Biology Reference
In-Depth Information
Blights
According to Webster, blight is “any disease or
injury of plants resulting in withering, cessation
of growth and death of parts, as in leaves, without
rotting.” The term is somewhat loosely used by
pathologists and gardeners to cover a wide vari-
ety of diseases, some of which may have rotting
as a secondary symptom. In general, the chief
characteristic of a blight is sudden and conspicu-
ous leaf and fruit damage, in contradistinction to
leaf spotting, where dead areas are definitely
delimited, or to wilt due to a toxin or other dis-
turbance in the vascular system. Fire blight,
discussed under Bacterial Diseases, is a typical
blight, with twigs and branches dying back but
holding withered, dead foliage.
hayfever. There are also parasitic forms causing
blights and leaf spots. Sometimes the disease
starts as a leaf spot, but the lesions, typically
formed in concentric circles, run together to
form a blight, the dark conidia making the surface
appear dark and velvety.
Alternaria alternata
Blight, Foliage
and
Pod
of pea.
Alternaria cassiae
Seedling Blight
of
Cassia
(sicklepod,
and coffee
senna),
and showy
crotalaria.
Alternaria cucumerina
Alternaria Blight
of
Cucurbits,
Cucumber Blight
,
Black Mold
, gen-
eral on cucumbers, muskmelon, watermelon, and
winter and summer squash. Symptoms appear in
the middle of the season, first nearest the center of
the hill. Circular brown spots with concentric
rings are visible only on upper surface of leaves,
but a black, moldy growth, made up of conidio-
phores and large brown spores, can be seen on
both leaf surfaces. Leaves curl and dry up, canta-
loupe foliage being more sensitive than that of
other cucurbits. The disease spreads rapidly in
warm, humid weather, and, with the vines drying,
the fruit is exposed to sunburn. Sunken spots
develop on the fruit, covered with an olive
green mass of conidia. Other species of
Alternaria
cause a decay of melons in transit
and storage.
Control
Purdue 44 and some other varieties of
muskmelon are rather resistant.
Alternaria dauci
Alternaria Blight
of carrot,
Carrot Leaf Blight
, general on carrot and parsley.
Affected leaves and petioles are spotted, then turn
Alternaria
Deuteromycetes, Hyphomycetes
Dark, muriform conidia formed in chains, simple
or branches, or sometimes singly, on dark, simple
conidiophores growing from dark hyphae (see
Fig. 1
). The apical portion of each conidium is
narrowed and often elongated, bearing at its tip
the next ovoid, tapering conidium. Species with
this characteristic formerly placed in
Macrosporium
are now in
Alternaria
; those with
spores rounded at both ends have been transferred
to
Stemphylium
.
There are many saprophytic species in
Alternaria
, the spores of which are wind-borne
for many miles and are a common cause of
