Temporal dynamics of Phytophthora blight on bell pepper in relation to themechanisms of dispersal of primary inoculum of Phytophthora capsici in soil

The effect of components of primary inoculum dispersal in soil on the tempo ral dynamics of Phytophthora blight epidemics in bell pepper was evaluated in field and growth-chamber experiments. Phytophthora capsici may potential ly be dispersed by one of several mechanisms in the soil, including inoculu m movement to roots, root growth to inoculum, and root-to-root spread. Indi vidual components of primary inoculum dispersal were manipulated in field p lots by introducing (i) sporangia and mycelia directly in soil so that all three mechanisms of dispersal were possible, (ii) a plant with sporulating lesions on the soil surface in a plastic polyvinyl chloride (PVC) tube so i noculum movement to roots was possible. (iii) a wax-encased peat pot contai ning sporangia and mycelia in soil so root growth to inoculum was possible, (iv) a wax-encased peat pot containing infected roots in soil so root-to-r oot spread was possible, (v) noninfested V8 vermiculite media into soil dir ectly as a control, or (vi) wax-encased noninfested soil as a control. In 1 995 and 1996, final incidence of disease was highest in plots where sporang ia and mycelia were buried directly in soil and all mechanisms of dispersal were operative (60 and 32%) and where infected plants were placed in PVC t ubes on the soil surface and inoculum movement to roots occurred with rainf all (89 and 23%). Disease onset was delayed in 1995 and 1996, and final inc idence was lower in plants in plots where wax-encased sporangia (6 and 22%) or wax-encased infected roots (22%) were buried in soil and root growth to inoculum or root-to-root spread occurred. Incidence of root infections was higher over time in plots where inoculum moved to roots or all mechanisms of dispersal were possible. In growth-chamber studies, ultimately all plant s became diseased regardless of the dispersal mechanism of primary inoculum , but disease onset was delayed when plant roots had to grow through a wax layer to Inoculum or infected roots in tension funnels that contained small volumes of soil. Our data from both field and growth-chamber studies demon strate that the mechanism of dispersal of the primary inoculum in soil can have large effects on the temporal dynamics of disease.