Biology Reference
In-Depth Information
Leaf symptoms are dark brown, circular to
oval spots, marked with concentric rings in
a target effect, appearing first on lower, shaded
foliage, with the spots growing together to blight
large portions or all of leaves, exposing fruits.
There may be a collar rot of young tomato seed-
lings, sunken spots or cankers on older stems,
blossom-drop with loss of young fruits, or dark
leathery spots near the stem end of older fruits.
Alternaria blight is the most common leaf spot
disease of tomatoes in the Central and Atlantic
States but is somewhat less important elsewhere.
Foliage symptoms on potato are similar to
those on tomato. Small round spots on tubers
afford entrance to secondary rot organisms.
Each leaf spot may produce three or four crops
of dark spores, which remain viable more than
a year. They are blown by wind, splashed by rain,
sometimes transmitted by flea beetles. The fun-
gus is a weak parasite, entering through wounds
and thriving in warm, moist weather, with 85
F
as optimum temperature. It can survive in soil as
long as the host refuse is not completely rotted; it
also winters on seed and on weed hosts.
Control
Plan, if possible, a 3-year rotation with
crops not in the potato family; dig under diseased
refuse immediately after harvest. Use seed from
healthy tomatoes, or purchase plants free from
collar rot.
Alternaria tagetica
Blight
of marigold.
Alternaria tenuissima
Alternaria Blight
,
Leaf
Spot
of violet and pansy. Spots vary fromgreenish
yellow to light buff with burnt amber margins.
Brown patches run together to form large, blighted
areas. Clean up and burn old leaves in fall.
Alternaria zinniae
Zinnia Blight
,
Alternariosis
on zinnia. Small reddish brown spots with grayish
white centers increase to irregular, large, brown,
dry areas. Similar spots on stem internodes or at
nodes may girdle the stem, with dying back of
upper portions. Dark brown to black basal can-
kers with sunken lesions are common. Roots may
turn dark gray, rot, and slough off. Small brown
flower spots enlarge to include whole petals,
causing conspicuous blighting. The
Ascochyta
Deuteromycetes, Coelomycetes
Pycnidia dark, globose, separate, immersed in
host tissue, ostiolate; spores two-celled, hyaline
ovoid to oblong.
Ascochyta asparagina
Stem Blight
of asparagus
fern. Small branchlets dry and drop prematurely;
small branches are killed if attacked at crown.
Ascochyta
chrysanthemi (
Mycosphaerella
ligulicola
).
Phoma chrysanthemi
(
Didymella ligulica
)).
Ascochyta Ray Blight
of
chrysanthemum, a conspicuous and rapid disease
of ray flowers.
Ascochyta fabae f. sp.
(see
▶
spiricia
Leaf Blight
of
vetch.
Ascochyta piniperda
Spruce Twig Blight
on
young shoots of red, Norway, and blue spruce;
apparently a minor disease.
Ascochyta pisi, A. pinodes, A.
pinodella
Ascochyta Blight
or
Mycosphaerella
Blight
of peas. All three fungi may be connected
with the disease complex known as Ascochyta
blight, are carried in infected seed and overwinter
in plant debris.
A. pinodes
has
Mycosphaerella
pinodes
as its ascospore stage so that the life
cycle can start from either pycnidia or perithecia
produced on plants or stubble. Lesions begin as
small purplish specks on leaves and pods. When
infection is caused by
M. pinodes
or
A. pinodella
,
the specks enlarge to round, targetlike spots,
which join together to form irregular, brownish
purple blotches.
M. pinodes
often withers and
distorts young pods;
A. pinodella
causes
a severe foot rot, a dark region at the soil line.
Elongated, purplish black stem lesions are com-
mon.
A. pisi
causes leaf spots with dark brown
margins, stem and pod spots, but no foot rot.
Control
Use western-grown seed, usually free
from the disease; clean up all pea refuse and use
a 3-or 4-year rotation. The host range now
includes many plants such as carrot, banana, and
foliage plants.
Phoma chrysanthemi (Telemorph, Didymella
ligulica) (formerly
Ascochyta
fungus
apparently winters on seed and in soil.
Control
Clean up refuse; use a long rotation if
growing plants commercially.
chrysanthemi
(telemorph,
Mycosphaerella
ligulicola
)).
