Biology Reference
In-Depth Information
result is less disease in the garden as a whole.
Protected corners in the garden where air circula-
tion is poor also increase the disease potentiality.
Spores are apt to be splashed farther when water
hits hard-packed soil without a mulch.
When old leaves drop to the ground, the myce-
lium continues a saprophytic existence, growing
through dead tissue with hyphae that are now
dark in color. In spring three types of fruiting
bodies may be formed: microacervuli or sperma-
gonia containing very small cells that perhaps act
as male cells; apothecia, the sexual fruiting bod-
ies formed on a stroma between the epidermis
and palisade cells and covered with a circular
shield of radiating strands; and winter acervuli,
formed internally and producing new conidia
in spring. The
Diplocarpon
or apothecial stage
is apparently not essential; it is known only
in northeastern United States and south-central
Canada. The shield over the apothecium ruptures,
and the two-celled ascospores are forcibly
discharged into the air to infect lowest leaves.
Where the sexual stage is not formed, primary
spring infection comes from conidia splashed by
rain to foliage overhead, from acervuli either in
overwintered leaves on the ground or in cane
lesions. New roses from a nursery sometimes
bring blackspot via these cane lesions
one bush to another during the season. Drastic
spring pruning, far lower than normal, reduces the
amount of inoculum from infected canes.
The importance of a dormant spray is debat-
able. Experiments have shown that as a true
eradicant, applied in winter, it has little value in
reducing the amount of blackspot the next sum-
mer. Use liquid lime sulfur after pruning, pro-
vided the buds have not broken far enough to
show the leaflets.
Summer spraying or dusting, weekly through-
out the season (from late April to early November
in New Jersey) is essential if you want to keep
enough foliage on bushes for continuous produc-
tion of fine flowers (it takes food manufactured in
several leaves to produce one bloom) and for
winter survival. Some strong varieties will, how-
ever, live for years without chemical treatment;
they are usually scraggly bushes with erratic
bloom. The idea that floribunda varieties do not
require as much spraying as hybrid teas is
a misconception. Some floribundas are quite
resistant; others are very susceptible. The same
holds true for old-fashioned shrub roses. All too
often blackspot gets a head start in a garden from
shrub roses we thought it unnecessary to spray.
Roses can be defoliated as readily by
chemicals as by the blackspot fungus; so the
fungicide chosen must be safe under the condi-
tions of applications as well as effective. There
are many chemicals that will control blackspot if
they are applied regularly and thoroughly. Choice
depends somewhat on climate. Some copper
sprays and dusts cause red spotting and defolia-
tion in cool, cloudy weather. Bordeaux mixture is
both unsightly and harmful, unless used in very
weak dilution. At strengths recommended for
vegetables it will quickly turn rose leaves yellow
and make them drop off. Dusts containing more
than 3 to 4 % metallic copper are injurious under
some weather conditions. Dusting sulfur fine
enough to pass through a 325-mesh screen has
been successfully used for years for blackspot
control, but in hot weather it burns margins of
leaves. Copper and sulfur have a synergistic
effect; a mixture of the two is more effective
than either used alone, but such a mixture also
combines injurious effects.
to
a garden previously free of disease.
Control
The importance of sanitation may have
been somewhat overstressed; it cannot replace rou-
tine spraying or dusting. It is certainly a good idea
to pick off for burning the first spotted leaves, if this
is done when bushes are dry so that the act of
removal does not further spread the fungus. Raking
up old leaves from the ground at the end of the
season makes the garden neater and may reduce
the amount of inoculum in spring, but, because the
fungus winters also on canes in most sections of
the country, removal of leaves cannot be expected
to provide a disease-free garden the next season.
Comparative tests have shown that fall cleanup
is ineffectual. A good mulch, applied after
uncovering and the first feeding in spring, serves
as a mechanical barrier between inoculum from
overwintered leaves on the ground and developing
leaves overhead. A mulch also reduces disease by
reducing the distance spores can be splashed from
