^pr. 21 ,1923 
Gummosis of Citrus 
207 
In the comparative inoculations with Fusarium alone on February 
20, 1913, and February 25, 1915, No. 3 and 8 (Table VII), only a narrow 
layer of tissue along the cuts was killed, without exudation of gum. 
A thin outer layer of bark subsequently died about one inoculation, but 
otherwise the ejffect was not different in either case from that produced 
hy the uninoculated cuts on the same trees. In a number of other 
inoculations with species of Fusarium associated with gummosis shown 
in Table VII, only slight effects in killed tissue and only slight to medium 
'effects in gum formation were obtained. 
Although the experiments on this phase of the question have been 
too few as yet to justify definite conclusions, the results suggest that 
the severity of the disease may be slightly increased by adding Fusarium 
sp. to Pyihiacystis citrophthora at the time of inoculation. The char¬ 
acteristics of the disease except in rapidity of development, however, 
were the same as when P. citrophthora was inserted alone. 
KKSISTANCB or DIFF^RieNt SP^lBS AND VARIETIES OF CITRUS TO PYTHIACYSTIS 
GUMMOSIS 
Among the Citrus species and varieties that have been tested, the 
common lemon has the lowest resistance to Pythiacystis gummosis and 
the sour orange the highest. The sour orange usually is so resistant 
to Pythiacystis attack that even when the most favorable conditions 
are given by inoculation in wounds there is only a slight gumming with 
rapid healing of the wounded tissue and with total failure to produce 
a diseased lesion. The sour orange is also highly resistant to all other 
infectious gum diseases of importance. Mere gum formation, however, 
may be induced by suitable stimuli in sour orange as well as in other 
species and varieties. Of the forms which have been most used for 
stocks, the trifoliate orange probably stands next to the sour orange 
in resistance and the sweet orange next to the common lemon in sus¬ 
ceptibility, with the pomelo and the rough lemon between these two. 
Because these stocks are grown from seed there is a large possibility of 
variation in resistance within each variety, due to differences between 
'‘strains,’* and observations have indicated that such variation actually 
•exists. The following observations and experiments indicate in a rough 
way the relative resistance of some of the species and varieties. 
A block of 5,000 sweet-orange seedlings about 2^2 years old, growing 
in nursery rows on medium heavy clay loam soil, had been planted 
^adjacent to a block of 15,000 sour-orange seedlings of the same age 
and having the same care. All the trees had been irrigated rather 
frequently and heavily. On October 21, 1914, four representative rows 
of sweet-orange trees showed the following percentage of Pythiacystis 
gummosis: 
Row No. 
Number of 
trees in row. 
Number of 
affected trees. 
Percentage 
affected. 
I..... 
222 
23 
21^ 
34 
— *0 
212 
i %J 
63 
20 
180 
29 
Total.. 
827 
241 
Average..... 
29 
30617—23 - 2 
