CITRUS SCAB : ITS CAUSE AND CONTROL. 13 



gaining entrance into sections hitherto free from this disease and in 

 many places increasing where abeady 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 periods 

 of scab outbreaks. Hundreds of sections 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-scab 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 (mycehal) 

 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 this 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 Kkely to 



