W.C. Nesmith and J. R. Hartman
BEAN YELLOW MOSAIC VIRUS
Viruses can cause serious damage to
Kentucky bean plantings. More than one virus is often involved but bean
yellow mosaic virus (BYMV) seems to be most frequently observed.
Although symptoms of this disease may
vary depending on time of infection, bean variety and virus strain, symptoms
generally include crinkling, downward cupping, yellow mottling, and dead
areas along the veins of infected leaves. Death of vine tips and new leaves
may occur on pole and half-runner bean types Vines may die back several
feet, thereby destroying the bean plants. Plants affected with root and
stem rot, manganese toxicity and bacterial blight may show some symptoms
similar to those of the virus so laboratory diagnosis is often advisable.
Observations of any adverse root and soil conditions should be made.
BYMV is carried to beans by aphids.
These insects pick up the virus mainly from red or white clover or other
legume weed hosts growing near the garden including earlier plantings of
beans. The following suggestions may reduce the damage caused by BYMV in
1.Avoid planting beans near clover
or other legumes.
2.Destroy legume weeds in fencerows
or other areas in the garden.
3.Plant a barrier of sweet corn or
other tall-growing crop upwind of the beans.
4.Plant bush-type beans if possible.
It appears that Kentucky Wonder Pole beans and White Half-Runner beans
are more susceptible to the virus.
5.Plant successive plantings of beans.
Infection levels may be high one week, then lower a few weeks later allowing
some of the plantings to escape infection. So keep on planting!
6.For commercial plantings, avoid planting
successive plantings near each other.
Stem and Root Rots
The fungi which decay the lower stems
and roots of green beans are present in almost all soils used for growing
beans but these fungi build to higher levels with continuous cropping of
beans in the same site. They can survive for long periods of time in the
soil and may spread from place to place via moving soil. Most root rots
are favored by cold or wet soils. Usually, these fungi are less of concern
under ideal growing conditions for bean crops. The fungi may do little
harm if plants are strong and vigorously growing. Virus-infected garden
beans may be damaged more than beans without virus by root rot fungi.
Rhizoctonia Root Rot (Rhizoctonia
solani)--The Rhizoctonia fungus attacks plants at almost any
age, causing seed rot, damping-off of seedlings, or stunting, yellowing
and killing of older plants. Elongate, sunken, red-brown lesions develop
on roots and stems at or below the soil line. Lesions may enlarge to girdle
the stem, killing the roots and weakening the top of the plant. Infected
plants may be stunted and the leaves may turn yellow and die.
Fusarium Root Rot (Fusarium solani
f. phaseoli) --Fusarium grows best in warm soils so this fungus
attacks beans later in the season. Plants are stunted or yellowed but not
usually killed. The taproot and lower stem show reddish lesions which later
turn brown to black. The red colored taproot tip and lateral roots may
decay, shrivel and die. Rootlets may develop above the lesion, enabling
the plant to survive.
Pythium Root Rot (Pythium spp.)--Pre-emergence
damping-off and seedling wilt of beans is often caused by the Pythium
fungus. Water-soaked lesions may appear on the stem and branches where
the affected tissue becomes soft and slimy. When the stem is girdled, the
plant wilts suddenly and dies. Older plants may develop dark brown lesions
instead of soft rot and may be stunted or die prematurely.
1.A rotation of 4 to 5 years between
bean crops is helpful--the longer the better.
2.Planting after the soil has warmed
should produce rapid, vigorous growth. Cool soils can increase the incidence
of root rots.
3.Shallow planting may be helpful.
4.Planting fungicide-treated seed can
reduce seedling rots.
5.Treat soils with appropriate fungicides
is available for commercial crops. (see ID-36)
6.Most bean varieties are susceptible
to root rots under certain conditions but some more so than others. Kentucky
Wonder Pole Beans and White Half Runner beans are among the most susceptible.
7.Be sure to allow time for cover crops
to rot before planting by plowing soil 4 to 6 weeks prior to planting.
The fungus Colletotrichum lindemuthianum
which causes anthracnose can reduce bean quality as well as yield. Losses
can be severe during cool, rainy weather.
The disease appears on all above ground
parts of the plant but rarely on the roots. Lesions generally are dark
brown and may contain pink spore masses during moist weather. Elongate,
angular spots appear on the lower leaf veins spreading slightly into surrounding
tissue and eventually appearing on the opposite side. Affected seeds are
discolored. Plants grown from infected seed will develop cankers on the
The most striking phase of the disease
occurs on the pod. Small brown spots appear and rapidly enlarge into dark
sunken cankers. Often the margin will be a dark brown while the center
portion of the canker is light in color.
1.Because the fungus can be carried
on the seed, be sure to plant only disease-free seed.
2.A crop rotation of at least 3 years
is important to reduce carry-over at site.
3.Follow a good weed control program.
4.Plow under or remove plants after
harvest to reduce overwintering of the fungus in the field/site.
5.Fungicide sprays can also be helpful.
(see ID-36 for commercial plantings)
Bean blights, caused by one or more
species of bacteria, occur in most of the bean growing areas of the world.
Under favorable weather conditions, these bacteria can spread rapidly through
a field causing defoliation and pod damage.
Common Blight (Xanthomonas phaseoli)
and Fuscous Blight (X. phaseoli var. fuscans)--Lesions on the leaves
first appear as small, watersoaked, light green areas. The spots become
dry and brown with a narrow yellow halo. As the disease develops, the spots
may grow, eventually killing the leaves. Similar watersoaked spots form
on pods and can develop into broad irregular blotches. In humid weather,
a yellow bacterial crust covers the surface of the diseased area. The margin
of the spot or the entire spot may be red-brown in color. In severe attacks,
the pods may shrivel and seeds may not develop.
Wilt (Corynebacterium flaccumfaciens)--The
symptoms of bean bacterial wilt are similar to those of common blight.
In addition, the plants are stunted and the leaves droop and appear wilted.
Halo Blight (Pseudomonas phaseolicola)--This
blight is similar to common blight except that there may be a large yellow
halo (up to 1/2 inch in diameter) surrounding the leaf spot (Figure 6).
Newly developing leaves may show yellowing due to systemic infection and
plants can die rapidly. Leaf symptoms without halos may develop if temperatures
are relatively high. Symptoms on the pods are also similar to those of
common blight. The bacterial crust on the surface of the spots may be white
instead of yellow.
Brown Spot (Pseudomonas syringae)--This
disease is more common on lima beans. Small, water soaked spots on the
leaves become a red-brown color, the spot center dries out, turns grey
and may fall away. Veins on the underside of the leaves may turn red-brown.
Spots on the stems and pods are more elongated than those on the leaves.
The bacteria overwinter in seed, plant
refuse or susceptible weeds. When infested seeds are planted, an early
outbreak may occur on the new crop. The' bacteria can spread to healthy
plants via splashing rain, wind-blown soil particles, or on tools and implements
moving through wet fields. The bacteria enter through natural openings
such as stomates or through wounds such as those caused by chewing insects
and blowing sand particles. The brown spot bacterium can overwinter in
other plants such as lilac or members of the Prunus group. Once
introduced, these bacteria can colonize the leaf without causing symptoms,
then cause sudden crop damage, following heavy rains. Therefore, control
will be centered around keeping the population of bacteria low and out
of the field.
1.Rotate beans with other crops leaving
2 to 3 years between bean crops.
2.Use commercially grown, disease-free
seed. Using locally saved seed is very risky because the pathogen is saved
with the seed.
3.Do not work fields when plants are
4.Spray bean plants at the first sign
of disease with a copper containing chemical bactericide. (see ID-36) ).
Bean rust is caused by the fungus Uromyces
phaseoli typica. The disease is most important on dry and pole snap
beans, but it also affects bush snap and lima beans. In Kentucky, bean
rust normally occurs in the late summer.
The rust pustules tend to be most numerous
on the underside of the leaves, less abundant on the pods, and they occur
sparingly on the stems. Infection is first evident as minute, almost white,
slightly raised pustules which later become distinct, reddish-brown, tiny
circular "cushions" that are typical of a true rust Each pustule is made
up of a powdery mass of rust-colored spores. When the leaf becomes heavily
infected, it shrivels and falls from the plant.
The fungus exists between crops in
the form of spores which initiate infections in the following crop. Cloudy,
humid days with temperatures between 60° and 75° F are favorable
for disease development. Under these conditions, an infection of bean rust
can produce a new crop of spores in 10 to 15 days. Although Uromyces
spores may blow long distances and infect plants where no beans have ever
been grown, it has been shown that when one crop of beans follows another
in the same field, the amount of rust inoculum is increased so that the
following crops may be damaged severely. In Kentucky, rust is much more
common in late summer and early fall than any other season.
1.Crop rotation is the first suggestion
2.Spraying with approved fungicides
at regular intervals, starting when the disease first appears or when advisories
are issued, will give effective control. (see ID-36)
3.Some bean varieties are resistant
to some races of the rust fungus. These varieties are listed as rust resistance
in seed catalogs. Consider them where space doesn't permit long rotations
and where fungicides will not be used, especially in late summer plantings.
NOTE: Contact your County Extension Office for the currently recommended