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 hyphse. 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. 



