Studies on the survival and transmission of Xanthomonas manihotis on cassava seed

SUMMARY Cassava seed which had been stored at 5 oC and 60% r.h. for 2–51 months was assayed for the presence of Xanthomonas manihotis by a leaf‐infiltration technique, using as inoculum the supernatant from seeds soaked in sterile water at 30 oC for 2–4 h. The threshold of sensitivity of the assay method was 105 cells/ml. Twenty out of 50 samples yielded the pathogen. The infested seed had been in storage for 2–18 months. Bacteria reisolated from infiltrated leaves were identical to X. manihotis in cultural characteristics, phage type and pathogenicity. Surface sterilisation or hot air treatment for 24 h at 65oC or lower did not eliminate the pathogen from infested seed. Soaking of infested seed in hot water at 60 oC for 20 min reduced the number of bacteria to less than the minimum detectable level without appreciably reducing germination. Cassava bacterial blight was observed in 8‐wk‐old seedlings which had been planted during the dry season at a site where infection from outside sources was unlikely. It is postulated that a low percentage of successful seed transmissions of X. manihotis can occur under favourable environmental conditions.