GENETIC ANALYSIS OF RESISTANCE TO ROSETTE DISEASE OF GROUNDNUT (Arachis hypogaea L.), ABSTRACT
Groundnut rosette disease (GRD), transmitted naturally by aphids, Aphis craccivora, is the most destructive viral disease of groundnut (Arachis hypogaea L.) in Nigeria and causes serious yield losses to farmers. The narrow genetic base among groundnuts has impeded efficient utilization for development of host resistance to GRD.
Studies were undertaken in Nigeria to: (i) ascertain farmers' knowledge of and preferences for rosette resistant genotypes; (ii) assess the genetic diversity among aphid and rosette resistant genotypes using microsatellite markers; (iii) exploit genotype x environment interaction towards improved selection efficiency to obtain high-yielding varieties; and, (iv) determine the mode of inheritance of resistance to groundnut rosette disease. A participatory rural appraisal (PRA) involving 90 farmers was conducted in two groundnut producing communities in Northern Nigeria. Early maturing genotypes and GRD resistance were the most important farmer preferred traits. Farmers ranked insect pests and inadequate rainfall as the most important causes of groundnut rosette disease. Majority of farmers across the study areas were doing nothing to avert the disease. Some farmers however rogue infected plants and use GRD resistant varieties when available. Genetic diversity and association of simple sequence repeat (SSR) markers with GRD resistance were detected in a set of 50 cultivated groundnut genotypes with different levels of resistance to GRD. Out of 170 bands amplified from 36 primers, 166 were polymorphic (97.65%). Each amplified 2 to 12 microsatellite loci, with an average of 4.74 loci per primer. The Polymorphic Information Content value of each marker ranged from 0.19 to 0.82. Average pairwise genetic distance among the 50 genotypes was 0.31. The largest distance was 0.51 (between ICGV – IS – 07812 and RS006F4B1 – 31) and the shortest distance was 0.05 between ICGV – IS – 07865 and ICGV – IS – 07864, all the four lines were GRD-resistant. Cluster analysis revealed seven clusters using disease reaction to GRD. The assessment of genetic diversity of GRD-resistant groundnut genotypes will help groundnut breeders to formulate crosses by choosing parents with different genetic backgrounds and in the development of gene-mapping populations with greater marker polymorphism. The 36 F2 populations generated from 9 x 9 half diallel mating scheme were infested with veruliferous aphids, Aphis craccivora and scored three times fortnightly following inoculation. General combining ability (GCA) and specific combining ability (SCA) effects for GRD resistance were highly significant, indicating that both additive and non-additive gene effects govern the inheritance of GRD resistance. Low narrow sense heritability for Area Under Disease Progress Curve (29.29 %) along with high broad sense heritability (94.78 %) further highlight the influence of non-additive gene action in controlling resistance to GRD, suggesting effective selection of superior genotypes at advanced generations when maximum homozygosity is fixed.