Morphological and genetic diversity in citrus genotypes to substantiate rootstock breeding for root rot resistance

Investigations were conducted on six citrus rootstocks, viz., rough lemon (RL), trifoliate orange (TO), Swingle citrumelo (SC), X 639 (X), Rangpur lime (RA) and Gou Tou (GT), to assess the morphological and genetic variability; and reaction against Phytophthora. Smooth trunk surface was recorded in all the genotypes. Three genotypes had erect growth habit, while it was spreading in Gou Tou (GT), Rangpur lime (RA) and rough lemon (RL). The leaves of GT, RA and RL were of unifoliate, while trifoliate orange (TO) had trifoliate leaf and Swingle citrumelo (SC) and X 639 (X) had multifoliate leaf division. No variation was recorded with respect to flowering season in all citrus rootstocks, i.e. mid February to last week of March. Fifty-five SSR markers were used for evaluation of genetic diversity amongst the six rootstocks. Twenty markers exhibited high polymorphism and showed wide allelic diversity. Capacity of each SSR to show polymorphic loci, varied from 0.29 (F98) to 0.83 (CCSME46 and CCSMEc4) with an average PIC value of 0.61. The resolving power (Rp) was highest for the primer CCSME43 (8.33) and was lowest for F90 (1.33). Significant differences were observed in the value of MI and were found to be the highest for primer F40 (9.25), while minimum MI was recorded for F98 (0.38). The PIC values of a marker vary with the crop and the set of the genotypes used. The reduction in the number of sporangia and lesion size in trifoliate orange and Swingle citrumelo indicated their tolerance against Phytophthora nicotianae var. parasitica. The number of sporangia counted after 48 h of incubation showed that all leaf baits of each rootstock were attacked by large number of sporangia. The number of sporangia on each leaf disc of rootstocks decreased after 48 h as the sporangia germinated into mycelium on the edges of leaf discs.