Alternaria black spot

Table of contents

    Important Tips for Best Management:

    • While all varieties of canola are susceptible to black spot disease to various degrees, B. napus varieties are less susceptible than B. rapa. B. napus varieties have more leaf wax, which provides more tolerance to black spot disease.
    • Alternaria is generally not damaging early in the season. However, when the disease spreads extensively onto green pods it may increase the green seed count and seed chlorophyll content, and cause premature pod shatter, shriveled seed, lower 1,000-kernel weight and reduced oil content.
    • Aerial fungicide application applied at 95% flowering provides economical control of this disease.


    About Alternaria black spot

    Overview

    Photo 1. Alternaria on leaf

    Photo by Beth Hoar

    Photo 2. Black spot on stem

    Photo by Beth Hoar

    Photo 3. Black spot on pods

    Figure 1. Alternaria disease cycle

    Alternaria black spot is caused by the fungi Alternaria brassicae, A. alternata and A. raphani. Although Alternaria is present every year on the Canadian Prairies, the severity of this disease varies considerably from year to year and area to area based largely on the moisture and temperature situation. The disease can infect all growth stages of the canola plant.

    Canola plants vary in their susceptibility to black spot attack throughout their development. Aging plants are more susceptible than young or intermediate-aged plants. Therefore, black spot epidemics intensify at flowering and reach their maximum intensity in ripening plants. A heavy infection on leaves and stems reduces photosynthetic potential of the plant, but is generally not damaging early in the season. However, when the disease spreads extensively onto green pods it may increase the green seed count and seed chlorophyll content, and cause premature pod shatter, shriveled seed, lower 1,000-kernel weight and reduced oil content.

    Seeds may become infected after development of alternaria on pods, adversely affecting seed quality and germination. Infected seeds are either killed or damaged and are generally non-viable. B. napus tends to have lower susceptibility to seed infection than B. rapa.
    The greatest yield loss from Alternaria black spot comes from pod shatter. Pod shatter results from uneven drying of the disease-infected pods causing them to split, particularly under dry, windy conditions in the swath. Research has reported substantial yield losses in B. napus under conditions favourable for disease development.

    Symptoms

    Alternaria black spot disease causes symptoms in the seedling stage on cotyledons and in later growth stages on leaves, leaf petiole, stem, flowers, pods and seeds. Infected seed may rot in the ground or produce seedlings with dark spots on the cotyledons. Some infected seedlings may also exhibit damping-off or wirestem disease. All above ground plant parts are susceptible to infection.

    The disease first appears on the cotyledons in the form of small light brown lesions that soon turn black due to the appearance of spore masses (under humid conditions) and act as a source of infection for other parts of the plant. The initial infections on lower leaves develop distinct brown to blackish lesions or spots with yellow halos around them (Photo 1.). The lesions vary in size (1 to 20 mm or 0.04 to 0.8") and colour, depending on environmental conditions. They may be entirely grey under moist conditions, and grey with a purplish or black border, or entirely black under less favourable conditions. These lesions can multiply rapidly and later spread to the upper leaves, stem and pods.

    On severely infected leaves, several lesions unite to cause defoliation under humid conditions. Stem and pod lesions first appear as small brown or black dots that may develop into conspicuous spots or longer and wider lesions of various shapes (Photo 2). The spots or lesions may be entirely black or dark-bordered with a greyish white centre. In severe outbreaks, the upper part of the stems and pods wither. Pods may show sunken, dark brown to black circular lesions (Photo 3). Deep lesions on pods cause infection in the seed. Diseased seed just beneath the black spots on the pods can be small, shriveled, and grey to brown. Grey or black seeds that are dark green inside are likely damaged by black spot and are unlikely to change colour in the swath. Infected pods may ripen prematurely and shatter while the crop is standing or in the swath.

    Disease Cycle

    Alternaria black spot fungi overwinter on infected crop residue, on cruciferous weeds and to a lesser extent on or in seed (Figure 1).

    Spring infections on plants begin directly from infected seed or from spores produced on crop residue or from infections on cruciferous weeds. Many cruciferous plants are hosts on which the fungi can survive from year to year. These include tame mustard, flixweed, hedge mustard, tumbling mustard and stinkweed. Spores, after landing on susceptible plant tissue, remain intact until exposed to moisture from dew or rain, then germinate, penetrate and cause lesions within a few days. The leaf lesions or spots are important because they produce more wind-borne or rain-splashed spores that may cause more infection on the same or neighbouring plants.

    Humid conditions and moderate temperatures favour the disease. Cooler temperatures (10 to 15°C) on days with rain and wind promote abundant spore production especially on days where leaves remain wet over longer periods. Wind spreads spores throughout the crop canopy.

    The cycle continues throughout the season under favourable conditions. Black spot epidemics intensify at flowering when heavy crop canopies promote humid conditions. Epidemics reach their maximum intensity in ripening plants. High concentrations of airborne spores are needed to initiate development of severe epidemics on canola leaves and pods. Early lodging and cool wet weather in the podding stage become critical factors that lead to major black spot outbreaks. Lodged canopies remain wet longer promoting greater spore production. Hot and dry conditions can interrupt black spot epidemics as the absence of moisture greatly reduces spore production. Black spot disease can be quite variable across a field with more serious infection levels on lower slopes than on upper slopes. Seeds may become infected following development of lesions on the pods. At harvest time, spores produced on stems, branches and pods may also infect the seed in the combine.


    Management

    Rotation

    Crop rotation has a limited effectiveness in controlling black spot disease. Frequently, fields that have not had canola grown for several years experience black spot, causing considerable damage despite the rotation. Black spot spores are excellent wind travelers and can spread to areas remote from the location of production. Small plot reseach has shown that three to four years is the time it takes for canola stubble to decompose on the Prairies. However, spores can still be blown in from stubble on neighbouring fields. Control cruciferous weeds and volunteer canola during the rotation.

    Tillage

    Buried stubble cannot release black spot spores. However, the amount of disease control is usually proportional to the effectiveness of residue burial. Unfortunately, under conditions favourable for disease spread, Alternaria black spot can spread even if less than 10% of crop residues are left unburied.

    Varieties

    While all varieties of canola are susceptible to black spot disease to various degrees, B. napus varieties are less susceptible than B. rapa. B. napus varieties have more leaf wax, which provides more tolerance to black spot disease.

    Seed

    Plant well-cleaned, high germination pedigreed seed. Use seed free of small, shrunken, and infected seed to reduce the seed-borne level of alternaria. In the laboratory, alternaria can be observed in the germination test for canola. It is identified by greyish-white mycelium that covers an infected seed or has spread to a nearby seedling. Presence of alternaria is often noted on the seed lot certificate.

    Fungicide Seed Treatments

    Registered seed treatments are effective in reducing seedborne inoculum and increasing seed germination. However, planting treated seed will not safeguard against black spot since alternaria also overwinters on plant residue and the spores are spread by wind.

    Foliar Fungicides

    Aerial fungicide application applied at 95% flowering provides economical control of this disease.

    Harvest

    Early swathing of badly infected crops may reduce serious losses from shattering. Swath fields infected with black spot as straight cutting extends the time period over which shatter damage from black spot occurs. Any practice that allows the crop to dry more quickly following a rain or dew may reduce the impact of this disease. Adequate balanced fertility and lower plant populations will help reduce early lodging. Adjust swathers to produce open rather than heavy swaths. Avoid the use of wide swathers in heavy stands and swathing in the rain. A lighter swath exposed to good airflow and rapid drying is less at risk.

    Plant Stress

    Factors that cause plant stress are known to increase the plant's susceptibility to black spot disease. A nutrient deficiency can cause stress. Follow soil test recommendations to ensure adequate fertility of both macro and micronutrients. Sulphur deficient plants tend to be more susceptible to black spot disease. Apply insecticides when warranted since insect injury predisposes plants to black spot disease.