May *6. Z913 
Graminicolous Species of Helminthosporium 
649 
Ascigerous forms have been reported for a relatively small number of 
species of Helminthosporium parasitic on grasses. These are all readily 
referable to the genus Pyrenophora Rab. or to Pleospora Fries, depending 
on the presence or absence of bristles on the perithecium—a basis for 
generic distinction, which, while regarded by Winter (iS9, P‘ 493 ) with 
considerable justification as inadequate, has been recognized as valid by 
Saccardo {128, v, 2, p, 2j8) and Lindau (82y p, 42^). Helminthosporium 
bromi, H. tricici-repentiSy and H, teres have associated with them in this 
country ascigerous forms belonging to the genus P3U'enophora that occur 
in great abundance on dead host material in spring, Pyrenophora Ures 
(Died). { = Pleospora teres Died.) being found on the spike and culm of 
barley straw, Pyrenophora tritici-repentis (Died.) {^Pleospora tritici- 
repentis Died.) on the culm, and in slight measure on the decaying leaves, 
of quack grass, while Pyrenophora bromi (Died.) { — Pleospora bromi 
Died.) is very abundant on the leaves of awnless brome grass, Bromus 
inermis L. In the latter two species the perithecia usually reach perfect 
development, asci and ascospores reaching maturity early in May near 
Madison, Wis. Material collected at that period ejdiibited abundantly 
the preliminary swelling and circumscissile rupture of the ascus preceding 
the simultaneous expulsion of the spores from near the base of the 
dehiscing structure—a mode of discharge observed by Porter (no) in 
species of Pleospora, and in the species under consideration by Diedicke 
{28) as well as byAtanasoff (2). 
Pyrenophora teres, on the other hand, probably does not form alto¬ 
gether normal ascospores, if the observations made in the spring of 1919 
and 1920 may be taken as typical. The ascus may remain undeveloped, 
showing no trace of young ascopores; or ascospores maybe delimited, but 
become arrested in their development before or after septation has oc¬ 
curred; or certain segments of the spore may develop normally, the 
others eventually collapsing, giving rise to the unsymmetrical spores 
shown in Plate 3, D. As Diedicke {29) and Noack (95) have shown, the 
developing perithecia or sclerotia begin to proliferate condiophores with 
the advent of suitable conditions; and a perithecium involved in the pro¬ 
duction of conidiophores is not likely to show any further development of 
its ascospores. The production of conidia usually is large in the case of 
the sclerotial form on barley; moderate in the case of Pyrenophora tritici- 
repentis; and small in the case of Pyrenophora bromi. The latter species 
is probably the only one in which the production of ascospores plays an 
essential part in the resumption of growth in spring; for, as a rule, in the 
forms on quack grass and barley, the conidia produced on the sclerotium 
or immature perithecium would appear to play a relatively larger part in 
effecting early dissemination on the young host plants. In ^e case of 
the forms with dark, thick-walled spores, as for example, H. sativum and 
H, vaganSy with which neither sclerotia nor perithecia are known to be 
associated, the conidia are found to germinate readily in spring, since 
exposure in winter does not bring about any very decided diminution in 
viability. Conidia of H, sativum, H. oryzae, and H, ravenelii will ger- 
menate well one year after their formation, whereas those of H, bromi, H. 
tritici-repentis y and H. teres are mostly dead within one or two months. 
The absence of an ascigerous stage from the life history of the former 
species, and its presence in the latter, are probably not without signifi¬ 
cance in relation to the longevity of the conidia. 
