44 SECOND REPORT ON ECONOMIC BIOLOGY. 



numerous minute black spots ; these are the conldia or fruit bodies of 

 this stage. At a later stage a second form of fruit body appears on 

 the dead leaf-stalks, known as the ascigerous form. 



After the mycelium of the fungus in the leaf-stalk has choked up 

 the vessels and the leaves have wilted, it passes from the leaf-stalk 

 into the root, ultimately reducing it to a blackened, rotting mass. 



PREVENTIVE AND REMEDIAL MEASURES. 



Wherever the disease is noticed, all roots that are attacked should 

 be burned. Where widespread, early lifting of the crop is advisable, 

 all leaves being burned. 



Diseased leaves and roots should not be buried, thrown on the 

 manure heap, or fed to pigs. 



The soil should be treated with 15 to 18 cwt. of ground unslaked 

 lime. 



Yellow varieties of mangels are said to take the disease more 

 readily than the red varieties. 



APPLE TREE CANKER. 



Nectria ditissima (Tul.). 



This disease, which has been particularly prevalent in Worcester- 

 shire and Warwickshire during 1911, is by no means confined to the 

 apple, being almost equally common on alder, ash, beech, dogwood, 

 gooseberry, hazel, hornbeam, horse-chestnut, oak, pear, plum, lime, 

 maple, etc. 



The fungus usually follows an attack of the Woolly Aphis 

 (Schizoneura lanigera), being what is termed a wound-parasite. Mr. 

 Massee 1 very pertinently remarks " perhaps it is not going too far to 

 state that if we had no American blight or woolly aphis, we should 

 have no epidemic of canker." There are, however, other causes of 

 wounds, such as frost and hail, various insects, and wounds caused 

 by artificial means. 



Once having gained admission the fungus spreads under the 

 bark, extending its area each season, portions of the bark crack, and a 



1 Diseases of Cultiv. Plants and Trees, 1910, p. 184. 



