Pona is the most popular cultivar of white yam (Dioscorea rotundata, Poir) cultivated in Ghana due to its early maturity, excellent eating qualities of the tuber and easy digestibility. Cornelius (1998) identify Pona as the most preferred cultivar of white yam by foreign markets and hence a source of foreign exchange for the country. Considerable employment, income and food security can be generated as opportunities provided by increasing Pona production.
Unfortunately, total yam production is far below requirement for export and home consumption throughout the year. A scarcity and expensive planting materials have been the major problem confronting yam production in general, which constitutes over 33% of production cost (Orkwor and Asadu, 1997). Solution to the problems of high cost and scarcity of yam planting materials has been offered through the introduction of yam minisett technique, which has increased the multiplication ratio from 1:5 to 1:30 (Orkwor and Asadu, 1997). Pona, unlike Dente and other white yam cultivars, has poor response to yam minisett technology, due mainly to the rotting of the minisetts in the sprouting medium caused by a complex of rot pathogens. Similarly, it has been reported that both the sprouting medium and the yam minisetts are sources of minisett rot pathogens. Various methods and recommendations had been made in order to control minisetts rot pathogens associated with yam cultivars; the use fungicides and wood ash for the treatment of yam minisetts are very popular (Otoo et al., 1987; Osai and Ikotun, 1994; Asare-Bediako, 2003).
This study attempted to evaluate the effectiveness of different disinfectants
in improving yam minisett sprouting and control the incidence of yam minisett
rot diseases using different disinfectant and protectants in roasted sawdust.
MATERIALS AND METHODS
Sterilization of sprouting medium: A metal drum; 85 cm in height and 55 cm in diameter was cut lengthwise into halves and used as troughs. Each was filled with moistened sawdust to about three- quarters full. The trough was then placed over fire to heat the sawdust and was periodically stirred with a piece of batten for an hour then allowed to cool (Heat sterilization). The roasted sawdust was allowed to stand for two days before use.
Preparation of yam minisetts: Pona and Dente yam minisetts were prepared;
a clean, healthy, medium sized mother seed yam that had sprouted was cut into
several short cylindrical pieces, each about 5 cm long. Depending on the circumference
of the pieces, each was cut longitudinally into 2, 3, 4 or more pieces such
that each piece had a periderm. Each piece termed minisett weighed between 25
to 30 g, according to the procedure described by Otoo et al. (1987).
The yam minisetts were treated with two disinfectants (wood ash and quicklime)
and three protectants (benomyl or Benlate, sodium hypochlorite and aqueous neem
leaf extract). All the treatments herein described were made simple for farmers
Wood ash: An amount of 120 g of Senna seamea wood ash was added to 4 L of water in a plastic bowl to form a suspension and small basketful of minisetts was dipped into the suspension for 3 min. The treated minisetts were then spread on palm fronds for 1 h to surface dry.
Quick lime: Lime suspension was prepared by adding 120 g of quicklime to 4 L of water. A small basketful of minisetts was dipped into the suspension for 3 min. The treated minisetts were then spread on palm fronds shade for 1 h to surface dry.
Benomyl (Benlate): Twenty five gram of benomyl was added to 4 L of water to form suspension and small basketful of minisetts was dipped into the suspension for 3 min. The treated minisetts were then spread on palm fronds for 1 h to surface dry.
Sodium hypochlorite (household bleach): The minisetts were surface sterilized by dipping a small basketful of the setts into 1:10 water dilution of household bleach (5.25% active sodium hypochlorite) for 3 min. The treated setts were then spread on palm fronds for one hour to surface dry.
Aqueous neem leaf extract: Fresh neem leaves were thoroughly washed and 1.2 kg were blended, soaked overnight in 4 L of water and the suspension filtered, small basketful of setts was dipped into the filtrate for 3 min. The treated setts were then spread on palm fronds for one hour to surface dry.
Control: For the control; the minisetts were only treated with sterilized distilled water.
Pre-spouting of minisetts: Treated minisetts were pre-sprouted in perforated boxes measuring 50x40x11 cm. Each box (experimental unit) contained 21 minisetts, comprising equal numbers of minisetts from head, middle and tail regions of mother seed yam, each for Pona and Dente cultivar. All the minisetts were laid out on a layer of moist heat sterilized sawdust in a box in such a way that their periderms touch the sawdust and spaced about 1 cm apart. They were then covered with another layer of moist heat sterilized sawdust. The boxes were watered as at when necessary in the screen house.
Experimental design and treatment: The treatment comprised; two yam cultivars (Pona and Dente) and six disinfectants and protectants (wood ash, lime, benomyl, sodium hypochlorite, aqueous neem leaf extract and control). The experiment was laid out in a 2x6 factorial design. The 12 treatment combinations were laid out as a Completely Randomized Design with three replications. Data were taken on the following parameters four weeks after planting; Percentage sprouted minisetts; percentage rotten minisetts; percentage sprouted but rotten minisetts and percentage unsprouted and unrotten minisetts. Square root data transformation was done before analysis of variance, mean values were separated using Duncans New Multiple Range Test (DNMRT).
The effect of different disinfectants and protectants on percentage sprouted minisetts of Pona and Dente white yam cultivars are presented in Table 1. Mean percentage sprouted minisetts recorded in both Pona and Dente yam cultivars were high. Dente minisetts produced slightly higher mean % sprouting than Pona minisetts. There was, however, no significant difference (p>0.05) between them. The interaction effects between yam cultivar and disinfectants or protectants on percentage sprouted minisetts was not significant (p>0.05). However, the mean percentage sprouted minisetts was significantly influenced (p<0.01) by the various disinfectants or protectants applied. Benlate-treated minisetts recorded highest mean percentage sprouting of 92.5%, which was not significantly different from 86.7% recorded for lime-treated minisetts. Benlate treated minisett were significantly higher than the minisetts treated with the other disinfectants and control. The percentage sprouting of lime-treated minisetts was significantly higher than the untreated minisetts (control) but was not significantly different from minisetts treated with wood ash, sodium hypochlorite and aqueous neem leaf extract with respect to percentage sprouted minisetts.
||Effect of disinfectants or protectants on percentage sprouted
Pona and Dente yam minisetts
|ns: Not significant at 5% level. Values bearing identical
letter(s) are not significantly different by DMRT at 1% level
||Effect of disinfectants on percentage rotten Pona and Denteh
|ns: Not significantly different at 5% level, Means bearing
identical letter(s) are not significantly different by DMRT (p>0.01)
Table 2 shows the mean percentage rotten minisetts recorded for Pona and Dente. These percentages were very low. Although the mean percentage rotten minisetts was slightly higher in Pona (11.9%) than in Dente (10.4%), they did not differ significantly from each other. Interactions effect between disinfectants and yam cultivars were not significantly different. However, there was significant difference in percentage rottened minisetts due to disinfectants or protectants applied. Minisetts protected with Benlate resulted in significantly lower mean percentage rot (4.11%) than those treated with other disinfectants or protectants and the control. Similarly, protecting minisetts with lime resulted in significantly lower mean percentage rotting (7.5%) than treating with wood ash, sodium hypochlorite, aqueous neem leaf extract and the untreated minisetts, having mean percentage rotting of 11.67, 14.17, 13.29 and 16.29% respectively. The percentage of minisetts which got rottened after disinfecting with sodium hypochlorate did not differ significantly from those of neem extract-treated minisetts but was significantly higher than wood ash-treated minisetts and lower than untreated minisetts.
Effect of disinfectants or ptotectants on percentage spouted but rotten minisetts of Pona and Dente white yam cultivars are shown in Table 3. Fewer minisetts sprouted but rotted at the end of the experiment. Pona minisetts recorded slightly higher percentage sprouted but rotten minisetts (5.83%) than Denteh minisetts (5.23%). However, they were not significantly different from each other. Yam cultivar and disinfectants or protectants interaction effects on percentage sprouted but rotten minisetts was not significantly different. Similarly, the mean percentage sprouted but rotten minisetts were not significantly influenced (p>0.5) by the disinfectants.
Mean percentage unsprouted and unrotten yam minisetts are shown in Table
4. The effect of disinfectants and protectants on percentage yam minisetts
were seen in the low percentages of the planted minisetts that were not rottened
nor sprouted. There was no significant difference due to yam cultivar.
||Effect of disinfectants on percentage sprouted but rotten
Pona and Dente yam minisetts
|ns = not significant at 5% probability level
||Effect of disinfectants on percentage unsprouted and unrotten
|ns: Not significantly different at 5% level
There was also no significant difference due to interaction between the yam
cultivars and disinfectants or protectants. Similarly, no significant differences
among the disinfectants regarding their influence on mean percentage unsprouted
and unrotten minisetts. All of the planted minisetts protected with Benlate
sprouted and did not rot. The untreated minisetts (control) recorded relatively
higher mean percentage unsprouted and unrotten minisetts (4.50%).
The observed significantly higher percentage sprouted minisetts and lower percentage rotten minisetts generally recorded could be attributed to the heat sterilized medium used for pre-sprouting the minisetts. Earlier studies revealed that heat sterilization was found to be the most effective method in controlling soil-borne minisett-rot pathogens (Asare-Bediako, 2003). This explains why the untreated minisetts recorded high percentage sprouting though it was the lowest compared to the treated minisetts. The significantly higher percentage rotting recorded for untreated minisetts suggested that, treatment of minisetts was necessary to prevent rotting and improve the percentage sprouting of the minisetts. Osai and Ikotun (1994) reported the importance of the disinfection of minisetts before planting because the minisetts themselves are sources of rot pathogens.
Benlate-treated and lime-treated yam minisetts produced significantly better sprouting. Thus, indicating that both disinfectants had the ability to slow down or prevent minisett rot pathogens thereby improving sprouting of the minisetts irrespective of the yam cultivar. The significantly higher mean percentage rotten minisetts observed in the lime-treated minisetts, than in Benlate-treated minisetts seems to indicate that protecting with Benlate was more effective in protecting yam minisett from rot pathogens than any of the disinfectant used followed by lime and wood ash. Benlate completely inhibited pathogens growth, while lime slowed down the growth rate of rot pathogens. This finding confirmed the earlier report of Martin (1973), that benomyl is a protective and eradicant fungicide with systemic activity and effective against a broad range of fungi. Lime could, however, be a better substitute for Benlate. The efficacy of lime in controlling growth of fungi had been reported by several researchers (Thompson et al., 1977; Ogali et al., 1991; Cornelius, 1998).
The non-significant difference recorded among yam minisetts treated with lime and wood ash for percentage sprouted minisetts and percentage rotten minisetts suggested that lime or wood ash can be used to treat yam minisetts to reduce rot and improve yam sprouting. The result of this study, confirms the earlier report of Adimora (1986) and Otoo et al. (1987); that the incidence of post harvest decay of yam tubers could be reduced by treating all cut and wounded surfaces with alkaline materials such as wood ash or lime in the complete absence of chemicals. Among the treated yam minisetts, those treated with sodium hypochlorite and aqueous neem leaf extract performed poorly with high rotting and low percent sprouting. Thus, sodium hypochlorite was not an important disinfectant to improve yam minisett sprouting ability. The main constraint to the Pona minisetts technology was rapid rotting of the minisetts in the sprouting medium. However, Pona cultivar had higher response to the minisetts technique as compared to Dente cultivar probably because both the minisetts and the sprouting media were disinfected against rot pathogens. This seems to suggest that under aseptic conditions Pona yam minisetts can sprout better than Dente minisetts.
It is concluded that, Benlate, quick lime and wood ash are the best disinfectant in that order in preventing or controlling yam minisett rot pathogens. They also improve yam minisett sprouting ability irrespective of the yam cultivars used.