Some Effects Hot Water Treatment

The role of ratoon stunting disease in causing yield losses under fully irrigated conditions in the lowveld is as yet undefined. Hot water treatment at 50:5OC for 2 hr was shown to adversely affect germination and reduce stalk population in certain varieties. The optimum age for hot water treatment of NCo 310 and NCo 376 seedcane was found to be 11-12 months. Exposure of hot water treated cane subsequent to treatment had no detrimental effect if the period did not exceed two days. No unsatisfactory stands resulted from planting hot water treated cane during a mild winter. Latent infections of culmicolous smut were controlled by thermotherapy. Introduction Though ratoon stunting disease (R.S.D.) has previously been recorded in NCo 376 at Chirundu (Whiteside and Herd7) the effects of this disease were not obvious in the Rhodesian lowveld until 1968. As a result of reduced irrigation applied over the 1967-68 season, poor uneven growth was seen to occur in many fields both on Hippo Valley Estates Ltd., and Triangle Ltd. Consequently a preliminary survey was carried out to determine the incidence of R.S.D. The results from this survey showed that R.S.D. occurs throughout the lowveld; however, the symptoms observed on longitudinally splitting the sampled canes were somewhat atypical, in that there was usually only one discoloured bundle (at the most two) per stalk in the nodes of affected canes. To confirm that the symptoms recorded in the survey were caused by the R.S.D. pathogen the following inoculation experiments welie initiated. Experimental Inoculation experiments Experiment 1 Maferials and methods Three-budded setts of four R.S.D. susceptible varieties CB 36-14, CP 43-47, Pindar and Q 58 were subjected to one of the following treatments. (a) Hot water treatment (H.W.T.) at 50S°C for 2 hr. (b) No H.W.T. (c) H.W.T., then inoculated with juice expressed from canes suspected of having R.S.D. (d) No H.W.T., but inoculated with suspect juice. The cane setts were inoculated using the pressure cup technique ( ~ e l l q . The experiment, which was a 4Vactorial design (two replications) with single line plots 6,7 m long containing 10 setts planted end to end, was cut at 16 months. Results H.W.T. significantly reduced the stalk population (P<0,01) when compared with the no H.W.T. plots; and the inoculation of setts with suspect juice also reduced the stalk population at 16 months No H.W.T. > No H.W.T. + inoculated (P<0,05). H.W.T. adversely affected the germination of both Pindar and Q 58, and reduced their stalk populations. (Table I).