Evaluation of the release and uptake of the fungicide tebuconazole from a novel microencapsulated formulation

Triazole fungicides, such as tebuconazole, have been pivotal in the control of both foliar and seed/soil-borne diseases of many crops, including cereals, since their introduction in the 1970s. Although routinely applied as a foliar spray, tebuconazole is also used as a seed treatment. The systemic nature of the fungicide may confer protection of plants from disease during the early stages of development. However, application rates are limited to between 1 and 7 g active ingredient (AI) per 100 kg of seed due to seed application causing reduced germination and stunting during early plant development. A novel technique whereby fungicides, including tebuconazole, are encapsulated using yeast cells as pre-formed microcapsules may provide a solution to the problem of phytotoxicity and allow seed treatments to be applied at higher rates, resulting in a reduction in total fungicide application required during the growing season. Several batches of microencapsulated tebuconazole were produced using yeast from different sources. These where tested for their potential for uptake, retention and subsequent delivery of the fungicide. The release of tebuconazole from the different yeast batches into water was assessed over a time course using liquid-liquid extraction (LLE) and GCMS. Results showed no differences in the release characteristics of tebuconazole observed between batches. Further to this the age of the batches tested was found to have no effect on release of tebuconazole into water, suggesting shelf-life would not be a problem with this technology. Repeated washing of a sample of microencapsulated tebuconazole with water suggested that the novel formulation could provide the gradual delivery of tebuconazole. An investigation of the phytotoxic effect of tebuconazole, applied as seed treatments at higher than recommended rates (40 g AI per 100 kg seed), showed the microencapsulated formulation had a significantly reduced impact on germination and early plant development, when compared to two commercial formulations of the fungicide, Mystique and Raxil. Further analysis, of the amount of tebuconazole accumulated by plants grown from treated seed, using solid phase extraction (SPE and GC-MS), showed there to be initially less fungicide in plants treated with the microencapsulated formulation. As plant development continued, the amount of tebuconazole recovered from plants treated with both the microencapsulated and conventional formulations became similar. Variation in the results, between replicate samples and also between growth stages led to a series of experiments investigating the possible effect of the growth environment and seed sowing density on tebuconazole uptake. However, no significant effects of these factors were observed. A comparison of the uptake of seed-applied microencapsulated and conventional formulations of tebuconazole between 2 and 19 weeks after sowing, showed there to be no significant differences between formulations used in the concentration of the fungicide accumulated in plants. However, a pronounced increase in plant accumulation of the fungicide was consistently observed between 17 and 19 weeks. It was suggested that this might be associated with roots becoming densely packed within the compost, encouraging fungicide uptake from the compost in the localised dressing zone around the seed. It was shown that tebuconazole became readily bound to the loam-based compost used during the research. Experiments conducted in 1m columns, containing tebuconazole applied to specific compost layers within the columns, confirmed the ability of wheat roots to recover the fungicide from the compost when they made contact with the specific layers.