Yellow is a fashionable colour in cereal crops this spring
Following an exceptionally mild and wet winter, and a slow start to crop development in the early spring, many crops are now reaching the famous “well on” growth stage. There is much anguish over yellow rust, where the offending fungus has enjoyed the mild wet winter, doing what fungi do over the winter months – reproducing and taking advantage of the cooler wet weather to attack crops. Unfortunately, the same weather meant crops were slow to grow away in the cold spring.
Disease ratings are good at representing what has happened in the recent past, but once you have used them to help identify which variety to grow, it is essential all crops are monitored. This is because disease-causing fungal populations can change, and varieties differ in their susceptibility to the disease at different points in their development. So, when does a variety move away from juvenile susceptibility to adult resistance? Sounds like a grim exam question with no clear answer. What is also likely to happen is the appearance of different populations in different areas and fields. Markets may dictate that we drill a single variety over a large area, but this approach exposes our single variety to a diverse population of fungi ready to attack it.
I’m sure the discussion about the merits of drilling a diverse selection of varieties to improve stability and minimise disease risk and 101 reasons why the market is less than happy with this approach will continue, but it is clear that a single stand of one variety in large blocks is a sitting duck when the pathogen, weather and declining fungicide activity conspire against it.
It is clear that some varieties have been hit harder than reflected in their resistance ratings, requiring protectant fungicides to be applied in the early spring. Now the focus for wheat has moved on to protecting crops from septoria tritici, azoles and SDHIs applied to this end will effectively protect crops from yellow rust, assuming doses and timings are well timed.
Another ‘yellow’ to look out for is Barley Yellow Dwarf Virus (BYDV) in wheat and barley. BYDV is a virus disease spread by aphids. It attacks many cereals, not just barley, and may not always cause yellow leaf symptoms – sometimes red, and late infections may not even cause shortening of the crop. Why is it so severe? The mild winter, once again, was the start of the woes, with weather conditions enabling the aphids to overwinter in cereal crops after migrating from grass, feeding on the emerging cereals, transmitting the virus. Patches of stunted plants with yellow or red tips to otherwise healthy leaves gives a clue to this migration. Later infections appear as aphids fly into crops, leading to random plants evenly affected through the field.
The bad news is there’s little that can be done now to treat affected crops. The good news is that it isn’t transmitted by seed, so you can plan ahead for the autumn and look at integrated management to reduce the risk. Early sown cereals following grass or grown in fields where volunteer cereals are common will be at greater risk. As far as Recommended List variety trials are concerned, we do apply a seed treatment to minimise early infections, but getting some trials with symptoms helps us to identify any varietal differences based on visual symptoms. The yields from our trials are a more important indicator of variety performance in a season where levels are higher than average.
Hopefully we will soon leave the yellow spring and, being positive, enjoy a healthy green summer. Septoria tritici and fusarium may yet come and spoil the party to give us a brown (septoria) then pink (fusarium) summer.