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 sp read in space of powdery mildew (caused by Erysiphe pisi) in pea (Pisu m sativum) cultivars differing in resistance to the disease. Disease s everity (proportion of leaf area infected) was measured in 19 x 23 m p lots of cultivars Pania and Bolero (both susceptible) and Quantum (qua ntitatively resistant). Inoculum on infected plants was introduced int o the centre of each plot. Leaves (nodes) were divided into three grou ps 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 f inal assessment 35 days after inoculation. Exponential disease progres s curves were fitted to disease severity data from leaves at middle no des. 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 di sease progress on leaves at lower nodes. Disease progress on Quantum w as delayed compared with Pania and Bolero. The average daily rates of increase in disease severity from Gompertz curves did not differ betwe en the cultivars on these leaves. Disease gradients in the plots from the inoculum focus to 12 m were detected at early stages of the epidem ic, but the effects of background inoculum inputs and the rate of dise ase progress meant that these gradients decreased with time as the dis ease epidemic intensified. Spread was rapid, and there were no statist ically significant differences between cultivar isopathic rates (Pania 2.2, Quantum 2.9 and Bolero 4.0 m d(-1)).