Winter Wheat Production Manual

Written by D. B. Fowler
Crop Development Centre
University of Saskatchewan

© University of Saskatchewan. All rights reserved.  No part of the Winter Wheat Production Manual may be reproduced in any form by any photographic, electronic, mechanical or other means, or used in any information storage and retrieval system without the written permission of the University of Saskatchewan or Ducks Unlimited Canada.

Root Rots

2. ROOT ROTS

A healthy, vigorous root system is necessary to ensure that the growing plant has an adequate supply of water and nutrients. There are many root diseases that can attack wheat and several of these are chronic problems on the Canadian prairies. Two types of root rot have mainly been responsible for crop losses in winter wheat in Saskatchewan.

 
  • Common root rot (dryland root rot) - Cochliobolus sativus and Fusarium species.
  • Take-all root rot - Gaeumannomyces graminis

In western Canada, crop losses from common root rot are normally associated with warm, dry, high pH soils. Take-all root rot usually causes greater damage under irrigation and in high moisture soils.


A. COMMON ROOT ROT

[ Images | Causes and Symptons | Losses | Control ]

a) Causes and Symptoms:

Cochliobolus sativus is the main fungus responsible for common root rot on the Canadian prairies. It is found on cereals and many grasses, but is most widespread in wheat and barley. Fusarium species that cause common root rot are also widespread and they are often found in combination with C. sativus. These are the same species that are responsible for Fusarium head blight (scab) of wheat.

The fungi that cause common root rot can survive as spores in the soil or on plant residues. They can be spread by wind, water or cultivation. They can also be carried on or in the seed from infected plants. However, most common root rot infections result from spores that are present in the soil. Spores can remain dormant in the soil for years until they are stimulated by root secretions from winter wheat plants or other hosts. Warm dry conditions that are often associated with periods of drought favor the development of this disease.

Infection by the fungi that cause common root rot can occur at all plant growth stages. Dark brown spots or blotches on the roots, subcrown internode (section of the plant between the crown and seed - see Figure 4, Chapter 10 or Figure 2, Chapter 12 ) and lower leaf sheaths are often the first signs of infection. A healthy subcrown internode is an unblemished creamy white color while a dark brown subcrown internode indicates that the plant has been severely infected by common root rot fungi. Color of the subcrown internode is often used as an indicator in the field assessment of common root rot infections; however, an extremely compressed subcrown internode usually limits the usefulness of this rating method for shallow seeded winter wheat.

Common root rot damage is most conspicuous shortly after the crop heads. At this stage, severely infected plants appear to ripen prematurely and, as a consequence, common root rot is often referred to as prematurity blight. Blighted plants are usually scattered throughout a field rather than appearing in patches and they are most noticeable in heavy winter wheat stands that have been subjected to drought stress near the time of heading. Blighted plants are normally stunted and they produce few, if any, seeds that are usually shrivelled.

The prematurity blight symptoms of common root rot have been especially evident in winter wheat produced in the Parkland (black and grey soil zones) region of Saskatchewan. Favorable early season growing conditions often produce stands of winter wheat with a higher yield potential than can be maintained throughout the late spring and summer. As a result, the winter wheat crop is normally subjected to drought stress once early summer temperatures start to climb. Plants with root systems that have been badly damaged by common root rot are the first to give up creating the characteristic scattering of blighted plants in an otherwise green field of fully headed plants.
[ Images | Causes and Symptons | Losses | Control ]


b) Losses:

Common root rot is a widespread disease on the Northern Great Plains of the U.S.A. and Canada. In fact, it is so prevalent that is often referred to as the "common cold" of wheat production on the Northern Great Plains. Average grain yield losses due to common root rot have been estimated to be 6 to 7% for spring wheat produced in the Canadian prairie/North Dakota/Minnesota region. However, because common root rot affects the crown and roots of the plant, damage to spring wheat often goes unnoticed.

Surveys conducted in the Parkland (black and grey soil zones) region of Saskatchewan when winter wheat is between Zadoks growth stages 60 to 80 (flowering to dough stages) routinely report common root rot (prematurity blight) symptoms in 40 to 90% of the field visited (see Chapter 26 and Chapter 27 ). Isolations made from winter wheat crown tissue collected during field surveys in Saskatchewan in 1986 found Fusarium species in 100% and C. sativus in 57% of the fields sampled. These observations suggest that winter wheat crop losses due to common root rot may be even higher than those reported for spring wheat on the Canadian prairies.
[ Images | Causes and Symptons | Losses | Control ]


c) Control:

1. Seed shallow. Deep seeding has been shown to significantly increase the severity of this disease.

2. Follow a soil fertility program that is well balanced and adequate for healthy plant growth and vigorous root development. Excessive nitrogen fertilization favors the development of common root rot.

3. Fungicide treatments may be used to reduce seedling infection. However, common root rot can infect the plant at all growth stages and seed treatments recommended in western Canada will not protect post-seedling stages from infection.

4. Plan rotations to include non-host crops such as canola, flax, mustard, and legumes. Common root rot inoculum levels are expected to drop to 50% after the first year and an additional 10% after the second year when fields are planted to non-host crops. The need for 2 or more years between wheat and barley crops usually means that crop rotation has limited practical value in controlling common root rot on the Canadian prairies.

5. Tillage operations, such as moldboard plowing, that bury crop residues prior to seeding are often recommended as a method of reducing common root rot infections. However, the need for a standing stubble snow trap to prevent winter crop damage eliminates this option for winter wheat production on most of the Canadian prairies. High operating costs and an increased likelihood of moisture loss and soil erosion also make the burying of crop residues an unattractive disease management option.

6. Grow varieties with a high level of resistance to common root rot. There are a number of spring wheat varieties that offer some resistance to this disease. However, the winter wheat cultivars presently registered for production in western Canada only have poor to fair common root rot resistance and none of the winter wheat breeding programs in western Canada are actively selecting for resistance to this disease.


B. TAKE-ALL ROOT ROT

[ Images | Causes and Symptons | Losses | Control ]

a) Causes and Symptoms:

The fungus responsible for take-all root rot can initiate its attack anytime during the growing season. Optimum conditions for its development are high soil moisture, soil temperatures between 10 and 20oC, nutrient deficiencies, and neutral to alkaline soil pH.

The take-all fungus can infect wheat, barley, rye, bromegrass, wheatgrass, and grassy weeds such as downy brome, wild barley, and quackgrass. Crop residues are the main source of inoculum, but take-all can also be spread by airborne ascospores. It is not spread by insects or seeds. The take-all fungus builds up slowly under field conditions and it is short-lived.

Once an infection has established, the fungus can move from root to root infecting the roots of other plants with which it comes in contact. Spread of the fungus by root bridges gives rise to the patches of diseased plants that are characteristic of take-all. Because moist soil conditions favor the development of this disease, the patches are often found in wetter, more poorly drained areas of the field. If conditions are particularly favorable, as occasionally happens with irrigation, whole fields can be severely damaged by this disease.

In most situations that give rise to severe damage, the take-all fungus infects the seminal roots of plants in the seedling stage (see Figure 4, Chapter 10 ). The root infection then progresses into the crown and tiller bases. Severely infected plants are stunted, develop few tillers, and often die prematurely. If seed is produced by blighted plants, it is usually badly shrivelled. Because root growth has been restricted, affected plants can be easily pulled from the ground. The roots and crowns of severely infected plants are black and removal of the lower leaf sheath usually reveals a shiny, coal-black discoloration of the tissue at the base of the stems. This discoloration is a key diagnostic symptom of take-all. Plants that are less severely affected, or are infected later in the growing season, may lack vigor and be less productive without showing typical text-book symptoms for take-all. Less severe infections may go unnoticed or may be attributed to poor fertility or other soil related problems.
[ Images | Causes and Symptons | Losses | Control ]


b) Losses:

Crop losses from take-all are most serious in the cool season cropping areas of the world (both the northern and southern hemispheres) where moisture is abundant, soil pH is above 6.0, and cereals and forage grasses are widely grown. Yield losses are usually significant once the take-all symptoms become obvious in a field.

In Saskatchewan, the most severe take-all damage has been observed in winter wheat produced on chemical fallow or under irrigation. Take-all damage is most frequently observed in the eastern part of the province where alkaline soils are common.

Even though no-till production systems should favor this disease (because crop residues are left on the soil surface), take-all is not considered to be a major problem in direct seeded winter wheat in Saskatchewan. However, this disease is chronically present and large patches of take-all are reported in a few fields in the eastern prairies in most high rainfall years.
[ Images | Causes and Symptons | Losses | Control ]


c) Control:

1. Crop rotation. The take-all fungus is short lived and rotation to nonhost crops or summerfallow usually reduces the risk of damage from this disease. Take-all builds up slowly. Consequently, once it has died back in a field, it often takes several years of continuous wheat and barley for the fungus to return to levels that will cause significant damage.

The most severe take-all damage that I have seen in Saskatchewan occurred in irrigated winter wheat sown into canola stubble and winter wheat seeded into chemical fallow. These were two isolated incidents, but they do suggest that a single year of rotation to a nonhost crop or summerfallow may not be sufficient to control take-all if the conditions for its development are favorable.

A form of biological control, which is referred to as "take-all decline", often occurs in continuously cropped fields. This decline is caused by parasites that attack the take-all fungus reducing the damage caused by this disease to tolerable levels. Take-all decline normally starts after the fungus has been allowed to build up for several years. A single year rotation to a nonhost crop is enough to reduce the control exercised by the parasites and allow the take-all to slowly return to peak levels.

2. Maintain adequate, balanced levels of soil nutrients. High rates of nitrogen fertilizer, especially in the nitrate nitrogen form, should be avoided in the fall. When planning late summer and early fall nitrogen fertilizer strategies, remember that ammonium nitrogen from urea and anhydrous ammonia is rapidly converted to nitrate nitrogen in moist, warm soils.

3. Operations that speed up the breakdown of wheat or barley crop residues help to reduce the survival time of the take-all fungus. Intensive tillage, weed free summerfallow, and the burning of straw can be used to accelerate the breakdown of crop residues and control grassy weeds that are hosts of take-all. However, none of these operations are options in the management system recommended for winter wheat production in Saskatchewan.

4. Winter wheat cultivars with resistance to take-all are not yet available in western Canada.

5. There are registered seed treatments available that may be applied by commercial applicators for take-all suppression.
[ Images | Causes and Symptons | Losses | Control ]

rootrot1
Figure 3. Prematurely ripened plants that have been damaged by common root rot. Blighted plants are usually scattered throughout the field. (Image Size = 45k)