Aug. 15 , 1914 
Head Smut of Sorghum and Maize 
343 
In respect to its optimum temperature, then, the head smut is quite 
unlike those smuts which infect chiefly from seed-borne spores. 1 It is, 
on the other hand, closely similar to those infecting intraseminally—i. e., 
the loose smuts of barley and wheat (Appel and Riehm, 1911, p. 364)— 
and also seems to resemble corn smut, UstUago zeae (Beckm.) Ung., 
which, while infecting extraseminally, has a late period of infection and 
shows a more or less localized development. Preliminary observations 
on com smut indicating a similar relatively high optimum tempera¬ 
ture were made at the same time as Nos. 11 to 19, inclusive, in Table I; 
and it is this analogy, rather than that with the loose smuts, which has 
been supported by the evidence of inoculations and other experiments, 
presented later. 
The fact that the head smut is indigenous to a host from subtropical 
climates should also be pointed out in this connection. At low tem¬ 
peratures, however, the organism can not be said to be injured, although 
it grows very slowly, if at all. Even severe freezing does not kill it. 
Both the spores and conidia have been frozen at St. Paul, Minn., at out¬ 
door temperatures which reached a minimum of —26° C., in both a wet 
and dry condition, and some were still found to be viable, though frozen 
for over three weeks. Similar tests at Amarillo, Tex., and at Wash¬ 
ington, D. C., were generally confirmatory of these results, although much 
weathering sometimes appeared to destroy viability. 
The writer has not found the spores readily germinable after several 
years, as did Brefeld (1883, p. 95). Furthermore, the conidia have not 
survived periods of drying, lasting from four to eight months at ordinary 
summer temperatures. The method used for determining the latter was 
to smear some cover glasses with conidia from carrot-agar culture and 
leave in a Petri dish or culture tube for the period mentioned before 
transferring to a culture medium for test of viability. 
The organism has been found to develop well on malt extract and 
beerwort agars—perhaps even better than on carrot agar. A synthetic 
dextrose agar is also favorable. Plate XXXIV, figs. 2 and 3, shows the 
characteristic, rugose conidial growth. Carrot agar gives a more rapid 
growth, but the darkened central area of the culture shown in Plate 
XXXIV, fig. 3, is becoming brown. This may be caused by differences 
in drying or by the influence of contaminations near it in the plate. A 
malt extract prepared from germinated Amber sorgo seed was tried, but 
did not prove to be as favorable a medium as the others. On a 3 per 
cent cane-sugar agar the growth was scant. Gelatin is liquefied readily. 
While the organism grows well in 1 per cent peptonized (1 per cent of 
peptone) solutions of saccharose, lactose, levulose, dextrose, and maltose, 
1 See Herzberg (1895, p. 23) on UstUago avenae. Dr. H. B. Humphrey, at present pathologist in the 
Office of Cereal Investigations, has found in unpublished experiments that Tittetia tritici has an optimum 
temperature of very close to 20° C. 
