DIE-BACK OF APPLE TREES, CAUSED BY 

 CYTOSFORA LEUCOSTOMA (PERS.) SACC. 



By Paul Andries van der Byl, M.A., D.Sc, F.L.S. 



{Plates 20-25 ^"^ fo\tr text figures.) 



Introduction. 



Aderhold (1) (1903J describes a very destructive disease 

 in cherry trees in the districts along the Rhine as due to Valsa 

 leucostoma, Fr. The fungus attacks the larger branches and 

 main stems of the trees, gaining access, in part at least, through 

 the injuries caused by late frosts. From these centres it spreads, 

 causing a one-sided growth of the twig and presenting a cancer- 

 like appearance, accompanied by a copious exudation of gum. 



The parasitism of the fungus is definitely established by 

 numerous inoculation experiments, and the relation of the fungus 

 to frosts discussed. 



Ellis and Everharl (2) record it on peach, plum, and almond 

 trees in Carolina, Pennsylvania, and New Jersey, and state that 

 it probably exists throughout the country where these trees are 

 found. It is also reported as parasitic on stone fruits in Aus- 

 tralia (3). 



Rolfs (4) in 1 907 contributed to om* knowledge of the 

 parasitic nature of the fungus, and showed it to be an active 

 parasite attacking the twigs, limbs, and trunk of peach, plum, 

 apricot and cherr}' trees. On the peach he found infection to 

 take ])lace at an}- time during the growing season through 

 wounds, being most noticeable in the s]:)ring months. Alternating 

 freezing and warm periods during the late winter months were 

 found favourable to infection. Twigs killed during the winter 

 months at first have a dark i)urplish skin, which later, on the 

 infected areas, becomes leathery and shades into a scarlet and 

 purple. The leathery, coloured areas finally change to drab, 

 and the skin on diseased tissue becomes loose and wrinkled. 



Black fruiting bodies {Cytospora riihescens Nitschke) 

 appear on the drab-coloured areas, enlarge and jnish a white 

 disc-like cap through a transverse slit in the epidermis. During 

 wet weather these black Cytospora bodies push out very fine red 

 threads, which are composed of masses of spores. These spores 

 are soon scattered by rain and insects, and start new points of 

 infection. 



The diseased portion of the twig soon becomes constricted, 

 making the division between the dead and living tissue ver^• 

 marked. Gum pockets, which rupture the epidermis and pro- 

 duce a copious flow of gum, also frequently form. During the 

 spring and summer months the foliage of infected twigs 

 frequently wilts and takes on a brown, blighted appearance. 

 This efifect is produced by the fungus girdling the twigs. A 



A 



