First Report of Dry Rot Disease of Yam Caused by Scutellonema bradys in East Africa

The yam nematode, Scutellonema bradys(Steiner & LeHew, 1933) Andrassy, 1958, appears to originate from West and Central Africa (Coyne et al. 2012), where it has only been found so far within Africa. Elsewhere, it occurs in the Americas and Asia (Bridge et al. 2005). It is the causal agent of dry rot disease on yam tubers, a major constraint to yam production (Dioscorea spp.). Infected tubers mostly have a cracked or flaky surface, under which nematodes feed endoparasitically, destroying cell walls as they migrate intracellularly, forming cavities and tissue necrosis. Field and storage losses vary but can be substantial, depending on level of infection, crop genotype, and conditions. The use of infected, untreated material perpetuates the disease into new crop cycles (Bridge et al. 2005). Recent studies demonstrated the pathogenic nature and threat of this nematode to potato (Solanum tuberosum) production (Coyne et al. 2011), while both cassava and sweet potato (Bridge et al. 2005) appear suitable hosts. Globally, the primary area of yam cultivation is West Africa, although the crop is cultivated across Africa and beyond. In East Africa, the crop is increasing in popularity with both local and immigrant West African populations. Locally cultivated yam tubers, exhibiting typical symptoms of dry rot, were collected in 2014 in Kenya and Tanzania. Flaky tuber surfaces revealed brown necrotic patches when scraped back with a knife. Extraction of the tuber peel produced low to high densities of S. bradys(up to 240/g) on D. cayenensis from farms in Coast Region, Rufiji District, Tanzania (7.72365[degrees]S, 38.93766[degrees]E) and on D. cayenensis from roadside market stalls near to the town of Mukuyu, Kenya (0.84763[degrees]S, 37.23257[degrees]E). The nematodes were morphologically identified as S. bradys and D2D3 28S rDNA and COI mitochondrial gene sequence data (GenBank Accession Nos. KT945157, KT945158 and KT945159, KT945160 respectively) confirmed this when compared with other data (Humphreys-Pereira et al. 2014; van den Berg et al. 2013). The East African populations formed for both genes a well-supported clade together with Nigerian Yam populations from GenBank. S. bradys, when inoculated onto yam plants at 500 per plant under greenhouse conditions, revealed similar symptoms (dry rot) as observed on the tubers collected. In addition, a nematode reproduction of 10.9-fold was recorded 2 months after inoculation, 1 month after storage. To our knowledge, this i