MAFF informed Biosecurity Australia in 2007 that there are no other citrus canker hosts in the designated export areas other than unshu mandarins, but citrus canker hosts may be present in the production area outside the four designated export areas. However, the number of host plants would be low, because horticultural production within the production area predominately consists of tea, unshu mandarin and bamboo plantations nestled amongst Cryptomeria forest (refer to Section 3.2).
Due to above reasons, the production area is considered as an area of low pest prevalence (ALPP) for citrus canker.
While the mountainous terrain that surrounds the production area acts as a natural wind break (Kuhara 1978), there is still concern that X. citri subsp. citri may be introduced to the production area during typhoons (Chapter 3).
Resistance of unshu mandarin to citrus canker
Resistance to citrus canker varies between citrus species and cultivars (Matsumoto and Okudai 1990; Koizumi and Kuhara 1982; Canteros 1992, 2004). Citrus species such as grapefruit, limes and trifoliata rootstock are of low resistance to X. citri subsp. citri infection. Unshu mandarin rates amongst the most resistant citrus species to this bacterium(Kuhara 1978; Danos et al. 1981; Koizumi and Kuhara 1982; Serizawa and Inoue 1983; Leite and Mohan 1984). Unshu mandarin fruit rarely show signs of infection (Kuhara 1978; Canteros 2004), although fruit infection does occur (Koizumi 1972; Serizawa et al. 1985). Goto (2005) provides an image of fruit infection of unshu mandarin.
Research on the inheritance of resistance to citrus canker was studied by Matsumoto and Okudai (1990). They showed that unshu mandarin is homozygous for a single dominant allele that confers resistance to citrus canker. These authors and Koizumi and Kuhara (1982) consistently found that unshu mandarin is moderately resistant to citrus canker. In this pest risk assessment, unshu mandarin is considered to be moderately resistant to citrus canker and this terminology is used in the report unless other terminology was used in the original source.
The moderate resistance of unshu mandarin fruit to citrus canker was supported by a 10 year (1995–2004) field research project in Argentina (Canteros 2004). Fruit of unshu mandarin exhibited the lowest citrus canker disease intensity of all tested species (i.e. grapefruit, lemon, Valencia orange and Murcott tangor). In fact, no fruit infection was recorded for unshu mandarin on all 25 observation dates over the length of the study, unlike the results for all the other species. Disease incidence for vegetative plant parts was slightly higher than for fruit, but unshu mandarin was again consistently the least susceptible species to citrus canker for each of the given 25 observation dates (Canteros 2004). Field research conducted on satsuma mandarin (C. unshiu) in Brazil provided similar results as fruit of this species was the least susceptible to X. citri subsp. citri infection compared with that of a wide range of citrus species/cultivars (i.e. mandarin, grapefruit, pummelo, natsudaidai, calamondin) (Leite and Mohan 1984).
Suitability of climate
Climatic information from the field research site in Argentina (Canteros 2004) and the production area in Japan is summarised in Table 4.3. Argentinian records were obtained from the closest weather station at Corrientes using data from Servicio Meteorologico Nacional (2008) and from Canteros (2004). Equally, the summers are warm and moist at the research site in Argentina, and at the production area in Japan, although the total rainfall in summer is lower in Argentina. Despite this, conditions for the expression of citrus canker symptoms are more conducive throughout the year in Argentina, as the monthly rainfall pattern is more even and average minimum temperatures above 14ºC occur for seven months, compared to four months in Japan (refer to Table 3.3). Proliferation of cells of X. citri subsp. citri commences at above 14ºC (Koizumi 1976, 1977, 1985).
Reports provided by MAFF indicate that leaf and fruit damage of unshu mandarin due to wind/weather events that would promote infection are not known to occur in the designated export areas. Unshu mandarin trees are thornless and are grown on the lower slopes in hilly terrain. This reduces the incidence of wounds as potential entry sites to citrus canker bacteria. The export orchards are well established and the mature unshu mandarin trees flush once per year during spring/early summer. This is the stage when leaf tissues are most susceptible to infection with citrus canker. However, leaf flush occurs prior to flowering and in most years prior to the commencement of the typhoon season in Japan.