A research to obtain tolerant new mutant genotypes via mutation breeding from fortune mandarin against alternaria brown spot disease and determination of some phytohormone levels in these mutant genotypes

Alternaria brown spot disease caused by the Alternaria alternata pv. citri factor affects the leaves, branches and fruits of many tangerine types and their hybrids. In this study, it is aimed to obtain tolerant genotypes by mutation breeding in Fortune tangerine (Citrus reticulata Blanco) which is known to be sensitive to the brown spot disease of citrus. Gamma rays were applied to Fortune mandarin budwoods at doses of 50 and 60 gray from 60Co as a physical mutagen in 2014. The irradiated budwoods were consecutivly grafted onto common sour orange by using 'T' grafting in order to obtain M1V1, M1V2 and M1V3 individuals, respectively. The tolerance status against Alternaria brown spot disease was examined in 111 candidate plants. The isolate used in this study was obtained from a commercial 'Minneola tangelo' orchard in Adana, Turkey. Disease tests were performed in leaves located within 1-2 cm length near the shoot tip of the plants in vitro conditions. For in vivo evaluations of Alternaria brown spot disease resistancy, in 9 tolerant and 2 sensitive mutant individuals and Fortune, Clementine Fina, Okitsu Wase and Minneola tanjelo commercial cultivars were used. As a result of both in vitro and in vivo testings and evaluations, it was determined that 9 mutant genotypes could be tolerant against the pathogenic Alternaria brown spot disease. Meantime, similar testing of commercial cultivars showed that Clementine Fina and Okitsu Wase mandarin were found to be tolerant while Minneola tanjelo and Fortune mandarin were found to be sensitive against the Alternaria brown spot disease. Leaves of the seedlings were cut in order to the conidial suspensions disease treatment when the young leaves reached to length of 2 to 3 cm after the pruning for abscic acid, gibberellic acid, indole acetic acid and zeatin analysis. Hormone analyzes of the leaves were carried out in order before the inoculation and after 24 – 72 hours from the inoculation. In most genotypes, gibberellic acid level was determined comparatively lower after the disease inoculation compared to pre-inoculation tests. The gibberellic acid level of commercial cultivars increased in the inoculated ones. Indole acetic acid levels decreased after inoculation in the some genotypes while it increased in some genotypes and cultivars. The zeatin level in the genotypes decreased after inoculation. However, the zeatin level increased in some of commercial cultivars whereas zeatin decrements were detected in some other commercial cultivars. Abscisic acid levels did not significantly change before and after the inoculations. The results of phytohormone analysis showed that the levels of abscisic acid gibberelic acid and zeatin were higher before the inoculations compared to after inoculation in the genotypes. However, indole acetic acid level increased after the inoculation compared to non inoculated ones. Nine tolerant genotypes were transferred to open field for the further evaluations.