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Dry Bean Diseases

Dry bean is very susceptible to a number of diseases and few chemical control measures are available – it’s important to use an integrated management program in dry bean production:

  • Use proper rotations: when yield loss occurs, use extended rotations of four years with non-susceptible crops, and avoid planting adjacent to last year’s bean field because many diseases are wind-borne and very mobile (shorter rotations are acceptable as long as diseases are not a problem)
  • Use certified seed: planting certified seed produced in arid regions is an effective technique for controlling bacterial blight
  • Use care when handling seed: mechanically damaged seed is prone to fungal diseases during germination, and young seedlings are less vigorous and more susceptible to fungal diseases – baldhead is also a symptom of mechanically damaged seed

The following are dry bean diseases of economic importance in Alberta.

Root Rot & Seedling Blight (Fusarium solani, Rhizoctonia solani and Pythium spp.)

  • root rot and seedling blight are common diseases of dry bean
  • failure of the plants to emerge from the soil indicates seed decay or seedling damping off
  • young plants may wilt and die after emergence or remain stunted and yellow (seedling blight) red to dark brown rotted areas appear on the taproot at or below the soil line – on larger plants, the disease often appears as a rusty brown discoloration of the tap root (root rot phase)
  • as the disease progresses, the discolored area spreads until the entire taproot and lower stem are reddish brown and decayed, and secondary roots are usually decayed as well
  • above-ground symptoms may not occur unless the degree of root rot is moderate to severe – then, there is likely to be stunting of the plants, yellowing of the foliage, wilting, some premature defoliation and a failure to produce normal, full pods
  • the causal fungi are all soil-borne, and plants may be attacked at any stage from seed through maturity (however, infection usually occurs early in the season and symptoms become progressively more pronounced)
  • fields that have grown dry bean for several years are likely to be the most severely affected by root rot and seedling blight diseases – follow a crop rotation with cereals, alfalfa, sugar beet or corn that includes bean only once every five years
  • choose fields that have good tilth and no compaction problems, so plants can form hardy roots, and moisture can move freely through the soil

White Mold (Sclerotinia sclerotiorum)

Description

Often called White Mold or pod rot, Sclerotinia rot is caused by Sclerotinia sclerotiorum. White mold occurs in all dry bean growing areas of western Canada. The critical infection period is during the flowering stage. While White Mold is of minor importance in most pulse crops, it can cause severe losses in bean, where the infection can spread quickly. Sclerotinia overwinters in infected crop debris and soil.

Causes

  • Cool temperatures
  • High humidity
  • Dense canopies
  • Growing pea and canola in the same rotation

Symptoms

The first sign of White Mold is the appearance of a light brown, water-soaked discolouration on the stem, leaves, or pods and a cottony threadlike growth in the collar region if the temperature and humidity levels remain high. A water-soaked area appears that spreads both upwards and downwards. Dark brown spots then develop on the stem, and the entire plant eventually turns brown. The growth and spread of infection is very rapid under favourable conditions, and whole plants may be killed in just a few days after symptoms are first observed (plant tissue invaded by the fungus becomes soft and slimy). Affected plants often appear wilted and ripen prematurely due to rotting stems; lodging is common in affected areas. Stems, when split open, exhibit characteristic white fungus growth – numerous, black, hard resting bodies (sclerotia) may be present in the pith. Affected plants yield poorly and often die prematurely.

Under favourable conditions during the growing season, Sclerotinia may germinate to produce mushroom-like structures that, in turn, produce spores – the spores are readily air-borne and may cause direct plant infection (generally, this mode of infection is more common than direct attack from mycelium in the soil).

Treatment & Prevention

  • Bury all crop residues.
  • In bean, avoid irrigating after the rows close over, and use wider row spacing and decreased seeding rates.
  • Sclerotia can persist in the soil for many years, so rotate with cereals and grasses – allow at least four years between susceptible crops such as legumes, canola, mustard and sunflower.
  • Fungicide use for control of Sclerotinia stem rot in the majority of pulse crops is not cost-effective because, once the canopy closes, the fungicides cannot reach their target.
  • In bean, a timely application of any of the available registered fungicides can help control the disease. Fungicide applications are most critical when target yields are high, when vine growth is heavy, and when moist weather occurs during flowering.
  • Heads Up® Plant Protectant has been approved for treatment of White Mold on dried beans.

Grey Mold (Botrytis cinerea)

Description

Grey mold is a common disease of bean in the Canadian prairies, especially under cool, moist conditions after flowering. Caused by Botrytis cinerea Pers. ex Fr., Grey Mold can be seed-, stubble- , air- and soil-borne and can attack at various stages of plant growth. It affects all parts of plants, usually starting on senescent organs such as cotyledons and flowers. Infection first occurs from spores dispersed by wind or rain. After infection, the disease spreads rapidly to other tissue by direct contact with diseased tissue. Infected seed produces infected seedlings, which die before emergence or soon after. On older plants, a greyish mold is observed, which quickly spreads under moist conditions. In addition to attacking senescent flowers, the disease is also often initiated through the infection of wounded young bean pods partially buried and bruised by the soil during hilling of bean rows.

Causes

  • Cool temperatures
  • High humidity
  • Wet soils
  • Dense canopies

Symptoms

On young plants that emerge, the Grey Mold appears as a grey moldy growth visible at the soil surface. On older plants, it appears as a fuzzy grey or dirty white moldy growth on flowers, pods, or lower areas of the stem. The infected sites first develop small water-soaked lesions that expand to form large brown lesions with concentric zones. Under humid conditions, massive greyish-brown spores are produced to cover the infected tissues. Sometimes, black sclerotia may be formed on old infected tissues. These lesions spread to the entire lower foliage. As the disease progresses, wilting, premature ripening, failure of pods to fill, and dead infected crop areas occur.

Treatment & Prevention

  • Treat seed or plant disease-free seed.
  • A thin crop canopy may offer some disease control.
  • Risk of grey mold can be reduced by the use of clean seed, field sanitation and crop rotation.
  • Widen row spacing or lower seeding rates to improve air circulation in the crop.
  • Cereals, forage grasses and corn are rarely affected by grey mold and thus are ideal crops for rotation with bean.
  • No fungicides are registered for control in many of the pulse crops, and control products in bean are not considered economical.
  • Potassium fertilizer in potassium deficient soils reduces the severity of Grey Mold.

Rust (Uromyces appendiculatus)

  • rust occasionally occurs in Alberta and is found primarily in pinto bean
  • it tends to infest late-maturing crops
  • rust first appears as small, white spots on the lower surface of the leaves – these spots break open within a few days to expose rust-colored fungus spores on both upper and lower leaf surfaces
  • severely infected leaves turn yellow, then brown and soon die (pods and stems may also be attacked)
  • rust fungus can survive on infected bean crop residue and is also spread by wind-borne spores (rust fungus is not transmitted as a seed-borne disease)
  • follow a crop rotation that allows a minimum of three years between bean crops
  • after harvest, turn under all bean residue as completely as possible

Common Blight (Xanthomonas phaseoli) and Halo Blight (Pseudomonas phaseolicola)

  • blight diseases cause leaf lesions, defoliation, pod lesions and shrunken, discoloured seed
  • the most prevalent blight is common blight
    • large irregular shaped lesions are surrounded by a distinct yellow zone
    • veins near the lesion are darkened
    • infected pods develop greasy-appearing spots surrounded by a chlorotic halo
    • lesions exude yellow ooze when wet
  • halo blight is less prevalent, and pinto, great northern and red Mexican varieties have some resistance
    • halo blight first appears as small water-soaked spots
    • these spots soon die, leaving chocolate brown lesions
    • during cool weather, the lesions are surrounded by light green halos up to 1/2 in. (1.3 cm) in diameter
    • the bacteria can cause pod lesions, and these exude a white/cream ooze
    • all bacterial diseases are spread by rain splash, and water aids bacteria penetration into leaf pores and wounds
  • these pathogens are seed-borne but can survive in non-decomposed bean trash for at least one year
  • always plant high quality seed free of bacteria
  • bury bean trash and use a three-year (or longer) crop rotation
  • avoid cultivation when bean plants are wet

Bean Yellow Mosaic & Bean Common Mosaic Virus

  • infected plants are often stunted and spindly, with few pods set and seeds that are off-colour and small
  • infected leaves have irregular areas of yellowish tissue intermixed with areas of green – leaves may be puckered, twisted and elongated
  • plants are seldom killed prematurely and yield loss depends on time of infection
  • the disease spreads by plant sap contamination of wounds, insects and infected seed
  • many commercially available seed varieties have resistance to virus diseases – plant certified seed and control aphids when they appear in large numbers

Anthracnose

Description

Caused by Colletotrichum lindemuthianum on bean, Anthracnose can be seed- or soil-borne. Spores can survive on stubble for up to four years.

Causes

  • Warm temperatures
  • High humidity
  • Recurrent rains (spores are spread by splashing rain)
  • Dense canopies

Symptoms

Anthracnose first appears as grey to cream-coloured spots on leaves and tan to brown lesions on stems. The entire lower stem may become covered in lesions, giving it a brown, rough appearance. Pod lesions are circular and sunken with reddish-brown margins and reddish centres; close examination of pod lesions often reveals fruiting bodies with orange-pink spores. Leaf and stipule lesions are oval, with brown margins and greyish centres, and stem lesions are elongate, appearing as a copperish colour when moist and greyish when dry. Severely diseased plants have an overall reddish-brown appearance, and die back of leaves can occur.

Treatment & Prevention

Apply various registered fungicides prior to flowering, and repeat if conditions for disease development are favourable. Seed treatments can be used for seed-borne infections of this disease. On beans, resistant cultivars are available.