Coupling disease-progress-curve and time-of-infection functions for predicting yield loss of crops

A general approach was developed to predict the yield loss of crops in rela tion to infection by systemic diseases. The approach was based on two premi ses: (i) disease incidence in a population of plants over time can be descr ibed by a nonlinear disease progress model, such as the logistic or monomol ecular; and (ii) yield of a plant is a function of time of infection (t) th at can be represented by the (negative) exponential or similar model (zeta( t)). Yield loss of a population of plants on a proportional scale (L) can b e written as the product of the proportion of the plant population newly in fected during a very short time interval (X'(t)dr) and zeta(t), integrated over the time duration of the epidemic. L in the model can be expressed in relation to directly interpretable parameters: maximum per-plant yield loss (alpha, typically occurring at t = 0); the decline in per-plant loss as ti me of infection is delayed (gamma; units of time(-1)); and the parameters t hat characterize disease progress over rime, namely, initial disease incide nce (X-0), rate of disease increase (r; units of time(-1)), and maximum (or asymptotic) value of disease incidence (K). Based on the model formulation , L ranges from alpha X-0 to alpha K and increases with increasing X-0, r, K, alpha, and gamma(-1). The exact effects of these parameters on L were de termined with numerical solutions of the model. The model was expanded to p redict L when there was spatial heterogeneity in disease incidence among si tes within a field and when maximum per-plant yield loss occurred at a time other than the beginning of the epidemic (t > 0). However, the latter two situations had a major impact on L only at high values of r. The modeling a pproach was demonstrated by analyzing data on soybean yield loss in relatio n to infection by Soybean mosaic virus, a member of the genus Potyvirus, Ba sed on model solutions, strategies to reduce or minimize yield losses from a given disease can be evaluated.