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IRISH GARDENING. 



Scab on Apple Twigs. 



By GEORGE H. PETHYBRIDGE, Ph.D., B.Sc. 



NEXT perhaps to canker, the scab or spot 

 disease of apples is one of the most 

 troublesome pests with which the grower 

 has to contend in this country. The appearance 



of affected fruits is, 

 unfortunately, too 

 familiar to require a 

 detailed description 

 here, especially 

 since an illustrated 

 account of the 

 disease was given in 

 Irish Gardening 

 for February, 1907, 

 p. 27. 



The fungus which 

 causes the trouble 

 {Fusicladiuni den- 

 driticum) not only 

 gives rise to the 

 spots, cracks, mal- 

 formation, &c., of 

 the fruits, but is also 

 found on the leaves 

 where it produces 

 dark, more or less 

 velvety, patches 

 which of course 

 affect adversely the 

 normal work of the 

 green leaf — viz. , 

 food manufacture. 

 At these spots the 

 conidia or spores, 

 by which the fungus 

 spreads, are pro- 

 d u c e d in large 

 numbers. In ad- 

 dition to these 

 conidia, however, 

 the fungus has a 

 second form of 

 fructification — a 

 more complex one — 

 which may be found 

 on the dead tissues, 

 such as fallen leaves. 

 These are minute, pear- 



Twig of Apple (Lord Suffield 



Badly attacked with Scab (twice 

 natural size). 



&c., in the spring. 



shaped structures, each opening by a pore, and 

 containing a number of small bags of spores. 

 In this form the above fungus, known now as 

 Vetituria incpgualts, is able to pass unharmed 

 through the rigours of winter, and these fructifi- 

 cations therefore serve the purpose of carrying 

 it over from one season to the next. 



It is, however, only within comparatively 

 recent years that the connection between the 

 V^ntiiria and Fusicladium forms has been estab- 



lished, and the Venturia form seems to be com- 

 paratively rare. Nevertheless the scab is ever 

 with us, and the question arises as to whether 

 there is any other method by which the fungus 

 can remain alive over the winter. It has been 

 suggested that it hibernates in the form of 

 mycelium on the fallen fruits, and this may be so ; 

 but from a case which has recently come before 

 my notice I am much more inclined to think 

 that hibernation occurs on the timgs. Most of 

 the text-books state that the fungus may occur 

 on the twigs as well as on the leaves and fruit, 

 but no special stress is laid on the point. Dr. 

 Aderhold, indeed, says that it is very seldom 

 seen on apple twigs., although the closely allied 

 pear-scab is common enough on the twigs of 

 certain varieties of pears. 



The accompanying illustration is a photograph, 

 enlarged about twice the natural size, of an 

 apple twig which is badly attacked by the fungus. 

 The scabby nature of the bark is very well shown, 

 and, owing to the growth of the fungus within 

 and beneath it, portions of it become from time 

 to time detached and cast off". In a compara- 

 tively short time the smaller twigs and spurs 

 become completely killed, and I have lately seen 

 many such dead twigs on apple trees, particu- 

 larly on the variety " Gladstone." On examining 

 sections of the bark, under the microscope, the 

 fungus is seen in the form of dense masses of 

 mycelium closely resembling the resting stages 

 of certain fungi known as " Sclerotia." From 

 these, large numbers of the characteristic conidia 

 were being produced this spring just at a time 

 when the foliage was developing and the flowers 

 opening. Without doubt such twigs are a rich 

 source of infection for the coming fruit crop. 

 I am inclined to doubt whether spraying would 

 kill these mycelial cushions, so thick are some 

 of them, and so deeply embedded in, and well 

 protected by, the dead portions of the bark. 

 Such affected twigs should be carefully searched 

 for where apple scab is troublesome, and when 

 found they should be cut off and burned. By 

 this means the risk of continuing the disease 

 from season to season should be considerably 

 diminished, since it seems likely that the fungus 

 hibernates largely in this way on the twigs 

 rather than as mycelium on fallen fruits, or as 

 an ascus fructification on dead tissues. 



^?* C^^ 9^^ 



" HiBERNiA," writing' in the Gardeners Magazine, re- 

 commends the use of the male fern [Lastria Felix-mas) 

 for the indoor decoration of fire-places and other dark 

 recesses in rooms. The plants may be lifted in June, 

 and potted firmly in a soil made of half leaf-mould and 

 half loam. After keeping' in a shady position for a couple 

 of days the plants may be brought indoors. They must 

 be well supplied with water. We have ourselves used 

 the male fern for the purpose sugg'ested, and can 

 recommend it as a most effective decoration. They 

 may be replanted in open ground at end of season. 



