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
2.1. Experiments with simulated rain in still air 2.1.1. Rain simulator
A rain simulator similar to that used at IACR- Rothamsted Mann et al., 1995 was made at INRA,
Grignon. It consisted of a 1 m square frame contain- ing ten 1 m long plastic tubes 20 mm in diameter
in parallel rows. Hypodermic needles were inserted 11.5 cm apart along the length of each tube, giving
an array of 80 drop sources. This array was supplied by a peristaltic pump linked to a reservoir of sterile
water. Used as a linear source, the simulator pro- duced a uniform distribution of drops over an area
of about 0.2 m
2
. Drop size depended on the gauge of the hypodermic needles; four needle sizes were
used to produce drops of 2.5, 3.4, 4.2 or 4.9 mm in diameter. Drop diameter was estimated from the mea-
sured volume of a known number of drops. The drops were allowed to fall 9 m and should have reached
speeds close to their terminal velocity before hitting source plants Fitt et al., 1986b. Released drops were
protected from air turbulence by vertical thin plastic walls. The intensity of simulated rain was measured
by placing 12 collectors 180 ml capacity in a ran- dom pattern under the rain simulator for 5 min periods
and measuring the volume of water collected.
2.1.2. Measurement of dispersal gradients Spore dispersal gradients Gregory, 1973 resulting
from rain splash were studied by measuring the hori- zontal dispersal of spores from infected source plants
placed under the rain generator Fig. 1. The source plants were exposed to simulated rain consisting of
drops of uniform size. Spore dispersal was assessed by trapping the spores on ‘trap plants’ placed at differ-
ent distances from the source and measuring disease symptom development see further.
The inoculum source consisted of infected wheat seedlings in 22 pots diameter 7 cm, filled with
soil-less compost with added nutrients. Each pot contained up to six wheat seedlings cv. Michigan
Amber which had been grown in a growth room until the unfolding of the second leaf Fig. 1. The source
plants were inoculated in a settling tower, using a density of 540 ± 60 spores cm
− 2
of either P. recondita f. sp. tritici or P. striiformis, and placed in a growth
room at 17
◦
C and light intensity of 250
m
E m
− 2
s
− 1
Geagea et al., 1997. The plants usually developed sporulating lesions after 10–12 days.
The trap plants were healthy wheat plants cv. Michigan Amber at the primary leaf stage, grown in
pots in the same way as the source plants. 22 pots were placed in two parallel rows 84 cm long and
L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66 55
Fig. 1. Schematic layout of source and trap plants in the exper- iments in still air. Shaded boxes: source plants with sporulating
lesions, clear boxes: trap plants.
20 cm apart at right angles to the source plants; the nearest pots were 14 cm from the source plants Fig.
1. After each experiment, the pots were removed, placed in closed polyethylene bags and incubated in
darkness for 24 h at the optimal temperatures for in- fection by each fungus; 15
◦
C for brown rust Clifford and Harris, 1981 and 8
◦
C for yellow rust Rapilly, 1979. The pots were then placed in the growth room
at the same conditions to allow the lesions to de- velop. Symptom development was monitored at regu-
lar intervals by visual examination of four leaves per pot. The number of brown rust lesions was counted
after 10 days and the number of yellow rust chloroses was counted after 8 days on 2 cm
2
areas per leaf. For yellow rust, the rapid growth of the lesions and
their coalescence did not allow a precise assessment of the number of successful infections. Therefore, the
pale yellow chloroses that precede the appearance of individual lesions were counted and sporulation was
used to confirm that the spots were yellow rust. The number of brown rust lesions or yellow rust chloroses
recorded per unit area gave a measure of the number of spores deposited at each distance from the source.
Horizontal dispersal gradients were quantified by fitting an exponential equation to the relationship
between disease symptoms and distance Fitt et al., 1987:
y = a exp−bd 1
where y was the number of either brown rust lesions or yellow rust chloroses cm
− 2
, recorded at the distance d cm from the source. The coefficients a and b were
estimated by linear regression from the linearised form of Eq. 1 lny = lna −bd. The exponent b can
be expressed as a ‘half-distance’ α=0.693b, that is the distance in which y decreased by half Fitt et al.,
1987. The ‘half-distance’ was calculated only if the coefficient of determination r
2
of the regression was ≥
0.8 and the coefficient of variation cv was ≤0.3. 2.1.3. Effect of drop size and rain duration on total
numbers of spores dispersed Simulated rain was allowed to fall for periods of 5,
10 or 15 min onto source wheat plants with sporulating P. recondita f. sp. tritici brown rust or P. striiformis
yellow rust. In each experiment, drops with diame- ters 2.5, 3.4, 4.2 or 4.9 mm were used. At the end of
each rain period, both source plants and trap plants were replaced by new ones Table 1. As an estimate
of the total number of spores dispersed, the numbers of lesions or chloroses found were summed over all
distances observed. There were five tests with a du- ration of 10 min for the drops of 3.4, 4.2 and 4.9 mm
diameter and two tests with drops of 2.5 mm diameter.
2.1.4. Source depletion Wheat source plants, with either sporulating brown
rust or sporulating yellow rust, were exposed, succes- sively, to three, four or five sequential 10 min periods
of rain. 30 min of rain was used for drop diameters 4.2 and 4.9 mm, 40 min for drop diameter 3.4 mm, and
50 min for drop diameter 2.5 mm. After each 10 min period of rain, trap plants were replaced by new ones
but the same source plants were used. Thus the experi- ment measured spore dispersal for each 10 min period
of exposure to rain Table 1.
56 L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66
Table 1 Summary of the experimental conditions used in the experiments at Grignon and Rothamsted
a
Location Physical
Biological Incident drop
Rain Source
Spore parameters
variables diameter mm duration
plants trapping
Grignon, France Rain duration Disease dispersal
2.5, 3.4, 4.2, 4.9
5, 10 or 15 min Changed for each
rain period Plants changed for
each rain period Grignon, France Rain duration
Source exhaustion 2.5, 3.4, 4.2,
4.9 5 × 10 min
4 × 10 min 3 × 10 min
Not changed Plants changed
every 10 min Grignon, France Interrupted
rain
b
Source restoration 4.9
periods of 30 min with
interruptions of 2, 4, 6 h
Not changed Plants replaced
after the first rain treatment
Rothamsted, UK Rain duration Wind speed
Wash-off of air- borne spores
4.9 5, 10, 15 or
20 min with wind speed = 1
or 4 m s
− 1
Changed for each rain period
Rotorods changed for each rain period
a
The rain intensities were 8 for 2.5 and 4.2 mm drops, and 12 mm h
− 1
for 4.9 mm drops.
b
Sets of source plants were placed in controlled conditions temperature 17
◦
C and light intensity 250
m
E m
− 2
s
− 1
for interruptions between rain periods.
2.1.5. Source recovery Six sets of source plants were used in this exper-
iment; they are referred to as A, B and C when in- fected with brown rust and D, E and F when infected
with yellow rust. Each set was exposed to 30 min of rain with a drop diameter of 4.9 mm. After exposure
to the rain, each set of plants was placed in a growth room at 17
◦
C and a light intensity of 250
m
E m
− 2
s
− 1
for different times. Sets A and D were kept in the growth room for 2 h, sets B and E for 4 h and sets C
and F for 6 h. The range of rain interruptions from 2 to 6 h was chosen so as to obtain a variation in the bi-
ological responses to rain interruption change. After these time intervals, rain was allowed to fall on each
set of plants for 30 min and any spores released were detected on trap plants as before. New pots of trap
plants were used for a second 30 min period of rain Table 1.
2.2. Experiments with simulated rain and wind Experiments were done in the rain tower and wind
tunnel complex at Rothamsted Fitt et al., 1986b. The rain tower was 11 m high and linked to the horizontal
open ended wind tunnel; both the rain tower and the wind tunnel had 1 m square cross-sections. Air was
drawn through the wind tunnel by an axial fan located 12 m upwind of the rain tower. The rain simulator was
of a similar design to that used in the experiments with simulated rain only Mann et al., 1995.
2.2.1. Source plant, inoculation procedure and spore trapping
Wheat cv. Michigan Amber was used, as in the experiments at Grignon, except that the plants were
grown in 12 cm pots 12 seeds per pot and 3 mg of spores suspended in 6 ml of a mineral oil Soltrol
170
TM
, Phillips Petroleum, Boulogne-Billancourt, France were used to inoculate the plants. Experiments
were done only with plants infected with P. recondita f. sp. tritici. The spore source consisted of seven pots
of seedlings with sporulating lesions placed at the base of the rain tower in a line perpendicular to the air
flow. The pots were placed on stands such that their bases were 43 cm above the floor of the wind tunnel.
Spore concentrations downwind of the source were measured using rotating-arm spore traps McCartney
et al., 1997. The spore traps had ‘U’ shaped rotat- ing arms which collected spores on the leading edges
of the vertical arms which were covered by a trans- parent tape coated with a thin layer of wax to re-
tain the spores McCartney and Lacey, 1990. The ra- dius of the rotating-arms was 3.9 cm and the collect-
ing surface was 0.16 cm wide by 6 cm long. As the arms rotated at 3500 rpm, the nominal sampling rate
was about 160 l of air per min. After exposure, the
L. Geagea et al. Agricultural and Forest Meteorology 101 2000 53–66 57
tape on each arm was cut into four sections, removed and mounted on microscope slides McCartney and
Lacey, 1990. Spores were counted in five horizon- tal traverses of each slide at a magnification of 200.
The spore samplers were placed 2 m downwind of the source plants and 50 cm from the floor of the wind
tunnel, at about the same height as the infected plants. As the fall speeds of the brown rust spores were likely
to have been in the order of 1 cm s
− 1
Gregory, 1973, the vertical line of the spore plume at the spore trap
would only have varied by about 3–4 cm over the wind speeds used. The spore traps sampled air from a vol-
ume several times the diameter of the arms. Therefore, it was unlikely that variations in the position of the
spore plume, caused by different wind speeds, would have affected the proportion of the plume sampled by
the spore trap.
2.2.2. Combined effect of rain and wind An experiment was done to measure the effect of
wind alone in removing spores from infected leaves. Infected plants were placed at the base of the wind
tunnel and exposed to successive 5 min periods wind of different speeds 0.5, 1, 2, 2.5, 3, 4 and 5 m s
− 1
. The rotating-arm traps were changed after each 5 min
period. The experiment was repeated three times with different source plants each time.
In a second experiment, source plants were exposed for 20 min to simulated rain of drop size 4.9 mm and
a constant wind of either 1 or 4 m s
− 1
. During the tests, the rotating-arm traps were changed every 5 min.
The change in concentration with time reflected the changes in spore removal rate from the source. The
tests were replicated three times at each wind speed Table 1.
2.3. Kinetic energy of simulated rain drops The relationship between fall speed of drops and
vertical travel from rest given by Huber et al. 1998 suggests that the water drops in these experiments
would have reached close to their terminal velocities before impact. The terminal velocity of a water drop
was calculated from V =386.6 D
0.67
m s
− 1
, where D is the drop diameter in m; Ulbrich, 1983, and the ki-
netic energy E
K
D =12ρπ D
3
6 V
2
J, where ρ is the drop density in kg m
− 3
of the drop is given by: E
K
D = 391 × 10
5
D
4.34
2 Eq. 2 gives E
K
in J when D is measured in m. This equation was used to investigate the relationship be-
tween rain drop kinetic energy and the potential for splash dispersal of the two pathogens.
3. Results
