Causes And Control Of Tuber Rots Of White Yam (Dioscorea Rotumdata Poir Varieties Araba, Asana And Puna)

ABSTRACT

Studies were undertaken to identify the pathogens associated with rotting of white yam

(.Dioscorea rotundata var. Araba, Asana and Puna) in Ghana, to identify exportable white

yam varieties with superior shelf life and to evaluate the effectiveness of lime and neem

wood-ash in controlling rots in wounded yam tubers.

A field survey was conducted among yam retailers and exporters in Accra. The results

revealed that: a major constraint to yam production and marketing in Ghana is the high

percentage of tuber rot of 10-30% for retailers and 10-40% for exporters; Puna is the

most preferred white yam variety for export and local consumption; Some handling

practices predispose tubers to rot diseases; traditional rot control measures is not widely

practiced; yams shipped abroad are kept in poorly ventilated containers.

Identification of the three yam varieties based on tuber characteristics, showed that the

furrow markings on the corky periderm (skin) of the three white yam varieties are

consistent with variety and therefore a reliable index for their identification.

Isolations made from tubers of the three white yam varieties obtained from three yam

markets in Accra and a yam barn at the Crop Science Department, University of Ghana

Legon, and subsequent pathogenicity studies with the isolates revealed that storage rots

were caused by Aspergillus flavus, Aspergillus niger, Aspergillus oryzae, Botryodiplodia

theobromae, Erwinia carotovora, Fusarium culmorum, Fusarium oxysporum, Fusarium

sp Penicillium brevi-compactum, Penicillium sp., Rhizopus stolonifer and Scutellonema

bradys. This is the first record of the ability of A. oryzae and F. culmorum to cause rot in

yam. Symptoms aiding the identification of A. niger, E. carotovora, P. brevi-compatum,

R. stolonifer and S. bradys are described. Puna had the highest mean frequency of

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infection by pathogens (41.9% and 41.3%), followed by Asana (34.4% and 30.7%) and

then Araba (23.7% and 28.0%) in the market and barn respectively. After ten months of

storage Araba had the lowest weight loss of 39.31%, and a lower rotting percentage of

27.5% (P = 0.05). Even though at the 16th week all the yam tubers had sprouted, the rate

of sprouting of Araba was significantly lower (P = 0.05). Lime was significantly more

efficacious in suppressing decay in wounded tubers of the three yam varieties than neem

wood-ash (P= 0.05). Lime also inhibited growth of all the fungal isolates in vitro.