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A survey was carried out in 19 districts to investigate the prevalence and distribution of sweetpotato virus disease (SPVD) and its implication on the sustainability of clean seed system in Malawi. A total of 166 leaf samples were collected and tested for the presence of 8 viruses using nitrocellulose membrane enzyme-linked immunosorbent assay (NCM-ELISA). SPVD foliar symptoms were observed in 68.42% of the surveyed districts. There were significant variations in disease incidence and severity (p < 0.001) among districts, with the highest incidence in Mulanje (28.34%). Average SPVD severity score was 3.05. NCM-ELISA detected sweet potato feathery mottle virus (SPFMV, 30.54%), sweet potato mild mottle virus (SPMMV, 31.14%), sweet potato mild speckling virus (SPMSV, 16.17%), sweet potato C-6 virus (SPC6V, 13.77%), sweet potato chlorotic stunt virus (SPCSV, 22.16%), sweet potato collusive virus (SPCV, 30.54%), sweet potato virus G (SPVG, 11.38%), cucumber mosaic virus (CMV, 7.78%) either in single or mixed infections. Data from this study indicate a significant SPVD occurrence in the country, and the consequence implications towards national sweetpotato seed system.

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In Malawi the demand of the use of mineral acidifying fertilizer by farmers for sustenance of high crop yields is increasing. The soaring demand is a pointer to the loss of humic substances in the soil and the resultant poor soil health. There is potential however to reduce the amount of mineral fertilizer used by the farmers and retain the applied nutrients within the plants rooting zone for increased use efficiency and productivity. This could be achieved through the combined application of humate based fertilizers with mineral fertilizer. Therefore, an experiment was conducted to evaluate the effect of different rates of NPK and humate based fertilizer (HBF) combinations on potato yield and yield components at Tsangano, Bembeke and Dwale Extension Planning Area (EPA) in the 2016/2017 cropping season. Ten treatments were laid out in a randomized complete block design (RCBD). Baseline soil data were collected and subjected to standard laboratory analytical procedure. Agronomic data collected in the experiment were analyzed in Genstat Discovery Edition 4 and were subjected to analysis of variance (ANOVA) at 95% level of confidence. The means were separated by the least significant difference (LSD0.05). Laboratory analysis showed that soils were strongly acid at the three sites with low amount of N, P, K except for Dwale EPA which had high P and medium content of K. Furthermore, results showed that different NPK and HBF combination significantly influenced potato tuber yield and yield components at the three sites. The recommended fertilizer rate of NPK 8:18:15 + 6S at 250 kg ha-1 + 60 kg N ha-1 produced the highest tuber yield at Tsangano (20,729 kg ha-1) and Bembeke (5,189 kg ha-1). At Dwale EPA, application of NPK 8:18:15 + 6S at 250 kg ha-1 produced the highest yield (13,956 kg ha-1). Nevertheless, different combinations of NPK and HBF fertilizer (Treatments 7, 8, 9 and 10) also gave comparably high yields and high number of big tubers. Therefore, the combined application of mineral fertilizer and humate based fertilizer potentially could increase potato yield in Malawi, sustainably. More studies however are required in order to confirm the results.

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