548 DIE-BACK OF APPLE TREES. 



These spores are readily washed away by water, and thus 

 further infection is brought about. 



Etiology. — This malady, like so many plant diseases, is 

 caused by a fungus that lives as a parasite on the affected tree. 

 This particular fungus on apple trees is known as Cytospora 

 leucostoma (Pers.j Sacc. 



The genus Cytospora is placed in the group of fungi — 

 " Fungi Impei'fecti'' — the complete life-history of whose mem- 

 bers are as yet imperfectly known. For many of the fungi of 

 this group later researches have traced their complete cycle, and 

 the majority of these have been referred to the Asconiycetes. 

 The conidia of Cytospora are produced in distinct pycnidia, and 

 the genus is thus placed in the " Splucropsidiales " of the 

 " Fungi Imperfecti." 



As mentioned in the introduction. Rolfs, by cross-inocula- 

 tion, has demonstrated that the perfect stage of the Cytospora 

 infecting stone fruits belongs to the genus Valsa of the x\scomy- 

 cetes. 



The only fruiting body of the fungus noticed on diseased 

 apple trees has l)een the conidial fructification, and the complete 

 stage failed to develop in cultures in the laboratory. The media 

 on which, and the cultivations under which, cultivated were 

 evidentlv not suited to the formation of perithecia, which, as far 

 as I am aware, has never yet for this genus been ol)tained in 

 pure cultures. 



Just how this parasite enters the trunks, etc., is not known. 

 Wounds would form favourable places of entrance, particularly 

 on the trunks and thicker branches, and it is not unlikely that 

 infection may also take place through the buds of young shoots. 

 This would explain the dying back of the branches. 



Conditions favouring the development of this disease have 

 not thus far been studied. As reported by Rolfs, it will pro- 

 bably be found that trees growing in unhealthy surroundings or 

 subjected to frequent extreme climatic conditions, or trees with 

 an unhealthy root-system will be particularly liable to infection. 



A section through a diseased twig shows the brownish- 

 coloured mycelial strands of the fungus ramifying in the cells 

 of the host (Plates 20 d, 21 a and Fig. i ). The strands, which 

 are septate and branched, measure between 3.08— 6.16/A* across 

 The individual cells are often peculiarly swollen, particularly 

 at the ends, and this leads to the shorter cells being somewhat 

 pear-shaped. 



The hypliK are at times closely attached to the cell-walls, 

 and they are also capable of invading the middle lamellae of the 

 cells, though this is not always easily made out. 



The ]wcnidia (PI. 24, a) are borne in distinct stromata, which 

 are lens-shaped, subcutaneous, and later erumpent, breaking 

 through with a ])lane whitish disc. (PI. 20, a-b). The begin- 



-■* A micron (» is .ooi of a millimetre, or .0004 of an inch. 



