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On the twigs. This form of the disease appears to be rare except on 
certain varieties like the Lady apple. In Maine and other very northernly 
apple sections it is not uncommon on other varieties. In the material 
provided, OBSERVE :— 
9. The rough blistered character of the lesions, confined to the 
growth of the current year. DRAW. ‘ 
ETIOLOGY 
-The apple scab is caused by the conidial stage, Fusicladium dendriticum 
(Wallroth) Fuckel, of an ascomycetous fungus known as Venturia inequalis 
(Cooke) Winter (= V. Pomi (Fries) Winter). It belongs to that group of 
the ascomycetes known as Pyrenomycetes which have their asci enclosed 
in a more or less globose fruit-body, called a perithecium. 
Life-history. It is in the conidial stage that this fungus exhibits its 
parasitic nature. It lives superficially on the host or nearly so, simply 
prying off the cuticle or upper part of thé epidermal cells, and applying 
its mycelium closely to the host-tissues. 
The Primary Cycle is initiated by ascospores from perithecia in old 
leaves on the ground. 
Pathogenesis. Crush in potassium hydroxide a bit of the old leaf 
provided, examine and OBSERVE :— 
10. The 2-celled olivaceous ascospores. DRAW. These ascos- 
pores are shot from the ascus which protrudes through the ostiolum of the 
perithecitum. Ascospores are discharged only during rains or very moist 
weather in spring. They are carried to the young leaves just emerging 
from the buds. Here they germinate. 
Study Cornell Bul. 335, pl. IX and X, and opsERvE:— 
11. That but one cell of the ascospore gives rise to a germtube. 
Which cell? pRaw to show three stages in the development of the germ- 
tube. This germtube pierces the cuticle of the leaf or young fruit and initi- 
ates the scab-spot. copy Cornell Bul. 335, fig. 185. Again examine the 
scab-spots on the leaf and with the hand-lens or low-power, MAKE OUT :— 
12. The radiating branched mycelial threads. Why do they 
radiate from a center? Make an enlarged prawiNG of a scab-spot to 
show this habit of the mycelium. 
Study the mycelium, as shown in the prepared slides of apple leaves 
which have been cooked in potassium hydroxide and the epidermis, bearing 
the fungus, peeled off. OBSERVE:— 
13. The form, size and septation of the mycelium. Its color and 
method of branching. No haustoria are sent into the host-cells and the 
mycelium does not at this stage penetrate beyond the epidermal cells. 
14. That the conidiopho-es arise in clusters or singly from this 
spreading mycelium. Note their form, length and color. 
15. The conidia lying about, which have been broken off from the 
conidiophores; their form, size and colorand point where they were attached 
to the conidiophore. Several conidia may be produced from near the same 
point on a conidiophore. How? See if you can find a conidiophore which 
shows this. ; 
Make prawincs to show the mycelium, conidiophores and conidia 
in their proper relations to each other and to the host. 
