Limited Occurrence of Tristeza in Yuzu and Possible Tristeza-free Cultivation in Northernmost Commercial Yuzu-producing Areas of Japan

Citrus tristeza virus (CTV) commonly infects yuzu (Citrus junos) in most parts of the citrus-growing areas in Japan. This study examined the occurrence of CTV in the northernmost commercial citrusproducing areas of Japan. Colonies of the brown citrus aphid (Toxoptera citricidus) were detected within the areas investigated. However, 23 of the 41 trees that were examined by RT-PCR and both or either subject to stem-pitting observation and immunostrip testing were free of CTV. Several CTVfree trees were apparently more than 60 years old. A field trial revealed that all eight uninoculated seedlings tested negative for CTV at 52 months after transplanting. These results suggest that CTVfree yuzu production is possible in the Rikuzen-Takata and Ofunato areas of Japan. Discipline: Agricultural Environment Additional key words: Citrus junos Sieb. ex Tanaka, Great East Japan Earthquake, Toxoptera citricidus Introduction Yuzu (Citrus junos Sieb. ex Tanaka) is a cold-hardy sour citrus species that can grow in northern areas (Kawase et al. 1983). The yuzu fruit is very popular due to its characteristic flavor and is widely used as a dressing in Japanese cuisine (Miyazawa et al. 2009). The northernmost commercial yuzu-producing areas in Japan overlap with the main apple-growing regions. Specifically, yuzu has been grown for many years in the Rikuzen-Takata and Ofunato areas in Iwate Prefecture (around 39°N and 142°E) (Fig. 1), primarily for ornamental purposes and production of the fruit, which is mainly consumed by the families of local farmers. However, in response to the recent increase in the popularity of yuzu, farmers in the Rikuzen-Takata and Ofunato areas have considered increasing the commercial production of yuzu, with potential financial benefits for these yuzu-producing areas. Yuzu is highly susceptible to citrus tristeza virus (CTV), which commonly infects citrus species (Moreno et al. 2008, Dawson et al. 2015, Harper & Cowell 2016). Yuzu trees infected by CTV grow poorly and often produce relatively small fruits with blemished skin (Ieki 2003). Many yuzu trees in the southern yuzu-producing areas of Japan are affected by tristeza. In Japan, CTV is mainly transmitted by the brown citrus aphid (Toxoptera citricidus) commonly found in the southern yuzuproducing areas (Ieki 2003). The conditions in those areas make it difficult to control the citrus tristeza *Corresponding author: ti207181@nodai.ac.jp Received 19 November 2019; accepted 27 February 2020. 307 JARQ 54 (4), 307-316 (2020) https://www.jircas.go.jp