The effects of pre-inoculation drought stress on the components of partial resistance of rice (Oryza sativa L.) to leaf blast (Magnaporthe grisea (Herbert) Barr)

last, caused by Magnaporthe grisea Herbert (Barr), is an important disease on Oryza sativa L. especially in upland rice culture where drought can alter the susceptibility of the plants to infection.;Pre-inoculation drought stress increased the relative infection efficiency of both susceptible- and resistant-type lesions. Ranking of cultivar partial resistance to blast infection was altered by drought stress. There was a stress threshold above which there was a dramatic increase in the relative infection efficiency. The age of the oldest leaf segment with at least one susceptible lesion was increased by drought stress. The distribution of lesions over different aged segments varied with lesion type. The maximum number of susceptible lesions occurred on 1-2 day younger leaf segments than resistant lesions. Susceptible-type lesion number decreased rapidly whereas resistant-type lesion number decreased slowly with increasing leaf segment age. The rate at which susceptible, but not resistant, lesion numbers decreased with leaf segment age depended on drought stress. The effect of drought stress was observed at all stages of vegetative development, although relative infection efficiency decreased with plant age.;In a simulation study, the effect of prolonging the susceptibility duration of a plant part was effective in increasing the rate of disease progress. The effect was more important for simulated pathosystems in which the host had a short initial susceptibility duration. The effect of increasing susceptibility duration was similar regardless of plant growth rates. The effect of increasing the susceptibility duration was as important as an equivalent proportional increase in the infection efficiency or maximum lesion size or decrease in the latent period. At low susceptibility durations, the effect of changes in susceptility duration on disease progress varied with plant-age resistance.;The effect of drought stress was mediated through an increase in both the asymptotic number of infections and the speed at which this asymptote was attained with increasing leaf wetness duration. This effect of stress on increasing the speed of infection was more important on older than younger leaves. Pre-innoculation drought stress had little or no effect on other epidemiological parameters. Latent period was significantly shorter on stressed plants than unstressed plants but the mean difference did not exceed 5 hours. There was no effect of pre-inoculation drought stress on either lesion size or sporulation.;Foilar infections reduced plant growth and affected root physiology as determined by reductions in both proton extrusion and oxidation of...