33 
13. Their form and structure. Distinguish the conidiophores 
from the large leaf-hairs. Note the peculiar swellings on the branches. 
14. The ovate conidia; their manner of formation and their 
attachment to the conidiophores. (See also photograph 5 or N. Y. (Geneva) 
Bul. 141, pl. XII.) 
If freshly diseased leaves are available, peel epidermis from lower 
surface of spct. Mount and study, MAKING ouT:— 
15. How the conidiophores emerge from the tissues of the leaf. 
(See also N. Y. (Geneva) Bul. 241, pl. XII, fig. 2; Maine Bul. 169, fig. 15 
or California Bul. 175, fig. 7.) 
Make a DRAWING to show the leaf-surface, conidiophores emerging 
and conidia attached both at the apex and side of the branch. 
These secondary conidia in turn give rise to other secondary infections 
on healthy leaves or they fall to the ground during rains and produce 
swarmspores in the soil. These find their way to the growing tubers which 
they penetrate by a germtube. This gives rise: therein to the mycelium 
that is to carry the pathogene over to the next season and invade the shoots 
and develop conidia for the primary infections. Thus are the secondary 
or at least the late secondary cycles completed. 
Saprogenesis. As already indicated, a true saprogenic develop- 
ment of this pathogene in nature is not known. It will, however, grow as 
a saprophyte on certain culture-media. Here it will sometimes produce 
not only conidia but also resting-spores, called oospores. These have 
never certainly been observed in the tissues of the host. One might 
expect to find them in the dead tops killed by the pathogene or in the 
rotting tubers since other species of Phytophthora regularly form oospores 
which are long-lived and serve to carry the fungus through the winter 
or drought-periods. Worthington G. Smith thought he had discovered 
them. (See Diseases of Field and Garden Crops p. 295-310.) No observer 
since Smith has been able certainly to find them in nature. Jones of Ver- 
mont, found in his pure cultures, bodies which are strikingly like the 
oospores figured by Smith. (See Science 29:271.) Finally Clinton of 
Connecticut, succeeded in growing the oospores of P. infestans on oat-agar, 
and has even produced what he believes are hybrid oospores of P. infestans 
with P. Phaseoli and also with P. Cactorum. (See Connecticut Ann. rept. 
1909-1910: 753-774, pl. XXXVIII-XL.) 
REPORT 
1. Discuss tuber-infection; where and how it occurs; two 
methods of prevention, explaining why each method is efficient. 
2. Give detailed directions for the control of the disease. 
