Dry-dispersal and rain-splash of brown (Puccinia recondita f.sp tritici) and yellow (P-striiformis) rust spores from infected wheat leaves exposed tosimulated raindrops

The dispersal of spores from lesions of brown (Puccinia recondita f.sp. tri tici) or yellow (P. striiformis) rusts of wheat by impacting drops was stud ied. Using a generator of uniform-size drops, drops of 2.5, 3.4, 4.2 and 4. 9 mm in diameter were released from rest at heights of 5, 50 and 100 cm abo ve horizontal and primary leaves uniformly covered with sporulating lesions . Dry-dispersal and rain-splash occurred simultaneously in response to drop impaction. A coloration technique allowed separate counting of dry-dispers ed and rain-splashed spores caught on slides. More spores were rain-splashe d than dry-dispersed. Neither removal mechanism affected in-vitro germinati on of spores, which was higher in brown than in yellow rust. For both rusts , the number of both dry-dispersed and rain-splashed spores, as well as the ir travel distance, increased with drop diameter and fall height. The fall speed of incident drops in relation to diameter and fall height was obtaine d by solving numerically the equation of vertical drop motion. The number o f spores removed by a given impacting drop was found to be a power function of the calculated kinetic energy of the impacting drop. Based on this expe rimental relationship, a simulation study showed the relevance of rain type in the removal of spores.