136 MANUAL OF FRUIT DISEASES 
rhizomorphic strands extending lengthwise of the twigs and 
petioles; (4) in later stages the leaves fall. 
Cause. 
The causal fungus, Hypochnus ochroleuca, hibernates as 
sclerotia on or near the terminal buds. As new twigs develop 
in the spring the fungus renews its activities by sending out 
rhizomorphs from the sclerotia. By this means the pathogene 
spreads over the lower surface of the leaves, causing them to 
droop and die, and finally to fall prematurely. In some cases 
the fruit is an object of attack. The rhizomorphs follow the 
petioles and main veins of the leaves, finally separating into 
single mycelial threads. Sometimes mycelial wefts are found 
on the lower sides of the leaves; these are always connected 
with the strands or rhizomorphs on the petioles and the strands 
in turn are in organic connection with the sclerotia on the twigs. 
Occasionally the weft of mycelium on the lower surface of the 
leaves assumes a more even texture and ultimately becomes the 
fruiting layer of the fungus. Here the weft consists of a very 
closely woven mass of hyphe. Erect hyphal threads become 
basidia, each with four spores. The spores are capable of re- 
producing the structures of the pathogene: the mycelium, scle- 
rotia, rhizomorphs, and finally the basidia. This fruiting stage, 
however, is rare, and the dissemination of the fungus is effi- 
ciently accomplished by the sclerotia and rhizomorphic strands. 
Throughout the summer the mycelium spreads from one leaf to 
another, fastening them together in a mat. This matting of 
leaves constitutes one of the peculiar and striking symptoms 
of Hypochnus leaf-blight. By midsummer the rhizomorphs 
form new sclerotia on or near the uppermost buds, where the 
winter is again passed. The rule seems to be that sclerotia 
develop in greatest abundance on the distal buds of the twigs 
where they can be of maximum use in perpetuating the fungus. 
Sclerotia occasionally develop on the fruit, but in general they 
are more prevalent on the lower, shaded side of the foliage. 
