Tohoku National Agricultural Experiment Station, Shimo-kuriyagawa, Morioka, Iwate, 020-0123, Japan

Background and objectives
Wheat yellow mosaic disease causes serious economic losses in Japanese wheat production. The pathogen, wheat yellow mosaic bymovirus (WYMV), is transmitted by the soil-borne fungus Polymyxa graminis Led. The use of resistant cultivars appears to be one of the most effective countermeasures to minimize yield losses. However, disease severity fluctuates from year to year, probably because of the variations in climatic conditions. Thus, field evaluation method of varieties for resistance can be time consuming. An alternative approach is to develop an evaluation method under controlled environment conditions. In this case, the optimum temperature for disease development regime needs to be determined. We have studied the dynamics of WYMV in field-grown winter wheat [1]. The virus infection occurs in autumn and propagation in the leaves follows through the winter. Visible symptom appear in early spring. While we obtained useful information on the effects of temperature on these processes from this study, two important questions still remain: whether exposure to cold temperatures is essential for virus propagation in the leaves and what temperature range is optimum for virus propagation and symptom development. The objective of this study is to clarify these questions.

Materials and methods
To examine the effects of cold temperatures on virus movement from roots to leaves, a replanting experiment was carried out under controlled environmental conditions. Wheat plants were grown in WYMV infested soil at 10C for 2 ;months. After their roots were washed free of soil, the plants were replanted into sterilized soil and grown at either 5C or 10C. Plants were sampled periodically to check for the presence of virus antigen in the roots and leaves by ELISA. Another experiment was carried out to examine the effects of temperature on virus propagation in leaves and symptom expression. Plants at the three-leaf stage were mechanically inoculated with WYMV and grown under controlled environmental conditions at 5, 10 and 15C. Periodically, the incidence of the virus antigen was determined by ELISA and the symptom expression was visually scored.

Results and conclusions
In the replanting experiment, the virus antigen was detected in the roots of more than 90% of plants, but not in the leaves. At the end of the test period, incidence of the virus antigen was higher at 10C than at 5C. The result showed that exposure to low temperature (5C) is not essential for virus movement from the roots to the leaves and propagation in the leaves. In the mechanical inoculation experiment, the incidence of the virus antigen increased more rapidly at 10C than at 5C. The symptoms were first observed at 10C, but they soon disappeared. At 5C, the symptoms appeared gradually but they were more severe than at 10C. No symptoms appeared at 15C. The results showed that the optimal temperature for virus propagation is around 10C, while the temperature conductive to the expression of symptom is lower than 10C.

1. Ohto Y, Naito S, 1997. Annals of the Phytopathological Society of Japan 63, 361-365.