Biology Reference
In-Depth Information
Blackleg
The term blackleg is used to describe darkening
at the base of a stem or plant. Blackleg of potatoes
and delphinium are described under Bacterial
Diseases; blackleg of geraniums is under Rots.
reported in France in 1849, and in the United
States in 1910. It is generally distributed east of
the Rocky Mountains and formerly caused from
50 to 90 % loss. With improved seed and seed
treatment it has become less important.
The first symptom is a sunken area in the stem
near the ground, which extends until the stem is
girdled and the area turns black. Leaves, seed
stalks, and seed pods have circular, light brown
spots. Small black pycnidia appearing on the
lesions distinguish blackleg from other cabbage
diseases. The leaves sometimes turn purple and
wilt, but there is no defoliation, as in black rot.
The fungus reaches the soil via infected plant
debris, remaining alive 2 or more years. Spores
are spread by splashing rain, or manure, on tools,
and perhaps by insects, with new lesions resulting
in 10 to 14 days. But the chief spread is by
mycelium wintering in infected seed. When
such seed is planted, fruiting bodies are formed
on cotyledons as they are pushed above ground,
and these serve as a source of inoculum for
nearby plants. A few diseased seed can start an
epiphytotic in wet weather.
Control
Use seed grown on the Pacific Coast,
which is usually, although not always, disease-
free. If the seed is infected, tie loosely in cheese-
cloth bags and immerse in hot water, held at
122
F for 30 min. It is sometimes possible to
buy seed already treated. Sterilize soil for the
seedbed; use a 3-year rotation; do not splash
seedlings when watering; do not transplant any
Cylindrocarpon
Rots
.
Cylindrocarpon obtusisporum Blackleg; on
grape.
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Phoma
Deuteromycetes, Coelomycetes
Pycnidia dark, ostiolate, lenticular to globose,
immersed in host tissue, erumpent or with short
beak piercing the epidermis; conidiophores short
or obsolete, conidia small, one-celled, hyaline,
ovate to elongate; parasitic on seed plants, chiefly
on stems and fruits, rarely on leaves.
Phoma lingam
Blackleg
of crucifers,
Foot Rot
,
Phoma Wilt
of plants of the mustard family,
including cabbage, cauliflower, Chinese cabbage,
brussels sprouts, charlock, garden cress, pepper
grass, kale, kohlrabi, mustard, rape, radish, ruta-
baga, turnip, stock, and sweet alyssum. The
teleomorph state,
Lystosphaeria maculans
has
been found on cabbage. The fungus was first
noticed in Germany in 1791; the disease was
