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.

生物防治红三叶草（Trifolium pratense L.）的根病可能是有效的，但需要优化其应用以平衡治疗效果和成本。我们的目标是在为期三年的田间试验中评估不同的Pythium oligandrum施用处理对根形态发育、植物根病评分和牧草产量的影响。红三叶草 开始在两个收获年的三割管理下进行管理，有四种处理：(i) 未处理的对照，(ii) 多次施用P. oligandrum（强化治疗），（iii）春季施用一次，以及（iv）秋季施用一次。评估每个地块的植物根系形态特征，并在每个秋季对根系病害进行评分。在每次切割中评估草料产量、压缩林分高度和茎密度。所有P. oligandrum处理增加了根干物质和根势指数，但与对照相比，只有强化处理显着提高了牧草产量（31%）。这种影响可能在很大程度上与植物生长刺激有关，导致切割前的林分高度增加。在单次施用处理中，秋季时间显着降低了植物根病评分，并且显示出比春季施用更高的立高。在本季晚些时候施用一次似乎对草料生长刺激有效，但可能至少需要施用两次才能产生最大效果。尽管前两年有一些干旱压力，但这项实地研究证实了P. oligandrum 改善根系发育和积极的产量反应，从而显示出这种生物防治处理在田间条件下的巨大潜力。