Jochum Wiersma, Small Grains Specialist
In the past week, I have had several calls about wheat fields that showed severe 'disease' on the flag leaf. The symptoms were always described as a dying back of the flag leaf from the tip of the leaf downwards (Photo 1 and 2).
Photo 1. Effect of hot, dry, and windy weather on young and tender flag leafs on the variety 'Glenn.'
This is not a disease but these symptoms are either caused by hot, dry, and windy weather (photo 1), a physiological phenomenon called leaf tip necrosis (photo 2), or the combination of both. Some may mistakenly identify this symptomology for barley yellow dwarf virus (BYDV). Both physiological injuries distinguish themselves from BYDV in the absence of the corn yellow color of the flag leaf but rather show a true necrosis in which the leaf tissue has died off.
The weather injury is caused when the tender flag leaf just emerges and is exacerbated with drought stress. The whole tip of the flag leaf tends to be necrotic and will often fold over. Leaf tip necrosis progresses from the margins of the flag leaf tip and lower leaves should show some signs of it as well. Also, leaf tip necrosis should be visible on all flag leaves, whereas the dying back due to hot, dry weather may appear on only a portion of the leaves. The severity of the leaf tip necrosis is dependent on both the variety and the growing conditions during flag leaf emergence and early grainfill and thus some varieties have a tendency to show a lot more leaf tip necrosis than other. The varieties 'Pioneer 2375' and 'Oklee' are known to show this leaf tip necrosis quite readily.
Leaf tip necrosis is observed to some extent in all wheat varieties containing the leaf rust gene resistance gene Lr34. Lr34 is a so called 'slow rusting gene' that confers durable resistance to leaf rust. Wheat cultivars that carry the gene display a longer latency period for infection and fewer and smaller rust pustules. It was first described in the cultivar 'Frontana' in 1966 and 'Chris' was the first University of Minnesota release that carried the resistance gene. It is present in many of our current HRSW varieties, like '2375', 'Oklee', 'Briggs', and others.
The presence of Lr34 carries a slight yield penalty in the absence of leaf rust, but the overall benefit of Lr34 outweighs both the cosmetic and yield penalties. The impact of the damage to the flag leaf caused by the heat stress is probably less than one my think at first glance. The remainder of the canopy has a great ability to compensate for this type of physiological damage. In either case, there is nothing producers can do to avoid or alleviate the symptoms.