572 
Journal of Agricultural Research 
Vd. XXIV. No. 7 
Table II gives a fair idea of the effect of the range of temperature, 
3° to 33° C., upon germination and seedling growth. The optimum lies 
at an average temperature of 19° C. 
If we compare the host and fungus in their temperature relations in 
the light of the above data, we find a rather close correlation. 'The 
fungus shows a tolerance for somewhat lower temperatures, especially 
as to germination and initial production df sporidia (essential to infec¬ 
tion), while the host shows a tolerance for higher temperatures. How¬ 
ever, their optima for germination and growth are practically identical. 
INFLUENCE OF SOIL TEMPERATURE AND SOIL MOISTURE ON 
INFECTION 
In order to visualize quickly this phase of the problem, it may be well 
to review briefly the salient points in the life history of the fungus. 
The spores of Ustilago avenae^ the loose smut of oats, are for the most 
part seed-bome. When the oat seed is sown the spores germinate, 
producing masses of sporidia which continue to bud and multiply much 
like yeast. As the oat kernel germinates, the sporidia send out germ 
tubes and infect the young seedling through the coleoptile. By the 
end of 10 days or 2 weeks the mycelium penetrates through the coleoptile 
and subsequently establishes itself in the meristematic tissue of the 
growing point. According to various investigators, infection is limited 
to the first few days following germination of the oat kernel and none 
occurs after the growing point emerges from the coleoptile. 
As the plant develops, the fungus is carried upward in the growing 
point of the primary shoot, or of the lateral shoots. Previous to the 
time of heading, it is impossible to see any difference between smut-free 
and infected plants, except for a slight lagging in vegetative growth, 
which is not a safe diagnostic feature. At heading time, however, the 
fungus appears to gain file ascendency and the mycelium spreads rapidly 
through the young ovaries, in which are produced the masses of black 
spores. At times the fungus appears in linear pustules on the leaves, 
resembling leaf smut of various grasses, and very often the glumes are 
striated with the black spore masses. The histological features of the 
fungus have been adequately described by Lutman.® 
Obviously the problem in hand is concerned with soil infestation and . 
the physical and chemical conditions of the soil at the time of infection 
of the oat seedling. Subsequent atmospheric conditions may influence 
the vegetative growth of the host and thus determine the ability of the 
fungus to maintain its position in a rapidly dividing meristematic tissue- 
It is interesting to note in this connection, however, that the most 
vigorously growing plants appear to be the most heavily smutted. Just 
why two heads on one plant may be entirely smutted and three other 
heads on the same plant remain free from smut, or conversely, offers an 
interesting problem of disease escape. Furthermore, it is not uncommon 
to see a single spikelet of a single head smutted and all the rest of the head 
smut-free. 
As indicated by the three preceding paragraphs, soil temperature and 
soil moisture may have a marked influence on infection. Consequently, 
numerous series of experiments were performed in order to determine 
the influence of these two factors upon infection of the host by the fungus. 
The data secured are presented in Tables III and IV. 
• rUTMAN, B. B. SOME CONTRIBUTIONS TO THE U?E HISTORY AND CYTOLOGY OF THE SMUTS. In TiaXlS. 
Wis. Acad. Sd., v. i6, p. 1191-1244. pl. 88-95. x9io. Literature cited, p. 1225-1228. 
