Diseases
Inputs/Production
Bacterial diseases of dry beans:
Every producer’s nightmare
HANGWANI MUEDI AND DEIDRE FOURIE, ARC-GRAIN CROPS INSTITUTE
Dry beans are one of the most affordable high protein sources in the
world, but its production is not without hassles. Some very small
microscopic organisms called bacteria, among other organisms, cause
diseases and brings about a negative impact on yield and profitability.
The most important bacterial diseases of dry beans in South Africa
are common bacterial blight, caused by
Xanthomonas axonopodis
pv.
phaseoli
(Xap) (Smith), halo bacterial blight, caused by
Pseudomonas
savastanoi
pv
. phaseolicola
(Psp), and bacterial brown spot, caused by
Pseudomonas syringae
pv.
syringae
(Pss).
The three diseases mainly affect the foliage and pods (including the
seeds) and are all seed-borne. They also penetrate a host plant through
wounds and natural openings on a plant.
Symptoms
Once inside the plant tissue, the bacteria start feeding and cause visible
damage (symptoms) to the plant. It must be said that these symptoms
deny the plant maximum generation and usage of the all-important
nutrients that plants require. In addition seeds look unattractive with less
chance of germinating or growing properly when planted.
Common bacterial blight
The symptoms associated with common bacterial blight first appear as
small water soaked spots on the underside of the leaves. These water-
soaked spots become dry (necrotic lesion) and are often surrounded by a
small zone of lemon yellow tissue (
Photo 1
).
The lesions enlarge and join together, making the plant appear as if it
has been burnt. When the bacteria have severely infected the plant, the
leaves die out and the dead leaves remain attached to the plants even
at maturity. Bacteria from infested material act as a source for new
infections on healthy plants and healthy plant tissues.
Pod symptoms consist of lesions that are generally circular, slightly
sunken, and dark red-brown. The sizes and shapes of the lesions differ.
After some rainfall, or heavy mist or lengthy overhead irrigation, the
lesions on pods become covered in bacteria. On white dry bean seeds,
butter yellow or brown spots can be seen throughout the seed coat.
When seeds are severely infected, they shrink and when planted, they
do not germinate properly, and those that germinate will appear weak.
Halo bacterial blight
Symptoms first appear as watery spots on the underside of the leaves.
These spots quickly dry up, becoming visible also on the upper side of
the leaves (
Photo 2
). The dry areas on the leaves generally remain small
in size (1 mm to 2 mm in diameter) and a green halo (from there the name
of the disease) surrounds the dry area.
When the disease is severe, the whole plant may generally appear
yellowish and eventually die. The general yellowish appearance is more
visible when the weather is cooler than when it is warmer. Bacteria
may need seven to ten days after first infection in order to cause new
infections.
Some red or brown lesions, that also look as if they contain water, may
appear on the pods. When the dry bean growing season approaches the
end and the pods mature, loosing their green colour, the diseased areas
on the pods remain green and some shining substance (bacteria) can be
seen on the area. Severe pod infections on the seeds cause the seeds to
shrink and loose their original colour.
Bacterial brown spot
Symptoms first appear as small watery spots on the underside of the
leaves, which become reddish brown and circular in shape, with different
sizes (
Photo 3
).
41
February 2014
Continued on page 43
Photo 1: Typical common bacterial blight symptoms on dry bean pods and leaves.
Photo 2: Typical halo bacterial blight symptoms on dry bean pods and leaves.
Photo 3: Typical bacterial brown spot symptoms on dry bean pods and leaves.
