Florida Agricultural Experiment Station


 CAUSE OF PSOROSIS
 Nothing definite is known concerning the cause of psorosis.
 The nature of this disease, its spread on the tree and to other
 trees in the grove, and the different degrees of susceptibility ex-
 hibited by different kinds of citrus trees strongly indicate that
 it is caused by an organism. Fawcett, working in California, has
 isolated many kinds of organisms from first-stage lesions but
 none have produced the disease when inoculated into citrus trees.
 He states that, in certain cases, it has been possible to transmit
 the disease to healthy trees by inoculations with bits of tissue
 from diseased bark but that most of the attempts have failed.
 In one of the successful cases about two years elapsed before the
 disease became visible. Efforts are also being made in Florida
 to isolate the causal organism from psorosis lesions.
 Observations, in both California and Florida, indicate that
wounds or injuries of various kinds may pave the way for the
development of psorosis. This disease has been observed in sev-
eral cases to start on the shoulder of the callus formation about
old pruning cuts. In many cases, however, the disease starts
in branch crotches and at other places where no bark injury is
apparent. Although primarily a disease of bearing trees, at
least from 10 to 12 years old, psorosis occasionally occurs on
much younger trees in Florida. A severe outbreak of this dis-
ease was recently observed on tangerine nursery stock in a nurs-
ery in a densely wooded, low hammock, where especially favor-
able conditions existed for disease development.

 CONTROL OF PSOROSIS
 Although the cause of psorosis is not known, a bark-scraping
method of treatment, which is described later, has been found
very effective for the control of this disease. Fawcett has con-
clusively demonstrated the effectiveness of this method of treat-
ment for the control of psorosis in California when the disease
has not progressed to the late stages. His experiments have
shown the advisability of thoroughly scraping not only the area
visibly affected but also well beyond this area to head off the
advance of the disease in bark not yet visibly affected. He has
tested a large number of disinfectants, both with and without
scraping the outer bark. Almost without exception, the diseased
trees on which the bark was carefully scraped,showed the best