202 
Journal of Agricultural Research 
Vol. XXIV, No. 3 
inoculations with diseased bark or fruit. The following is a typical 
example. 
On November 23, 1912, a lemon tree about 18 years old, having a 
trunk about 30 cm. in diameter, growing at Santa Paula, was inoculated 
by inserting a very small bit of mycelium from a culture of Pythiacystis 
citrophthora into a 2-cm. vertical cut made 15 cm. above the bud union. 
The surface of the bark had previously been washed with water and 
with alcohol and the inoculated cut was covered with oiled paper. 
No effect from the inoculation was evident in 37 days, but in 42 days 
an invaded surface area of 5 by 9 cm. was observed, and this increased 
to 8 by II cm. in 76 days and to 8 by 35 cm. in 7 months, with copious 
exudation of gum (PI, 3, A). A similar cut without inoculum, on the 
opposite side of the same tree, healed rapidly. 
A strip of bark 10 by 68 cm., as seen in Plate 3, B, was cut out at this 
time, for the purpose of cultural examination and for enzym experi¬ 
ments, to be discussed in Part III of this paper. Cultures made from 
this strip of bark at various places, by cutting bits of tissue from the 
inner side, yielded Pythiacystis citrophthora at both the upper and the 
lower margin of the invaded zone (see white line shown on the plates) 
but not at points 25, 8, and 16 cm., respectively, above this killed area, 
within the outer gummous zone. 
On removal of the bark, the cambium adjacent to the dark brown or 
blackened area appeared yellowish. This discoloration extended only 
slightly laterally but to a much greater distance from the margins of 
the cut upward and downward. In this outer gummous zone interior 
gum had formed in places 60 cm. or more upward from the killed and 
invaded margins. The causal organism, as indicated by isolation tests 
in numerous other cases, does not extend into this outer gummous zone. 
To remove the causal organism in treatment, therefore, it is necessary to 
cut away the bark only a short distance beyond the discolored region. 
Experiments have shown that when this is done the gum will cease 
forming, the further extension of the yellow, gummy zone will stop, and 
the bark over it will usually return to normal condition. 
The cut-out area and surrounding bark shown in Plate 3, B, was 
painted with Bordeaux paste. Three weeks later, June 27, 1913, gum 
was seen to have continued to ooze out at the upper angle, but no further 
bark had died. The same tree is shown in Plate 3, C, in September, 1914, 
and in Plate 3, D, in June, 1920. No effect on the foliage or health of 
the tree, such as is usually seen in severe cases of Pythiacystis gummosis, 
was noticed in this tree. The organism was probably removed in time 
to obviate visible injurious effects. 
The results of a number of other inoculations on lemon trees with 
cultures of Pythiacystis citrophthora are summarized in Table III; others 
on lemon, orange, and other Citrus varieties, will be found described 
under ‘"Resistance” and “Mai di gomma.” In all, about 90 inocula¬ 
tions with pure cultures resulted in Pythiacystis gummosis, from 20 of 
which the organism was reisolated. 
