Ability of systemic insecticide dimethoate to prevent aphid colonisation and the spread of aphid transmitted viruses in Solanum tuberosum Lin.

Seven potato (Solanum tuberosum Lin.) varieties previously indexed against potato leaf roll virus (PLRV), potato virus Y (PVY), potato virus S (PVS) and potato virus X (PVX) were grown in the fieid for two seasons at Namulonge representing warm, mid-altitude tropics and, for one season at Kalengyere representing cool highland areas. One set of the seven potato varieties was sprayed with dimethoate at a rate of 1.19 kg a.i. ha-1 at weekly intervals to prevent aphid infestation and spread of aphid transmitted viruses. The second set was not treated. Seed tuber indexing before planting showed the absence of aphid transmitted viruses (PLRV, PVY and PVS) in seed potato obtained from Kalengyere during both seasons. Weekly collection of aphids at both sites showcrl a predominance of Myzus persicae (Sul.), the principle vector of most potato viruses. Dimethoate controlled aphids at Kalengyere but not at Namulonge. As a result, seed potatoes obtained from Kalengyere and grown at Namulonge acquired the aphid-transmitled PLRV in the fint season of growth whether they were treated with the insecticide or not. However, all the varieties were free from PVY and PVS. At Kaleogyere, all the varieties grown in the field were free from aphid-borne viruses irrespective of the insecticide fre:ltment. These results indicate that there·is probably M. persicae resistance to dimethoate at Namulonge.Key words: Dimethoate, virus latent infection, aphid-borne viruses, Myzus persicae, ELlSA

Effect of mother plant applied nitrogen and potassium on the sowing value of true potato seed

No Abstract

In vitro microtuberisation of two improved Ugandan Solanum potato varieties

The performance of explants derived from three and six week-old green house-grown potato (Victoria and Kisoro) mother plants was evaluated. The mother plants were fertilized with different combinations of nitrogen, phosphorus, and potassium fertilizers. Explants were cultured on modified Murashige and Skoog's(MS) basal salts, supplemented with appropriate vitamins and growth regulators, for shoot and microtuber induction. Microtubers were induced directly from nodal cuttings inoculated on tuberisation medium and from defoliated in vitro shoots previously grown on shoot propagation medium, before transfer onto the tuberisation medium. In vitro shoot length and node number were significantly (P<0.05) increased by sole N application and retarded by sole P, or K application. Application of N along with P or K showed intermediate effects, but improved shoot growth. Single N application was inimical to microtuber forn1ation, whereas P and K promoted early tuberisation, characterised with significantly (P<0.05) laql'e diameters and fresh weights. Victoria produced bigger and fewer microtubers than Kisoro. Increased number of micro tubers occurred due to combined application ofN,P and K. Microtubers derived from defoliated in vitro shoots showed no significant (P>O.OS) difference in size due to mother plant pre-treatment, unlike the number of microtubers per explant. The six week-old nodal cuttings barely regenerated (5-15%) on the tuber induction medium, and had 30-70% regeneration on shoot induction medium. Better regeneration (60-70%) occurred in theN fertilized mother plants than with P and K (30-50%). Microtubers derived from defoliated in vitro shoots. irrcspecth·e of genotype and pre-treatment produced very few, tiny microtubers and others did not tuberize. Attempts to chemically break dormancy using gibberellic acid gave eratlc results. No significant variation was observed between the conventional mother clones and the in vitro cultures and microtubers.Keywords: Solanum tuberosum, nitrogen, phosphorus, potassium, microtubers, dormancy, gibberellic acid