Identifying Leaf Spots on Wheat
Unusual weather conditions this spring have led to unusual signs of leaf spot on cereal plants. Listed below are the out of the ordinary signs to look for when trying to identify the signs.
This has a wide range of symptons. Lesions on the adult plant are brown and often restricted by veins giving a rectangular shape. Pycnidia are normally always present from the time lesions become visible. If there are no signs of Black Pycnidia within the lesion, chances are that it is not Septoria tritici. Latterly lesions will join together to form a large area of brown dead tissue. Symptoms of Septoria tritici are usually more marked on the lower leaves of the plant although occasionally the flag leaves can also become infected even though leaf 2 may be unblemished.
Although most people think they can recognise Yellow Rust, there are a number of symptoms, caused by the disease that can be easily missed or misdiagnosed.
In the Autumn and early Spring, it very rarely produces clear stripes but instead produces bands of infection across the leaf. Where crops have been sprayed, symptoms can be very unusual with the spray sometimes prevention sporulation, leading to chlorotic spots. Often symptoms will appear soon after spraying so the crop can actually look worse after spraying than before, making farmers and advisors very concerned about poor control. Under hot dry conditions sporulation stops so striping can occur without any obvious spores.
Although it is rare for this disease to reach high enough levels to cause concern, we have occasionally had severe outbreaks in sprayed crops. Typically, it is found in min-till systems.
Initially small tab-brown spots or flecks appear on the lower leaves which expand into oval lesions often with a yellow halo. These lesions can darken at the centre giving it an eye-shaped appearance and under ideal conditions, these lesions will come together to produce one large area of dead tissue. Normally symptoms are more severe in the lower canopy. The fungus does not produce pycnidia and the symptoms are very similar to those caused by transient drought – oval lesions with a dark centre can be caused by drought rather than tan spot.
Fine chlorotic spotting can occasionally be seen on resistant varieties. Yellow Rust resistant varieties for example which are exposed to high levels of the disease spores like we have at the moment. This is where the plant is resisting infection by killing off the cells around the fungal hypha that are trying to grow into the leaf. This sometimes produces a dark brown necrotic spore with a chlorotic halo. This can also happen with Mildew and Brown Rust.
MICHRODOCHIUM LEAF SPOT
Although well known as an ear disease and more importantly a seed-borne disease, Michrodochium can in cool wet seasons affect the leaf too.
Symptons are normally seen as oval or elliptical grey/green blotches with pale centres and are often seen alone in fields already treated with fungicides where other diseases have already been controlled. The lesions can have very small black dots in the centre which can be mistaken for the pycnidia of Septoria tritici. Under warmer conditions, these lesions can dry out and bleach in the centre which can cause the leaf to split.
PHYSIOLOGICAL LEAF SPOTTING
This can be caused by many factors including drought and high levels of radiation during the leaf emergence stage in the summer. Spotting often occurs after prolonged cloudy or rainy conditions which are then followed by bright sunny days with high levels of radiation. This is often associated with low temperatures followed by high temperatures, which lead people to think it is a temperature effect. It is thought to result from Activated Oxygen Species (AOS) in the leaf tissue. These are oxygen molecules that are more active than the molecular oxygen in the atmosphere and can promote damage at high level, causing problems when the plant undergoes stress.
In cloudy weather the plants phytochrome system activity is increased to help the plant during low light intensities. However, levels of AOS must drop quickly when light levels increase to avoid photo-oxidation. When temperature and light levels change rapidly from cold to hot enzyme activity can be slow to respond leading to this increased AOS activity which can be damaging to the leaf. This manifests itself as chlorotic or even pale nectrotic spotting on the leaf but unlike diseases, the leaf spotting often occurs on upper leaves with the same leaf layer often affected throughout the crops. The lesions are frequently small and rarely spread, and have darker centres with chlorotic halos. These can be similar to hypersensitive reactions to rust or mildew. Early symptoms can look very much like tan spot and there are varietal differences in susceptibility too.
Fungicides are not normally the cause of leaf spotting.