22 BULLETIN 1118,' IT. S. DEPARTMENT OF AGRICULTURE. 
on the widespread impression thai for a number of years this disease 
was confined to the sour orange and rough lemon, later "adapted" 
itself to grapefruit, and still later to the round orange. This alleged 
adaptation on the part of the scab fungus has been explained in 
several ways. Some claim the grapefruit and round orange, as a 
result of cultural practices, are rapidly losing their original immunity; 
others are of the opinion that the scab fungus is gradually adapting 
itself to the more resistant species; still others would explain the 
condition on the ground that very virulent strains of the causal 
organism capable of infecting grapefruit and orange have resulted 
as natural mutations; and a few do no more than make the prediction 
that within a short time the orange will be as seriously affected as is 
now the case with grapefruit. 
In the case of citrus scab there appears to be no sound basis for 
such claims in spite of the fact that such a phenomenon has been 
noted for other pathogens. Within a few years after citrus scab was 
first recorded in America, Underwood (30) reported the commercial 
species as being susceptible to infection in essentially the same 
proportion as is the case to-day. His report was made before the 
grapefruit as a fruit, or rough lemon as a root stock, began to attract 
very much attention from commercial growers. Since then these 
species have gained a great deal of popularity, resulting in the set- 
ting out of millions of trees susceptible to infection from scab. This 
has increased immeasurably the number of foci of infection and also 
the mathematical chance for infection of all species. 
If the scab fungus is adapting itself to the more resistant species, 
this transition is going on very slowly, so much so that it can not be 
measured even when viewed over a period of 30 years. The causal 
organism isolated from round orange, grapefruit, and avocado has 
shown itself by actual inoculation tests to be neither more nor less 
pathogenic to grapefruit than was the same organism isolated from 
sour orange and grapefruit. Cultural studies of the causal organism 
isolated from various hosts also failed to reveal any difference in the 
fungus, and field studies over a period of five years fail to give the 
slightest indication that the round orange is losing its original re- 
sistance. There is no valid reason for presuming that at least within 
the life of trees now planted the orange will be as susceptible to infec- 
tion as is grapefruit at present. There is no evidence that an inher- 
ently more virulent strain of the scab fungus is developing. 
INOCULATION EXPERIMENTS. 
In order to determine accurately the period of development dur- 
ing which fruit and foliage are likely to become infected and the 
relative susceptibilities of some citrus species to scab, a rather ex- 
tensive series of inoculation experiments was conducted on young 
