Vergara - 1976 - Physiological and morphological adaptability of ri

400 ci.ixi.»\'r|-: AND Rjtfl-I
Another insight into the retarding effects of low temperatures on disease
development was provided by a comparison of constant vs. fluctuating 12-hour
data/night temperature regimes. Lesion size was greater at a constant 25°C
incubation temperature than at fluctuating temperatures of 31° daywl20° night
and 31° dayf22° night when compared at five dew periods and dew temperatures
of 18°, 20°. and 22°C. Exposure to drastically sub-optimal temperatures more
than negated the effects of time periods at highly favorable colonization temperatures.
These results gain added interest considering that lesion size at 25°C
is considerably below‘ the median between 20° and 31°C when virtually any comparison
is made at any combination of dew period and dew temperature (Table
l .
)Other related studies revealed that a lower RH during colonization markedly
reduced lesion size. Furthermore. lesion expansion was slowed measurably
when plants were removed from dew periods for infection to lighted growth
chambers as compared with darkened chambers.
Sporulation
A iota] of 20-25 lesions on the four plants in each pot were selected from the
third basal leaf for sporulation studies. These lesions xvere marked for future
identification and their size was detennined to permit use of the data to reflect
either total sporulation per lesion or sporulation per mm= of lesion.
Sporulaticin was determined by collecting spores from lesions on intact plants
immediately after the plants had dried after being removed from their designated
dew periods for sporulation. The large opening of a glass eye dropper was plugged
with a piece of filter paper and attached to a rubber hose. which in turn ‘N85
attached to a vacuum source. Conidia, as well as some conidiophores and
mycelial fragments, were sucked from the upper and lower surfaces of lesions
and trapped in the filter paper. The filter paper containing the conidia was placed
in 5 ml of 0.05% 11,0 agar containing 0.5% copper sulfate to inhibit germination.
The contents were agitated to remove the conidia from the filter paper and
the filter paper removed. Following a second agitation to disperse the conidia
evenly vrith the solution. 0.1 ml of solution was distributed evenly on a thin piece
ofagar placed on a glass slide which was divided into sections by a ivaxed pencil
to facilitate counting. All of the conidia present in the 0.1-ml aliquot were
counted, and the average of five such {ll-ml samples was used to determine
sporulation per lesion. Early in these studies, microscopic examination of the
lesions after spore harvest and the data obtained from spore counts indicated
that the efficiency of the collection techniques exceeded 95%.
Eflrect 0fdew period and dew temperalztre on sporulation. Plants at the 3-4 leaf
stage were inoculated and exposed to 8 hr of dew at 22°C a climatic regime
known to permit adequate infections. The plants were then incubated in a
growth chamber at 28°C for 4 days prior to inducing sporulation. Dew periods
of l2. l6, 20, and 24 hr of darkness and dew temperatures of l2, l6, 20, 22. 24,
26. 28. and 30°C were used for sporulation studies.