Influence of simulated rain on dispersal of rust spores from infected wheat seedlings

Spores of both Puccinia recondita f. sp. tritici and P. striiformis (brown rust and yellow rust of wheat) are thought to be primarily dispersed by wind. The results of experiments, using a rain simulator with uniform drop sizes (2.5, 3.4, 4.2 or 4.9 mm), on the effect of rain on dispersal of brown (leaf) rust and yellow (stripe) rust spores are reported. Experiments on both pathogens were done in still air; additional experiments were done on brown rust with simulated wind and rain. Spore dispersal was estimated by trapping spores on wheat plants and assessing the disease symptoms which subsequently developed under optimum conditions. Simulated rainfall of each the four drop sizes tested dispersed spores of both pathogens. In still air spore dispersal patterns were similar to those usually found for pathogens which are characteristically splash-dispersed. Rain exhausted the source of spores in about 20 min for the four drop sizes. When the plants were kept under optimal conditions for sporulation, the source of brown rust spores available for dispersal was restored to its initial numbers in about 2 h after depletion. For yellow rust, spore numbers in the source had not been restored to their original value after 6 h, even under optimal conditions. In the wind tunnel experiments, simulated rain did not inhibit the dispersal of brown rust spores by wind. Large incident drops dispersed more spores of both pathogens than small drops. A simulation study based on the experimental relationships obtained was done. Although these experiments clearly show that rainfall has the potential to spread both brown rust and yellow rust of wheat, the understanding of the exact role of rain dispersal in the epidemiology of both diseases requires further investigation.