164 Wormald . — ‘ Brown Rot ’ Diseases of Fruit Trees, II. 
placed in water. In January, 19 17, during a period of severe frost, conidia 
of M. cinerea were taken from pustules, growing on an apple tree in the 
open (the thermometer at the time showing a temperature of — 2° C.) and 
mounted in a drop of water ; the slide was placed in a damp chamber in the 
laboratory and within 24 hours 50 per cent, of the conidia had developed 
germ tubes. 
These observations confirm conclusions arrived at by Ewert ( 9 ), who 
states that ‘im Winter auf den Pflaumenmumien die Sporen der M. cinerea 
stets lebendig, die der M. fructigena stets tot sindh 
(d)' Cultural Studies. 
That M. fructigena and M. cinerea can be distinguished by their mode 
of growth on sterilized culture media has already been pointed out, and 
further study has confirmed previous observations. Prune juice agar has 
hitherto proved the best substrate for distinguishing the two species when 
growing vegetatively. On this medium, in Petri dishes, M . fructigena grows 
out uniformly to the edge of the plate in the form of an almost regular 
circular disc of mycelium (Fig. 6), while M. cinerea produces a more 
irregular growth with a tendency to form lobes (Fig. 5). Another 
characteristic of the latter species is its habit of zonate development ; when 
the culture has made some growth (usually when it has reached about half- 
way to the edge of the plate, but often before this) further development is 
temporarily inhibited, but is soon renewed in the form of flabelliform out- 
growths of mycelium which eventually coalesce to form a definite zone 
(Figs. 4 and 5). In some cultures three or four distinct zones are produced 
in this way. 
In these plate cultures of M. cinerea f. mali , one or more dark brown 
zones invariably appear, the first at 5-10 mm. from the point of inoculation 
in about 12 days (Fig. 4) ; the brown zones are less regularly produced by 
the Primus form and some strains remain quite hyaline (Fig. 5). This 
coloration is more pronounced in tube cultures, the zones becoming more or 
less confluent, so that in the case of the apple form the cultures may become 
almost black throughout. 
Sterilized potato is another useful medium for diagnosing the species, 
particularly when they are found within the tissues of the host as barren 
mycelium only. On potato both species develop fructifications, each pro- 
ducing conidial tufts of the colour peculiar to the species, i. e. M. fructigena 
buff yellow pustules, M. cinerea grey ones. The two biologic forms of 
M. cinerea grow equally well on sterilized potato, but the conidial tufts of the 
apple form are generally less numerous and not so well developed as those 
of the Primus form, and often some difficulty is experienced in obtaining 
sufficient conidia for inoculation experiments. On the other hand strains 
