Waxy bloom on capsules is a major determinant of early infection by gray mold (Amphobotrys ricini (NF Buchw.) Hennebert) in castor (Ricinus communis L.)

Gray mold is a major disease of castor caused by the pathogen Amphobotrys ricini. It mainly affects the inflorescence resulting in severe yield losses. Role of qualitative characters of inflorescence (spike) and capsule viz. spike compactness, capsule type, waxy bloom intensity on capsules (visual), capsule spine length, spine texture and spine density in predisposing castor crop to gray mold was investigated. A set of 26 castor lines differing for presence and intensity of bloom was characterized for spike and capsule characters. The lines were screened in vitro (detached spike and capsule techniques) and in vivo by raising the plants in poly house and field conditions for disease reaction. Spearman's rank correlation analysis revealed significant positive correlation (r(s) = >0.5; p = <0.05) between disease severity and capsule type, capsule waxy bloom, spine texture, spine length. Principal component regression analysis revealed that waxy bloom intensity is the major factor, among the evaluated, that predisposes castor lines to the fungus infection promoting disease severity. Quantification of epicuticular wax from capsules of different waxy bloom lines revealed that there was an increase in severity of infection with increase in wax quantity on capsules irrespective of nature of bloom substantiating the role of waxy bloom quantity in promoting the infection. High quantities of wax (1.50-3.00 mu g/mg) were extracted from lines showing high per cent disease severity (40%-95%), and low quantities of wax (0.00-1.50 mu g/mg) were extracted from lines showing low per cent disease severity (1%-40%). A disease scoring scale considering differences in waxy bloom quantity as a biochemical marker has been proposed. It may not be a replacement to phenotypic screenings but helps breeders in selecting early generation material for low disease severity when huge populations are to be screened against gray mold by choosing low waxy bloom varieties.