Soybeans: Phytophthora 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: Soybeans: Phytophthora Root Rot
6. Related links...

Incidence

Phytophthora root rot is a potential problem in heavy clay soils in Southwestern Ontario. It remains one of the most destructive diseases of soybeans in Ontario. Losses to Phytophthora root rot have occurred in the province since the 60s. The disease has also been observed in Central and Eastern Ontario. As soybean production continues to expand in those areas, so will this disease.

Appearance

The disease can affect soybeans at any stage of development but is often most damaging when it occurs early in the season. Plants infected at the primary leaf stage display typical "damping-off" disease symptoms. Seeds may fail to emerge, or infected seedlings are killed shortly after emergence. Infected areas of the stem are water-soaked or "bruised" and disintegrate easily (soft rot). Since it causes a "wet rot," it is difficult to distinguish Phytophthora root rot from Pythium root rot at this stage. Both diseases cause taproot and lateral root pruning or rotting resulting in yellowing of the leaves, wilting and even death. Infected plants are easily pulled from the ground since the plants are not well anchored. Older plants can be affected any time before maturity. A purple or dark-brown discolouration of the stem may extend from the roots (just below the soil line) to the lower nodes of wilted plants. Dead plants may appear a few in a row or as patches in low areas of fields. Leaves will often remain attached to the plant even after death.

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Plate 64. Phytophthora root rot causes water-soaked lesions on seedlings and purple or dark-brown discolouration of the stem. Begins at the soil line and progresses into the lower nodes.

Disease Cycle

Cool, wet weather favours disease development. Low areas, poorly drained and slow-drying areas of the field are most prone to the disease. Heavy clay soils, reduced tillage and monoculture of soybeans may increase the damage due to the disease. Phytophthora and Pythium are unique organisms, in that they produce mobile spores that can swim in the water film between soil particles to locate soybean roots. The fungus colonizes the root tissue and will plug the water-conducting tissues of the plant, resulting in wilting of the plant. See the section Diseases, for more information concerning the uniqueness of Phytophthora.

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Management Strategies

Control of Phytophthora root rot requires a combination of soybean variety selection, seed treatment and good soil management. Soybean varieties with resistance or tolerance to Phytophthora root rot are available. Some varieties have both resistance and tolerance. Select soybean varieties that have both specific resistance (Rps genes such as 1K and 1C) and good partial resistance (tolerance) to all races of Phytophthora. Varieties containing only the Rps 1a source of Phytophthora resistance may not be very effective in many parts of the province. Many Phytophthora races have developed over the years that can bypass the Rps 1a resistance. Consult with your seed company for variety profiles, especially with new varieties or technologies such as herbicide resistance. Consult the current report, Ontario Soybean Variety Trials, available on the Ontario Soybean Growers Web site at http://www.soybean.on.ca for plant loss ratings for Phytophthora root rot and resistance genes in varieties.

1. Resistant varieties. The Phytophthora fungus is present in Ontario soils as a series of races (or "strains"). Resistance in any one soybean variety is effective against some but not all of the races. Root rot is controlled in a particular field when the variety grown is resistant to all of the Phytophthora races that happen to be present in that field. Resistance will "break down," however, should another race appear to which the variety is not resistant. If this occurs, switch to a variety that is resistant to the new race or use a tolerant variety or a different gene for resistance. Varieties with different resistance genes should be grown in rotation. To determine which races may be present in a field, plant strips of several varieties with known race resistance.
2. Tolerant varieties. Some disease develops in these varieties when grown in infested soils, regardless of which races of Phytophthora are present. However, yields of tolerant varieties usually are not seriously reduced by the disease.

Any soil management practice that reduces soil compaction or waterlogging will decrease the incidence of Phytophthora root rot. On clay soils where the disease may be a problem, the following procedures are recommended:

• Choose a variety with a low percentage of infected plants. (See the Ontario Soybean Variety Report.)
• Rotate with corn and wheat. A short rotation will result in a higher population and an increase in the number of races present in the field.
• Do not work the field when the soil is wet.
• Use good soil management practices to improve soil structure and drainage (rotation, manure, cover crops, reduced tillage, etc.).
• Tile inherently slow-draining fields.
• A small amount of tillage will help warm soil and increase surface drainage.
• Plant when soil temperatures are above 13°C (55°F).
• Refer to OMAFRA Publication 812, Field Crop Protection Guide, for recommended seed treatments.
• Inspect each soybean field for dead plants in late July or early August to determine whether the variety has enough resistance/tolerance to provide adequate protection under local conditions. (Order OMAFRA Publication 812)

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Updates on Soybeans: Phytophthora Root Rot

• Field Crop Protection Guide - Publication 812
  • Soybeans: Phytophthora Root Rot

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Related links...

• Disease
• Soybeans

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