LEAF-BLISTER OF OAKS 
This is a very common leaf-disease of oaks. Frequent epiphytotics, 
with more or less serious effects on the host, occur in the southern portion 
of the United States, while in the north very little damage is done. 
SYMPTOMS 
The leaves only are affected. In the dry material, NOTE—- 
1. The blisters; more or less circular; varying greatly in size. 
2. That in some cases the spots have become confluent and the 
entire leaf is much curled. 
3. That the blister is usually convex on the upper surface of the 
leaf and concave on the lower; however, this is not a constant character. 
4. That the upper or convex surface of the blister is, in young 
spots, lighter green than the normal leaf-tissue but, that in the older spots, 
it becomes more or less flecked with brown areas. 
5. That the lower or concave surface of the blister is a bluish 
gray in the fresh condition but changes on drying to a dirty or brownish 
gray. The coloration and velvety appearance of the under surface is due 
to the fruiting stage of the causal fungus. 
Make a SKETCH to show the symptoms of leaf-blister. 
ETIOLOGY 
The pathogene causing the leaf-blister of oaks is Taphrina coerulescens 
(Desmazieres and Montagne) Tulasne, an ascomycetous fungus belonging 
to the order Protodiscales. No fruit-body is formed by these primitive 
ascomycetes. The asci are developed directly from the mycelium in a 
hymenium on the surface of the host. The peach leaf-curl pathogene, 
Exoascus deformans (Berkley) Fuckel, is closely related to T. coerulescens 
(Desm. and Mont.) Tul. 
Life-history. As is the case with the peach leaf-curl fungus, the life- 
history of T. coerulescens is imperfectly known. Ascospores, borne on 
the blisters in the spring and summer, bud in the asci and form numerous 
secondary spores. These spores are supposed to hibernate in some way 
and initiate primary infections the following spring. 
The Primary Cycles are initiated in the early spring by the over- 
wintered inoculum (kind unknown). 
Pathogenesis. ‘The pathogenic activities of this fungus are exhib- 
ited entirely by the sexual or ascigerous stage. In fact no true conidial 
form of this pathogene or, for that matter, of any of the entire order, 
the Protodiscales, to which it belongs, is known. 
Examine the stained cross-sections of a blister and NoTE:— 
6. The mycelium. To what portion of the leaf is it confined? 
7. The layer of more or less angular asci standing at right angles 
to the lower epidermal cells. 
8. The wedge-shaped root-like projections pushed down between 
the lower epidermal cells and which serve as hold-fasts for the asci. 
9. The spores within the asci. They are small and either eight 
in number or numerous (probably in some cases as many as a hundred). 
Some authors have suggested that the budding of the eight ascospores, 
176 
