Verifications of Resistance to Phytophthora spp. in 2-year-old Citrus junos Cultivars and Related Species

Abstract

Yuzu (Citrus junos) gummosis disease, caused by Phytophthoranicotianae, was first reported in 1997. As known in citrus, Phytophthora is the most fastidious soil-borne pathogen to control. In order to minimize its damage to Citrus spp., integrated pest management (IPM) approach, including fungicide chemicals and resistant cultivars, is necessary. Therefore, in this study we tried to evaluate tolerance of yuzu cultivars and its related species against yuzu Phytophthora. Trifoliate orange was evaluated as a susceptible host to yuzu Phytophthora by both mycelial growth onto extract media and immature fruit inoculation. However, in zoospores spray-inoculation on 2-year-old cuttings tree, trifoliate orange appeared to have a resistant property as showing less than 6% diseased leaf rate. Among yuzu cultivars only ‘Namhae No. 1’ appeared resistant property against both P. nicotianae and P. citrophthora. The ‘Namhae No. 1’ showed 5.7% and 10.6% diseased leaf ratio by P. nicotianae and P. citrophthora, respectively. Clearly, in order to reduce damages caused by two yuzu Phytophthora, we suggest that growers may utilize a trifoliate orange as a rootstock and ‘Namhae No. 1’ as a scion for fruit production. Disease response of 2-year-old cuttings of yuzu cultivars and related species against yuzu Phytophthora spp. †It represent percentage of infected leaf number out of total leaves a tree 4 days after spray-inoculation of zoospores in concentration of 103 per ml. ‡Lesion size was measured by leaf area meter (LI-3100, LI-COR, USA). §Mean separation within columns by the Duncan’s multiple range test at p = 0.05.