Use of gamma rays to induce mutants resistant to cocoa swollen shoot disease in Theobroma cacao L.
Vegetative buds of three cocoa varieties, Amelonado (P30), Trinitario (K5) and Upper Amazon (T85/799) were irradiated with 15, 20 and 25 Gy of γ-rays, respectively, and budded on to rootstocks to generate MV1 shoots. The terminal buds of the shoots were removed to induce the formation of MV2 shoots, from which MV3 shoots were similarly derived. The MV3 plants were screened for resistance to cocoa swollen shoot virus [cacao swollen shoot badnavirus] (CSSV) by patch-graft inoculation of the rootstocks. Only a few plants from each of the 3 cocoa varieties were symptomless after indexing. These plants were multiplied by budding to the MV4 and MV5 stages and screened at each stage for CSSV resistance by inoculation with viruliferous mealybug (Planococcoides njalensis) nymphs. At the MV5 stage, some plants still remained symptomless and this was confirmed with enzyme-linked immunosorbent assay (ELISA). The study reveals that γ-rays can be used to induce genetic variability for resistance to CSSV in cocoa, as well as for other traits such as chlorophyll deficiency.