210 
Journal of Agricultural Research 
Vol. XXIV, No. 3 
^ The lesions here, though small at the time of observation, give some 
indication of differences in resistance. The common lemon was most 
afifected, followed by the sweet orange (Valencia and navel) and the 
citron. Sour orange, as before, was quite resistant, though showing 
some effect, but was already beginning to form callus at the cut. 
Experiments to test the effect of Pythiacystis citrophthora on small 
roots of different species of seedlings were also made.® 
On November 15, 1915, mycelium of Pythiacystis and lemon fruits 
affected with Pythiacystis in different tests were placed on the healthy 
roots (with and without punctures) of the common lemon, sweet orange, 
pomelo, and sour orange. When last examined, on May ii, 1916, only 
the roots of the common lemon, where inoculated with Pythiacystis 
citrophthora (with punctures) and where inoculated with diseased lemon 
fruit (without punctures), were killed. The fungus was reisolated from 
the lemon root which had been inoculated with P. citrophthora. This 
experiment gives additional weight to the correctness of previous observa¬ 
tions that even small and medium sized roots of the common lemon are 
frequently attacked by P, citrophthora, but that the small roots of other 
varieties are quite resistant. 
Inoculations reported in other sections of this paper also give further 
indication of differences in resistance, especially as between the common 
lemon and sweet orange. The reasons for these differences in suscepti¬ 
bility is an interesting question in itself, which has not as yet been 
investigated. Our inoculation experiments would indicate that the 
differences in resistance in sour orange, sweet orange, and lemon, at least, 
can not be confined to the superficial layers of cells. If it were so limited 
the insertion of the parasite into cuts should cause equal effects in each 
variety; 
MAI, DI COMMA IN RELATION TO PYTHIACYSTIS GUMMOSIS 
Mai di gomma, due to Phytophthora terrestria Sherbakoff, is a gum 
disease with close relationships to Pythiacystis gummosis. It has 
previously been pointed out that certain phases of the Pythiacystis 
gummosis occurring on or near the main roots of sweet-orange trees 
are similar to those of mal di gomma, or footrot. 
For this reason, certain footrot-like forms due to Pythiacystis citroph¬ 
thora in California have previously been known by the name of mal di 
gomma according to Smith and Butler (56), 1908, and Fawcett (26). 
Since the name mal di gomma was first used in Florida to designate a 
common Florida gum disease, which is now known to be due to Phytoph¬ 
thora terrestria, it is proposed to restrict its use (in this country at least) 
to the disease due to this fungus. 
This type of gum disease affects, for the most part, the bark on the 
lowest portion of the trunk and the upper portion of the first main roots, 
mostly below the surface of the soil. Gum usually forms on the trunk 
of the tree above the soil. The inner bark and finally the wood under- 
* Boxes were constructed with one side consisting of glass plates inserted into grooves and covered by 
a removablewooien door fastened by hooks to hold it against the glass. After the young trees had become 
established in these boxes, the glass plate was removed to make inoculations on roots that had grown out 
against it and was then replaced in its former position. The assistance of Mr. E. E. Thomas, for whom 
similar experiments with Eusarium were made, is here acknowledged. 
