Apr. 19,1924 Relation of Environmental Factors to Citrus Scab 247 
have entered the tissues, the small and slow amount of growth which the fungus 
makes necessarily lengthens the period of incubation. The period of initial 
infection for citrus scab is a question of hours and not minutes. A summary of 
Fawcett's ( 4 ) infection experiments shows that the period of incubation is also 
long, six days to two weeks being required even under optimum conditions 
for infection. 
Primary infection in the spring usually occurs on the unfolding buds, while 
secondary infection takes place on the young fruits very soon after the petals 
drop. It is rather unusual under Alabama conditions for the pathogene to 
attack half-grown leaves or fruits. It is true that scab is found on leaves and 
fruits of this age, but the length of the incubation period indicates that initial 
infection takes place while the leaves and fruits are quite small. 
The fact that the disease appears on either side of the leaves, precludes the 
possibility of stomatal infection as no stomata are present on the upper surfaces 
of Citrus plants. Infection is accomplished, for the most part, by the direct 
penetration of the epidermis by the fungus hyphae. As the leaves mature the 
firmer leaf texture would necessarily become less penetrable by the hyphae of 
the fungus. 
Initial infection is not dependent on rapidly growing tissue so much as on the 
fact that the leaves must be young, whether rapidly growing or not. However, 
after initial infection takes place, rapidly growing infected tissues produce more 
malformations and consequently larger and more misshapen leaves and fruits. 
Any conditions favoring the rankness of the first growth would also contribute to 
produce larger scab spots. 
In figure 1, the minimum, optimum, and maximum temperatures for scab 
infection, as determined under controlled conditions, are indicated. While 
these points may not be strictly applicable to what actually occurs in the field, 
they are sufficiently accurate for the purpose of our discussion. During the 
early part of some years, the weekly mean temperatures rise above the minimum 
for infection for short periods, but in most instances, they are followed by sudden 
drops in temperature. No scab has ever been found during these periods, as 
the other two conditions for successful infection are not always at hand; namely, 
sufficient moisture and the presence of young foliage. Temperatures suitable 
for slight infection occur at different times during the various seasons, as indicated 
in figure 1. The period during which optimum temperatures for infection prevail 
is usually from the middle of April through most of May. By the third week 
in April the temperature during most years reaches the optimum and by the 
first of June the prevailing temperatures are above the maximum for infection. 
If the temperature factor is considered by itself, we should expect the same 
amount of scab each year, as the optimum temperature for infection is reached 
at practically the same time. There is only one week or at the most, two weeks' 
difference in the time required for the temperatures to reach the maximum. 
We should also expect that the earlier the season the more prevalent scab would be. 
However, it is exactly the reverse, as will be brought out later. 
The presence of moisture over a long enough period for initial infection to 
take place has been continually stressed as one of the essentials for successful 
infection. The weekly rainfall, including the number of rainy days per week for 
the spring season of 1914 to 1920, was tabulated, but no positive correlations 
between the relative prevalence of scab and the amount or frequency of rainfall 
could be made. Under Alabama conditions, there is rarely a week in which rain 
does not occur during the development of the first spring growth. In the majority 
of cases, when temperatures suitable for infection are at hand, moisture sufficient 
for initial infection is present. Thus, as far as Alabama is concerned, we can take 
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