L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66 61
Fig. 4. Restoration of spore populations on brown rust 4a and b and yellow rust 4c and d lesions after a rain event. Infected source plants were exposed to simulated rain consisting of 4.9 mm diameter drops for 30 min, then incubated under optimal conditions for spore
development for 2 h
d
, 4 h
j
or 6 h
m
, before being exposed to simulated raindrops again for another 30 min. Numbers of lesions or chloroses produced on trap plants at all distances observed from the source before 4a and c and after 4b and d the incubation periods
are presented.
source plants, K
t
, was estimated for each drop size and the three exposure periods 5, 10 and 15 min, see Sec-
tion 2.1.3. The relationships between the total num- ber of brown rust lesions or yellow rust chloroses, Y,
and total kinetic energy K
t
of the rainfall is plotted in Fig. 6. For both pathogens, Y was almost proportional
to the total kinetic energy; for brown rust, Y = 0.70 K
t 1.09
r
2
= 0.8 and for yellow rust, Y = 0.24 K
t 1.15
r
2
= 0.8. These relationships mask the decrease in
the rate of spore dispersal with longer rain exposures. However, they suggest that for short durations of rain
the potential for dispersal of rust spores may be re- lated to the total kinetic energy of incoming splash,
assuming all rain droplets can contribute to splash.
The kinetic energy of drops falling at their termi- nal velocity is proportional to their diameter raised to
the power 4.34 Eq. 2. As the size distribution of raindrops depends on the type of rain Ulbrich, 1983,
the potential for disease spread may be influenced by the type of rain, as well as its duration. The relation-
ship for ND given by Ulbrich 1983 see Huber et al., 1998 and the above relationships between Y
and K
t
were used to evaluate the number of lesions or chloroses potentially produced by raindrops of a given
diameter D Fig. 7b for a 5 min duration of rain of a given type thunderstorm, shower and widespread
rain of intensity of 10 mm h
− 1
Fig. 7a.
4. Discussion
The results clearly show that spores of P. recon- dita and P. striiformis may be removed and dispersed
by the action of falling water drops alone. There- fore, rain-splash could be an effective mechanism for
short-range dispersal of both rusts. Dispersal gradients in still air were steep and the dispersal half-distances
were similar to those for spores of pathogens which are characteristically splash-dispersed Fitt et al.,
1989. Although most brown rust spores apparently travelled less than 80 cm in the experiments in still air,
in the wind tunnel experiments appreciable numbers of spores were trapped 2 m from the source plants.
This suggests that these spores were either ‘dry’ or
62 L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66
Fig. 5. a Change in the flux of brown Puccinia recondita rust spores measured 2 m downwind of infected source plants with increasing wind speed. Wind speed was increased in seven successive steps of 5 min each. b Change in flux of brown rust spore measured 2 m
downwind of infected source plants with duration of ‘rain’ and wind speeds of 1 m s
− 1
open squares, dotted lines or 4 m s
− 1
filled squares, solid lines. The rotating arm spore traps were changed every 5 min. Means and standard errors of three replicates are presented.
associated with very small splash drops. It is possible that the water drops removed some of the spores by
dislodging them on impact. Dry dispersed spores of Passalora personata late leaf spot of groundnut have
been shown to be removed from lesions by the action of rain splash Wadia et al., 1997, and Savary and
Janeau 1986 concluded that air-borne dispersal of Puccinia arachidis groundnut rust spores was asso-
ciated with rain. Furthermore wind has been shown to enhance dispersal of characteristically rain-splashed
pathogens such as Colletotrichum acutatum Yang et al., 1990 and Ascochyta fabae f. sp. lentis Peder-
sen et al., 1994. Although no wind tunnel experiments were done with plants infected with P. striiformis, it
is likely that this fungus would have exhibited similar behaviour to P. recondita.
In the wind tunnel experiments, spores of P. recon- dita were removed by wind speeds as low as 0.5 m s
− 1
.
L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66 63
Fig. 6. The relationship between the total kinetic energy of incident drops, K
t
Ln transformed and the numbers Ln transformed of brown rust lesions or yellow rust chloroses subsequently produced
on trap plants. Data for rain durations of 5, 10 and 15 min are combined. The regression line equations are given in the text.
Srivastava et al. 1987 also found that these spores could be released at this wind speed. However, in ex-
periments using a miniaturised wind-tunnel, Geagea et al. 1997 found that no spores were removed by
winds of less than 0.7 m s
− 1
, although they were ap- plied for a much shorter time 10 s compared to 5 min.
There was no apparent threshold velocity needed to remove the spores, in contrast to spores of some other
wind-dispersed fungi such as Helminthosporium may- dis Aylor, 1975 and Erysiphe graminis Hammett
and Manners, 1974. Geagea et al. 1997 found that the retention force of brown and yellow rust of spores
is lognormally distributed, which could explain why spores were released over a range of wind speeds. It
may be noted that wind speed U affects the horizontal flux density Q =UC of spores not only directly lin-
early but also indirectly by altering the source strength and therefore the concentration C.
The decline in the rate of spore dispersal with time Fig. 5 for both rusts suggests that the source of spores
may quickly become depleted when exposed to rela- tively short periods of ‘heavy’ rain. Thus the decline
in source ‘strength’ with rain duration implies that short showers are likely to be particularly effective in
spreading brown rust and yellow rust as they are for barley leaf blotch Fitt et al., 1986a. Exhaustion of
spore sources with time has been observed by other workers Madden et al., 1992.
The time needed to restore the spore source after spore removal by rain was different for brown rust and
yellow rust. For brown rust, spore populations recov- ered in 2–4 h under optimal conditions, but for yel-
low rust spore numbers had not fully recovered by 6 h after depletion. Rapilly et al. 1970 found that 5 h
was necessary to restore spore numbers in yellow rust chloroses. In the experiment with brown rust, after
6 h in the growth room, spore availability indicated by the number of lesions which developed on the trap
plants was about double than that before depletion. This increase may have occurred because 6 h under
favourable conditions would have induced new spores and also allowed spores located at the bottom of the
sorus to mature and detach their pedicels and so be easily removed. Thus a series of short showers sep-
arated by a few hours may encourage the spread of brown rust, but not yellow rust.
Using the raindrop size distribution and the ex- perimental relationships between kinetic energy of
drops and disease severity on trap plants, the effect of rain type was analysed by simulation. For the size
range 1–1.5 mm, raindrop concentration was highest for widespread rain and smallest for a thunderstorm.
For this diameter category thunderstorms are slightly more effective in spreading disease than the two other
types. In the size range 1.5–2.0 mm, thunderstorms have higher drop concentration than widespread rain
or showers Fig. 7a, leading to higher potential dis- ease Fig. 7b. There is a significant difference be-
tween rusts due to differences in removal efficiency. In the size range 2.0–3.2 mm, thunderstorms have
more incident drops and consequently much higher potential for disease dispersal by rain-splash.
64 L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66
Fig. 7. Simulated variation in potential disease severity as a function of drop diameter for three types of rain thunderstorm, shower and widespread rain: a distribution of drop sizes and number of lesions or chloroses produced on trap plants by a single drop of diameter
D, b simulated number of lesions or chloroses potentially produced by raindrops of diameter D. Rain duration and rain intensity were assumed to be 5 min and 10 mm h
− 1
, respectively. Brown rust lesions are shown as thin lines, and yellow rust chloroses as thick lines.
L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66 65
The experimental results suggest that large drops dispersed more spores numbers of lesions and
chloroses found on trap plants were greater than small drops and that these spores travelled greater
distances. Other studies have also shown that large drops are more effective in dispersing splash-borne
inoculum Huber et al., 1996. Thus rain drop size distribution may play an important role in potential
dispersal of these two pathogens as well as its ki- netic energy. Simulations, based on the relationship
between rain kinetic energy and disease symptom development show a compensation effect between
number of drops of a given diameter and their kinetic energy. Small drops are very numerous but they have
reduced kinetic energy and they do not contribute to disease spread at large scales. Large drops are rare in
a natural rainfall but they have great kinetic energy and thus lead to more spore dispersal Fig. 7a and b.
5. Conclusions
