Root Rot

Table of contents

    Important Tips for Best Management

    • Use B. napus varieties where possible, as they are relatively less susceptible. Plant breeders are making good progress on the development of B. rapa varieties with resistance to brown girdling root rot. 
    • Use recommended seed treatments for control of the root rot complex, however, these fungicides do not provide season long control. 
    • Use balanced fertility and proper seeding depth to help guard against root rots; weaker plants are significantly more susceptible than healthy robust plants. 


    About The Root Rot Complex

    Overview

    The root rot complex is caused by soil-borne fungi that affect the roots of adult plants. Rhizoctonia solani, Fusarium species and Pythium species are the primary pathogens in the root rot complex, which includes foot rot, late root rot, root rot and brown girdling root rot. Different strains of these pathogens can also cause seedling diseases early in the season. (See Section 4.4 Seedling Disease Complex).

    The fungi causing these symptoms are potentially serious, and the incidence, as well as intensity, is increasing throughout the prairies. Brown girdling root rot is the most serious root rot disease and is believed to be caused primarily by the fungus Rhizoctonia solani, with secondary infections by Fusarium spp. In most of western Canada, the disease manifests itself primarily at the seedling stage, while in the Peace River region both seedling loss and the much more serious brown girdling root rot occur. [1]

    Brown girdling root rot affects all varieties, although B. napus (Argentine) varieties are more resistant. Losses are highest when wet soil conditions occur at early flowering, followed by dry weather later in the season. Infection levels may reach 80 to 100% in some fields, with losses approaching 50%. In the 2010 disease survey [2], brown girdling root rot (BGRR) was found in 20% of canola crops in Alberta with a mean incidence of 3%. The regions with the highest prevalence (percentage of fields affected) of BGRR were in the Peace River Region, with some fields having incidences (percentage of plants infected) as high as 60%.

    Research by Agriculture and Agri-Food Canada (AAFC) Beaverlodge, AB Research Centre [3] found that nearly half of the microorganisms isolated from brown girdling root rot infected root tissue were identified as Rhizoctonia solani. Rhizoctonia solani exists as several strains with anastomosis group AG2-1 commonly identified in the Peace River region and the less virulent AG4 in other regions.

    Disease Cycle

    Brown Girdling Root Rot

    The brown girdling root rot pathogens are soil borne. Light brown lesions with irregular margins first appear at or after the onset of flowering, usually 7.5 cm (3") or more below the soil surface on the taproot or main lateral roots. Later lesions may appear anywhere on the taproot. As the lesions develop they expand, grow together, become sunken and eventually girdle the taproot, causing poor pod development, shriveled seeds and uneven maturity. The sunken lesions become dark brown. Roots below the girdling rot off. The lesion continues to develop upwards, sometimes to the soil surface but never moves into the stem. Root tissues above the infected part become swollen.

    In moist conditions, the whole taproot may be destroyed up to the soil surface. Girdled plants may survive and even set some seed if they are not uprooted or blown over by wind.

    In dry soil, a sound taproot stub remains. The above ground parts of the plant remain turgid as long as there is any root connection with moist soil. Taproot stubs that retain a few roots are sometimes capable of regenerating main laterals as well as fibrous roots. Plants with girdled taproots wilt, dry up and shrivel, even though soil moisture may be adequate for plants with normal root systems.

    Yield losses will depend on the amount of root system lost by girdling. If only root stubs are present or brown lesions girdle the taproots, the disease will result in considerable yield loss. If brown spots are present, but do not girdle the root, actual disease losses may be minimal. Losses are a result of increased pod sterility, loss of seed weight, and seed shriveling and plant death from desiccation or wind toppling. Girdled plants that survive ripen prematurely and tend to be pulled out of the ground rather than cut during swathing, increasing shatter losses.

    Disease development appears to be favoured by fine-textured heavy clay soils with high levels of copper (4 to 20 ppm). Research at the Agriculture and Agri-Food Canada (AAFC) Research Centre at Beaverlodge, AB [3] found that disease severity was similar for all tillage systems. Brown girdling root rot can be more severe when canola follows canola or in canola following clover or fescue than on canola following summerfallow. Decomposing fescue sod, for reasons not clear, is conducive to the development of this disease. It is unpredictable because it can also be severe where canola has never been grown previously, and even on freshly broken land that has never been cropped. Several cruciferous weeds, including stinkweed, shepherd's purse and ball mustard, also suffer from root rot.

    Another study conducted at Beaverlodge, AB in 2000 [4] showed that on average, seed yield was not reduced on plants with roots with nongirdling lesions and superficial nonsinking girdling lesions, but was reduced by 17% on plants with roots with girdling and sinking lesions, and by 65% on plants with decayed taproots. Over 2 years, yield losses in growers' fields ranged from 1 to 5%.

    Foot Rot 

    In the growing season, brown, hard, clearly defined lesions occur near the stem bases of canola plants. The lesions may be black-bordered, and during periods of humid weather, pink spore masses may develop on diseased root tissues. Discolouration of the upper part of the taproot above the lateral roots may occur. In severe cases, the stem is girdled, killing the plant. Yield loss may occur when the stem is one-half girdled. Yield losses are light when lesions occur late in the season. Affected plants may ripen prematurely as scattered plants or in patches in the field. Where foot rot develops late in the season, the earlier maturing B. rapa varieties may escape infection or lesions may not have time to develop to the point that yield is seriously reduced.

    Root Rot 

    The root rot pathogens are soil borne. Losses are highest when wet soil conditions occur at early flowering, followed by dry weather later in the season. The root rot complex disease causes brown lesions on the main taproot near the time of flowering. 

    Influence of Environment

    The root rot pathogens are soil borne. Losses are highest when wet soil conditions occur at early flowering encouraging extensive root rot development, followed by high temperatures and dry windy conditions. Root rot complex diseases are destructive, but sporadic, and sometimes linked to root maggot infestations or high rainfall and waterlogged soil, particularly during flowering.


    Identify Root Rot Complex

    Symptoms

    Brown girdling root rot:

    Early symptoms consist of light-brown lesions on the taproot or main lateral roots well below the soil line. These enlarge and coalesce, become sunken and girdle the taproot. Only a short taproot stub may be left. Plants ripen prematurely in the field, often before any seed has been set. Girdled plants are subject to death from desiccation or uprooting by wind.

    • Light brown lesions on taproot and at bases of larger roots
    • Tap root finally girdled, leaving a stump


    Foot Rot Symptoms

    • Hard brown lesions at stem base
    • Salmon coloured spore masses often present in lesion
    • Losses tend to be minor as lesions develop late in season; early lesions can cause premature ripening and reduced yields


    Root Rot Symptoms

    The symptoms are variable in colour and shape but can be grouped into four general types:

    • a light grey oval lesion of the upper taproot
    • a dark grey discolouration of the lower taproot and internal tissue, later becoming black 
    • a light brown, soft, diffuse taproot lesion
    • a dark brown, sunken, sharply defined taproot lesion


    Affected regions

    Root rot symptoms occur sporadically throughout canola growing areas. Brown girdling root rot is an extremely widespread in the Peace River region of Alberta and British Columbia, but occurs only occasionally in the northern parkland zone. It tends to be a more serious problem in B. rapa varieties as they have greater susceptibility. In the Peace River region, brown girdling root rot causes greater yield losses than all other diseases. 

    Testing to Confirm

    BGRR is rarely seen outside of the Peace region, so tests will need to be done to confirm. 


    Prevent Root Rots

    Crop Rotation

    • Risk of root diseases can increase with increasing canola cropping intensity. Consider including field peas in the rotation (to improve the nitrogen level) to reduce disease severity.


    Field Management (cultural control)

    Consider these measures for control of root rot:

    • Control volunteer canola and cruciferous weeds in rotation to help prevent a build-up of root pathogens in the soil.
    • Use clean seed
    • Maintain recommended N, P, K and S fertility levels in the soil. Nitrogen decreases disease severity. Liming the soil may be beneficial in reducing disease severity.
    • Use B. napus varieties since they are only moderately susceptible to root rot, compared to the more susceptible B. rapa varieties.
    • Use the management practices outlined for the seedling disease complex to establish a vigorous, uniform growing crop.


    Manage Root Rots 

    Chemical Control

    • Use a recommended seed fungicide treatment to control the seedling blight stage of this disease. 
    • No economical chemical controls are available for brown girdling root rot.


    Genetic resistance

    Polish (B. rapa) varieties are generally more susceptible to brown girdling root rot. Use Argentine (B. napus) varieties in suitable climatic regions, as they are only moderately susceptible, but under the right conditions brown girdling root rot can still be a problem. 

    Plant breeders are making good progress on the development of B. rapa varieties with resistance to brown girdling root rot. Researchers at Agriculture and Agri-Food Canada (AAFC) Research Centre in Beaverlodge, AB are continuing with screening B. rapa lines for resistance in the field. Also, advanced lines with resistance have been incorporated into hybrids/synthetics and these are to be field tested in upcoming field trials.


    Research

    [1] Woods, D. L., Turkington,T. K., McLaren, D. and Davidson J. G. N. 2000. Breeding summer turnip rape for resistance to brown girdling root rot. Can. J. Plant Sci. 80: 199-202. 

    [2] The Canadian Phytopathological Society. 2011. Canadian Plant Disease Survey: Disease Highlights. (http://phytopath.ca/download/cpds-archive/vol91/cpds_vol_91_2011.pdf)

    [3] Soon, Y. K., Klein-Gebbinck, H. W. and Arshad, M. A. 2005. Residue management and crop sequence effects on the yield and brown girdling root rot of canola. Can. J. Plant Sci. 85: 67-72. 

    [4] Klein-Gebbinck, H. W., and Woods, D. L. 2002. Yield loss assessment in canola: Effects of brown girdling root rot and maggot damage on single plant yield. Plant Dis. 86:1005-1010.