A leaf spot is a rather definitely delimited dead area in a leaf. When
dead areas are large and indefinite, the disease goes under another
name-anthracnose, blight, blotch, or scorch. Leaf spots are present
in our gardens by the thousand. They are so common we seldom notice
them, and comparatively few are injurious enough to cause concern or
to warrant control measures beyond removal of disfiguring foliage.
Leaf spots caused by fungi often have fruiting bodies, small black
dots, in the center; those caused by bacteria lack dark pustules but often
start as water-soaked areas. Some spots are due neither to fungi nor
bacteria but to sun and wind, the cells collapsing because water cannot
be drawn up from the roots quickly enough.
BLACK SPOT OF ROSE (Diplocarpon rosae).
This is undoubtedly the leaf
spot of most concern to gardeners even though the disease is strictly
limited to roses. The black spots are more or less circular with fringed
margins made from the dark mycelium crisscrossing just under the
epidermis. There will be one to several dozen spots on a leaf which may
or may not turn yellow and drop. Very susceptible varieties, including
most of those with yellow and orange flowers, are rather quickly defoliated; tolerant varieties hold their foliage and suffer little from black
spot. Summer spores are formed in acervuli which break through the
cuticle and are splashed by rain to nearby leaves. Infection takes place
in any period when the leaves are continuously wet for 6 hours.
The fungus winters either in small dark lesions on the canes or in
old leaves on the ground. Fruiting bodies formed in the latter produce
sexual spores in spring which are shot up to unfolding foliage overhead.
This is the reason why we are always admonished to pick off all diseased
leaves and rake up and burn all fallen leaves in autumn. But no matter
how scrupulous one is in sanitation, black spot is practically inevitable,
unless you live in the arid Southwest, without a summer spraying or
dusting program. It comes into the garden on canes of new roses from
the nursery and winters on canes of your old bushes.
Control. Sulfur, copper, and ferbam effectively control black spot
provided they are applied often enough to keep susceptible leaf surfaces
protected. This means weekly in most instances, perhaps oftener in
rainy weather when roses are growing rapidly. Spraying every 2 weeks
is not satisfactory, nor can one be a good enough weather prophet to
spray just before rains.