Vergara - 1976 - Physiological and morphological adaptability of ri

EPIDEMIOLOGY or SOUTHERN coax LEAF BLIGITT 397
Experimental periods were I5 min. with an air speed of 33. 45. or 9O cm sec"
(0.75. 1.0. or 2.0 mph). Relative humidities used were: constant high (II—II)
91-10094». constant low (L—L) 20—36%; varying high to low (ll-L). and varying
low to high (L-H). except at the lowest speed, when the highs and lows were
74-99 and 33-44 %, respectively". The temperature was 23—24°C.
At times no spores were deposited on the slide. The question then was. were
the experimental conditions responsible for the lack of spores. or were the spores
in a physiological state inhibiting release‘? Of the criteria tested. H-L humidity
air at 9O cm sec" caused the maximum deposit of spores. Therefore. a standard
procedure was adopted. to count as valid the results obtained only ifa succeeding
trial on the same leaf section with H-L humidity air at 90 cm sec" deposited at
least 100 spores. To reduce variation, the number of test spores is expressed as a
ratio of the number obtained on the succeeding reference trial. For instance, the
test xvith Il-l. humidity air at 90 cm sw“ was followed by a reference trial with
the same conditions after high humidity was reestablished. This was done without
releasing spores by flowing humid air at 20 cm sec“ over the leaf section for
an hour or more.
Means of six replications were compared for significant differences by the “t"
test for unpaired observations and unequal variances.
The number of spores trapped increased with increase in air speed (Fig. l).
Maximum numbers were trapped with Il-I. humidity air at 9O cm sec”. To
reduce variation. spores trapped during a IS-min test period are expressed relative
to the spores trapped. with the same leaf section. during a succeeding l5-
min reference period of HeL humidity air at 9t) cm sec-l. Relative numbers of
spores trapped were: about 1V; times as many at H-l. humidity air at 90 cm sec“;
1/3 as many" with either L-L humidity air at 9O cm sec‘ or II-l. air at 45 cm
sec‘; less than 1f l0 as many with the other combinations. Thus relative humidity
favoring spore release apparently can compensate for a lower air speed.
Infection
Dew temperatures of l2. 14. l6. l8, 20. 22. 24. 26. and 28°C and dew periods of
4. 6. 8. 10. and 12 hours were used. Preliminary studies revealed negligible. if
any’. increase in number tifinfections with dew periods greater than l2 hours. A
dew period of24 hours consistently resulted in fewer infections, probably due to
the deleterious effects. of free moisture on post-germination or pro-infection
sequences. For these reasons, we chose arbitrarily to restrict the dew period to
a maximum of l2 hours. (‘irowth chamber temperatures of 20. 25. 28. 31°C were
titilized to evaluate post-dew temperature effects.
Lesion counts were taken 48 hours after inoculation. Comparisons of lesion
number 48 and 72 hours after inoculation indicated little. if any. differences
between time periods. thus permitting us to use the shorter period. Statistical
analyses. accomplished by an IBM 360/67 computer. were based on lesions per
plant for greater accuracy‘.
Data submitted to an analysis of variance showed differences significant at