178 A Blossom Wilt and Canker of Apple Trees 



striking, for the conidia of the blossom-wilt Monilia are then consider- 

 ably larger than when they are produced on spurs and cankers. 



Four strains obtained from dead spurs and cankers from February 

 to April, 1916, were subsequently grown on steamed potato and in each 

 case 100 conidia were measured^ with the result as shown in the table. 

 The conidia taken from the sterihsed potato were measured when the 

 cultures were 6-8 days old. 



Average of 100 conidia Average of 100 conidia 



Strains of the Blossom Wilt Monilia taken from spurs taken from cultures 



from spurs and cankers of apple trees or cankers on steamed potato 



Strain XI from dead spur, Feb. 1916 12-0 x 8-5 /u. 19-0 x 150m 



„ XII from dead spur, March 1916 11-.5 x 85 ^ 190 x 15-5 m 



„ XIIT from a canker, April 1916 11-.5 x 8-5^ 180 x 14/* 



XIV from another canker, Aprill91 6 11-0 x 80m 18-0 x 13-5 m 



Strain XIV was subsequently used in inoculations on apple blossom 

 in the open, and conidia obtained from an infected flower stalk in July 

 had an average size (for 100 conidia) of 15-5 x 12-0/^, but conidia taken 

 during the following winter (February 1917) from a spur killed by 

 inoculation with this same strain gave an average of 11-0 x 8-5/x which 

 is a close approximation to that obtained from conidia taken from the 

 original canker. 



A strain obtained from young apples soon after setting (June 12, 1915) 

 was found to have conidia the majority of which were from 17 to 20/i, 

 in length but sufficient measurements were not made for an average to 

 be taken comparable with the above. Another strain obtained from 

 young apples in July 1916 gave an average of 18 x 13 /^ for 100 conidia 

 taken from the fruit itself, and when grown on steamed potato the 

 average was about the same, i.e. 17-5 x 14-0/a. The other cultural 

 characters of these two strains, e.g. habit of growth and brown coloration 

 on prune-juice agar, and grey conidial tufts on potato, resemble those 

 of the typical blossom-wilt strains. 



It has not yet been ascertained what factors determine the output 

 and the size of the conidia, but probably temperature and the degree 

 of moisture have some influence. The conidia are invariably smallest 

 if obtained in winter when the fungus is living on dead wood and bark ; 

 they are much larger when produced on young apples in the summer 

 or on potato at the temperature of the laboratory (15°-18° C). 



Therefore when a Monilia obtained from an apple tree is examined 

 with a view to its identification it is essential that the conditions under 



^ The readings were taken correct to 0-5 m and the averages were calculated also 

 correct to the nearest 0-5 m- 



