248 
Journal of Agricultural Research voi. xxviii, no. 3 
it for granted that the rainfall is sufficient during the greater part of the time in 
which temperatures suitable for infection prevail. 
We come now to the most important variable factor and by all means the most 
difficult to present, namely, the development of the early spring growth and its 
relation to scab. 
In the preceding pages, it has been pointed out that the various types of Citrus 
plants start growth in the spring when different weekly mean temperatures are 
reached. These points vary during different years so that in some cases they 
may occur as much as six weeks apart. Again, not all plants of the same variety 
start at the same time. Furthermore, during certain seasons all plants may 
start at practically the same time, or at different times, during years when the 
weekly mean temperatures are low. The amount of first spring growth and the 
rapidity with which it matures are also dependent on weather conditions. Thus, 
all these points must be taken into consideration in duscussing the relative prev¬ 
alence and susceptibility to scab of various plants. 
Any environmental factor or factors which induce a slow or slight spring 
growth and rapid maturation or late starting of susceptible Citrus trees favor 
scab escape; while any environmental factor or factors which induce a large 
amount of spring growth and subsequently slower maturation, especially during 
the period of optimum infection, favor scab attacks. Thus, we must not 
only consider the internal factors influencing the development of spring growth 
of the Citrus trees, but also the external factors which may inhibit or stimulate 
these processes. 
To illustrate, grapefruit plants, as a rule, start rather early in the season 
and during normal years complete their growth before optimum conditions 
favorable for scab are at hand. However, plants may go into the winter in a 
devitalized condition and so start extremely late in the spring and thus escape 
infection. It is only when environmental conditions are such that the develop¬ 
ment of the first spring growth coincides with the optimum conditions for scab 
development that the disease is at all serious on grapefruit plants. 
As is well known, Satsumas are generally susceptible to scab. The somewhat 
higher temperature necessary for the forcing of active growth of Satsuma usually 
means that it starts later in the season and first spring growth is well developed 
at the time optimum conditions for infection are at hand. 
Until more data are at hand, we can roughly divide the susceptible commercial 
Citrus species and varieties into three groups, according to their normal develop¬ 
ment (internal) in the field as follows: 
1. Varieties which start early in the season and make a slow and slight growth, 
which matures rapidly. 
2. Varieties which start somewhat later, and produce a larger amount of 
spring growth, which matures slowly and about the time optimum conditions 
for infection are at hand. 
3. Varieties which start late in the season after the optimum for infection has 
occurred. 
Plants falling in Groups 1 and 3 are generally free from scab, while those in 
Group 2 are usually easily infected and badly attacked by scab. 
External factors, however, play an equally important part in determining the 
type of spring growth developed and in its scab susceptibility or escape. After 
a thorough consideration of the numerous factors involved, it is not surprising 
that no successful infections have been made in the field with this disease. How¬ 
ever, it should not be difficult to do when all factors are taken into consideration. 
Evidently some essential requirements for successful infection in the experiments 
so far reported have been omitted. 
