126 
sphaera leucotricha (E. and E.) Salm., the mycelium may winter in a semi- 
dormant condition within the host-buds on the embryonic leaves. As 
these buds open and the leaves and shoots develop in the spring, the myce- 
lium grows out over them and produces conidia, which, scattered to healthy 
shoots, may initiate primary infections. (See U.'S. Agr. Dept. Bul. 
120:9-10.) { . . ; 
Ascospores are, however, responsible for the primary infections in 
the case of most powdery mildews and often even in those in which hiber- 
nating mycelium is known. ery 
Pathogenesis. Remove with the scalpel, several perithecia of one 
of the pathogenes indicated by the instructor. Mount in a drop of potas- 
sium hydroxide. Cover and, while examining it under the low-power, 
crush the perithecium by gently pressing on the cover-glass with the point 
of the scalpel. ORSERVE:— ; 
25. The irregular crack in the perithecium from which one or 
more asci are forced out. How many in this case? adi 
: 26. The large ellipsoidal ascospores usually remaining within 
the asci; number in each ascus; color; contents. 
27. The thin place in the wall of the ascus at the apex. At 
maturity this dissolves as the perithecium cracks open and the ascospores 
are forcibly ejected into the air. 
Make a diagrammatic DRAWING of a cracked perithecium with protrud- 
ing asci discharging spores. 
Borne by the breeze, these ascospores fall wpon young shoots or leaves 
of the host and germinate. The germtube grows out over the surface 
and sends a haustorium into the epidermal cells, from which point the 
branching mycelium develops. Study germinating ascospores or illustra- 
tions provided, especially Bot. Gaz. 29, pl. XI-XII. Make a diagram- 
matic DRAWING of a germinating ascospore with haustorium. 
To study the mycelium, scrape some from the surface of young spots; 
tease apart in water or potassium hydroxide; cover and examine. oB- 
SERVE -— 
28. The broken pieces of irregular, branched mycelium; septa, 
color and contents. 
29. The large, ellipsodial or ovoid conidia; color and contents. 
Several may be found attached in a chain or even still on the conidiophore. 
30. The short conidiophores,—upright branches which bear the 
conidia ina chain. (See the demonstration specimen under the binocular 
microscope; or U.S. Agr. Dept. Bul. 120, fig. 2.) 
Make a diagrammatic DRawING to show the vegetative structures 
in position on the leaf-surface. — 
The conidia are produced in great quantities. They give the powdery 
appearance so characteristic of these mildews. They are scattered by the 
wind and initiate secondary cycles. 
After a period of conidial production, the mycelium begins to form the 
sexual fruit-bodies,—-the perithecia. These usually begin to develop 
toward the end of the growing-season but before the leaves fall. In some 
cases as in the apple mildew-pathogene, P. leucotricha, the perithecia 
are formed on the twigs. The detailed study of the development of the 
sexual bodies and the formation of the perithecium cannot well be under- 
taken in this exercise. (See de Bary, Morphology and Biology of the 
Fungi. p. 226, fig. 107.) 
