Discussion

Many recent studies have shown the importance of experimental investigations of traditional seed storage methods. Given the traditional application of ash to protect grains from pests during storage [16]; lately, the importance of concentration used has been of some interest. Golob recommended quantities at least above 5% by weight should be applied if ash is used as a protectant [22]. However, quantities in excess of 20% or more are likely to result in greater protection [22]. To the best of our knowledge, effects of ash on L. siceraria, T. cucumerina and C. maxima have never been assessed previously. The results of this study showed that ash is really a powerful biological substance enhancing the seed viability of these species during storage. As expected, there was a general increase in percentage of seeds survived with increase in ash concentration for all three species tested (Figure 1).

In general, we found 5% concentration of ash was statistically less effective in seed storage compared to 10% and 20% concentration (P < 0.05) for L. siceraria and T. cucumerina species after 12 months of storage. Seeds of these two species at lower ash concentration failed to germinate adequately suggesting that seed losses are intensified at lower ash concentration in some species. This result clearly supports the early recommendations by Golob [22]. In addition, this finding is also consistent with some of the early empirical investigations in cowpea and maize [23,24], where 20% concentration of ash was reported to result in better storage. Experiments reported in Tanzania and Malawi revealed higher dose of ash is essential for maize storage against Sitophilus [24,25]. Similarly, Wolfson, et al. found that a minimum ratio of 3 parts of ash to 4 parts of cowpea seeds prevented population growth of Callosobruchus maculates (Fabricius) (Coleoptera: Bruchidae) [19], but addition of 3 cm layer of ash above the stored cowpea seeds prevented infestation by adults. In a previous survey of the same region, Karthikeyan, et al. found that Sorghum bicolor seeds are stored with ash at the ratio of 4:1 in airtight jute gunny bags to prevent the seed loss by rice weevil (Sitophilus oryzae) rodents (Tatera indica) and mite (Oligonychus indicus) for a period of 6 months [26]. It is however interesting to note that nearly 70% of C. maxima seeds stored with 10% ash concentration germinated after 12 month storage (Figure 1c).

Our main hypothesis included whether or not ash can be used to store seeds of L. siceraria, T. cucumerina and C. maxima under the storage conditions farmers’ use. In order to stimulate the traditional storage conditions, bags with seeds were undisturbed during storage. This process hindered us from specifically identifying the types of insects and fungus that lead to seed deterioration during storage. However, we found insects like Dacus ciliates, Rhyzopertha dominica F., Tribolium castaneum Herbst. were main contributors for seed damage, as they were still present in controls at the end of storage. Insecticidal properties of ash controlling these insects need specific experimental analysis. A casual literature survey shows ash obtained from wood can potentially control Bruchidae Aspergillus flavus [19,23], A. niger, A. fumigatus, Rhizopus [20], Callosobruchus maculates [19,27]. In a more detailed study, Hakbijl reported the use of ash as an insecticide against beetles Sitophilus granaries (Linnaeus) [16], Cryptolestes ferrugineus (Stephens), Tribolium castaneum (Herbst), larvae of Tenebrio molitor (Linnaeus) and Calliphora vomitoria (Linnaeus). Misra reported the use of cow dung ash completely inhibited the oviposition of Callosobruchus chinensis in stored black gram seeds [28].

In the present study area (and in many tropics), neem woods can be easily obtained and dried in the sun. Since many farmers rely on cattle for additional income, dung can also be collected any time. The easy availability of resources makes the technique more readily practical and cheap (at almost no cost) compared to the other pesticides and storage techniques, which are expensive and sometimes unavailable. However, whether or not ash produced by other plant materials maintains viability of these seeds remain unknown and further works are called to answer this question. The two-year storage experiments (data not shown) showing less than 10% seed germination in all three species provides evidence to show that ash can serve as a protective agent for only one year. One might assume ash added to seeds that had been sun dried might induce a drying effect thereby leading to viability. However, this assumption does not explain the viability loss during the first year (control) and/or second year, as if the dryness of the seeds was the reason for viability a longer storage time might be expected. Thus, we conclude ash has some (yet unidentified) properties that contributed to successfully seed storage in these species for 1 year.

While the experiments reported in the present and previous studies reveal that ash treated seeds can be kept healthy and free from insects, fungi etc. for at least 12 months (Figure 1), precise rationale behind the mechanism of action is still obscure. It has been suggested that dust particles such as ash blocks the air spaces between grains thereby inhibiting the insect movement during storage [22]. In addition, the cessation of insect movement also results in less likelihood of oviposition directly onto the seeds [23]. Insects that attack seeds coated or submerged in ash are possibly killed as a result of desiccation of water damaging the cuticle [22]. Besides insects, spores of storage fungi may also damage the seeds, because these storage fungi can grow in highly complex environments such as low moisture content and high osmotic pressure that inhibit the growth of normal fungi. It is probable that the ash maintains the seeds in a lowest possible dry state, a property that could prevent the growth of storage fungi [20,22]. In contrast, storing seeds without ash equilibrates to the environmental RH (values are often high in Tropics) at a favorable temperature all-round the year resulting in higher insect multiplication rate.

Alternative traditional storage methods for L. siceraria, C. maxima and T. cucumerina are not well established, but Karthikeyan, et al. in their survey reported L. siceraria seeds embedded in cow dung are stored for up to one year [26]. In our ongoing survey, we also found some farmers keep fruits in storage and collect seeds before sowing. The efficacy of this method needs more study, but there is a chance that these fruits are infected. Being the largest groups in the vegetable kingdom, Cucurbitaceae crops are more commonly cultivated in Asian countries. However, the high cost required for storing seeds using insecticides becomes frustrating. Our findings indicate, if used at right amount, ash can have a great potential in storing seeds of L. siceraria, T. cucumerina and C. maxima seeds for up to 12 months. We believe the method described here could have a significant application in minimizing the post-harvest losses of these species and supply high quality of seeds for next sowing season. More importantly, the low cost and less health hazards of ash will encourage its wider application especially in tropics.