Field surveys of chrysanthemum-infecting viruses and viroids in Akita Prefecture, Japan

A study to check for virus and viroid was conducted on a total of 709 chrysanthemums (Dendranthema grandiflorum) from chrysanthemum fields in Akita Prefecture, Japan during the 1996-2005 growing seasons. This revealed the epidemic characteristics of five chrysanthemum-infecting viruses and viroids in the prefecture as follows. In 1996, an RT-PCR assay identified Chrysanthemum stunt viroid (CSVd) as the causal agent in chrysanthemums with stunting symptoms, which have been a problem from before. After constructing nested PCR primers to make the CSVd assay more sensitive, an outbreak monitoring of the viroid was conducted. It revealed that CSVd occurs continuously and is distributed throughout chrysanthemum fields, and infected plants were detected in about 60% of tested cultivars. It is believed that the viroid is more deleterious than the other four viruses and viroid toward producing chrysanthemums. An RT-PCR assay was designed to detect part of the gene for the coat protein of Chrysanthemum virus B (CVB), and the virus was detected in 1998. Though CVB causes almost no symptoms, it cannot be ignored, as this assay revealed that approximately 20% of chrysanthemum cultivars harbor the virus. Tomato spotted wilt virus (TSWV), whose outbreak had received attention in the prefecture, was detected in chrysanthemum plants in 1999 and occurred considerably in the following two years. However, occurrence of TSWV has decreased and has not spread since that time. At present, TSWV occurs only occasionally, but because TSWV is difficult to detect, there is a possibility that it is being overlooked. Since Tomato aspermy virus (TAV) was isolated from a chrysanthemum cultivar in 2003, a primer pair was designed for an RT-PCR to detect the virus. A survey of TAV revealed that the virus occurs in only a few cultivars and districts, which differs from cases of the other four viruses and viroids. Although Japan had long been alarmed about the invasion of Chrysanthemum chlorotic mottle viroid (CChMVd) from overseas, the viroid was first detected in the prefecture in 2003. A field survey of the viroid in various chrysanthemums revealed that CChMVd had invaded by 2002 and approximately 20% of cultivars harbored CChMVd, including more symptomatic type than nonsymptomatic. Large-flowered cultivars were less frequently infected than small-flowered and spray-type ones. CChMVd-infected chrysanthemums are increasing and have spread throughout major chrysanthemum-producing districts. Hereafter, it must continue to take great precaution against occurrence of new virus and viroid infecting chrysanthemum in the prefecture.