H. WORMALD 177 



Other differences have also been detected between the two types; 

 thus the blossom-wilt form produces conidia less freely than the hyaline 

 form from plums when urown on steamed potato. Again when inocula- 

 tions are made on mature apples the former soon causes the fruit to 

 become dark brown to black with conidial pustules absent or very few 

 in number, while with the latter the dark colour is less pronounced and 

 numerous pustules are readily developed. 



Four strains have been obtained from flowers and twigs of acid 

 (Morello) cherry trees and these on prune- juice agar produce a brown 

 coloration which however is not so uniform as that of the strains from 

 the apple trees. 



Whether the hyaline form is less virulent than the brown is at present 

 uncertain, and it is proposed to carry out inoculation experiments with 

 that object in view, but it is significant that all the strains obtained 

 from wilted trusses and cankers of the apple tree produce on the agar 

 cultures a coloration which eventually becomes almost black. 



(b) DimeTisions of Conidia. 



Apart from the colour of the pustules, the size of the conidia is the 

 character most generally quoted as determining the species of Monilia 

 (in the absence of the ascigerous stage) found on cultivated fruit. Of the 

 strains of the blossom-wilt fungus at first studied only a few conidia 

 were measured ; it was soon evident however that not only did the 

 average size of the conidia vary with the conditions under which they 

 were produced, but also that there was considerable variation in the 

 dimensions of conidia taken from the same pustule. The number taken 

 therefore in order to obtain a fair average was in each case 100, for those 

 strains obtained more recently. 



It has already been stated that the Monilia of the apple blossom wilt 

 has in general smaller conidia than M. friictigena and this difference is 

 particularly well marked if a comparison is made of the dimensions in 

 the two forms when each is producing pustules most abundantly under 

 natural conditions. Thus if conidia of the blossom-wilt fungus are 

 taken from cankers or spurs in the spring when this form is most con- 

 spicuous, the typical conidia are found to measure 11-12 x 8-9 /x. 

 M. frucfigena on the other hand is most vigorous in late summer and 

 early autumn when the majority of its conidia reach or exceed 20 /x in 

 length. If, however, the two are grown under the same conditions, as, 

 for example, on sterilised potato in the laboratory the difference is less 



