The disease sclerotina 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 weeds. Cereal crops and grass weeds do not host the disease. The fungus can be soil-borne or carried with seed. 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 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 percent under Australian conditions, depending on the percentage of plants infected and the crop growth stage when infection occurs. 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.
Symptoms of sclerotina in canola include fluffy white fungal growth on stems and bleached stem lesions, (Figure 11.7). 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.
Figure 11.7 Canola stems infected by sclerotina, showing formation of the bleached stem lesions
Sclerotina survives as hard, black bodies resembling rat droppings called ‘sclerotia’. Sclerotia require prolonged periods of moist soil to germinate and form golf tee-shaped fruiting structures that release air- borne spores in late winter, (Figure 11.8). The spores infect and colonise canola flower petals under humid or wet conditions. The disease spreads to the plant stems when the infected flower petals fall and become lodged within the junctions of the main stem and side branches accompanied by humid or wet weather. Once in contact, the infection then spreads from the petal to the stem, (Figure 11.9).
Sclerotes develop inside stems and sometimes on the surface of infected tissue, (Figure 11.10). The sclerotia will be later released onto the ground during harvesting or collected in the harvested seed, (Figure 11.11). It appears that if the onset of the infection occurs late enough in the season, yields may be unaffected.
Figure 11.8 Apothecia fruiting structure that releases windblown fungal spores
Figure 11.9 Typical sclerotina infection at the junction between the side branch and the main stem. Note that the plant is healthy below the infection point
Figure 11.10 Sclerotia survival body outside of stem. These fungal bodies also commonly occur inside the stem within the pith. Slice stems open to confirm if sclerotia are present
Figure 11.11 Disease cycle of sclerotina on canola. Illustration by Kylie Fowler
A canola crop is at risk of developing sclerotina 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
- sclerotina has been present within the past three years in that paddock or adjacent paddocks.
The following conditions are conducive for a Sclerotina 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 (20 – 25°C) temperatures.
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.
Avoid sowing canola into or next to paddocks that were heavily infected with sclerotina in the previous three years. The spores are airborne and can be blown some distance into surrounding paddocks. Although rotation does not effectively control sclerotina, 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.
The use of wider row spacings and lower seeding rates can increase ventilation within the crop canopy and reduce moisture retention within the canopy microclimate required for infection by sclerotinia. Avoid the temptation to sow crops at high seeding rates and follow the recommended plant population targets for your region.
If favourable environmental conditions occur (see Warning signs) fungicides are the only available option for managing sclerotina stem rot after sowing. A number of products are currently registered in Australia to manage sclerotina 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 percent flowering, a second foliar spray may be required 10 days later if conditions favourable to the disease persist. If the crop is not growing in an area prone to sclerotinia, it is unlikely that a foliar fungicide application will be economic.
ALTERNARIA LEAF AND POD SPOT (BLACK SPOT, DARK LEAF SPOT, ALTERNARIA BLIGHT)