Soybeans: Rhizoctonia Root Rot

Excerpt from Agronomy Guide for Field Crops (Chapter 4)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Incidence
2. Appearance
3. Disease Cycle
4. Management Strategies
5. Updates on Soybeans: Rhizoctonia Root Rot
6. Related links...

Incidence

Rhizoctonia root rot has been found in most of the soybean-growing areas of the province. In most fields, stand losses range from less than 5% to over 50% in severe cases. The disease has been increasing in importance and can result in substantial yield losses. It is most prevalent on seedlings and young plants, causing a root and stem rot, particularly during prolonged wet periods.

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Appearance

Rhizoctonia root rot causes preemergence (seed rot) and postemergence (seedling blight) damping-off on affected seedlings. A characteristic reddish lesion is produced on the stem at or just below the soil line. These firm, dry, brick-red lesions can join, forming a sunken girdling of the stem that may move down the taproot, pruning roots along the way. Symptoms of the disease are very similar to plants infected with Phytophthora root rot. Affected plants are pale yellow, which is often confused with nitrogen deficiency symptoms or poor nodulation. Severely infected plants may lose their leaves. Wilted and/or dead plants often occur in small patches. Stem lesions girdle the stem and weaken the plant, often causing infected plants to break at the soil line under stormy conditions. Stressful growing conditions favour this disease. Rhizoctonia root rot is most damaging when cool, wet conditions in the spring are followed by hot, dry conditions.

Plate 65. Rhizoctonia root rot causes reddish lesions on the stem at or just below the soil line.

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Disease Cycle

This disease affects a large number of field and horticultural crops. It occurs on all soil types and environmental conditions. Disease losses are greatest when a wet spring is followed by dry conditions. The fungus is primarily a soil inhabitant and survives as resting mycelium or sclerotia. Under optimum temperature conditions (25°C-29°C or 77°F-84°F), disease severity will be greatest in fields that have a history of the disease. Over time, smaller infected areas increase in size.

Management Strategies

Few management options exist since no resistant and few tolerant varieties are presently available. Crop rotation with corn and small grains can help minimize the disease. Maintain good soil drainage and avoid planting under cool, wet conditions. Fungicide seed treatments offer some measure of protection and increase emergence. Further information on fungicide seed treatments can be found in OMAFRA Publication 812, Field Crop Protection Guide. (Order OMAFRA Publication 812)

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Updates on Soybeans: Rhizoctonia Root Rot

• Field Crop Protection Guide - Publication 812
  • Soybeans: Rhizoctonia Damping-off

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Related links...

• Disease
• Soybeans

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