476 
der certain’ climatic conditions the dis- 
ease is not uncommon on full grown trees 
in the orchard. It usually attacks only 
the tips of actively growing branches but 
may occur in spots on the underside of 
otherwise unaffected mature leaves. 
This disease is caused by a fungus 
known technically as Sphaerotheca mali, 
| Ove. 
éy wh g 
Fig. 1. Branch of Apple Attacked by Powd- 
ery Mildew. 
the mycelium of which develops mainly 
as a covering on the surface of the grow- 
ing twigs and leaves and sends feeding 
threads only into the external cells. The 
twigs present a white, mouldy appearance, 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
due to the growth of the threads of the 
fungus in considerable quantity on the 
surface. (See Fig. 1.) Sometimes this 
is so copious as to resemble felt. Spores 
are produced in great. abundance under 
favorable conditions and give the affected 
twigs and leaves a frosty appearance. 
Such twigs are usually more or less 
thickened and shortened, and frequently 
distorted. The leaves present a wilted 
appearance and are smaller than normal. 
When the disease is abundant, the nor- 
mal functions of the tree are interfered 
with on account of the reduction in the 
foliage. Affected trees may fail to form 
blossom-buds, or the fruit may not be of 
good size and quality. 
The fungus hibernates as mycelium on 
the affected twigs. This is the principal 
method by which it is carried over the 
winter. Sexual spores may also be formed 
in protected fruit bodies partly buried in 
the felted mass of mycelium. 
Treatment 
It has been found that this fungus, un- 
like most powdery mildews, is a difficult 
one to control. Since the fungus winters 
on the twigs, it might be expected that a 
dormant spray applied late in the spring 
would control the disease; but this has 
not been found sufficient where the dis- 
ease is abundant. Investigations carried 
on in California by Volck indicate that a 
special spray, the iron sulphide, will hold 
the disease in check so that it will not 
cause serious damage, if the spray is ap- 
plied as soon as the foliage buds open 
and is repeated at frequent and regular 
intervals. Where lime-sulphur can be 
safely used as a remedy for apple scab, 
it should not be necessary to use the iron 
sulphide. 
When the disease is present only in 
slight amount, pruning out the diseased 
twigs, removing them from the orchard 
and burning them, will usually be found 
sufficient to hold the disease in check. 
Bibliography 
1909. B. M. Duggar, Fungus Diseases of 
Plants. 
1910. 
New York Experiment Station, 
Bulletin No. 328. 
