PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Black spot of rose
Example of a fungal leaf spot
The most common and serious disease of roses.
Scientific name
Black spot (Marssonina rosae, Imperfect Fungi =
Diplocarpon rosa, Phylum Ascomycota).
Host range
Roses. Some cultivars are more susceptible than
others. Most fungal leaf spots are host specific –
see page 321 for more species
Symptoms
Leaves.
More or less circular black spots with fringed
margins up to 12 mm across develop usually on
leaf uppersurfaces (Fig. 196).
Leaf spots vary in number from 1-20 per leaf
and may coalesce to produce large, irregular
black areas.
During damp weather, examination of the
feathery spots with a hand lens shows small
black blisters (fruiting bodies or acervuli) which
contain spores (conidia).
In susceptible varieties, the appearance of black
spots is soon followed by a yellowing of portion
or the entire leaflet and then by defoliation. The
leaf tissue around the lesions turns yellow and
often entire leaves become yellow and fall
prematurely.
Sometimes new leaves are produced which also
become infected.
Flowers. Continual defoliation results in a
reduction in the size and number of flowers.
Young canes of susceptible varieties may also
develop spots. Lesions are indistinct black areas,
slightly blistered without fringed margins and as
raised, purple-red blotches on immature wood of
1 st year canes. There will be a reduction in size and
number of flowers as well as dieback of stems.
General. Repeated defoliation weakens the plant
and may lead to dieback of stems and reduction in
size and number of flowers. If the plant is
continually defoliated in this way dieback and
death may follow.
Diagnostics.
Do not confuse the feathery black spots on roses
with anthracnose (Sphaceloma rosarum) or other
minor leaf spotting fungi, eg Mycosphaerella
which have smooth margins.
Small dark fruiting bodies can be seen with a
hand lens or under a compound microscope.
Some fungal leaf spotting fungi can be identified
by DNA analysis, eg Mycosphaerella nubiloa on
Eucalyptus globulus.
Fig. 196. Black spot (Marssonina rosae) on rose.
Rose leaves showing typical symptoms of black spot.
The large black spots have a feathery margin. Severely
affected leaves yellow and fall prematurely. PhotoCIT,
Canberra (P.W.Unger).
Fig. 197. Anthracnose (Sphaceloma rosarum) on rose.
Rose leaves showing typical symptoms of anthracnose.
Spots are ash-gray with well defined margins. Leaves may
become tattered at the tips. Defoliation does not occur to the
same extent as with black spot. PhotoCIT, Canberra (P.W.Unger).
Fungal diseases - Examples of fungal diseases 355