Witches' Broom disease of Lucerne. III. A greenhouse study on symptom expression and growth in relation to frequency of watering and cutting

Abstract

The effects of frequency of watering and cutting on symptom expression and growth of witches' broom-diseased lucerne (Medicago sativa L.) were studied in a greenhouse experiment. Clones of two strains of diseased lucerne and one clone of non-infected lucerne were given four treatment combinations of two rates of watering and two frequencies of cutting. The cultural treatments caused pronounced differences in the growth of diseased plants and small differences in the growth of non-infected plants. Symptoms of the two strains of witches' broom-diseased lucerne maintained their characteristic identities in all four treatments. For both strains, symptoms were consistently more severe in plants grown in soil of low moisture content and cut at intervals of 6 weeks rather than 12 weeks. In the spring some diseased plants of one strain, which were grown in soil of high moisture content and cut at 12-weekly intervals, showed partial masking of symptoms. A high mortality of diseased plants was associated with frequent cutting. The seasonal patterns of dry weight yield and moisture content were essentially the same for each cultural treatment and for both diseased and non-infected plants. The yield of non-infected plants was significantly greater than that of diseased plants. The total yield of all three clones was increased by cutting at 12-weekly intervals rather than at 6-weekly intervals. The greatest reduction in yield of diseased plants as compared with non-infected plants was in treatment combinations in which there was a low soil moisture and cutting was at 6-weekly intervals.