© Benaki Phytopathological Institute
Al-Shehadah
et al.
50
in Table 1. Germinability of
R. secalis
conidia
was significantly (
P
<0.001) reduced by up to
94% after 8h of exposure to direct sunlight
in comparison to the non-exposed controls
(Table 2). Results showed a highly significant
decrease in the germination of conidia with
increasing exposure time to direct sunlight
(Table 2). In addition, significant differences
(
P
<0.05) in the virulence between exposed
and non-exposed to sunlight conidia were
observed (Table 3).
Survival of conidia is of paramount im-
portance in the build-up of
R. secalis
inoc-
ulum, as conidia transported from infected
plant residues
via
wind or rain can germi-
nate and infect healthy barley plants. Very
little is known about the effect of sunlight in
the survival of these conidia. In the present
work, conidia collected on coverslips and ei-
ther exposed or not exposed to solar radi-
ation were used to represent conidia nat-
urally deposited on the upper surface of a
barley leaf. Although there may have been
differences in the ability of conidia of differ-
ent ages to germinate
the germination of
non-exposed conidia were used as a base-
line (control) to compensate these differenc-
es. Under the conditions of our study, ger-
minability of
R. secalis
conidia was reduced
significantly by exposure to solar radiation.
These results agree with those of earlier
studies that have demonstrated a significant
effect of light on the germinability of plant
pathogens in other pathosystems (Rotem
and Aust, 1991; Ben-Yephet and Shtienberg,
1994; Braga
et al.,
2015; Cordo
et al.,
2017).
Our findings clearly indicate that solar ra-
diation significantly affects
R. secalis
conidial
viability, which mainly depends on the dura-
tion of exposure. Considering the impacts of
these findings on the epidemiology and the
spread of
R. secalis
conidia with a potential
of causing infection at a given location, the
later could be reasonably predicted through
detailed analyses of the available solar radi-
ation data along with the travel path of the
conidia (Isard
et al.,
2005). On the other hand,
conidial pigmentation has been reported to
play an essential role in solar radiation pro-
tective mechanisms (Swan, 1974; Ignoffo
and Garcia, 1992; Butler and Day, 1998; Full-
er
et al.,
2015), which might be a possible ex-
planation of
R. secalis
tolerance to solar ex-
posure up to 8 h.
UV-Cexposure (3.2±0.7Wm
-2
) significant-
ly (
P
<0.001) reduced the germinability of
R.
secalis
conidia (Fig. 1). The reduction in ger-
minability increased with increasing time of
exposure to UV irradiation. It is well known
that the UV-C wavelength is highly efficient
Table 1.
Environmental conditions that pre-
vailed in the field during the exposure of
Rhynchosporium secalis
conidia to direct
sunlight.
Variable
Range (Unit)
Air temperature
26-38 (ºC)
Relative humidity
30-47 (%)
Average wind speed at
sample height
0.9-4.1 (m/s)
Irradiance of incident solar
radiation
670-860 (Wm
-2
)
Table 2.
Germination (%) of
Rhynchosporium secalis
conidia exposed (Gs) and not exposed
(G
NS
) to direct sunlight in the field for periods ranging from 0.5 to 8 h.
Treatment
Time (h)
0.5
1
2
3
4
5
6
7
8
G
S
A83.3b
x
B74.4b C65.7b C60.3b D50.1b E37.9b E32.2b E25.6b F11.1b
G
NS
A94.3a A94.6a A97.5a A98.5a A98.8a A98.9a A98.9a A98.1a A98.3a
(G
NS
-G
S
/G
NS
)
0.12
0.21
0.32
0.39
0.49
0.62
0.67
0.74
0.89
x
Means preceded by different capital letters (column) and followed by different lowercase letters (row) differ
significantly at P<0.001 according to Newman-Keuls test.
