Apr. 21,1923 
Gummosis of Citrus 
197 
be detected without a microscope, and even then it is somewhat difficult 
to see because of the hyaline nature of the vegetative hyphae. The in¬ 
vading hyphae frequently die out rapidly behind the marginal fringe of 
advance, and quite often they die out completely over a part or all of 
this outer margin, so that progress of the disease is checked or entirely 
arrested. Such cases are often found in trees having some resistance, 
’especially in orange and pomelo trees, or where the weather conditions 
subsequent to infection become unfavorable to the parasite. 
In trees on which the disease has been present for a long time, the 
dead bark over the invaded portions dries, shrinks, and cracks. The 
larger cracks are mostly vertical, with smaller horizontal cracks (PI. i, 
D; 2, D, E; 5, E, F). A thin layer of the wood immediately under the 
invaded bark will usually be found to be infiltrated with hardened reddish 
brown gum which protects the under layers from rapid drying out and 
appears to protect the wood to a considerable extent against the entrance 
of wood-rotting fungi. 
On old sweet-orange trees, the invaded areas are usually less extensive 
and more restricted laterally than on the common lemon. There is 
usually less gum than on the lemon. With the orange there is greater 
tendency than with the lemon for the invading fungus to die out—for 
the invaded area to become self-limited. Frequently the invaded areas 
on old sweet-orange trunks extend upward from the soil surface as narrow 
tongues of killed bark. On younger and frequently on older vigorous 
orange trees growing on heavy clay soils the disease may assume much 
the same characteristics as on the common lemon. 
ISOLATION AND IDENTIFICATION OF PYTmACYSTlS CITROPHTHORA 
Culture tests were made from different parts of gummosis lesions, 
beginning at the center of a lesion and taking samples about i cm. apart 
upward, downward, and laterally from the center. A number of organ¬ 
isms were found, as would be expected, in the dead and discolored bark 
tissue. From recently killed and dying bark tissue at the advancing 
majrgin of the darker areas (invaded zone) a fungus was obtained which 
was afterwards found to be identical with that producing brownrot 
mentioned by Smith and others (57) of lemon fruits and which, even 
before its identity with Pythiacystis citrophthora was known, was shown 
to be capable of reproducing the same type of bark disease. Beyond the 
margin of the killed areas, however, all through the outer gummous 
zone the inner bark tissue was usually found to be entirely free from 
microorganisms, although internal gumming had set in 15 to 60 cm. 
above and somewhat less below the margin of the killed tissue. 
Pythiacystis citrophthora was first isolated from the bark of diseased 
lemon trees in September, 1912, at Whittier, Calif., but its identity was 
not recognized at that time. 
The method which the author finally found most useful for determining 
the fungus was to cut out with a sterile scalpel small bits of diseased 
tissue from the margin of the invaded portion and place them in tubes 
of corn meal agar. After two to four weeks’ growth on this medium 
the characteristic sporangia were formed. By this method pure cultures 
were readily obtained, and when other organisms were present as con¬ 
taminations Pythiacystis citrophthora could be identified by its character¬ 
istic sporangia. The Pythiacystis may readily be separated from con¬ 
taminating fungi by transferring the mixed culture to a jar of water 
