Etiology Of Storage Rot Of Sweet Potato ( Ipomoea Batatas (L.) Lam.) And Its Control By Curing

ABSTRACT A survey was carried out to assess the extent of rot, and susceptibility of different types of sweet potato, and methods of storage in Ghana. Pathogens were isolated, identified and their pathogenicity established. The effectiveness of curing in extending the shelf-life of sweet potatoes was also studied. The market survey revealed that there are four types of sweet potato tubers (based on external colour). The local names of three of the sweet potato types are “Fante” (yellow skin with yellow flesh), “Kwahu” (yellow skin with white flesh) and “Ayigbe” ( red skin with white flesh). The fourth type, which is relatively scarce, is the one with red skin and yellow flesh. Sweet potatoes are transported mainly by road from the areas of production to the markets in Accra and this takes 1-2 days. Deliberate curing of sweet potatoes before storage is not practised in Ghana. Nine methods for storing sweet potato are practised in Accra. They are (a) Packed in roofed and open-sided sheds (b) Heaped in airy place and covered with tarpauline (c) Packed in sacks in a room (d) Exposed on the floor in a room (e) In baskets under a shed (f) In sacks in open-sided shed (g) In sacks in open place (h) In wooden boxes under a shed (i) In sacks in airy place. The maximum period sweet potatoes can be stored is one month. The major insect pest associated with sweet potato storage is the sweet potato weevil, Cylas sp. All respondents have experienced sweet potato tuber rot and there are differences in the degree of rot between different types of sweet potato. Between 1-100% of tubers per sack get rotten depending on the storage conditions. From the laboratory studies, five fungal species namely Aspergillus ochraceus, Botryodiplodia theobromae, Pusarium moniliforme, Fusarium oxysporum and Rhizopus stolonifer, were isolated and proved to be pathogenic. Botryodiplodia theobromae had the highest percentage(85 .8%) of occurrence both in the markets and in the bam. Aspergillus ochraceus, Fusarium momliforme, Fusarium oxysporum and Rhizopus stolonifer had the same frequency of occurence (6.7%) in the markets. Rhizopus stolonifer was the second most frequent organism in the bam (6.1%). Aspergillus ochraceus and Fusarium oxysporum were not encountered in the bam. With respect to both the markets and the bam, Rhizopus stolonifer was the most frequent, after Botryodiplodia theobromae. Aspergillus ochraceus and Fusarium oxysporum had the same frequency of occurrence in both the markets and the bam. Fusarium moniliforme was the third most frequent in both the markets and the bam. When the virulence of the five pathogens were determined on the “Fante” and “Ayigbe” sweet potato types they were all virulent. Based on the size of the necrotic area, the most virulent organisms were Aspergillus ochraceus in the “Ayigbe” type (5.2cm2) and Fusarium oxysporum in the “Fante” type (3.2cm2) and the least virulent were Botryodiplodia theobromae in the “Fante” type and Fusarium oxysporum “Ayigbe” type. Curing experiments involving incubator-curing, polyethylene-curing and solar-curing revealed that there was wound periderm formation in all the treatments except the control which was wounded but not cured and the other control that is unwounded, uncured, which had normal periderm. There were significant differences (P= 0.05) in the thickness of the periderm formed in all the treatments. When the cured and uncured tubers were stored in the bam at temperature and relative humidity ranges of 25-34 °C and 46-80% ERH, respectively, for 18 weeks, the incidence of rot of incubator-cured tubers (35.5%) was significantly lower (P= 0.05) than the values for unwounded and uncured (89.2%), wounded and uncured (82.1%), wounded and sun-cured (100%) and wounded and polyethylene-cured (94.6%). Fresh weightloss in incubator-cured tubers was significantly lower (P= 0.05) than those of the other treatments and the controls from week 1 to week 8. Sprouting in incubator-cured tubers reached 98.1% by the 8th week and this was significantly higher (P= 0.05) than the other treatments and the two controls. It can be concluded that, proper curing (incubator-curing) leading to formation of a thick periderm is important, in prolonging the shelf-life of sweet potatoes provided weevil damage is taken care of