Septoria avenae blotch
Septoria avenae blotch is the most common oat disease in Western Australia. It occurs throughout the cereal growing areas and is most severe in the high rainfall areas.
The disease is caused by the fungus Parastagonospora avenaria f.sp. avenaria (synonym: Phaeosphaeria or Stagonospora avenae f.sp. avenaria). It is not one of the septoria diseases of wheat, which are caused by different species.
The fungus infects leaves (Figure 1), sheaths and stems (Figure 2) and may also infect heads. Symptoms begin as mottled light and dark brown blotches, with dark brown centres. They are restricted and distinct at first but may enlarge to cover most of the leaf.
Figure 1 Leaves affected by septoria avenae blotch
Figure 2 Stems affected by septoria avenae blotch
Lesions in the leaf sheath extend into the stem causing death and blackening (Figure 2) which may lead to lodging. The fungus sometimes causes a dark discolouration of the grain (Figure 3) when unseasonably late rain occurs.
Figure 3 Grain showing discolouration from septoria avenae blotch
Septoria avenae blotch may cause up to 50% yield loss and crop lodging in extreme cases but losses of around 10% are more common in high rainfall areas. Tall or slow maturing oats are less likely to be affected by the disease than short (dwarf) or fast maturing varieties (Figure 4).
Figure 4 Tall, late maturing oat varieties (right) are less prone to septoria avenae blotch than short, early maturing varieties (left)
Infected stubble is the main source of carryover infection from one season to another. The sexual stage of the fungus occurs on infested stubble and produces ascospores which are spread moderate distances by wind. Oat stubbles in paddocks rotating from oat probably contribute most of the inoculum to nearby paddocks. In multiple cropped oats where stubble is not destroyed, ascospores land on the new crop in much larger quantities, resulting in the development of earlier and more severe outbreaks.
During the season, the fungus on diseased plants produces splash-borne pycnidio-spores which spread the disease onto new foliage during rain. These spores do not move between paddocks but may also be produced by infested stubble residues and contribute to the development of new disease in multiple cropped oats.
Control
Use more resistant varieties in disease-prone areas if suitable agronomic types are available. For current disease resistance tables see: Oats: choosing a variety.
Sow at the time appropriate for the maturity of the variety.
Foliar fungicide registrations exist for control of this disease, refer to Registered foliar fungicides for cereals in WA.
Septoria avenae blotch can be minimised by not growing continuous oat crops. In continuous oat cropping, stubble from diseased plants should be destroyed by burning or ploughing. Burning is not advised on light soils subject to wind or water erosion. Heavy soils, that is, soils which are 50-60% covered with clods of 2-3cm diameter, may be considered for burning. Following crops should be sown at low speeds into moist soil.