34 A " Wither Tip " of Plnm Trees 



continuous zones, being produced towards the upper end of the potato. 

 In this respect these strains differ from those obtained from the apple 

 since the latter have always produced but a scanty supply of conidia 

 when grown on sterilized potato. 



(c) Dimensions of the Conidia. 



The conidia obtained from a leaf bearing pustules in the summer of 

 1916 were found to range from 10 x 8-5/x to 23 x 14-5/>t, the majority 

 however were 16-18 x 12-14/x. In subsequent cases 100 conidia were 

 measured and an average taken. In two instances conidia were obtained 

 in summer from twigs killed during the previous season ; the averages for 

 100 conidia of each weje 14-0 x 10-5/x and 14-0 x 10-0/x respectively. 

 When conidia were taken from dead twigs in the winter or early spring 

 their dimensions were considerably smaller; thus the average of 100 

 conidia taken from a twig in winter was 11-0 x S-O/x, and from another 

 in early spring 10*5 x 7*0/*, the size varying from 5-5 x 5-0^ to 

 17-0 X io-O/x. 



By analogy with observations made on Monilia cinerea as occurring 

 on the flowering spurs of the apple it was considered that the variation 

 was not due to specific differences in the various strains but was the 

 result of modifications in the substratum (particularly the amount of 

 moisture present), and perhaps also of differences in temperature accord- 

 ing to the season during which the conidia were in process of development. 

 To confirm this supposition the various strains were subsequently in- 

 duced to produce conidia under approximately the same conditions by 

 cultivating them on sterilized potato in the laboratory, and an average 

 was obtained of the dimensions of 100 conidia taken from each culture. 

 The size of the conidia formed under such conditions was found in each 

 strain to approximate to that of conidia taken from infected leaves in 

 summer. One strain, already referred to as producing in early spring 

 conidia averaging 10-5 x 7'0/x, was also inoculated into plums in the 

 plantation while the fruit were green and still growing ; the average size 

 of 100 conidia taken from one plum was 16'5 x* 12-5 /a, from another 

 •16-0 X 11-0/Li. 



The variation in the dimensions of the conidia of the various strains 

 according to the substratum or to the time of the year is shown here in 

 tabular form. 



I 



