CITRUS MELANOSE AND ITS CONTROL 
21 
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intensity of the outbreaks expressed on a percentage basis are shown 
in Figure 2. 
Figure 2 gives marked evidence of the effect of several maintained 
temperatures upon infection where the plants were subjected to the 
inoculum for 48 hours and then surface disinfected. At 15° C. (59° 
F.) and 30° C. (86° F.) the infection became visible on the sixth day 
and reached 100 per cent on the twelfth and thirteenth days, respec- 
tively. The plants 
exposed at 20° C. 
(68° F.) and 25° C. 
(77° F.) did not show 
signs of infection un- 
til the eighth day 
and reached 100 per 
cent on the thirteenth 
and fourteenth da}^s, 
respectively, whereas 
those exposed at 10° 
C. (50° F.) showed 
no signs of infection 
until the ninth day 
and reached only 56 
per cent on the thir- 
teenth day. The de- 
gree of infection ob- 
tained is indicated on 
the curve for each 
temperature. 
From these tests it 
is seen that melanose 
infection may occur 
at temperatures as 
low as 10° C. in 48 
hours as well as at 
30°. The rate of de- 
velopment and the se- 
verity of outbreak of the disease does not appear to be very different 
at temperatures within the range of these tests. A 5 per cent cresol 
solution proved to be equally as effective as mercuric chloride for 
disinfecting surfaces of citrus leaves. 
In order to determine further the effect of several temperatures 
and both shorter and longer exposures upon infection by the melanose 
fungus, a series of holding tests was run, using potted sour orange 
and grapefruit plants held at definite maintained temperatures con- 
tinuously during the infection period. These plants were inoculated 
by the swab method and placed in various compartments for varying 
lengths of time. Later they were taken from these compartments, 
swabs removed, leaves immersed in a 1 to 1,000 corrosive-sublimate 
solution or 5 per cent solution cresol compound for one minute, rinsed 
in running tap .water, and transferred to slat shade to await develop- 
ments. Final readings were made three weeks later, and the findings 
are given in Table 2. 
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Fig. 2. — Percentages of grapefruit leaves developing 
melanose infection after being inoculated and incu- 
bated for 48 hours at various maintained temperatures 
(10°, 15°, 20°, 25°, and 30° C), and then held for 
15 days under equal conditions favorable for melanose 
infection. The number of grapefruit plants in each 
lot is indicated by numbers in parentheses, and the 
average intensity of infection as varying from very 
slight to severe 
