LAUSR

Abstract Biological control of root diseases of red clover (Trifolium pratense L.) could be effective but optimization of its application is needed to balance treatment efficacy with costs. Our objectives were to evaluate the effects of different Pythium oligandrum application treatments on root morphological development, plant root disease score and forage yield in a three-year field experiment. Red clover cv. Start was managed under a three-cut management in two harvest years with four treatments: (i) untreated control, (ii) multiple applications of P. oligandrum (intensive treatment), (iii) a single spring application, and (iv) a single autumn application. Plants from each plot were evaluated for root morphology traits and scored for root diseases in each autumn. Forage yield, compressed stand height and stem density were evaluated in each cut. All P. oligandrum treatments increased root dry matter and root potential index, but only the intensive treatment significantly improved forage yield (by 31%) in comparison with the control. This effect could be largely associated with plant growth stimulation resulting in increased stand height before cutting. Among single application treatments, autumn timing significantly reduced plant root disease score and showed higher stand height than spring application. One application later in the season seems likely to be effective for forage growth stimulation, but probably at least two applications are needed to produce maximum effect. Despite some drought stress during the first two years, this field study confirmed an effect of P. oligandrum on improved root development and a positive yield response, thus showing a high potential of this biological control treatment under field conditions.