Biology Reference
In-Depth Information
Blotch Diseases
Cercospora
Diseases designated as blotch have symptoms
that are intermediate between blights, where the
entire leaf or shoot dies, and leaf spots, where the
necrotic lesions are definitely delimited. Blotches
are irregular or indefinite large or small necrotic
areas on leaves or fruit.
Blights
.
Cercospora
▶
Myrovellosiella
concors
).
Potato Leaf Blotch
. An unimportant
disease; leaflets turn yellow with small blackened
dead areas or larger, irregular brown areas.
Cercospora purpurea (see
▶
concors (see
▶
Pseudocercospora
purpurea
).
Avocado Blotch
,
Cercospora Spot
,
considered the most important avocado disease
in Florida with no commercial variety entirely
resistant.
Myrovellosiella concors (formerly
Cercospora
concors
).
Potato Leaf Blotch
. An unimportant
disease; leaflets turn yellow with small blackened
dead areas or larger, irregular brown areas.
Pseudocercospora purpurea (formerly
Cercospora purpurea
).
Avocado Blotch
,
Cercospora Spot
, considered the most important
avocado disease in Florida with no commercial
variety entirely resistant. Leaf spots are angular,
brown to chocolate brown, scattered and distinct,
less than 1/16 inch or coalescing to larger
patches. With a hand lens, grayish spore groups
can be seen on both sides of the leaf. Successive
crops of spores are produced in moist periods
throughout the year. Fruit spots are 1/4 inch or
less in diameter, brown to dark brown, irregular,
sunken, with cracked surfaces and grayish spore
tufts. Lesions are confined to the rind so that the
Alternaria
Blights
.
Alternaria porri
Purple Blotch
of onion, also
on garlic, and shallot, a problem in southern and
irrigated areas. Small, white, circular to irregular
spots increase to large purplish blotches, some-
times surrounded by orange and yellow bands, on
leaves and flower stalks. Leaves often turn yellow
and die beyond the spots; girdled stalks die before
seeds mature. Brown muriform spores form
a dusky layer on the blotches. Varieties with
a waxy foliage are more resistant than those
with glossy leaves. The fungus winters as myce-
lium and spores in crop refuse. Rotation, cleaning
up plant debris,
▶
and seed treatment
are
recommended.
Two other species of
Alternaria, A. alternata
and
A. tenuissima
, may cause purple or brown
blotches on onion, and there are physiological
races as well.
