May 17 ,1924 
Resistance of Oat Varieties to Stem Rust 
711 
by Stakman and Levine ( 16 , p. 5) and their associates have been used in these 
studies. They are as follows: 
0. Immune—N o uredinia developed; hypersensitive flecks usually present, but sometimes there is 
apparent absolutely no trace of mycelial invasion in the host tissues. 
1. Very resistant. —Uredinia minute and isolated; surrounded by sharp, continuous, hypersensitive, 
necrotic areas. 
2. Moderately resistant. —Uredinia isolated and small to medium in size; hypersensitive areas 
present in the form of necrotic halos or circles; pustules often in green, but slightly chlorotic, islands. 
3. Moderately susceptible.— Uredinia medium in size; coalescence infrequent; development of rust 
somewhat subnormal; true hypersensitiveness absent; chlorotic areas, however, may be present. 
4. Very susceptible. —Uredinia large, numerous, and confluent; true hypersensitiveness entirely 
absent, but chlorosis may be present when cultural conditions are unfavorable. 
X. Heterogeneous.— Uredinia very variable, apparently including all types and degrees of infection 
on the same blade; no mechanical separation possible; on reinoculation small uredinia, may produce large 
ones, and vice versa. Infection ill defined. 
Table I shows that the great majority of the varieties tested are susceptible 
to the oat stem rust used (usually grading from 3— to 4), a few are highly resist¬ 
ant (grading from 1— to 1+), and not one is fully immune (grade 0). 
The oats fall naturally into two well-defined groups on the basis of rust resist¬ 
ance, and there are few intermediates. Some of the slender varieties with weak 
straw may not show as large uredinia or as abundant spore production as the 
stouter-stemmed varieties with greater substance, but they may be quite as 
susceptible. It is quite possible, too, that of two varieties showing equal sus¬ 
ceptibility in the seedling tests in the greenhouse one may mature earlier than 
the other and so escape the worst ravages of the disease. This is quite distinct 
from the question of susceptibility. 
Examples of the susceptible group are given in Plate 1. A is a specimen of 
Avena sativa L. (Silvermine, I 20) and B is Avena saliva orientalis L. (Short 
Tartarian, C 1027), a type resembling Sparrowbill. In both varieties the fungi 
make an abundant growth and produce well-developed uredinia often confluent 
into broad streaks several inches long and bearing urediniospores in abundance. 
In sharp contrast to this are the resistant forms. The specimens photographed 
(PL 2) belong to Avena sativa orientalis and are of the White Tartar (White 
Russian) type. A was listed as Long’s White Tartar (C 1026) and B as White 
Tartar (C 1020). The fungus enters these hosts as readily as the susceptible 
varieties. In fact, each specimen is peppered with hundreds of minute uredinia, 
giving it a speckled appearance, but the development of each fungus is sharply 
limited, the sorus remains minute, and its spore output is negligible. Examina¬ 
tion shows that it frequently contains teliospores. 
Resistant varieties were found only in Avena sativa L. and its subspecies A. 
sativa orientalis L. All the available lots of A. brevis Roth., A . fatua L., A. nuda 
L., A. sterilis L., and A. strigosa Schreb. were found to be susceptible. 
For convenience the data on the resistant varieties of Avena sativa t from Table 
I, are presented separately in Table II. The two lots of the variety Richland are 
similar. The plants are erect, with fine, short culms, narrow leaves, and small 
sparsely-branched panicles of the general type of Kherson. The glumes are 
short, the kernels light yellow and awnless, and there is an occasional three- 
kerneled spikelet. The reaction to rust is 04- to 1, so far as observed. 
Four lots of Green Russian are included in the varieties tested but only one lot 
proved uniformly resistant to stem rust. The first of these lots (see Table I) 
contained only susceptible plants; the second and third contained a small per¬ 
centage of markedly resistant ones in addition to those susceptible; and the 
fourth, Iowa 91-4, appears to be a selection containing only the resistant strain. 
In the subspecies Avena sativa orientalis L., 15 lots proved resistant. They 
were received from several sources under seven different names. Close examina- 
