Effect of soil amendments on bacterial wilt incidence and yield of potatoes in southwestern Uganda

Potato bacterial wilt caused by Ralstonia solanacearum is a major threat to potato production in Sub-Saharan Africa. It is believed that yield losses due to bacterial wilt increase with decreasing soil fertility. A soil amendment experiment was therefore conducted for 3 consecutive seasons, 1998A, 1998B and 1999A at Kachwekano at an altitude of 2200 meters) in southwestern Uganda. Organic materials: Sesbania sesban (S) and Leucaena diversifolia (L.) were applied in amounts sufficient to supply 100 kg N ha-1 either singly or combined with P and PK. Also added were NP and NPK from inorganic sources. The organic materials were incorporated into soil one week before planting, while the inorganic fertilisers were side-dressed at planting all at rates that would supply 100 kg ha-1 of N, P and K. Nitrogen in the form of urea was split-applied at planting and one month after. Bacterial wilt incidence differed with treatments and seasons. Disease incidence was lowest with treatments NP and S+ PK and highest with the control. Application of organic manures alone did not necessarily result in reduced wilt incidence except in a few cases. Both marketable and total tuber yields were consistently highest with S + PK and differed significantly from the control in all seasons. A combined analysis over the three seasons showed that the treatment S + PK gave a significantly higher yield (20.8 ha-1) than all other treatments, while the control yielded significantly lower (9.7 ha-1) than the other treatments. Sesbania as an organic manure performed better than Leucaena and potassium was found to be a useful nutrient for crop performance. When K was applied with NP, LP and SP, it brought about marketable yield increases of 11, 23 and 37%, respectively. Generally, the rate of wilt development, expressed in wilt incidence per unit time, was highest at early stage of growth, thereafter, it declined and stabilised during much of the tuber bulking stage The interaction between soil fertility and bacterial wilt incidence merits further studies in different environments. Key Words: Organic materials, inorganic fertilisers, polyphenols, ware yields, wilt incidence (African Crop Science Journal 2001 9(1): 267-278)