Agricultural and Forest Meteorology 101 2000 53–66
Influence of simulated rain on dispersal of rust spores from infected wheat seedlings
L. Geagea
a,b
, L. Huber
a,∗
, I. Sache
b
, D. Flura
a
, H.A. McCartney
c
, B.D.L. Fitt
c
a
Institut National de la Recherche Agronomique, Unité de recherche en Bioclimatologie, BP 01, 78850 Thiverval-Grignon, France
b
Institut National de la Recherche Agronomique, Laboratoire de Pathologie Végétale, BP 01, 78850 Thiverval-Grignon, France
c
IACR-Rothamsted, Harpenden, AL5 2JQ, UK Received 15 March 1999; received in revised form 29 October 1999; accepted 6 November 1999
Abstract
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. ©2000 Elsevier Science B.V. All rights reserved.
Keywords: Rain; Simulation; Wheat; Rust; Spore; Dispersal
1. Introduction
Rain-splash is an important mechanism for disper- sal of spores of plant pathogens Fitt et al., 1989.
When a raindrop strikes an infected plant, splash droplets can entrain and disperse spores and thus can
introduce inoculum into areas where the pathogen
∗
Corresponding author. Tel. +33-1-30-81-5555; fax: +33-1-30-81-5563.
E-mail address: secretarialbcgn.grignon.inra.fr L. Huber.
is not present Fitt and McCartney, 1986. The use of rain towers has provided a tool for understand-
ing the dispersal of plant pathogenic fungi with spores which are usually splash-dispersed Fitt et
al., 1986b; Madden et al., 1996. Although it has been shown that typically wind-dispersed spores e.g.
those of Botrytis cinerea, Madden, 1992 can be re- moved from host tissues by impact of single incident
drops, rain simulators have not been used to study the role of splash in dispersal of wind-dispersed
spores.
0168-192300 – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 9 9 0 0 1 5 5 - 0
54 L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66
Spores of Puccinia recondita f. sp. tritici and P. striiformis brown and yellow rust of wheat are known
to be dispersed by wind Nagarajan and Singh, 1990; Hau and de Vallavieille-Pope, 1998, but they have
been shown to be released from host tissue by the impact of water drops, and subsequently dry-dispersed
Rapilly, 1979, and, in wheat crops, the dispersal of these spores has been linked to occurrence of rain
Rowell and Romig, 1966; Park, 1990. Geagea et al. 1999 have shown that spores of P. recondita and
P. striiformis can be entrained by rain-splash droplets and dispersed with them, and that dry-dispersal and
rain-splash dispersal occurred and were not mutually exclusive.
The potential for rain splash to disperse plant pathogen spores depends largely on the characteris-
tics of natural rain Reynolds et al., 1987; Fitt et al., 1989, since the size and velocity of individual inci-
dent raindrops and rain duration affect the splash dis- persal process Yang et al., 1991; Ntahimpera et al.,
1997. However, little is known about the influence of rain drop characteristics on the removal and dispersal
of spores which are usually wind dispersed.
This paper reports the effects of rain on dispersal of P. recondita and P. striiformis spores. The main aim
was to study the influence on dispersal of rain param- eters such as drop diameter and rain duration. The ex-
periments were done using a rain simulator and dis- persal was measured by assessing disease severity on
wheat ‘trap’ plants. Rain simulation experiments were also done in the presence of wind to evaluate the ef-
fect of a horizontal wind flow on rain-splash disper- sal. Knowledge of physical properties of rain, such as
drop size distribution and total kinetic energy of rain were used in an attempt to quantify the influence of
natural rain on spore dispersal from infected plants to healthy trap plants.
2. Materials and methods
