DITHANEM-45 RESIDUES IN TOMATOES
ON UGANDAN MARKETS MAY BE ABOVE SAFE LEVELS

Kaaya NA ¹

The wide use of dithiocarbamate salts as agricultural fungicides on food crops may be attributed to their activity against a wide range of plant diseases and their low phytotoxicity [1]. The most common dithiocarbamate fungicide used in Uganda is DithaneM-45 (mancozeb). This is used mainly on tomatoes and potatoes to control preharvest fungal diseases especially blight [2].

It is common experience that fruits like tomatoes and watermelons purchased from Ugandan markets have obvious signs of DithaneM-45 contamination. Pesticide residues in agricultural commodities are highly discouraged because they are potential health hazards to consumers. Most countries have set safety levels for almost all agrochemicals and, in fact, consumers are increasingly demanding organically grown produce. For example, effective January 1998, European Community set guidelines on maximum allowable dithiocarbamate fungicide residue limits for maneb, mancozeb and others to be 0.05 mg CS2/kg [1].

In order to achieve recommended residue levels, care must be taken to apply the right dose of the chemical while spraying crops in the field, and also to give ample time recommended for the chemical to degrade before harvesting the produce. These are clearly indicated in the manufacturers' manuals which farmers are encouraged to follow. For DithaneM-45, the dose recommended by manufacturers is 2.5 g/litre, sprayed at 7-10 day's interval. Farmers are also encouraged to avoid contact with the chemical during spraying.

During the studies conducted in the year 2000 to investigate the postharvest handling of fresh horticultural crops in selected markets of Uganda, it was observed that the majority of tomatoes had observable DithaneM-45 residues on their surface [3]. Retailers indicated that they need to observe these residues before purchasing tomatoes from transporters because such fruits store for a longer time. Thus, farmers apply excess levels of DithaneM-45 to fruits in the field, and, others spray the fruits after harvest to ensure that DithaneM-45 residues appear on the surface of the fruit. This scenario, together with recommendations on the levels and use of dithiocarbamates in fresh produce, prompted me to carry out a survey at farm level in Wakiso District. This is one of the districts that grow most of the tomatoes in Uganda. The survey involved twenty commercial tomato growers in two sub-counties, ten from each sub-county. The concentration of DithaneM-45 applied by each farmer during the dry and wet seasons was investigated (Table 1). Information on the frequency of application, safety precautions and whether farmers spray the fruit after harvest was also obtained.

According to the findings, farmers were found not to follow the recommended procedures and safety precautions while applying DithaneM-45. They do not use protective measures while mixing and applying the chemical in the field. Over 50% were found to apply a concentration which was five to six times the recommended dose. They all reported that if they follow the recommended dosage of 2.5 g/1, they experience total crop loss especially during the rainy season. This may imply that fungal pathogens are resistant to DithaneM-45, which could have originated from excessive application of the chemical by farmers.

All farmers commended DithaneM-45 for its role in controlling tomato diseases and good colour development of the fruit, as long as high dosages are applied. Four out of twenty farmers indicated that they spray the fruit with DithaneM-45 after harvesting. There was no consistent time between field application of DithaneM-45 and harvesting such that some farmers may spray in the morning and harvest fruit in the evening.

The frequency of application depends on the season and stage of development of the crop. Six out of twenty (30%) farmers apply the chemical daily while 16 out of 20 (80%) apply every 2 days. The majority of farmers (80%) apply once a week during the dry season. Farmers also apply the chemical more after fruit set and less after 2-3 harvests.

This article concludes that the excess doses used by farmers, the frequency of application, the inconsistency between application and harvesting, and the spraying of the fruit after harvest are likely to increase DithaneM-45 residues in the harvested tomato fruit. It is therefore recommended that analysis of DithaneM-45 residues in these fruits be done to establish whether the current levels are above or below internationally recommended levels. In addition, there is need for research to establish whether tomato diseases in Uganda are resistant to recommended doses of DithaneM-45 and also a study on the profitability of applying the chemical excessively.

REFERENCES

1. Schwack W and S Nyanzi Second-Derivative UV Spectrometric Microdetermination of Dithiocarbamate Residues as Methyl Xanthate J. AOAC Int. 1995; 78: 458 – 462.

2. Hakiza JJ, Adipala E, Namada S, Turiho-habwe G, Kakuhenzire R, Olanya M and AS Bhagsari Biological and Economic Benefits of Integrating Plant Resistance and Minimum Fungicide Use for Management of Potato Late Blight in South-Western Uganda. IPM CRSP Integrated Pest Management Collaborative Research Support Program 9th Annual Report, 2002: 181 – 186.

3. Mathooko FM, Kiiyukia C, Koaze H, Kyamuhangire W, Kaaya AN and J Kikafunda Surveillance on Postharvest Handling of Fresh Horticultural Crops: Part IV, A Case Study of Selected Retail Markets in Three Regions in Uganda. JICA Report, 2000.

¹Department of Food Science and Technology, Makerere University, P.O. Box 7062, Kampala.