Wormald. — ‘ Brown Rot ’ Diseases of Fruit Trees. II. 159 
mail) which causes a blossom wilt and canker disease of apple trees secretes 
an enzyme, easily detected in liquid culture media, in infected apples, and in 
infected flowering shoots ; this enzyme responds to the guaiacum test and 
accelerates the absorption of oxygen by tannins. 
The fact that those strains of Monilia cinerea which in inoculation 
experiments were unable to cause a blossom wilt of apples secrete this 
enzyme far less readily suggests that the virulence of the apple form may be 
due to this property of secreting an enzyme capable of acting on substances 
present in the tissues of the host plant. 
In this connexion it is to be noted that Percival ( 21 ), in his investiga- 
tions on the ‘ Silver Leaf’ disease of fruit trees, found that the disease was 
associated with the presence of an oxidase which is easily extracted from 
diseased tissues ; later, however, Brooks ( 5 ) stated that 4 until the present 
I have been unable to confirm Percival’s view that the disturbing agent is 
an oxidase which is secreted by the fungus \ 
Whether the secretion of an oxidase is a factor determining the degree 
of virulence of the Brown Rot fungi is yet to be determined, but the fact 
that two biologic forms morphologically similar are in some respects 
physiologically distinct under laboratory conditions is of considerable 
interest, and further investigations along these lines may throw some light 
on the mode of parasitism of such biologic forms. 
III. Morphology and Taxonomy. 
ilt 
(a) Colour and Size of Pustules. 
When Monilia fructigena and M. cinerea are growing under the same 
conditions , they can, as a rule, be readily distinguished by their conidial 
fructifications. Thus the pustules of the former are larger than those of the 
latter and are yellow in colour, while the pustules of M. cinerea are, as its 
specific name implies, ashy grey. These characters may, however, show 
some variation according to the conditions under which the fructifications 
are produced. The difference between the two is most striking when they 
are growing vigorously on recently infected fruit. Thus in an experiment 
where plums were simultaneously inoculated with the two species, M. fructi- 
gena produced pustules of a maximum diameter of 1-5 mm. (mostly about 
1 mm.) and ‘ Light Buff’ 1 in colour, while those of M. cinerea only reached 
a maximum diameter of o-8 mm. (mostly about 0-4 mm.) and were ‘ Smoke 
Grey ’ 1 in colour. The pustules had, in this instance, developed during 
a period of dry weather and were on that account comparatively small. 2 
When produced in a moist atmosphere the pustules are larger, but the 
relative size for the two species remains approximately the same. 3 Pustules 
of M. cinerea on apple spurs and cankers may under favourable conditions 
1 Ridgway’s scheme of colours. 2 Vide Part I, Figs. 8 and 9. 
3 Vide Part I, Fig. 2. 
M 
