2.4.1
EPIDEMIOLOGY OF CMV IN GENTIAN PLANTS

K SUZUKI1, A OKUMURA1, T KURODA1, Y MIURA1and Y HIKICHI2

1Iwate Biotechnology Research Centre, Kitakami, Iwate 024, Japan; 2Faculty of Agriculture, Kochi University, Nankoku, Kochi 783, Japan

Background and objectives
A perennial plant, gentian, is one of economically important ornamental flowers in Iwate prefecture, Japan, but recently its cultivation has been severely damaged by the occurrence of viral disease. We have reported that cucumber mosaic virus (CMV) is important as the causal agent of viral disease of gentian plants in Iwate prefecture and suggested aphids, which multiplied on gentian plants only after flowering time, as main factor of CMV transmission in gentian fields [1,2]. In this study, we investigated the behaviour of CMV in identical infected gentian plants and the spread of CMV infection in gentian fields precisely by immunodetection assay.

Materials and methods
Leaves and overwintering buds were sampled from identical gentian plants at several grower fields in Iwate prefecture at appropriate intervals. Infection of CMV was detected by ELISA and/or tissue printing immunoassay from these samples.

Results and conclusions
The behaviour of CMV in infected gentian plants was investigated in two fields using 20 gentian plants from December in 1994 until October in 1995. Regardless of the frequency of infected plants in two fields, detection rates of CMV from overwintering buds of infected plant were high and absorbance of ELISA of these buds did not change considerably during winter. In spring of the following year, CMV was detected at a high frequency from leaves of plants on which CMV was detected from overwintering buds in winter and symptoms were clearly observed in leaves of CMV-positive plant. When the distribution of CMV was examined from the bottom to the top of the stem of infected plants, CMV was detected in parts of stem and leaves. While symptoms were not obvious after flowering, the detection rates of CMV from the leaves of infected plants were high until the cutting stage after harvest. Furthermore, CMV was detected from overwintering buds that were newly formed at that stage.

Spread of CMV infection in gentian fields was monitored at another field just after transplanting using about 200 gentian plants from August in 1994 until March in 1997. In the year of transplanting, no CMV infection was detected. CMV infected plants were first detected after flowering in 1995. In the infected plants, CMV was detected in only one or two stems and symptom were not clearly observed. In 1996, however, CMV-infected plants increased significantly. The frequency of infected stems became high and symptoms were clearly observed in gentian plants from which CMV was detected in autumn of 1995. Furthermore, CMV was detected from overwintering buds of infected plants at high frequency on November in 1996 and March in 1997.

We also confirmed the infectivity of saps of overwintering buds sampled in which CMV was detected in winter by inoculation test. These results indicate that infection of CMV is carried over to the next year through overwintering buds in gentian plants. We consider that translocation of CMV to overwintering buds is important for the establishment of the infection cycle in a gentian plant. Conversely, horizontal spread of CMV from infected plant to uninfected plant appeared to be mediated mainly by aphid transmission [2] and sap transmission at cutting after harvest.

References
1. Chaumpluk P, Sasaki Y, Saito A, Koiwa H, Hikichi Y, Suzuki K, 1994. Annual Report of the Society of Plant Protection of North Japan 45, 88-92.
2. Suzuki K, Saito A, Chaumpluk P, Sasaki Y, Hikichi Y, 1994. Annual Report of the Society of Plant Protection of North Japan 45, 84-87.