Sometimes plant pathologists assess disease intensity when they are primarily interested in other response variables, such as yield loss or toxin concentration in harvested products. In these situations, disease intensity potentially could be considered a surrogate of yield or toxin. A surrogate is a variable which can be used instead of the variable of interest in the evaluation of experimental treatments or in making predictions. Surrogates can be measured earlier, more conveniently, or more cheaply than the variable of primary interest, but the reliability or validity of the surrogate must be shown. We demonstrate ways of quantifying two facets of surrogacy by using a protocol originally developed by Buyse and colleagues for medical research. Coefficient-of-determination type statistics can be used to conveniently assess the strength of surrogacy on a unitless scale. As a case study, we evaluated whether field severity of Fusarium head blight (i.e., FHB index) can be used as a surrogate for yield loss and deoxynivalenol (DON) toxin concentration in harvested wheat grain. Bivariate mixed models and corresponding approximations were fitted to data from 82 uniform fungicide trials conducted from 2008 to 2013. Individual-level surrogacy—for predicting the variable of interest (yield or DON) from the surrogate (index) in plots with the same treatment—was very low. Trial-level surrogacy—for predicting the effect of treatment (e.g., mean difference) for the variable of interest based on the effect of the treatment on the surrogate (index)—was moderate for yield, and only low for DON. Challenges in using disease severity as a surrogate for yield and toxin are discussed.

有时，植物病理学家主要对其他响应变量（例如收获产品中的产量损失或毒素浓度）感兴趣时，会评估疾病的强度。在这些情况下，疾病强度可能被认为是产量或毒素的替代物。替代变量是可用来代替目标变量的实验评估方法或预测方法。替代指标可以比主要关注变量更早，更方便或更便宜地进行测量，但必须显示替代指标的可靠性或有效性。我们演示了如何使用Buyse及其同事最初为医学研究开发的协议来量化代孕的两个方面。确定系数类型统计信息可用于方便地以无单位规模评估代孕的强度。作为案例研究，我们评估了镰刀菌枯萎病的田间严重程度（即FHB指数）是否可以替代收成小麦籽粒中的产量损失和脱氧雪腐烯（DON）毒素浓度。对2008年至2013年进行的82项均匀杀菌剂试验的数据进行了双变量混合模型和相应的近似拟合。个人水平的代孕-用于从采用相同处理的地块中的代孕指标（指数）预测目标变量（产量或DON）-非常低。试验级代孕（基于治疗对替代变量（指数）的作用来预测目标变量的治疗效果（例如，均值差异））对产率中等，对DON较低。