Development and Application of Attenuated Plant Viruses as Biological Control Agents in Japan.

In 1929, it was reported that yellowing symptoms caused by a tobacco mosaic virus (TMV) yellow mosaic isolate were suppressed in tobacco plants that were systemically infected with a TMV light green isolate. Similar to vaccination, the phenomenon of cross-protection involves a whole plant being infected with an attenuated virus and involves the same or a closely related virus species. Therefore, attenuated viruses function as biological control agents.

The entry of cucumber mosaic virus into cucumber xylem is facilitated by co-infection with zucchini yellow mosaic virus.

We investigated the synergistic effects of co-infection by zucchini yellow mosaic virus (ZYMV) and cucumber mosaic virus (CMV) on viral distribution in the vascular tissues of cucumber. Immunohistochemical observations indicated that ZYMV was present in both the phloem and xylem tissues. ZYMV-RNA was detected in both the xylem wash and guttation fluid of ZYMV-inoculated cucumber.

Effectiveness of an Attenuated Zucchini yellow mosaic virus Isolate for Cross-Protecting Cucumber.

To cross-protect cucumber plants from Zucchini yellow mosaic virus (ZYMV), we used cold treatment to obtain an attenuated isolate of ZYMV, designated ZYMV-2002. ZYMV-2002 was obtained from a virulent ZYMV isolate after repeated low temperature treatment at 12.5 to 15°C followed by five cycles of single-plant transfer.

A New Technique to Select Mild Strains of Cucumber mosaic virus.

An efficient technique to select a good attenuated virus to control Cucumber mosaic virus (CMV) disease was developed. Preliminary screenings were conducted to assess the virulence of virus recovered from dark-green islands and yellow tissues of mosaic leaves of Nicotiana rustica after co-inoculation with an attenuated mutant P2bR46C of CMV and its original severe isolate Pepo. All single-lesion isolates (SLIs) obtained from dark-green islands had the attenuated P2bR46C phenotype, but the SLIs from yellow tissue had either the virulent Pepo or the P2bR46C phenotype.

Cucumber mosaic virus 2b protein compensates for restricted systemic spread of Potato virus Y in doubly infected tobacco.

Tobacco plants (Nicotiana tabacum cv. Xanthi-nc) inoculated with a necrotic strain of Potato virus Y (PVY, T01 isolate) developed necrotic symptoms in some systemically infected leaves, but not in younger leaves. However, PVY expressed distinct symptoms not only in the older leaves, but also in the younger leaves, of plants that had been doubly inoculated with PVY and with Cucumber mosaic virus (CMV, strain Pepo).