Forages: Phytophthora Root Rot

Excerpt from Agronomy Guide for Field Crops (Chapter 5)

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

Incidence

Phytophthora root rot is an important and common disease of alfalfa. The disease shows up in Ontario mainly on poorly drained soils or on clay loam soils during extended periods of wet weather.

Appearance

Infection occurs as plants emerge. Therefore new seedlings are most at risk. As the stand gets older, the risk declines somewhat. Infected seedlings are stunted, grow slowly due to a reduced root system and eventually begin to wilt. A girdling, pinching or damping-off of the stem, at the soil line, may be seen causing the seedling to fall over and die. The field is often affected by the disease in circular or irregular patches. Damping-off of seedlings is a problem in heavy, wet soils during cool periods. In older seedlings or on established plants, a reddish-brown, water-soaked lesion may develop on the roots. In severe cases, root lesions become black, and the taproot may rot entirely. Since the plant is unable to supply water and nutrients, the plant wilts and dies. Lower leaves are yellow at first and as the disease progresses may turn reddish-brown.

Plate 96. Phytophthora root rot infection begins as the plants emerge. Infected seedlings are stunted and begin to wilt.

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Disease Cycle

Phytophthora root rot is a soil-borne disease that can result in root injury or the death of alfalfa plants. The fungus survives as thick-walled spores (oospores) that produce mobile spores in the spring that migrate and infect the plants' roots. Water is important since these mobile spores (zoospores) move in the water film between soil particles. Disease development is favoured when moderate to high temperatures occur (21°C-32°C or 70°F-90°F) during humid or wet conditions. Fields that are compacted or poorly drained are especially prone to the disease. Infection occurs as the plant emerges, therefore new seedings are most at risk. Risk declines somewhat with the age of the stand. The fungus is able to survive for many years in infected plant tissue as oospores.

Management Strategies

For fields with a history of Phytophthora root rot, use highly resistant varieties and seed treatments. Consult the current edition of the report, Ontario Forage Crop Variety Performance, available from your OMAFRA Resource Centre, online at www.plant.uoguelph.ca/performance_recommendations/ofcc/ofcc.htm for variety ratings for Phytophthora root rot. Refer to the OMAFRA Publication 812, Field Crop Protection Guide, for fungicide seed treatment recommendations. Other management practices that help in managing this disease include maintaining good soil fertility, which will promote lateral root growth, removing excess moisture through improved tile drainage and ensuring reduced compaction, avoiding other stresses such as leaf-feeding insects, weeds and untimely cuttings that may stress the plants making the plant more susceptible to Phytophthora. Crop rotation has little effect on this disease. For additional information, see the OMAFRA Factsheet, Phytophthora Root Rot of Alfalfa, Order No. 94-021, or visit the Web site at www.omafra.gov.on.ca/english/crops. (Order OMAFRA Publication 812, Field Crop Protection Guide)

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Updates on Forages: Phytophthora Root Rot

• OMAFRA Factsheet, Phytophthora Root Rot of Alfalfa, Order No. 94-021 is no longer available.
• Field Crop Protection Guide - Publication 812
  • Forage: Phytophthora Root Rot

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Related links...

• Diseases
• Forages

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