192 
Journal of Agricultural Research 
Vol. XXIV, No. 3» 
conditions which favor infection and invasion of the host by causat 
parasites. 
Since, however, a nonparasitic explanation for these gum diseases in 
California had become so firmly established in the literature of the sub¬ 
ject, it has been necessary to carry on a large series of carefully con¬ 
trolled experiments before concluding that fungi are necessary factors^ 
in the initiation and development of certain of these diseases. For the 
same reason the author deems it necessary to present his data in consider¬ 
able detail. 
. The failure to recognize the parasitic factor in previous investigations^ 
as regards Pythiacystis gummosis at least, is probably due in part to the 
following facts which will be brought out in more detail later: (i) The 
fungus initiating the disease is found in the bark in its vegetative stage 
only; (^2) it dies out rapidly in tissue already invaded, leaving only a, 
fringe or band of live mycelium at the advancing edges of the invaded 
tissues; (3) under adverse temperature and moisture conditions the 
organism frequently dies completely in the tissue, especially on resistant 
varieties, giving the appearance of a pure physiological effect; (4) im 
advanced stages the stimulus to gum formation spreads out far in advance 
of the band of invading mycelium, producing an outer gummous zone 
in which localized gum pockets are frequently formed. This outer 
gummous zone is usually free from the causal organism. Any attempt 
to find the organism in regions other than the narrow band at the outer 
rim of the invaded zone would result in failure. 
HISTORICAL REVIEW OF CITRUS GUMMOSIS 
In this historical review of the literature of Citrus gummosis it has beea 
found impossible to separate with certainty the different forms of gum¬ 
mosis which recent investigations have shown to be distinct. It is. 
believed, however, that most of the more destructive, rapidly developing; 
forms mentioned by various writers were similar to those represented by 
Pythiacystis gummosis in California and mal di gomma in Florida^ 
Two of the earliest European writers on Citrus, Ferrari (29, p. 156-158) 
in 1646® and Sterbeek (do, />. 777) in 1682, briefly discuss certain forms 
of Citrus gummosis occurring in the orange plantings of Europe. Two* 
other early writers on Citrus referred to by Savastano {52) as mentioning 
some minor form of gummosis were Clarici in 1726 and Corrado in 1787. 
The first highly destructive type of gummosis of Citrus on record 
appeared in the Azores in 1834. Sweet-orange trees which had grown 
to the age of 200 to 300 years, and which were producing 6,000 to 20,000 
oranges apiece, were found by Fouque {32, p, 837) to be affected with 
a very destructive form of gummosis (i). Yellow gum is mentioned as 
exuding on the trunks near and sometimes benea^ the surface of the 
grdund. The trees put on heavy crops of fruit, and the leaves turned 
yellow and fell off in great quantities. This brief description appears 
to indicate a form of disease resembling Pythiacystis gummosis. It was 
supposedly transferred from the Azores to the vicinity of Lisbon, Portu¬ 
gal, where a similar disease appeared in 1865. 
A similar type of Citrus gummosis, according to Savastano (52), was 
extremely destructive in many Mediterranean localities. It was estab¬ 
lished in Messina, Italy, in 1863 and was reported near Reggio in 1864. 
• Reference is made by number (italic) to “ Literatxxre cited," pp. 232-^35, 
