Effects of fluquinconazole and silthiofam, applied as seed treatments to single or consecutive crops of wheat, on take-all epidemic development and grain yields

Seed treatments containing fluquinconazole, silthiofam or a standard fungicide mixture with no activity against take-all were compared in all combinations of sequences in successive second and third winter wheat crops in five field experiments and second to fourth crops in a sixth experiment. Compared with the standard treatment, silthiofam decreased take-all more effectively than fluquinconazole when crops were sampled at tillering. In samples taken in summer, during grain filling, silthiofam often decreased the incidence of take-all (percentage of plants with root symptoms) more than fluquinconazole, but fluquinconazole more effectively decreased the incidence of severe take-all (percentage of plants with more than 75% of their root systems blackened). It is suggested that these differences are a consequence of more effective control of primary infection of roots by silthiofam and of secondary, root-to-root, infection by fluquinconazole. Silthiofam usually increased yield more than did fluquinconazole, perhaps as a consequence of better early protection during tiller and/or spikelet formation. Treatment with either of the fungicides affected epidemic development in the treated crop and in crops grown subsequently. In particular, decreased take-all had the effect of delaying the year-to-year epidemic, so that nontreatment of a subsequent crop resulted in an upsurge in disease. Treatment with either take-all fungicide of a crop grown after a treated crop was relatively effective if the epidemic in the comparable nontreated crop sequence was continuing to increase. It was, however, detrimental if the disease was approaching its peak in the first treated crop, particularly if a treated (fourth wheat) crop was being compared with a similar crop in a nontreated sequence in which take-all decline had developed. These results provide a basis for recommendations for the use of seed treatment fungicides in sequences of wheat crops.