Apr. 21,1933 
Gummosis of Citrus 
219 
detail in a subsequent bulletin of the California Agricultural Experiment 
Station. 
Injuries of various kinds to the bark, not only near the soil but any¬ 
where on the trunk or large branches, may lead the way to infection and 
development of Botrytis gummosis when the conditions of moisture and 
temperature are also favorable. This disease is frequently severe on 
living tissue of trees that have been injured by frost. The fungus may 
become established in such trees first in a small portion of dead or dying 
tissue and then advance rapidly into tissue which appears to be sound. 
A desquamated condition of bark, fairly common on old lemon trees 
in the California coastal regions, is also frequently accompanied by 
Botrytis gummosis. It furnishes dead outer bark tissue from which the 
fungus may advance. This desquamated condition is similar in appear¬ 
ance to that which usually follows inoculations with Botrytis cinerea 
on sound tree trunks and with which it is often confused, but which is 
thought to be due to other causes. 
The previous use of “neaCs-foot oil'* in the treatment of gummosis 
also encouraged the growth of this fungus. Lemon trees previously 
treated at Santa Paula by scoring the bark and painting with neat's-foot 
oil were observed in February and March of 1912 to have their trunks 
fairly well covered with a gray coating consisting of the sporophores and 
spores of Botrytis. The bark on these trees was found to be in various 
stages of soft decay with the exudation of large masses of gum. Experi¬ 
ments also showed that this fungus grew better on bark treated with 
neat*s-foot oil either before or after infection by the organism than on 
bark free from this oil. More recently the application of neat*s-foot oil 
to Citrus trees has been largely given up, and the more severe stages of 
this disease, such as were previously seen, have not been observed. 
CONTROL 
The principles governing the control of this type of gum disease are 
similar to those of Pythiacystis gummosis: (i) The prevention of infec¬ 
tion by avoiding injuries, and the use of fungicides, and (2) if that is not 
done, the elimination of the invaded tissue. As the result of many 
different experiments in which growers took a prominent part, a method 
consisting largely of scraping off the outermost layers of bark proved 
to be best adapted for treatment of this disease. The portion where the 
bark is totally killed is cut away, but beyond this, where only the outer 
layers of bark are dead, these outer layers only are scraped off, leaving 
the live inner layer next to the cambium intact. To prevent further 
invasion of bark it is usually found necessary to scrape the sound bark 
several inches beyond the margin of the affected region. A sharp 
curved tool made on the principle of a box scraper is in general use for 
this purpose. Where both types of gummosis are present on the same 
trees, as is frequently the case, this method is applicable to the combined 
lesions produced. The cut or scraped portions are then painted with a 
fungicide. Bordeaux paste, and some of the coal-tar products which 
contain only the heavier oils, have given good results in experimental 
work and in practice. A discussion of the experiments on which these 
control methods are based will be reported in a bulletin of the California 
Agricultural Experiment Station. 
