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 ru st 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 (le af) 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 trappi ng spores on wheat plants and assessing the disease symptoms which subseque ntly developed under optimum conditions. Simulated rainfall of each the fou r drop sizes tested dispersed spores of both pathogens. In still air spore dispersal patterns were similar to those usuary found for pathogens which a re characteristically splash-dispersed. Rain exhausted the source of spores in about 20 min for the four drop sizes. When the plants were kept under o ptimal conditions for sporulation, the source of brown rust spores availabl e for dispersal was restored to its initial numbers in about 2 h after depl etion. For yellow rust, spore numbers in the source had not been restored t o their original value after 6 h, even under optimal conditions. In the win d tunnel experiments, simulated rain did not inhibit the dispersal of brown rust spores by wind. Large incident drops dispersed more spores of both pa thogens than small drops. A simulation study based on the experimental rela tionships obtained was done. Although these experiments clearly show that r ainfall has the potential to spread both brown rust and yellow rust of whea t, the understanding of the exact role of rain dispersal in the epidemiolog y of both diseases requires further investigation. (C) 2000 Elsevier Scienc e B.V. All rights reserved.