Double-stranded RNA (dsRNA) viruses in some fungi are associated with hypovirulence and have been used or proposed as biological control agents. We isolated 7.5-kb dsRNAs from 13 of 286 field strains of Fusarium graminearum isolated from maize in Korea. One of these strains, DK21, was examined in more detail. This strain had pronounced morphological changes, including reduction in mycelial growth, increased pigmentation, reduced virulence towards wheat, and decreased (60-fold) production of trichothecene mycotoxins. The presence or absence of the 7.5-kb dsRNA was correlated with the changes in pathogenicity and morphology. The dsRNA could be transferred to virus-free strains by hyphal fusion, and the recipient strain acquired the virus-associated phenotype of the donor strain. The dsRNA was transmitted to approximately 50% of the conidia, and only colonies resulting from conidia carrying the mycovirus had the virus-associated phenotype. Partial nucleotide sequences of the purified dsRNA identify an RNA-dependent RNA polymerase sequence and an ATP-dependent helicase that are closely related to those of Cryphonectria hypovirus and Barley yellow mosaic virus. Collectively, these results suggest that this dsRNA isolated from F. graminearum encodes traits for hypovirulence.Double-stranded RNA (dsRNA) mycoviruses have been described for a wide variety of fungi and plant-pathogenic fungi (7,9,27,42,44,55,58). These dsRNA mycoviruses are classified into three families based on number of genome segments, capsid structure, and nucleotide sequences, with some dsRNA mycoviruses remaining unclassified (8,20,24,31,39,45). The isometric dsRNA mycoviruses are classified into two families, Totiviridae and Partitiviridae, consisting of viruses that are enclosed in nonenveloped isometric particles of 25 to 50 nm in diameter and typically cause latent infections in their host fungi (20). Totiviruses have a nonsegmented genome, while partitiviruses have segmented genomes. Hypoviruses (family Hypoviridae) are the unusual exception in that they can result in considerable morphological and physiological changes, including cytological alterations, changes in colony morphology and growth rate, and persistently attenuate novel virulence-related phenotypes (hypovirulence) (2,7,13,25,28,34,42). Hypoviruses lack conventional virions, and their dsRNAs are enclosed in host-encoded vesicles (17).Among Fusarium species, dsRNA mycoviruses have been reported to be present in F. poae and F. solani f. sp. robiniae (14,39,40). dsRNA elements of the same electrophoretic mobility isolated from vegetatively compatible strains of F. poae were similar or identical in many cases. Similar-sized dsRNA elements of vegetatively incompatible strains of the fungus could contain both homologous and nonhomologous dsRNAs (14). Morphological alterations or signs of degeneration of F. poae, however, were not observed in any of the dsRNA-containing isolates.We have isolated Fusarium graminearum Schwabe [telemorph: Gibberella zeae (Schwein.) Petch] and occasionally fou...

Fusarium graminearum is the causal agent of a serious scab disease of small grains in Korea. We screened 827 isolates of F. graminearum from diseased barley and maize and tested for the presence of double-stranded RNA (dsRNA) mycovirus. Of them, 19 isolates contained various sizes of dsRNAs. A dsRNA associated with pronounced morphological changes including reduction in mycelial growth, increase in dark orange to red pigmentation, reduced sporulation and virulence was previously observed in nine dsRNA-containing Fusarium isolates (Chu et al., Appl Env Microbiol 68, 2529-2534, 2002). Ten additional isolates were found infected with dsRNA mycoviruses. These mycoviruses contain 2-4 different segments of dsRNAs with the size-range of approximately 1.7-10 kbp in length. The presence of dsRNAs did not affect colony morphology and were transmissible through conidia and ascospore with incidence of 30-100%. Interestingly, dsRNA mycovirus found in F. graminearum isolates, JB33 and JNKY19, that show the pattern of mixed infection of two different viruses were transmitted to all progeny conidia and ascospores. These results indicate that there is genomic diversity of dsRNA mycoviruses that infect F. graminearum isolates and that impact of virus infection on host's morphology and virulence is determined by the interaction between dsRNAs and the fungal host, not by the mere presence of the dsRNAs.

Wheat streak mosaic virus (WSMV), a member of the genus Tritimovirus in Potyviridae, severely impacts wheat and corn seed worldwide, but has yet to be detected in Korea, and hence, every effort should be made to prevent its introduction. To prevent WSMV from entering the country, it is necessary to prepare a specific, sensitive, simple, and fast detection method for routine application to plant quarantine procedures. For this reason, a two-step diagnosis system consisting of RT-PCR and nested PCR is being used for WSMV detection. In addition, a novel positive control was developed for use with the system. WSMV has been detected in seed sweet corn from Japan and seed wheat from USA by a two-step diagnosis system, the details of which are described in this study. After sequence analysis, similarities of 80.6 and 100.0% with other isolates were determined by BLAST. They showed the same topology, which was classified as 4 genotypes by various phylogenetic trees, using a poly protein encoding sequence amplification. In this analysis, WSMV-JSweet-corn2868 (JX845574) is classified as clade B, while WSMV-Uwheat1944-1 (KC754959) and WSMV-Uwheat1944-2 (KC754960) belong to clade D.