The population of SDV in Mie Prefecture was found to consist mainly of isolates belonging to the CiMV strain. The SDV strain was the second most common strain, and isolates belonging to the NIMV strain were also detected. In terms of the composition of SDV strains in Japan, the SDV strain predominates in Shizuoka Prefecture, while the CiMV strain predominates in western Japan, including Kyushu (Tomimura et al. 2013). The situation observed in Mie Prefecture, which is located between Shizuoka and western Japan, is similar to that of western Japan.
Combined infection with SDV and CiMV was confirmed in ‘Miyagawa Wase’, an early maturing Satsuma mandarin planted in 1966. As all neighboring trees were of the same variety, and no other varieties had been grafted, it was unclear how this combined infection occurred.
An isolate of the NIMV strain was detected for the first time in Mie Prefecture. The NIMV strain had been reported only in Wakayama Prefecture in Japan (Imada 1977; Inuma and Tomimura 2016; Iwanami et al. 2001), followed by one report in Ehime Prefecture (Shimizu et al. 2005). It has also been reported once in Sichuan province in China (Yan et al. 2021). The NIMV strain confirmed in Mie Prefecture infected the very early maturing satsuma mandarin ‘Sakikubo Wase’ and caused boat-shaped leaves on spring shoots; however, because the trees were cut down to prevent the spread of infection, gangrenous spots on adult leaves and mottled symptoms on fruits were unconfirmed (Fig. 5). Combined infection with CiMV and NIMV was confirmed in the very early maturing satsuma mandarin ‘Yamakawa Wase’ and caused boat-shaped leaves on spring shoots and atrophy of the entire tree. However, no gangrenous spots on adult leaves or mottled symptoms on fruit were observed. This is the first report of combined CiMV and NIMV infection of citrus.
The CiMV strain is the causal pathogen of citrus mosaic disease, which is observed as mosaic-like mottling on satsuma mandarin fruit and is well known for ‘Tora Miakan’ (called for satsuma mandarin fruits with mosaic symptoms) riots that spread along with the early maturing satsuma mandarin ‘Miyamoto Wase’ (Imada 1977; Yamamoto and Yamaguchi 1980). Therefore, it was previously thought that trees with foliar symptoms of satsuma dwarf disease but without mosaic symptoms on the fruit were infected with the SDV strain, and that the occurrence of the CiMV strain was rare. Of the 18 CiMV-infected citrus trees, only a small proportion (22.2%) showed mosaic symptoms on the fruit, suggesting that isolates of the CiMV strain that do not cause mosaic symptoms on fruit may be widely distributed in Mie Prefecture. In a survey of SDV strains conducted in Wakayama Prefecture, CiMV accounted for more than 80% of the total number of isolates, and no mosaic disease symptoms were observed on those trees (Inuma and Tomimura 2016). In Korea, many trees were confirmed to be infected with the CiMV strain but did not show typical symptoms (Hyun et al. 2020). Infection with the CiMV strain results in boat-shaped leaves and crowded branches, symptoms that are similar to, but milder than, those caused by the SDV strain (Usugi et al. 1986). These findings suggest that CiMV strain(s) that do not cause mosaic symptoms on the fruit may be spreading to other citrus production areas in Japan.
In this study, we made detailed observations of citrus trees that showed symptoms of satsuma dwarf disease, such as leaf atrophy and loss of vigor. However, healthy very early maturing satsuma mandarin trees have a more dwarfed appearance than early maturing satsuma mandarin trees, and also have smaller leaves, shorter internodes, and extremely rounded leaves (Iwamasa et al. 1984). In some trees, it was difficult to distinguish infected trees only by leaf atrophy because the new shoots of healthy trees also had an atrophied appearance. Such characteristics of very early maturing satsuma mandarin, including dwarfing of the tree and a small leaf size, are similar to mild symptoms of satsuma dwarf disease, making it difficult to identify infected trees in the early stages of infection.
The transmission of SDV strains through soil takes at least 4–5 years (Isoda and Gyotoku 1990; Katagi and Ushiyama 1990). In this study, we monitored the disease status of trifoliate orange seedlings planted around diseased trees. Our results show for the first time that SDV and CiMV can be transmitted through soil within 1 year. The soil transmission rate of CiMV from the tree in the P-1 plot was 17.7% at 370 days after planting and 36.7% at 721 days after planting, suggesting that this isolate may have a very high transmission ability. Conversely, the soil transmission rate of SDV from the tree in the K-1 plot was as low as 1.0% at 309 days after planting and 1.1% at 660 days after planting. Previously, Kageyama et al. (2011) reported soil transmission rates of 50% after 3 years and 100% after 9 years of planting of trifoliate orange trees around citrus trees infected with SDV, indicating that the soil infection rate of SDV strains is not always low. In the present study, infection of trifoliate orange seedlings was not confirmed during the survey period in the remaining six test plots. Whether the difference in soil transmission rate observed between plots P-1 and K-1, where infection was confirmed, was caused by differences in viral strains or by environmental factors in the field is still unknown. Trifoliate orange is widely used as a rootstock of citrus trees. Infection of trifoliate orange rootstocks may occur in a similar manner as that in healthy citrus trees planted in a field affected by satsuma dwarf disease. The diseased trees in the P-1 and K-1 plots were shaded by other citrus trees and adjacent forests, and the soil was wetter than that in the other six plots; however, the effect of these factors on soil transmission was not clarified. The vectors of the SDV and CiMV strains remain unknown. Further research is required to identify the vectors for all the strains and to determine how environmental conditions affect soil-borne infection.