© Benaki Phytopathological Institute
Effect of solar radiation on
Rhynchosporium secalis
conidia
49
to the length of the conidium (12.5-14.3 μm)
(Manners, 1966). For calculating the per-
centage of conidial germination, the follow-
ing formula was used:
Germination (%) = (Number of germinated
conidia x 100)/total number of conidia exam-
ined.
Meteorological data
Solar irradiance, air temperature, relative
humidity (RH) and wind speed were mea-
sured in the field during the whole exper-
imental period. Solar irradiance (Wm
-2
) was
measured with a pyranometer (LI 200 SB,
Li-COR. Int., Lincoln, NE 68504) located at a
height 1.1m above the ground. Air temper-
ature and RH were measured with a plati-
num resistance-thermometer sensor (Pt100)
of DIN EN 60751, Class B (ES Electronic Sen-
sor GmBH, Germany) and wind speed with a
cup anemometer (model 014A, COMBILOG,
Theodor Friendrichs & Co. Hamburg, Germa-
ny). All instruments were located at the same
height above the ground as the coverslips
carrying the conidia (i.e. 1.1 m). The meteoro-
logical data were recorded at 1h intervals us-
ing a data logger (COMBILOG 1020, Theodor
Friendrichs & Co. Hamburg, Germany).
Virulence tests
The virulence of conidia exposed for 8h
to solar radiation and those that were not
exposed was tested in the field using three
barley cultivars; WI 2291 from Australia, and
Arabi Abiad and Tadmor from Syria. Those
cultivars were selected for their different lev-
els of resistance to scald disease (Arabi
et al.,
2010). The experiment was located at a site
with favorable environmental conditions for
the development of the disease. Seeds were
sown under rainfed conditions (500 mm an-
nual rainfall) in a completely randomized
block design, with three replicate plots for
each treatment (50 plants per replicate and
cultivar). Each replicate plot was 1 × 1 m,
with 1m wide buffer zone around it. Each
plot consisted of five rows, 25 cm apart, with
10 seeds sown per row. Inoculum of patho-
type Rs22 was prepared from conidia pro-
duced on 2- to 3-week-old cultures grown
on Lima Bean Agar (LBA) medium. Conidia
were suspended in water and their concen-
tration was adjusted with a hematocytom-
eter to 0.5 × 10
6
conidia/mL. A surfactant
(polyoxyethylene-20-sorbitan monolaurate)
was added (100 μL/L) to the conidial suspen-
sion to facilitate dispersion of the inoculum
over the leaf surfaces.
Inoculations were timed to correspond
with periods of free moisture when possi-
ble. Three inoculations were performed us-
ing a fine mist hand-held sprayer. The first
inoculation was applied when plants be-
gan to tiller (GS 31) and the last when plants
were at least 30 cm height (Zadoks
et al.,
1974). Non-inoculated plants (control) were
sprayed with distilled water. Disease severi-
ty was assessed as a percentage of the sec-
ond leaf lamina showing symptoms, 17 days
post-inoculation, according to the method
used by Ceolini (1980).
Statistical analysis
Data were analyzed using the STAT-ITCF
programme (MICROSTA, realized by ECO-
SOFT, 2
nd
Version, Institut Technique des
Céréales et des Fourrages Paris). Analysis
of variance (Newman-Keuls test) was con-
ducted to test for differences among expo-
sure periods in test sets. The germination for
conidia exposed to sunlight was calculated
as G
S
/G
0
, and for conidia that were not ex-
posed to sunlight (controls) as G
NS
/G
0
where,
G
S
: the average of absolute germination per-
centage for exposed conidia, G
NS
: the aver-
age of absolute germination percentage for
non-exposed conidia, and G
0:
the average of
absolute germination percentage for non-
exposed conidia at the beginning of the ex-
posure period (time 0). The germinability of
conidia exposed to sunlight was compared
to that of conidia not exposed to sunlight
using the formula: G
NS
-G
S
/G
NS
.
Results and Discussion
The environmental conditions that prevailed
in the field during the exposure of
R. seca-
lis
conidia to direct sunlight are presented
