Root Rot and Seedling Blight are common diseases of dry bean.
Pathogens (various fungal and fungus-like organisms) associated with root rot often appear as a complex, where more than one pathogen is present, making identification of the primary causal agent difficult.
These diseases are caused by several fungi including Pythium, Rhizoctonia, and Fusarium species.
Fusarium is the most common and most problematic in drier years.
Dry beans have a variable response to Aphanomyces. They are not resistant, and can be a host, but will only produce a few oospores and have variable symptoms from the disease
These soil-borne fungi are seed or soil-borne and can infect the plant at any stage between germination and maturity, and any part of the root system up to a short distance above the soil surface.
These fungi are common in the soil, and infection is more likely if the soil around the seed is excessively wet. Warm and moist conditions generally favour these diseases, but cold and wet is also detrimental because the cool temperatures slow plant development and add additional stress.
Other factors, including abiotic conditions such as flooding and soil oxygen depletion can result in root cell death.
Failure of the plants to emerge from the soil indicates seed decay or seedling damping off.
Symptoms include dark brown lesions on the tap root that can spread to all roots.
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 (Seeding Blight), wilting, with red to dark brown rotted areas appearing on the taproot at or below the soil line, as well as some premature defoliation and a failure to produce normal, full pods
On larger plants, the disease often appears as a rusty brown discolouration of the tap root (root rot phase).
Young plants may wilt and die after emergence or remain stunted and yellow (seedling blight).
Rhizoctonia presents with chocolate to red coloured spots that are more distinct, and may have small black or brown sclerotia develop inside the stem.
As the disease progresses, the discoloured area spreads until the entire taproot and lower stem are reddish brown and decayed, and secondary roots are usually decayed as well.
Typically occur in patches and may expand if conditions are favourable for the pathogens over several growing seasons. Symptoms are often associated with areas of flooding or waterlogging.
PREVENTION AND CONTROL
As a prevention measure, seed treatments offer protection to the developing seedling, especially under cool, wet conditions when emergence may be delayed.
Longer rotations can help reduce disease pressure, however many of the pathogens will affect other crops as well (such as Rhizoctonia, Fusarium, and Pythium), so crop rotation will have less effect.
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.
Taking note of disease presence in previous years’ crops can help tailor seed treatment selections.
Use seed with high vigour, practice good soil fertility, liming of acid soils and ripping to reduce soil compaction.
Choose fields that have good tilth and no compaction problems, so plants can form hardy roots, and moisture can move freely through the soil.
Understanding the disease, identifying the risks for root rot infection, and thorough planning for prevention are the only current options.
Root Rot diseases have the second highest impact in economic importance in dry beans in Alberta.
Botrytis (Grey Mould)
Caused by Botrytis cinerea Pers. ex Fr.
Botrytis Stem and Pod Rot is also known as Grey Mould.
Grey Mould can be seed-borne, stubble-borne, air-borne and soil-borne and can attack at various stages of plant growth.
Grey Mould favours cool, moist conditions and a thick plant canopy, such as found under irrigation.
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.
Grey Mould first appears as a grey mouldy growth visible at the soil surface. On older plants, it appears as a fuzzy grey or dirty white mouldy 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, watch for wilting, leaves become shriveled and dry, premature ripening, failure of pods to fill, and dead infected crop areas.
PREVENTION AND CONTROL
Treat seed or plant disease-free seed.
A thin crop canopy may offer some disease control.
Risk of Grey Mould 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 Mould 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 Mould.
This disease is less common than other pathogens impacting Alberta producers.