Spatial and temporal spread of powdery mildew (Erysiphe pisi) in peas (Pisum sativum) varying in quantitative resistance

A field experiment was conducted to assess the progress in time and spread in space of powdery mildew (caused by Erysiphe pisi ) in pea ( Pisum sativum ) cultivars differing in resistance to the disease. Disease severity (proportion of leaf area infected) was measured in 19 × 23 m plots of cultivars Pania and Bolero (both susceptible) and Quantum (quantitatively resistant). Inoculum on infected plants was introduced into the centre of each plot. Leaves (nodes) were divided into three groups within the canopy (lower, middle, upper) at each assessment because of the large range in disease severity vertically within the plants. Disease severity on leaves at upper nodes was less than 4% until the final assessment 35 days after inoculation. Exponential disease progress curves were fitted to disease severity data from leaves at middle nodes. The mean disease relative growth rate was greater on Quantum than on Pania or Bolero, but it was delayed, resulting in an overall lower disease severity on Quantum. Gompertz growth curves were fitted to disease progress on leaves at lower nodes. Disease progress on Quantum was delayed compared with Pania and Bolero. The average daily rates of increase in disease severity from Gompertz curves did not differ between the cultivars on these leaves. Disease gradients in the plots from the inoculum focus to 12 m were detected at early stages of the epidemic, but the effects of background inoculum inputs and the rate of disease progress meant that these gradients decreased with time as the disease epidemic intensified. Spread was rapid, and there were no statistically significant differences between cultivar isopathic rates (Pania 2.2, Quantum 2.9 and Bolero 4.0 m d⁻¹).