Septoria Tritici Blotch
· Triticale · Wheat · Rye
The initial symptoms of septoria tritici blotch are small chlorotic spots on the lower leaves that appear soon after the seedlings emerge. As they enlarge, these spots become light to dark brown blotches with an oval or stripe shape that can stretch across the leaf blade. They also appear on the stems and heads,however to a lesser extent. Tiny black fruiting bodies within the lesions give them a characteristic speckled appearance. Later, entire leaves may be engulfed by large, brown rusty lesions and only some islands of green tissue remain, surrounded by a yellow halo. Finally, the leaves dry and die. In the absence of the black fruiting bodies, similar blotching symptoms may be caused by another disease or nutritional disorders such as aluminum toxicity or zinc deficiency. Symptoms are first evident at later stages of plant growth, two to three weeks after infection.
The disease is caused by the fungus Mycosphaerella graminicola. It overwinters on plant debris on the soil surface, in grass hosts, in volunteer plants and in autumn-sown crops. The spores are transmitted via rain splashes and wind over long distances. First symptoms are visible on older leaves and as the spores disperse upwards, lesions start to appear in the upper foliage. If the flag leaf and the two leaves underneath are affected, yield reduction occurs. The life cycle of the fungus takes 15 to 18 days to complete, depending on temperature. Optimal conditions are temperatures between 15°C and 25°C and free water or prolonged high humidity. Below 4°C the life cycle is stopped. For a successful infection, at least 20 hours of high relative humidity neccessary. Wet springs and summers are ideal.
Biocontrol agents have been used successfully against M. graminicola in controlled conditions. Fungi belonging to group Trichoderma and some species of pseudomonads and bacillus have been shown to protect wheat plants against leaf spot diseases or to hinder the progression of the disease.
Always consider an integrated approach with preventive measures together with biological treatments if available. Many populations of M. graminicola have rapidly evolved resistance to fungicides, especially to the strobilurin class of chemicals. The economic threshold depends on expected yield loss, the market value of the wheat and the costs of the fungicide application. Fungicides of the group of the azoles are commonly used as folliar sprays. Alternating fungicides such as carboxamide, morpholine or benzophenone helps mitigating the development of resistance.