32 
Pathogenesis. The fungus passes the winter as mycelium im 
diseased fibers. Make thin tangential sections of the diseased tubers, 
stain with eosin and OBSERVE :— : 
7. The mycelial threads between the cells. Their densely 
granular contents. Note the ay of haustoria. (Read Connecticut 
Ann. rept. 1904:364, last paragraph. : : : 
° 8. The general absence of septa in the mycelium; this fungus is a 
cenocyte. ‘Toxines secreted from this mycelium kill the adjoining host- 
cells and their soluble contents are set free for the fungus. 
DRAW, or copy a part of fig. 6 in California Bul. 175 to show character 
of the mycelium and its relation to the cells of the tuber. 
The mycelium in the diseased tuber grows out into one or more of the 
shoots resulting in a systemic invasion. The diseased shoot is weak 
and dwarfed. From such shoots conidia are produced in great abundance. 
Such plants become centers of dissemination or sources of inoculum for the 
primary infections. After these conidia are produced and dispersed the 
fungus in these systemically invaded shoots dies with the host. (Read in 
Jour. Agr. Research 5:99-102.) copy pl. VI, fig. 1. 
The primary infections occur on the leaves; are relatively few and serve 
to afford inoculum for the secondary cycles. With the death of the leaves 
the pathogene in them perishes and the primary cycle is not completed. 
The lesions and pathogene-structures are like those resulting from the 
secondary infections in the leaves. : 
The Secondary Cycles arise from the conidia produced on the primary 
lesions. These conidia are distributed by wind during rainy or foggy 
weather, to nearby healthy plants where they germinate. : 
Pathogenesis. Make mounts of these conidia from material 
provided (freshly blighted leaves or cultures, if available) and OBSERVE :— 
_ . 9. The shape, contents and color of conidia. These conidia upon 
germination usually act as sporangia, giving rise to swarmspores. 
Study germinating spores, if available; N. Y. (Geneva) Bul. 141, pl. 
XII, and California Bul. 175, fig. 8; or photograph 5. MAKE OUT:— 
10. The manner of germination; number and form of swarmspores 
and their motility by flagella. 
DRaw to show points observed or copy from plates or photographs 
studied. 
After swimming about for a short time in the drop of rain or dew on 
the host-leaf, the swarmspores come to rest, round up, invest themselves 
with a thin cellulose wall and germinate by a germtube. Study slides; 
figures in the bulletin; or photographs 5 and 6. OBSERVE:— 
11. The relative size of the germinating swarmspore and the 
conidium. The difference in form of the active and germinating swarm- 
spores. 
12. The two methods of entrance of the swarmspore-germ- 
tube into the leaf. copy to show this. 
This germtube gives rise to mycelium within the leaf-tissues like that 
observed in the tuber. Haustoria-like branches are often sent from the 
intercellular mycelium into the leaf-cells. From this mycelium, branches 
are put forth on the underside of the leaf to form the conidiophores. 
Scrape some of these conidiophores from the leaf and mount in water. 
OBSERVE — 
