IOWA ACADEMY OF SCIENCE 
17 
premature dropping. Frequently other soft parts are also attacked 
which renders the injury to the host much greater. The loss to 
farmers and horticulturists through this source is often much 
greater than is at first apparent. The weakening of the vitality 
of the plant brings a less abundant harvest. 
The choice of hosts by the various species of Erysiphacese is 
varied. Some, e. g. Uncinula geniculata on Morus rubra, are con- 
fined to one host plant. Others, e. g. Uncinula circinata on species 
of Acer, are confined to a single genus. Still others, e. g. Uncinula 
salicis on Salicacese, are confined to one family. Lastly, some grow 
on a great variety of host plants, e. g. Erysiphe polygoni. 
Sometimes perithecia, when crushed, instead of emitting the us- 
ual asci emit a stream of small oblong spores 6.5 — 10.5x3.5 — 6 
micra immersed in a colorless gelatinous substance. In such cases 
smaller bodies oval to pyriform in shape will also be found. These 
rupture irregularly and emit the same kind of spores. Often these 
bodies are found where no perithecia have formed. They belong 
to a plant parasitic on the mildew. By careful examination the 
delicate hyphae of the parasite may be seen within the hyphae of 
the mildew. If the mildew attacked is in the perithecial stage 
the perithecium is filled with its fruits. If the mildew is in the 
conidial stage the attack of the parasite seems to prevent the for- 
mation of perithecia but the fruits of the parasite are often found 
in abundance. This parasite is Ampelomyces quisqualis Caseti. 
(See plate I.) It undoubtedly serves to hold the mildew in check. 
It seems to attack all species of Erysiphaceae. During the past 
two years it has been abundant in Decatur County, infesting the 
mildews on the following hosts: 
Aster cordifolius L. 
Aster laevis L. 
Aster multiflorus Ait. 
Aster salicifolius Lam. 
Adicea pumila (L) Raf. 
Ambrosia trifida integrifolia (Muhl.) T. & G. 
Carduus altissimus L. 
Cucumis sativa L. 
Hydrophyllum virginicum. 
Lactuca canadensis L. 
Lactuca ftoridana (L.) Gaertner. 
Lactuca sagittifolis Ell. 
Monarda fistulosa L. 
Physalis heterophylla Nus. 
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