selected at juvenile age. Any resistances to three groups of loquat canker seemed not to associate with fruit quality and it suggested that inheritance of these resistances was independent on that of fruit quality. Therefore, it would be possible to develop canker-resistant cultivars with high fruit quality by selection of resistant seedlings at juvenile age. 2. To produce canker-resistant seedlings effectively, the inheritance of resistance to loquat canker Group A was investigated by using needle-inoculation with the bacterial suspension at the midribs on the abaxial surface of young leaves. In a seedling population from crosses among genetic resources, crosses between resistant cultivars resulted in two types of segregation: one cross combination produced only resistant seedlings, whereas another combination yielded progenies with the segregation ratio of 3 resistant: 1 susceptible. Progenies derived from crosses between resistant and susceptible cultivars segregated into 1 resistant: 1 susceptible, except for five crosses derived from 'Champagne' or 'Fukujuin', in which all seedlings were resistant. All seedlings in the crosses between susceptible cultivars were rated susceptible. These results indicate that the resistance to loquat canker is controlled by a single dominant gene, Pse-a. Except for 'Champagne' and 'Fukujuin' which are dominant homozygous plants, the genotype of resistant cultivars in this experiment is heterozygous, whereas that of susceptible cultivars is recessive-homozygous. In a breeding population, segregation for resistance of 14 crosses among 15 crosses fit a single-dominant-gene model, which supported the hypothesis above mentioned. 3. The inheritance of resistance to loquat canker Group C was investigated to development resistant cultivars to not only Groups A and B but also Group C effectively. First, the inheritance of resistance to loquat canker group C in 'Shiromogi' was determined. Cross combinations between resistant parents 'Champagne' and 'Shiromogi', and selfing of 'Shiromogi', produced all resistant seedlings. Most crosses between 'Shiromogi' and susceptible parents either produced only susceptible seedlings or segregated for resistance in a ratio of 1 resistant: 1 susceptible. Seedlings produced by selfing two of the susceptible parents segregated in a ratio of 1 resistant: 3 susceptible. These results indicate that the resistance to loquat canker group C of 'Shiromogi' is conferred by a single recessive gene, designated pse-c. Based on the crossing tests, we conclude that resistant parents 'Shiromogi' and 'Champagne' are homozygous for pse-c; the susceptible parents 'Fukuharawase', 'Fusahikari', 'Gold Nugget', 'Kusunoki', Nagasaki No. 2, 'Tanaka', 'Tsukumo' and 'Yougyoku' are homozygous for Pse-c; and the other susceptible parents in this experiment ('Mogi', 'Nagasakiwase', Nagasaki No. 3, 'Taisho', and 75-142) are heterozygous. Based on the pedigree of 'Shiromogi' and the results reported here, pse-c is probably derived from 'Mogi', a major cultivar in Japan. Second, the inheritance of this resistance in progenies of crosses between 'Champagne' and susceptible cultivars was examined. 'Champagne', which is distantly related to 'Shiromogi', is another cultivar with resistance to Group C. All of the hybrid progenies between 'Champagne' (pse-c/pse-c) and Pse-c/Pse-c parents showed two types of resistant and susceptible. The proportion of resistant progenies showed great differences significantly, depending on the hybrid combinations. And it ranged from 0.203 to 0.596, with an average of 0.399. It indicated that the resistance was controlled by one or more additional genes or loci other than the Pse-c (pse-c) locus. In addition, the proportion of resistant progenies from crosses between 'Champagne' (pse-c/pse-c) and Pse-c/pse-c parents ranged from 0.463 to 0.701 (and averaged 0.594), which seriously deviated from the segregation of 1:1, indicating that the segregation was both Mendelian and polygenic. The proportion of resistant seedlings cannot be predicted by the phenotype and the genotype in the Pse-c (pse-c) locus. Therefore, it was estimated the general combining ability of 'Champagne' due to the additional gene effect, which was 0.407 and 0.094 for 'Champagne' * Pse-c/Pse-c and 'Champagne' * Pse-c/pse-c cultivars, respectively. The gene effect of susceptible cultivars ranged from -0.204 ('Yougyoku') to +0.189 ('Togoshi') for 'Champagne' * Pse-c/Pse-c cultivars, and from -0.131 ('Taisho') to +0.096 ('Nagasakiwase') for 'Champagne' * Pse-c/pse-c cultivars. 4. Fifty four loquat cultivars which released in Japan and introduced from foreign countries in recent years were evaluated for resistance to loquat canker Groups A, B and C for the promotion of canker resistant breeding. For released cultivars in Japan, 'Harutayori', 'Natsutayori', 'Reigetsu' and 'Ryoho' were resistant to Groups A and B, but other four cultivars evaluated were susceptible to two groups. Whereas, 'Harutayori' was only a resistant cultivar to Group C but the others were susceptible. 'Harutayori' was an epoch-making cultivar as a first commercial cultivar resistant to all groups of loquat canker. Furthermore, it was very valuable not only as a cultivar which was combined resistance and high fruit quality but also as a resistant resource. Among foreign cultivars, 23 cultivars were resistant and 18 cultivars were susceptible to Group A. For Group B, 25 cultivars were resistant and 16 cultivars were susceptible. On the other hand, only three cultivars, 'Xia lou bai mi', Vietnamese loquat No. 4 and Greece loquat 87-58, were resistant to Group C but the others were susceptible. These three resistant cultivars, especially 'Xia lou bai mi', seemed to be very valuable as a resistant resource. Evaluations of the resistance to Groups A and B coincided with in all of the 46 cultivars except for 'Xi ye xiang dun', which suggested mechanism of resistance to two groups was identical or very similar. It was also suggested that resistant cultivars to Group A or Group B were distributed in not a specific cultivar category but every group category around the world.