EXAMINATION BY GRAFTING OF THE EXTREME RESISTANCE TO COTTON LEAF CURL DISEASE IN TWO WILD GOSSYPIUM HIRSUTUM L CULTIVARS
R. Ullah, K.P. Akhtar, P. Moffett, F. Akbar, I. Hassan, S. Mansoor, R.W. Briddon, H. Mumtaz Hassan, M. Saeed
Cotton leaf curl disease (CLCuD) is a serious threat to cotton production across the Indian subcontinent. In the Punjab province of Pakistan the disease is associated with a distinct strain, the Burewala strain, of Cotton leaf curl Kokhran virus (CLCuKoV-Bu) and a distinct strain of the symptom-determining betasatellite, Cotton leaf curl Multan betasatelliteBurewala (CLCuMuBBur). At this time all commercial varieties of Gossypium hirsutum are susceptible to CLCuD. Two exotic G. hirsutum wild accessions AS0039 and AS0099 from Caribbean, were found to be naturally resistant to CLCuD. However, the nature of the resistance has remained undefined. Graft-inoculation and whitefly transmission assays were conducted as a preliminary analysis of the resistance of these cultivars to CLCuKoVBu/CLCuMuBBur complex. By whitefly transmission under conditions where all plants of the susceptible cultivar CIM-496 became symptomatic AS0039 and AS0099 plants remained non-symptomatic and no virus was detected. Graft-inoculation of AS0039 and AS0099 with severely infected scions from G. hirsutum cv. CIM-496 showed that these accessions could support the replication and long-distance spread of the virus/betasatellite-complex. Several leaves developing at the time of grafting close to and distal to the grafts in these plants showed mild symptoms with low amounts of viral/betasatellite DNA; significantly less than the levels seen in susceptible CIM-496 plants. Although symptoms spread to leaves away from the initial point of infection, the youngest leaves at the apex of both AS0039 and AS0099 plants remained symptomless with very low virus/betasatellite-complex. The results showed that the resistance of AS0039 and AS0099 plants is either due to reduced virus/satellite replication or perhaps interference in the short distance (cell-to-cell) spread of the virus/betasatellite-complex.