Cold weather can cause a physiological response in crops leading cereal leaves to turn yellow and barley to develop brown spots. These can be easily confused with disease symptoms and there can be a temptation to respond with a fungicide. It is important to confirm the presence of disease before breaking out the sprayer. Often crops grow away from these problems with warmer longer days.
Dr Steven Simpfendorfer, NSW DPI, in the article, Think before you spray, compared physiological damage with similar symptoms observed for disease.
Key points are that:
- Damage from cold weather and frosts tends to be concentrated towards the leaf tips of wheat and barley. This characteristic is most likely to be confused with Barley Yellow Dwarf Virus (BYDV).
- Physiological leaf spots that occur in many barley cultivars are often in a circular pattern and may be called ‘thumb spots’.
- Yellow (Leaf) Spot in wheat and Spot Form of Net Blotch in barley tend to have a yellow margin around a central lesion.
For those who prefer audio, Steve discussed the content of this article in more detail during the first 6 minutes of the GRDC Radio (Northern Update) 090 from July 2015.
Barley varieties vary in their susceptibility to physiological spot development. DAFWA compares physiological spotting in barley with boron toxicity and Spot Form of Net Blotch. Physiological spots are initially orange to brown with no rim, but yellowing of tissue between the spots may develop with time. There is no benefit in applying fungicides to physiological spots.
AGT investigated yellowing in a number of wheat varieties including the related Kord CL Plus, Grenade CL Plus, Justica CL Plus, Axe, Gladius and Correll. It was described as often forming blotches, but sometimes stripes. Samples were examined for disease and virus by Dr Hugh Wallwork, SARDI, and nutrient deficiency but no obvious cause for the yellowing was found.
In 2013 there were several reports of yellowing in the wheat variety Mace across Western Australia. Reports described chlorotic blotches scattered throughout the crop canopy, but particularly on the lower leaves. Dr Caroline Moffat, CCDM, found that this yellowing was not caused by Yellow Spot or any other disease
The GRDC Wheat & Barley Leaf Symptoms Back Pocket Guide provides a starting point to diagnosing problems with a pictorial guide to the leaf symptoms and a list of possible causes including disease, nutrient deficiency, herbicide and weather conditions.
Online tools can help to short list possible causes of unusual crop symptoms: