Root Rot Diseases – Seed Rot, Root Rot, Seedling Blights; Rhizoctonia Root Rot, Fusarium Root Rot, and Damping off. 

CAUSED BY

• These are soil-borne diseases caused by species of Pythium, Rhizoctonia and Fusarium.
• Pathogens (various fungal and fungus-like organisms) associated with root rot often appears as a complex, where more than one pathogen is present, making identification of the primary causal agent difficult. 
• Fusarium is the most widespread disease of faba bean around the world, though it is not currently found in Alberta.
• These pathogens can infect and kill individual seedlings and affect 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.
• Other factors, including abiotic conditions such as flooding and soil oxygen depletion can result in root cell death. 
• The pathogens overwinter on crop residue and in the soil.

SYMPTOMS

• Infected areas are greyish-brown to black – infection often begins on the feeder roots and progresses gradually towards the main root (in some cases, all roots are destroyed).
• Symptoms on foliage are also progressive, ranging from a few yellow leaves to pronounced yellowing of the top growth and severe stunting.
• If infection occurs early, it can cause seed rot and pre- and post-emergence damping-off – seedlings will sometimes show a constriction of the stem at or near the soil line.
• Infected seedlings usually die, resulting in poor stands.
• 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

• Follow a crop rotation that does not include faba bean or other legumes (pea, lentil, bean, alfalfa, clovers) more than once in four years will reduce the build-up of soil-borne pathogens, however, many of these pathogens can survive as saprophytes in the absence of a susceptible host. Therefore, crop rotation may have a limited effect in managing seedling blight and root rot.
• Ensure good soil fertility to produce a vigorous crop stand
• No fungicides are presently registered for control of this disease in faba bean.
• Seedling stress or damage due to environmental or herbicide injury can lead to an increase in the incidence of seedling blight.
• A seed test can determine if faba bean seed is infected with a seed-borne disease, such as Anthracnose, Ascochyta, Botrytis, Sclerotinia, or Fusarium. Consider using a seed treatment: (1) if disease is detected (2) if there is a history of soil-borne diseases or, (3) if seeding under cool moist soil conditions. If a seed treatment fungicide is used, ensure compatibility with rhizobia inoculants.
• As prevention measure, seed treatments offer protection to the developing seedling, especially under cool, wet conditions when emergence may be delayed.
• 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.

IMPACT

• Root Rot diseases in faba beans have less economic importance than foliar diseases.