Diagnosing physiological leaf spot in barley

Physiological leaf spotting occurs in many forms that may be easily mistaken for spotting symptoms caused by pathogens or herbicides. It is not thought to affect yield. Barley varieties vary in susceptibility to spot formation and the type of spot formed. Initially spots are orange to brown with no rim, but yellowing of tissue between the spots may develop with time. Abiotic factors such as nutrient deficiencies or adverse weather conditions can also cause abnormalities in barley leaves and it is important to distinguish if spots are caused by disease or not before jumping to apply fungicides.

What to look for

Variety specific spot symptoms include

Very small pale to orange pinprick spots (for example, variety Stirling).
1-2 millimetres brown to dark brown spots that may increase in size (for example, variety Vlamingh).
Brown fingerprint type blotches that may have distinct colours and boundaries or have a water-soaked appearance (for example, variety Baudin).

Generic plant symptoms

Older leaves are affected first and most severely.
Spots most commonly develop from stem elongation to flag leaf emergence.

It can be very hard to determine the cause of spotting symptoms in barley- can be physiological, abiotic (nutrient deficiency/toxicity) or disease. Assistance with disease identification can be obtained from DDLS - Plant pathology services (phone +61 (0)8 9368 3721), this is a chargeable service, and can be worthwhile to avoid unnecessary application of fungicide.

What else could it be

Condition	Similarities	Differences
Diagnosing boron toxicity in barley	Numerous brownish lesions on older leaves	Boron toxicity occurs on certain highly alkaline soils, and the lesions are spread more evenly on the edges and end of the leaf.
Diagnosing spot type net blotch	Small brown lesions with yellow margins on older leaves	However spot type net blotch usually occurs earlier in the season, is not soil type specific, and lesions enlarge and destroy the leaf.