The disease sclerotinia stem rot is caused by the fungi Sclerotinia sclerotiorum and Sclerotinia minor which can occur on many broadleaf plants including canola, pea, bean, lupin, sunflower, pasture species and broadleaf weeds. Cereal crops and grass weeds do not host the disease. In Australia, the disease is highly sporadic requiring specific environmental conditions to develop. Disease incidence can vary greatly from year to year but is most damaging with prolonged wet conditions, resulting in high humidity leading up to and during flowering.

The sporadic nature of the disease and its severity make it difficult to reliably make foliar fungicide application decisions. Several forecasting tools developed overseas have been evaluated in Australia but have been found to be inappropriate due to differences in climate and length of flowering. Yield loss is often difficult to predict but can be up to 24 per cent under Australian conditions, depending on the percentage of plants infected and the crop growth stage when infection occurs. Recent research has shown that stem infection results in more yield loss than lateral branch infection. Current management options before sowing are limited to sowing clean seed, isolating canola from last year’s infected paddocks and crop rotation. The use of foliar fungicides at flowering is the only post-sowing management option.

Warning Signs

A canola crop is at risk of developing sclerotinia stem rot if it is:

• grown in a high rainfall area (especially if the crop has been sown early at high seeding rates).
• grown in low lying parts of the landscape such as the floor of valleys which stay wetter for longer than nearby hill slopes.
• grown in intensive rotation with other broadleaf crop species, including summer crops of sunflower and soybean.
• sclerotinia has been present within the past three years in that paddock or adjacent paddocks.

The following conditions are conducive for a Sclerotinia outbreak in canola. All three must occur for infection to take place:

• wet conditions for at least 10 days at the soil surface in mid to late winter and temperatures of 11–15°C to germinate sclerotia and trigger spore release.
• extended wet periods during flowering for petal infection.
• extended wet periods during petal drop, the lodging of petals on stems and subsequent stem infection. Stem lesion development is favoured by humid/wet conditions and mild temperatures.

What to Look For

Symptoms of sclerotinia in canola include fluffy white fungal growth on stems and bleached stem lesions. Initial symptoms are water soaked, light-brown discoloured lesions on stems or leaves that expand and become greyish-white. If a lesion completely girdles the main stem, the plant quickly wilts and dies prematurely. Infected canola plants will ripen earlier and stand out among green plants. The bleached stems tend to break and shred. In wet or humid weather, a white growth resembling cotton wool can develop on infected plant tissue.

Disease Cycle

Sclerotinia survives the summer season as a sclerotia. Sclerotia are simply very hard, dry mycelium; sclerotia have the appearance of rat droppings. With favourable moisture and temperature conditions in winter the sclerotia germinate in the soil and produce apothecia; small, golf-tee-shaped fruiting bodies, 5 to 10 millimetres in diameter. The apothecia then release airborne fungal spores. Spores of the sclerotinia pathogen cannot infect canola leaves and stems directly. They require flower petals as a food source for infection. infected petals then drop into the canopy and lodge on leaves, leaf axils or stem branches. Under moist conditions the fungus will spread from the petal, with infection initially commencing as a tan-coloured lesion resembling a watermark. Infected plant parts wilt and turn mouldy. Leaves infected by petals may also fall and lodge further down the canopy as well as spread infection through direct contact with other plants. Stem lesions will result in the production of sclerotia within the stem, which are then returned to the soil after harvest.

Management Options

Before Sowing

Clean Seed

Sow only good quality seed that is free of sclerotia. If using ‘farmer saved’ seed for sowing it should be graded to remove any sclerotia. Carefully inspect seed before sowing. Ungraded seed used for sowing can inadvertently transfer sclerotia into the soil, which can later initiate the disease.

Crop Isolation and Rotation

Avoid sowing canola into or next to paddocks that were heavily infected with sclerotinia in the previous three years. The spores are airborne and can be blown some distance into surrounding paddocks. Although rotation does not effectively control sclerotinia, close rotation of susceptible crops such as lupin may increase fungal inoculum build-up. In addition, it is preferable that crops be sown on the western side or ‘up wind’ from old canola stubbles.

After Sowing

Consider Fungicide Use

If favourable environmental conditions occur (see Warning signs) fungicides are the only available option for managing sclerotinia stem rot after sowing. A number of products are currently registered in Australia to manage sclerotinia stem rot of canola.

Due to the sporadic nature of the disease, it is uneconomical to apply fungicides routinely; to be effective they need to be applied before the plant becomes infected. This can be difficult as fungicides should be applied before petal infection occurs.

Research has shown that strategically applied foliar fungicides (1 or 2 applications) can be effective in reducing the level of sclerotinia stem rot and subsequent yield loss in crops with a high yield potential and at high risk of developing the disease.

If you decide to spray, the current recommendation is to apply a foliar fungicide between 20 and 30 per cent bloom, a second foliar spray may be required 10 days later if favourable disease conditions persist. If the crop is not growing in an area prone to sclerotinia, it is unlikely that a foliar fungicide application will be economic.

For fungicide decision support, please consult the SclerotiniaCM app that is available from GRDC. This app can be used to identify factors that trigger high Sclerotinia risk and therefore potential yield returns from applying fungicides.

 

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