242 
Joumal of Agricultural Research voi. xxviii, no 3 . 
to assume that scab has been introduced into these countries on nursery stock. 
Further, Reinking (8) does not mention the presence of scab in the Citrus-growing 
regions of Siam and Indo-China. 
Fawcett (2) summarized the California situation as follows: “This disease 
(scab) has never been found in California. Before the strict quarantine laws 
were passed, thousands of sour orange trees with their leaves affected with scab 
were brought into California, but the new foliage came out free from attack. It 
would appear that this fungus is unable to persist in a climate like that of Cali¬ 
fornia.’ 7 
According to Swingle and Webber {9), scab appeared in Florida about 1884 
and spread rapidly over the State and into Louisiana. They state that it was 
probably introduced into America from Japan. Since that time it has been 
introduced from Florida and Louisiana, or direct from Japan on the many im¬ 
portations of nursery stock, especially Sai^uma (Citrus nobilis unshiu (Makino) 
Swingle, into the Gulf Coast States of Alabama, Mississippi, and Texas. Thus, 
at the present time, scab is one of the more important diseases of Citrus fruits 
in the Gulf Coast States, although fortunately it can be controlled by spraying. 
Scab has also been reported from India, Formosa, the West Indies, Paraguay, 
the Canal Zone, Yucatan, and Hawaii. 
Scab is a rather serious disease in the Gulf Coast States, South China, Japan, 
and parts of the West Indies. So far it has not been reported from the Citrus¬ 
growing regions of the Mediterranean countries, California, and the Philippine 
Islands. The absence of scab in these localities is an interesting phase of the 
scab situation. That scab has been introduced on nursery stock into California 
and the Philippine Islands, and possibly the Mediterranean countries without 
gaining a foothold, indicates that there are certain factors which prevent the 
successful propagation of citrus scab in these localities. 
INFLUENCE OF ENVIRONMENTAL FACTORS UNDER CON¬ 
TROLLED CONDITIONS 
ON THE PATHOGENE 
Fawcett (I, 2) has continually emphasized the fact that Cladosporium citri 
differs from other species of Cladosporium, and especially C. herbarum Lk., which 
is usually associated with C. citri on old scab spots. In fact, C . herbarum has in 
some cases been mistaken for the causal organism. He has so well described 
the difference in appearance of these organisms both in pure culture and on the 
plants themselves, that it will be sufficient to state here that C. citri is an un¬ 
usual Cladosporium, which makes an extremely small and slow growth in pure 
culture. 
The optimum temperature, as worked out by Fawcett (4) for the best develop¬ 
ment of the fungus in pure culture, is 21° C. (69.8°F.). Growth occurs in cul¬ 
ture within a range of temperatures between 13.5° and 32° C. (56° and 89.6° F.). 
Judging from Fawcett’s results, spore production in pure cultures seems to be 
limited to temperatures between 16° and 27.5° C. (60.8° and 81.5° F.).^ 
It has also been observed by Fawcett (1) that the spores of this fungus germi¬ 
nate readily both in tap water and on agar; germination of the spores beginning 
in from 5 to 24 hours. It is not known whether the spores will germinate in 
a saturated atmosphere or at lower humidities. 
ON THE HOST PLANTS 
The results obtained by the senior writer (6) at the University of Illinois 
indicate that each type of Citrus plant reacts differently under similar environ¬ 
mental conditions. Under controlled conditions, grapefruit plants (Citrus grandis 
