Survey for potato leafroll virus (plrv), evaluation of solviepotato clones for resistance to the virus and comparison of elimination methods

Abstract

A survey of Potato leafroll virus(PLRV) conducted in the major potato growing districts of Kenya namely Meru, Kiambu, Nyamira and Kisii revealed the highest incidence of PLRV in Kisima location, Timau division of Meru district and absence of PLRV in leafrolled samples from Githongo location of the same district. Meru had the highest incidence while Kisii had the lowest. Direct tissue blotting assay (DTBA) was used to detect potato leafroll virus (PLRV) III leaves and sprouts of potato. Virus was detected in the phloem tissues of the plant parts. Immunosorbent electron microscopy CISEM) was used to detect PLRV in crude sap of infected potato plants. The decorated virus particles revealed a hexagonal outline and were thus easy to locate. Studies conducted on the effect of acquisition access period (AAP), inoculation access period (lAP) and the latent period of PLRV in its vector Myzus persicae Sulz. suggested that the shortest AAP and lAP was 10 min for each and a latent period of 3 hours for a successful infection of the plants. Thermotherapy was more sensitive, faster and less expensive than thermotherapy followed by tissue culture when used in elimination of PLRV from infected potato tubers and plants. PLRV was eliminated in two Kenyan potato varieties namely Pimpernel and Annet by use of both methods. Seven promising potato clones from International Potato Center and one local cultivar (800224) were evaluated for tolerance to potato leafroII virus (PLRV) in the field and for resistance to infestation by Mvzus persicae Sulz. and Macrosiphum euphorbiae Thorn. in (he greenhouse. The differences in yieldloss that each clone incurred was the criteria used in evaluating the clones for tolerance to PLRV and those in the number of apterae aphids Ihal each clone supported, resistance to aphid infestation. Two clones 381381.20 and 387792.5 were found to be as tolerant as the local cultivar (clone no.800224) with a reduction in plant height but not in number and weight of marketable tubers. Clones 378699.2 and 800224 supported the highest aphid counts while clone 381381. 20 supported the lowest. Clone 381381.20 was found to be resistant to aphids as pests but not as vectors. There was a weak positive correlation (r= 0.281, P = 0.05) between leaf area and mean aphid colonisation. Resistance to aphid infestation was not related directly to tolerance to PLRV for instance clone 800224 was very susceptible to aphid infestation yet it was tolerant to PLRV. The effect of time of inoculation with potato leafroll virus on the yield of three Kenyan potato varieties was studied. The loss in yield was very significant in plants infected early (2 and 5 weeks after emergence). However, there was no significant difference in yieldloss between plants infected late (8 weeks after emergence) and the control plants. It was concluded that the risk of early infection with PLRV with late planting dates should be considered in areas where PLRV has been observed in the previous seasons