FROMME: THE CULTURE OF CEREAL RUSTS 503 
(see TABLE 1) has determined *the inhibiting action of different 
concentrations of various acids and salts on the germination of 
aecidio- and uredospores of P. graminis. Carleton finds that com- 
pounds containing Hg, Cu, Fe, Pb, Cr, and strong acids inhibit 
the germination of uredospores of Puccinia rubigo-vera, P. graminis 
and P. coronata and that those which contain O, Na, K, Mg, S, C, 
and NH; in large proportions are favorable to germination. 
The effect of temperature on spore germination has also 
received attention. Eriksson (9) found that aecidiospores of 
Aecidium Berberidis, A. Rhamni, and Peridermium Strobi and 
uredospores of Puccinia glumarum, P. graminis and P. coronata 
often germinated more readily at a few degrees below zero C. 
and on melting ice than at higher temperatures. Uredospores of 
P. dispersa according to Marshall Ward (37) germinate after 
freezing in ice for ten minutes. He attributes any increase in 
vigor obtained in this way to the drying action of the freezing 
and not to the low temperature. They germinated also at 27° 
but failed to do so at 30° and were killed at 65° to 70°. The 
optimum is near 20°. They germinate readily, if the spores are 
properly ripened and fresh and the temperature does not rise 
above 25°, in light, darkness, or red light, but less readily in blue 
light. Gibson (14) reports tests with uredospores of Puccinia 
Chrysanthemi as follows: Fifty per cent germination at 6°-6.5°, 
free germination between 7° and 21°, all at 21°-25°, one eighth 
at 24°-25°, and none at 30°. If kept dry at 35° for eighteen hours 
and then removed to 17° they germinate freely. Johnson (18) 
has determined the minimum, optimum, and maximum tempera- 
tures for germination of uredospores of the cereal rusts. These 
are: for Puccinia graminis on wheat, oats, and barley, 2° to 31°; 
for P. rubigo-vera on rye, 2°-30°; and for P. coronata on oats, 7°-8° 
to 30°. The optimum was determined by the general vigor of 
the germination tube and for all forms studied lies between 12° 
and 17°. This is somewhat lower than for P. dispersa as deter- 
mined by Marshall Ward. Johnson suggests that these low 
cardinal temperatures may explain the difficulty of obtaining 
infections in very warm greenhouses and on hot summer days 
and may account for the observation that rust epidemics are 
favored by subnormal temperatures at critical infection periods. 
