THE ROLE OF THE APHIDS, BREVICORYNE BRASSICAE AND MYZUS PERSICAE, IN THE SPREAD OF XANTHOMONAS CAMPESTRIS, THE CAUSAL ORGANISM OF BLACK ROT OF TURNIPS

Abstract

Turnip plants, heavily infested with the cabbage aphid, Brevicoryne brassicae (L.), and the green peach aphid, Myzus persicae (Sulz.), were inoculated with a virulent strain of Xanthomonas campestris (Pammel), the causal organism of black rot disease of turnips, and placed in special cages together with healthy turnip plants. Some aphids of both species were allowed to migrate naturally, and others were transferred mechanically from the diseased to the healthy plants, but no symptoms of black rot ever appeared in any of the latter plants. X. campestris was isolated from only 15% of the B. brassicae and from less than 1% of the M. persicae taken from the diseased plants. Aphids of both species were artificially contaminated externally with X. campestris by exposures to Petri plate and broth cultures of the pathogen. Internal contamination of other aphids of both species was accomplished by feeding them 0.2% aqueous dextrose solutions containing X. campestris. The pathogen was isolated from individuals m both groups of aphids thus contaminated, but none of the aphids in either group was able to infect caged, healthy turnip plants with black rot.