160 W or maid. — ‘ Brown Rot ’ Diseases of Fruit Trees. II. 
reach a diameter of 1*5 mm., while M. fructigena when growing on maturing 
apples and on apple spurs frequently produces pustules 2 mm. in diameter. 
Old pustules of M. cinerea when redeveloping on mummied plums in 
winter and spring often coalesce, and from their general appearance at that 
time of the year might easily be mistaken for M. fructigena (cf. Figs. 2 
and 3). 
(b) Dimensions of Conidia. 
The dimensions of the conidia of the two species are generally quoted as 
a distinguishing character. The great variation in the size of the conidia of 
M. cinerea according to the conditions under which they are developed 
renders this distinction somewhat unreliable unless the environmental factors 
are taken into consideration. This point has been discussed in previous 
papers in connexion with strains occurring on apple trees and plum twigs ; 
it was there shown that the average size of 100 conidia of each of a number 
of strains of M. cinerea found on cankers, spurs, twigs, and mummied plums 
was from irox8-o/x to 1 2-0 x 8-5 pi, but when the strains were grown on 
fruit in summer or on sterilized potato in the laboratory the dimensions of 
the conidia were about one and a half times as great. 
Further observations and experiments have confirmed those results as 
shown in the following tables. In each case the conidia were mounted in 
distilled water ; 100 were then measured and the average determined. To 
avoid unconscious selection,/ small groups were taken and all the conidia of 
each group measured, though isolated conidia occurring between the groups I 
were also included ; the range of variation was obtained by examining the I 
whole of a slide and selecting for measurement the largest and smallest that 
could be found. The readings were taken correct to 0-5 pi and the averages 
were calculated and recorded also correct to the nearest 0*5 pi. 
Dimensions of Conidia of Monilia cinerea. 
Source of Conidia. 
Date. 
Range of 
Variation. 
Average. 
Apple tree — Brown Rot canker 
Mar. 8- 
6.5 x 5.0- 
-T4.0 x 8.5 fx 
10.5x7.5 p 
Apple spur 
Apr. 5 
6.0 x 4.5- 
15*5 x I2 *5 
11-5x8-5 
Apple tree — Brown Rot canker 
June 19 
8-o x 6*o- 
-i9-ox 12.5 
T 3*5 x 9-5 
Apple twig 
June 29 
8-o x 7.5- 
-22-5 X 14.5 
14.0 x 10.5 
Plum twig (Kent) 
Mar. 14 
5 -° x 4 - 0 “ 
-17.0 X 1 1.5 
11*5x8.5* 
Mummied plum (Cambridge) 
Mar. 19 
6.0 x 5.0- 
-19.0 x 1 1-5 
1 1.5 x 8-o 
Mummied plum (Worcestershire^ 
Apr. 8 
5.0 X 4.0- 
-17.5 x 10-5 
1 1*5 x 7*5 
Mummied plum (Devonshire) 
Apr. 8 
7.0 X 5.0- 
-18.5 x 1 r-o 
n-5 x 8.0 
Mummied cherry (Kent) 
Mar. 4 
6-o x 5.0- 
-15.0 X I 1-0 
1 1.5 x 8-o 
ditto ditto 
Apr. 19 
7 ’ c x 5 * 5 - 
-21-0 X 15-0 
13.0 x 1 0.0 
Plum (Ireland) 
Aug. 2 
10.5 x 7.0- 
-27.5 X 16.5 
18-5x11.5 
Plum (Kent) 
Sept. 8 
9.5 X 7.0- 
-23.0 x 15*0 
16.5x9.5 
Plum (Kent) 
Sept. 15 
8.5 X 6.5- 
- 22-5 X 18 . 0 
17.0 x 10.5 
That this difference in the average size of the conidia was due to 
environmental factors was shown by cultivating certain strains under various 
conditions and noting the variation induced by transferring any one strain 
from one set of conditions to another. A strain found on a mummied 
