The effect of barriers and mixed cropping on the epidemiology of Sweet potato virus disease (spvd) in central Uganda

Field studies were conducted at Namulonge Agricultural and Animal Production Research Institute (NAARI) AND Makerere University Agricultural Research Institute Kabanyolo (MUARIK), both located in central Uganda. The studies aimed at: a) determining the effect of a virus resistant sweet potato variety and non-host tall crop when used as barriers on the incidence of sweet potato virus disease and whitefly population in the susceptible variety, b) assessing the spread of sweet potato virus disease in a mixture of resistant and susceptible sweet potato varieties, and c) comparing the suitability of SPVD affected and un-affected plants for oviposition by the whitefly vectors. The experiments were carried out during the short rains of 2002 (September –December 2002) and long rains of 2003 (March – August, 2003). Two plantings were done each season of 2002, the second 60 days after the first for 2002 and one planting was done in 2003 respectively. There was significantly (P=0.01) higher adult whiteflies population in the early-planted crop than in the late-planted crop of 2002 and early plantings of 2003. The barriers affected the mean monthly disease (SPVD+SPCSV) incidences significantly (P, 0.01) only in the early-planted crop during the short rains of 2002; the plots enclosed by bare soil were the most affected. These plots also supported the highest number of adult whiteflies in 2002 and the new Kawogo resistant cultivator enclosed plot were the least colonized accompanied with low disease incidences. The highest number of plants with SPVD+SPCSV was recorded at 3 MAP while at 4MAP most plants had SPCSV symptoms alone. New Kawogo reduced SPVD incidence in the cultivator Tanzania in all the plantings. The peak whitely population density was highly (P<0.001, R = 92%) associated with the peak SPVD incidence. The high whiteflies populations were highly (P<0.001, R = 0.92) with the disease incidence while the root tuber yield was negatively correlated (r= -0.974, R = 0.93) disease incidence. The barriers did not affect the storage root yield of the enclosed susceptible cultivator in the early planted crop of 2002 and first season crop of 2003 whereas a significant (P=0.027) effect was recorded in the late planted crop of 2002. There was no significant (P>0.05) effect of crop stands on the population of whiteflies except in the late planted crop of 2002. The diseases status of the sweet potato crop did not influence the activities of the whiteflies (P<0.001, r = 0.4) using two sample t-test. The mixing of New Kawogo and Tanzania cultivators resulted into a highly significant (P<0.001> delay on the onset of SPVD infection in both plantings of 2002 with the highest incidence recorded at five months after planting. The highest incidence was recorded in pure stand of the cv. Tanzania compared to the mixed stands. There was a strong (P=0.002) relationship between the population of adult whiteflies and disease incidence but the two parameters were negatively weakly correlated (r= -0.70). The total amount of disease recorded in cv. Tanzania represented by the area under disease progress curves, was high in pure stands but not statistically (P=0.172) different. The Implication of these results in the control of sweet potato virus disease is that mixtures were beneficial in terms of enhanced yield and reduced disease incidence among plants of the susceptible variety but a more susceptible variety should be used.