RAIN AND GROWTH OF PSEUDOMONAS SYRINGAE

Authors: HIRANO, SUSAN - UNIVERSITY OF WISCONSIN; BAKER, L STUART - UNIVERSITY OF WISCONSIN; Upper, Christen

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/5/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Experiments conducted in the field assessed the role of the weather in causing large increases in population sizes of P. syringae pv. syringae, the cause of bacterial brown spot disease of bean. Intense rainfall events were found to precede periods during which bacterial population sizes on bean leaves increased by 10- to 1,000-fold. The microclimate around bean plants in the field was modified by constructing portable polyethylene shelters to shield plants from rain and fine-meshed inert screens to modify the momentum of raindrops before they hit the bean leaves. Following each of three separate intense rains, large increases in population sizes of P. syringae occurred on plants exposed to the rains but not on plants in the shelters or under the screens. These experiments clearly demonstrate that rain triggers the onset of large increases in population sizes of P. syringae. The window screens decreased the velocity of the raindrops but not the volume or quality of rainwater that fell through the screens onto the plants. Only the kinetic energy or momentum of the water striking the plants under the screens was altered. Thus, the absence of increases in population sizes of P. syringae on plants under the screens suggests that raindrop momentum plays a role in the growth-triggering effect of intense rains on populations of P. syringae on bean plants under field conditions.

Technical Abstract: Field experiments were conducted to determine the role of the physical environment in affecting large increases in phyllosphere population sizes of P. syringae pv. syringae, the cause of bacterial brown spot disease of bean (Phaseolus vulgaris L.). Comparisons of daily changes in population sizes of P. syringae on four plantings of snap bean with weather records identified a pattern in which rainfall preceded periods during which bacterial population sizes increased by 10- to 1,000-fold. The microclimate of bean plants in the field was modified with polyethylene shelters to shield plants from rain or with inert screens to modify the momentum of raindrops. After each of three intense rain events, increases in P. syringae population sizes greater than 10-fold occurred on plants exposed to the rains but not on plants in the shelters or under the screens. Sheltering plants from natural rains demonstrated clearly that rain triggers the onset of large increases in population sizes of P. syringae. The screens decreased the velocity, and thus, momentum of raindrops that fell on plants beneath them without affecting the volume or quality of rainwater. Thus, the absence of increases in population sizes of P. syringae on plants under the screens suggests that raindrop momentum plays a role in the growth-triggering effect of intense rains on populations of P. syringae on bean.