EFFECT OF NECROTIC AND NON-NECROTIC STRAINS OF BEAN COMMON MOSAIC ANDBEAN YELLOW MOSAIC-VIRUSES ON NODULATION AND ROOT-TIP NECROSIS OF BEAN (PHASEOLUS-VULGARIS)

Nodulation of beans (Phaseolus vulgaris) decreased as a result of infe ction by bean common mosaic virus (BCMV) and bean yel low mosaic virus (BYMV). Nodulation decreased progressively in infected plants as the plants aged. There was a greater decrease in nodulation in BCMV infect ed cv. Sanilac plants than in BYMV infected cv. Black Turtle Soup plan ts. This difference corresponded with symptom severity. Severely disea sed plants yielded fewer nodules than did less severely diseased ones. Necrotic strains (BCMV-NL8 and BYMV-N) caused more reduction in nodul ation than mosaic strains (BCMV-NY15 and BYMV-t). Nodulation in infect ed bean plants was affected by temperature. The effect was more eviden t in temperature-dependent necrotic strains (BCMV-NL8 and BYMV-N). For example, BCMV-NY 15 infected plants yielded an average of 205 and 242 nodules/plant at 24 degrees and at 28 degrees C, respectively, but BC MV-NL8-infected plants produced an average of 68 and 128 nodules/plant , respectively. Infected plants developed more severe mosaic and necro tic symptoms at 24 degrees than at 28 degrees C and consequently reduc tion in nodulation was also more drastic at 24 degrees than at 28 degr ees C. When plants infected with the necrotic strains were examined, t hey had fewer nodules and also more root tip necrosis. Root tip necros is was significantly higher in plants infected with necrotic strains t han in those infected with mosaic strains and was higher at 24 degrees than at 28 degrees C. Root tip necrosis decreased when virus infectio n occurred in older plants. This phenomenon was most apparent in plant s infected with the necrotic strains.