Witches' Broom disease of Lucerne. II. Field studies on factors influencing symptom expression, disease incidence, and mortality rate

Abstract

Field surveys and ratings of individual lucerne plants (Medicago sativa L.) for severity of symptom expression in the Lachlan and Murrumbidgee Valleys, N.S.W., showed that each year there was a seasonal variation in observed incidence of the disease. This was due to masking of symptoms of diseased plants in the spring. Shoots from plants with masked symptoms contained virus. Maximum symptom expression developed in the late summer and early autumn. Rainfall and temperature (for which day length and light intensity are possible alternatives) appeared to be factors influencing both time and intensity of symptom expression; during the growing season severe symptoms were associated with hot dry conditions and mild symptoms with high rainfalls. Green flowers were associated with the disease in midsummer in some but not all infected plants. Disease incidence and rate of infection were influenced by local environment; more diseased plants and more mortalities of diseased plants occurred on river terraces than on river flats. Disease incidence and rate of infection were greater in old stands and in young thin stands than in young dense stands. Witches' broom was an important factor associated with mortality of plants in 10-year-old lucerne stands in the Lachlan Valley. The minimum observed life of infected plants was less than 2 months. The maximum was 33 months in the field and 54 months in the greenhouse. The economic importance of the disease in Australia is discussed.