Blackspot - Westcott's Plant Disease - page 91

Blackspot

In common usage the term black spot without
qualifying adjectives has come to mean but one
disease, rose black spot, with the two words cur-
rently written as one, blackspot. This section is
limited to the rose disease. Delphinium black spot
will be found under Bacterial Diseases, elm black
spot under Leaf Spots, other black spots under
Black Mildew.

transferred to Marssonina . The blackspot fungus
was first reported in the United States in 1831,
from Philadelphia, and in 1912 Wolf made the
connection with the teleomorph state, so that the
correct name became Diplocarpon rosae .
Blackspot is probably the most widely distrib-
uted and best known rose disease. It is confined to
roses, garden and greenhouse, and may affect prac-
tically all varieties, although not all are equally
susceptible. There has been some progress made
in breeding resistant varieties, but recent investiga-
tion disclosing many physiological races of the
fungus explains why roses that are almost immune
to blackspot in one location may succumb in
another. Rosa bracteata is the only species thus
far shown to be reasonably resistant to all the
different races tested. Roses with the Pernetiana
parentage, which has given us the lovely yellows,
coppers, and blends, are especially prone to
blackspot. Some roses, like Radiance, are tolerant
of blackspot, usually holding their leaves, even
though they cannot be considered resistant.
Symptoms are primarily more or less circular
black spots, up to 1/2 inch in diameter, with
radiating fimbriate or fringed margins (see
Fig. 1 ). This fimbriate margin is a special diag-
nostic character, differentiating blackspot from
other leaf spots and from discolorations due to
cold or chemicals. The spots vary from one or two
to a dozen or more on a leaf, usually on the upper
surface. With close examination you can see
small black dots or pimples in the center of the
spots. These are the acervuli, bearing conidia, and
they glisten when wet (see Fig. 2 ).

Diplocarpon

Ascomycetes, Helotiales,
Dermateaceae (Mollisiaceae)

Apothecia innate, formed in dead leaves, but
at maturity rupturing overlying tissues; horny to
leathery with a thick margin or outer wall
(excipulum) of dark, thick-walled cells; spores
two-celled, hyaline; paraphyses present. Anamorph
state is a Marssonina with two-celled hyaline spores
in an acervulus.
Diplocarpon rosae Rose Blackspot, general
on rose but less serious in the semi-arid

Southwest; reported from all states except

Arizona, Nevada, and Wyoming.
For nearly 100 years the fungus was known
only by its anamorph state, which has had about
25 different names. The first definite record is by
Fries in Sweden in 1815, under the name
Erysiphe radiosum , but the first valid description
was by Libert in 1827 as Asteroma rosae . Later
Fries called it Actinonema rosae , and that term
was widely used until Actinonema species were