_ 874. REPORT OF 
(haustoria) which also serve to absorb the nourishment required fo 
fora time upon the surface of the host—most usually upon the 
-capable of germinating at once when favorable conditions are found. 
rs Ste % ates 
mm 
OF AGRICULTU 
4 
\ 
[H COMMISSIONER 
the further growth of the mildew.* After the mildew has grown — %, 
1s 
foliage—it begins to send up vertical branches from the net-work “ata 
horizontal filaments. These upright threads quickly form small |, 
oblong cells by division walls across the filament near the tip (Plate 
XI, Fig 1). These cells are the summer or asexual spores and are 
These conidial spores furnish a very rapid method of propagation 
for the mildew, and on account of their small size and powdery ~ 
nature they may be carried for long distances by the wind and thus — 
spread the parasite with startling speed. ays! 
Up to this point in the life history of the Hrysiphee one speciesis 
so much like that of the others that, unless something further is — 
known, the form is not classified, or at least only provisionally. ~ 
Oidium is an old genus which was used to include the conidial forms 
of the Hrysiphee, and to this time this is still true in some measure. ~ 
For example, Oidiwm Tuckeri is the conidial form of a mildew which |~ 
» 
has ravaged the vineyards of Europe, but of which the final state is 
not known. We have a mildew of the vine which bears its sexual 
7 eee and is classified as Uncinula spiralis, B.and C. It may be that 13 
this is the species which leads an incomplete existence in the EKuro- 
pean vineyards, owing to the changed conditions that there obtain. 
Any student of this group is quite certain to have an undetermined ~ 
ee of specimens that include only conidial forms. The writer — 
as recently collected some branches from a live oak in California, 
the whole surface of which, dwarfed leaves included, is heavily 
coated with a powdery mildew, but from a lack of the whole life his- 4 
tory the specimens remain among the unclassified, vo ae 
he genera and species of the Hrysiphew are founded uport the — 
sexual spores and the cases (perithecia) which bear them. The for- 
mation of these spores takes place usually after the season of most 
rapid growth of the mildewis closed. Two filaments unite theircon- 
_- tents (Fig. 3) and the invigorated protoplasm begins a new form of ~ ~ 
development, in which a sac is formed consisting of a nearly spher-. 
ical’ shell of thick walled cells (Fig. 4). In this hollow sphere are pro- 
duced the spores borne within one or more sacs. To repeat, the male 
element of one filament fertilizes the female cell contents, and the 
latter produce a number of spores which are borne within a sac,and 
this sac (or sacs) is contained within a thick-walled body, all of 
which is seen as asmall dark speck upon the surface of the mildewed 
plant. The fungus at this time is usually of a rusty brown color, in- ~ 
stead of white and powdery as in its earlier summer condition. These 
sexual or perithecial spores (ascospores) do not germinate at once, ~~ 
but remain within the protecting covering until spring, and then be-- 
gin a new series of developments and repeat the mildew of the pre- 
vious season. The genera of Hrysiphew are founded upon the num- _ 
ber of spore sacs (asci) within the perithecium and.the character of 
the arms or appendages which are developed upon the spore case. _ 
There is a third form of spore which is borne in large numbers within 
y’ 
4 
“ 
*In Erysiphe pannosa (Podosphera pannosa, Link), the Rose mildew, e. g., very 
thin tube-form projections appear on that side of the colorless, septate, mycelial. 
threads which rest on the upper surface cf the rose leaf, These projections bore 
through the outer wall of the epidermal cells, and these swell out in the interior 
into a bladder-shaped body. These bladder-form projections represent the com- 
plete haustorium.—Sorav_Er. ; 
