CITRUS SCAB: ITS CAUSE AND CONTROL,. 13 
gaining entrance into sections hitherto free from this disease and in 
many places increasing where already established. 
The exact part of the fungus concerned in dissemination is question- 
able. Most writers attribute it to spores produced in scab Lesions. 
In order to determine this point, a great many microscopic examina- 
tions were made of old and young scab lesions at various times during 
the past four years, especial attention being given to it during period- 
of scab outbreaks. Hundreds of -ections were examined, but in no 
instance were conidia of the causative organism found. That the 
fungus was present and alive in both the old and young lesions at the 
time of microscopic examinations was proved repeatedly by cultural 
methods. 
There appear to be no available data to show that the fungus in 
such lesions produces conidia in sufficient numbers to account for the 
widespread and rapid primary infections occurring under favorable 
conditions. At various times attempts were made to find a perfect 
stage of the causal organism. These efforts were futile. 
Attention is called elsewhere to the gross appearance of the fungus 
in citrus-^eab lesions on leaves and to the ease with which the fungal 
mass is divided into many thick-walled single-celled bodies called 
chlamydospores which are doubtless responsible for a great part of 
the primary infections occurring in early spring. 
In addition to old lesions abundantly supplied with a hold-over 
stroma, many scars occur in which the stromatic mass has weathered 
away. There is a marked flaking off of host and fungal (mycelial 
fragments from such lesions during the early spring, so much so that 
pure cultures can be isolated easily from such scales by holding an 
affected leaf over an open Petri dish and brushing the lesions. This 
indicates that spring infection in part at least may be attributed to 
fragments of host and parasite flaking from these lesions. 
Regardless of whether the disease starts in the spring from chlamy- 
dospores or fragments of host and fungus, there is no tenable evidence 
to show that citrus scab is spread for the most part by conidia pro- 
duced in these old lesions. 
CONDITIONS INFLUENCING INFECTION. 
Local and seasonal conditions play an important part in the dis- 
semination and propagation of tins disease. In years of rainy springs 
citrus scab is almost certain to be severe on grapefruit, provided the 
rainy periods occur while the leaves or fruit are in a stage of develop- 
ment susceptible to infection. It is a matter of common remark 
among growers that citrus scab is very likely to be serious if rainy 
spells occur during or soon after the blossoming period. 
If the vegetative flush precedes the opening of the bloom during 
weather that is then dry and warm very little infection is likely to 
