Apr. 19, 1924 
Relation of Environmental Factors to Citrus Scab 
243 
(L.) Osbeck) made an extremely slow growth at 18° C. (59° F.), while the 
other plants tested did not start until a temperature of 20° C. (68° F.), was 
reached. Even at this temperature C. mitis Blanco, a native of the Philippine 
Islands only made a slow growth. At 30° C. (86° F.) all plants tested developed 
rapidly. Above this point the growth of plants of the grapefruit type were 
inhibited, while Poncirus trifoliata (L.) Raf. made a good growth. 
It was also found that at 15° C. (59° F.), grapefruit plants, although making 
an extremely slow growth matured their foliage rapidly, in most instances within 
a week’s time, while at 30° C. (86° F.) growth was rapid, but the period of leaf 
maturation was extended over a period of about two weeks. The mature leaves 
at the low temperature were only from one-fourth to one-half the size of those on 
plants held at 30° C. (86° F.). The small leaves maturing at the lower tempera¬ 
ture retained their same size when transferred to higher temperatures. 
Plants held at 30° C. (86° F.) were inhibited in their growth when transferred 
to lower temperatures. The growth of citrus plants was inhibited when held at 
a high temperature during the day and at a low temperature during the night. 
ON INFECTION AND DEVELOPMENT OF THE DISEASE 
As far as the writers are aware, no one has successfully infected Citrus plants 
with Cladosporium dtri under field conditions. Even in the greenhouse, infec¬ 
tion has not always been successful. In practically all cases the only success¬ 
ful infections which have been produced under greenhouse conditions have 
been made on plants held under bell jars, or where abundant moisture on the 
surface of the leaves was supplied for a period of several days. In this connec¬ 
tion it should also be stated that only the young leaves have been infected. 
In the experiments reported by Fawcett (4), and in the observations made by 
the senior writer, the two factors of young growth and sufficient moisture for 
initial infection have always been supplied. Fawcett inoculated his plants and 
placed them in the temperature chambers, with sufficient moisture, for a period 
long enough for initial infection to take place. In the senior writer’s observa¬ 
tions on scab, the plants were held in temperature cases in a saturated atmosphere 
under bell jars. 
Fawcett (4) has shown that the temperature limits for successful infection 
of sour orange and pummelo plants under controlled conditions lies between 16° 
and 23° C. (60.8° and 74.3° F.). No infections were obtained at 14° (57.2° F.) 
or below, nor at 24.5° C. (76.1° F.) or above, so that infection is limited to a 
rather narrow range of temperature. The largest number of scab spots occurred 
at the temperatures-between 18.5° and 21° C. (65.3° and 69.8° F.). 
In the senior writer’s experiment with citrus canker (6) optimum conditions 
for scab development were afforded and typical scab spots did occur naturally 
on some of the plants growing in the temperature cases at 15° and 20° C. (59° 
and 68° F.). .No scab appeared on plants held at temperatures of 10° C. (51° F.) 
or below, nor at 25° C. (77° F.) or above. At 15° C. (59° F.) scab spots occurred 
only on the grapefruit plants showing new growth. It should be noted that 
grapefruit were the only plants making any growth at this temperature. At 
20° C. (68° F.) scab appeared on grapefruit, calamondin, and citrange plants. 
The spots were much more numerous on grapefruit than at 15° C. (59° F.). 
In all cases, scab was limited to the plants with young leaves. 
The results show that when abundant moisture is supplied and young growth 
is present, infection of Citrus plants by Cladosporium citri is limited to tempera¬ 
tures between 15° C. and 23.5° C. (59° and 74.3° F.), a range of less than 10 
degrees. The optimum for the best and greatest development of scab appeared 
to be about 20°-21° C. (68°-69.8° F.), which is also the optimum found by Faw¬ 
cett (4) for the fungus in culture. 
