Diseases of Vegetables - Bean

Common Blight.
Xanthomonas phaseoli

What to look for?
Common fuscous and halo blights occur on dry beans and are often difficult to distinguish from one another. Water soaked lesions appear on the leaves or may appear as brown areas that are surrounded by yellow halos.

Photo: Howard

Picture description Bacterial blight lesions.

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Management strategy


Halo Blight.
Pseudomonas phaseolicola

What to look for?
Seed-borne inoculum is the primary source of all three bacterial diseases. Epidemics of the disease in the bean crop follow wet, windy weather at temperatures of 20 to 30 ^oC.

Photo: Howard

Picture description Water soaking typical of bacterial lesions.

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Management strategy
Copper-based bactericides are registered for control in Canada. Purchase of reliable blight-free seed is an important control measure for these bacterial diseases.

Anthracnose
Colletotrichum lindemuthianum

What to look for?
A very destructive fungal disease of dry beans that can in some circumstances cause complete crop loss. Dark brown lesions may appear on any part of the growing bean plant including seeds. The disease is seed-borne.

The fungus will attack the seed directly, causing distinct brown lesions on the seed surface. The seed-borne fungus will remain viable as long as the seed is stored, over many years.

Photo: Howard	Picture description Anthracnose lesions on actively growing bean leaves - early stage of development.
Photo: Howard	Picture description Anthracnose lesions on mature pods.
Photo: Evans	Picture description Distinct brown sunken anthracnose lesions on the green pods are obvious and unmistakable signs of this fungus.
Photo: Howard	Picture description Bean seed heavily infected by the anthracnose fungus. Such infected seed is the primary means of fungus spread.

Management strategy
Disease control is primarily via genetic resistance to the six or so major races of this fungus. Crop rotation and fungicidal seed treatments are other means of control. Disease control via avoidance of infected seed sources, resistant cultivars and a precautionary seed treatment fungicide are musts for anthracnose control.

Dry Root Rot
Fusarium solani f. sp. phaseoli

What to look for?
This is a soil-borne fungus that can be most destructive on heavy clay soils. Disease build-up is favoured by short rotations for the bean crop, i.e. growing beans every second or third year.

Photo: Howard	Picture description Fusarium infected root.
. Photo: Howard	Picture description Seedling killed off by this fungus.

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Management strategy
The fungus does not infect the seed but fusarium spores may be carried in dust, on seed, or in seed bags. Fungicidal seed treatment will help in avoiding the introduction of this disease onto new bean cropland.

Gray Mold.
Botrytis cinerea

What to look for?
Gray mold can be widespread and destructive on beans in wet seasons particularly on crops damaged by wind, rain, hail or insects. Pods are often damaged by mycelium growing from infected flowers - gray powdery masses of spores and small black flattened sclerotia can form on the pods and stems.

Photo: Pepin	Picture description Botrytis damaged pods; bean are unmarketable.
Photo: Ormrod	Picture description Gray masses of fungal spores characterize the botrytis infection of the pods.

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Management strategy


Mosiac
Common mosaic

What to look for?
A seed-borne and aphid transmitted stylet-borne virus disease. This virus is generally confined to the to the genus Phaseolus. A 1% seed infection can result in a 100% stand infection at the end of the growing season. Seed infection by this virus can be as high as 30%.

Bean yellow mosaic virus can also affect beans but this virus disease is not seed-borne in Phaseolus beans. This virus is carried in perennial legumes and is rapidly spread by aphids in a stylet-borne manner. Outbreaks are rare in Canada. Bean yellow mosaic virus can be seed-bourn at trace to 5% in fava beans.

Photo: Howard	Picture description
. Photo: Howard	Picture description Bean common mosaic virus causing leaf distortion.

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Management strategy
Aphids can spread this disease very rapidly through the bean crop. The common mosaic infection causes significant loss of yield and quality in the beans. Many races of the common mosaic virus occur but there are resistant cultivars. If resistant cultivars are unavailable then use virus-free certified seed. Relatively speaking, very few plant viruses are seed-borne.

Rust
Uromvces phaseoli

What to look for?
Bean rust is wide spread on common bean cultivars but it is not of great importance since the disease usually appears too late in the year and yield losses are apparently fairly minimal.

Photo: Howard	Picture description Brown gori (lesions).
Photo: Ormrod	Picture description Numerous rust lesions particularly on the under surface of the bean leaf.

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Management strategy
Many bean cultivars are resistant to rust. Where rust is perceived to be a problem, rotate with other crops and be sure to eradicate volunteer bean plants.

White Mold
Sclerotina sclerotiorum

What to look for?
Sclerotinia white mold attacks a very wide range of host plants, over 300 species of dicotyledons. The fungus can attack all parts of the plant via ascospores, or via direct infection from germination of sclerotia on the roots or next to leaves at the soil surface. In wet humid growing seasons, losses in yield and quality in the bean crop can be huge.

Pods, when they touch the soil, may be infected directly by germinating sclerotia or pod infection may result from ascospore infection of the dead flower on the tip of the pod.

Following main stem infection by Sclerotinia, the whole bean plant may collapse and yield loss could be total for that plant.

Under very humid conditions, the white mold of the fungus can be seen growing on the outside of the pod.

Large quantities of irregularly shaped hard black sclerotes are produced in and on the pods and stems in the bean crop. These sclerotes can last for up to 5 years in the soil and may be particularly destructive to following sunflower, safflower and canola crops under favourable disease conditions.

Pyrthium spp. or Rhizoctonia spp. may cause seedling and root rotting conditions similar to that of Fusarium dry root rot infection.

Beans prefer a soil pH above 6 and are sensitive to manganese deficiency at pH levels above 7.5. Zinc deficiency is also common in beans, particularly in high pH soils. In heavily leached soils or soils below pH 6, molybdenum deficiency may occur resulting in poor or inefficient nitrogen fixation and metabolism.

Photo: Howard	Picture description White mold shown killing-off around 50% of a field bean crop.
Photo: Howard	Picture description Sclerotinia killed bean pods.
Photo: Pepin	Picture description Stem collapse.
Photo: Evans	Picture description White mold sclerotia forming on the outside of the pod.
Photo: Ormrod	Picture description Numerous sclerotia.

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Management strategy
For disease control see dry beans - field crop diseases.

Photographs and information assembled and prepared for ARD by Dr. Ieaun R. Evans Agri-Trend Agrology Ltd.

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