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In an attempt to find suitable indicator plants for citrus likubin (greening disease occurring in Taiwan) a series of experimental tissue-inoculation tests was made on plants of various citrus species and hybrids and other rutaceous plants grown and maintained in the greenhouse. Typical leaf symptoms in citrus were various chlorotic patterns including some resembling zinc deficiency, also blotchy mottle, yellowing, shortening of internodes and stunting (Fig. 1, A-D). In the inoculated seedlings and budlings irregular chlorotic mottle along the veins was produced in 2 or 3 months at 28-32 C, the most favorable temperature conditions for symptom development. At the earlier stage of infection the symptoms tended to appear irregularly in parts of the inoculated plants, then gradually progressed throughout plants, forming shortened internodes, small leaves and sometimes vein corking (Fig. 1, E, F and B). In this series of inoculation tests, however, consistently high rates of infection were obtained by inoculation with 2 or 3 inoculum buds in each seedling or budling (Table 1). Among the citrus varieties tested, sweet orange (Citrus sinensis), mandarins (C. reticulata) and Orlando tangelo (C. paradisi×C. reticulata) were found to be highly responsive to the likubin agent and likubin symptoms were clearly defined despite the presence of seedling yellows tristeza virus which is widespread in mandarin citrus trees in Asian countries. In contrast, Eureka lemon (C. limon), sour orange (C. aurantium), grapefruit (C. paradisi), and Sexton tangelo seedlings were much more sensitive to seedling yellows tristeza virus than to the likubin pathogen. The blotchy mottle which developed on leaves of inoculated rough lemon (C. jambhiri) was very similar to that of South African greening disease. Ponkan (C. reticulata) and Orlando tangelo seedlings showed especially conspicuous leaf symptoms in these tests (Fig. 1, A and Fig. 3). Therefore, it is suggested that these varieties be recommended as indicator plants for likubin, even if the inoculum carries seedling yellows tristeza virus. Kumquat varieties (Fortunella spp.) also developed chlorotic and yellowing leaf symptoms typical of likubin. No distinct symptom appeared on trifoliate orange (Poncirus trifoliata) seedlings; however, sub-inoculated seedlings were found to be infected by topgrafting them with susceptible citrus varieties. Murraya (Murraya paniculata), a favorite host plant for Diaphorina citri, the psyllid vector of greening disease, did not show any visible sign of infection following tissue-inoculation.

Experimentally-Induced Symptoms and Host Range of Citrus Likubin (Greening Disease)