G Aerial dispersal and drying of Peronospora tabacina conidia in ...

700 Applied Physical Sciences: Aylor and Taylor
Table 1. Average time for leaves to become half dry (t12) and
fully dry (tf)
Date Wall Location
t1/2P
min
tf,
min
May 22 Down
Up
N1
Si
163
160
178
179
Down N3 115 169
Up S3 123 174
Down N5 135 170
Up S5 138 168
July 1 Down N1 191 248
Up Si 164 230*
Down N2 171 236
Up S2 170 230
Down N4 181 246
Up S4 160 225*
Down N5 175 243
Up S5 160 235
Measurements were made at six locations after 0700 EST May 22
and at eight locations after 0800 EST July 1. Locations N1-N5 and
S1-S5 are shown in Fig. 1.
* Values for which tf was significantly less with the wall raised.
4 to about 12 m, but at 20 m Rc was still about 1.8. The percentage
of spores escaping the tent increased from 0.005% to
about 0.46%. When Kz increased exponentially, RC did not become
greater than 1.0 until 18 m from the source, and at 20 m
it was only 1.15. Meanwhile, the percentage of spores potentially
escaping the tent increased to fully 22%.
Drying. The times t1/2 and tf for leaves to become half and
fully dry are shown in Table 1. Generally, t1/2 was 70-90% of
On May 22, wind just outside the tent was north-northwest
and 0.3-1.2 m/sec. From 0735 to 1000 EST, air temperature
increased from 10'C to 24TC, while relative humidity decreased
from 87% to 51%. The tf was about 170 min. There were no
significant differences in the time for leaves to dry whether walls
were up or down.
On July 1, wind was north-northwest and calm to 1.4 m/sec.
From 0818 to 1155 EST, air temperature increased from 20TC
to 270C while relative humidity decreased from 86% to 72%.
The tf was about 230 min. There were slight differences in the
time for leaves to dry, depending on whether the wall was up
or down. In two offour locations inside the tent, the leaves dried
a significant 7-8% faster with the walls up.
To generalize, we calculated the effect of raising the walls on
the time for leaves to dry. Although the initial water on the leaf,
the radiation absorbed by the leaf, and leaf size were not known
precisely, these factors were not affected by wall position and
calculation provided a valid comparison of walls up and down.
Raising the end walls can increase air temperature, decrease
relative humidity and increase wind speed inside the tent. Depending
on the size of the plants, wind speed just inside the tent
can be increased 40-70% by raising the walls. The air is generally
about 1C warmer and about 5-7% drier outside than in.
Thus, raising the walls could at most change wind, temperature,
Proc. Nad Acad. Sci. USA 79 (1982)
and humidity inside the tent by these amounts, and, using these
values, we calculated an upper limit on the effect of raising the
walls. Penman's method for estimating evaporation from an extensive
wetted surface (1) indicates that these changes would
make essentially no difference in the time for the entire crop
to dry. We calculated the drying of an individual upper leaf and
found that the maximal changes in wind, temperature, and humidity,
had they occurred, might have caused upper leaves to
dry about 30 min faster on May 22 and about 40 min faster on
July 1.
Germination. Would a 30-min shorter drying time affect the
fungus? The critical stage in the disease cycle is germination of
spores and the penetration of the leaf. These stages require
water, whereas the subsequent stages can proceed in dry leaves.
Thus the significance of 30 min can be judged by the slope of
the curve relating percent germination to time after wetting
spores. Between 150C and 30TC, germination is most rapid during
the first 120 min after wetting and considerably slower
thereafter (16). Between 0 and 120 min a difference of 30 min
could increase germination by 16-18%. Thus, where leaves dry
during the first 120 min after the spores are first wet, a 30 min
faster drying might significantly reduce germination. Usually,
however, the spores and leaves will be wet considerably longer
than 120 min. Between 120 and 240 min a difference of 30 min
could increase germination by 2-6%. By 240 min germination
has reached its maximum and more time would not matter.
Thus, 30 min faster drying would generally decrease infection
less than 6%, a small matter.
Raising the end walls affects the wind and thus the dispersal
of spores from lesions near the edge of the crop but will have
little effect on drying. Blue mold often starts near the edge of
a tent where hedges or sheds shade the plants from the morning
sun and where wetter, cooler conditions encourage the fungus.
Thus, raising the walls could increase the inoculum deposited
on plants well into the tent and increase the number of spores
escaping the tent and spreading the disease afar.
The authors thank P. A. Halbert and K. T. Paw U for technical assistance
and G. J. S. Ross of Rothamsted Experimental Station for help
with the statistical analysis of leaf drying.
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