• Print
• Share

Is it Physiological or Disease in my Wheat?

This is a question many wheat producers and agronomists are asking themselves today and being able to distinguish between the two could either make you money or cost you a bundle. I know some of you are saying "What are you talking about, Tenuta!" Think of it this way - if you are using a fungicide for disease control in most cases you will make money especially if you are targeting economical important diseases such as leaf rust, Septoria and Fusarium. On the other side, if the damage is a response to an environmental stress such as physiological fleck (spot), a fungicide will not remove the symptom and may very well cost you money.

Although disease pressure continues to remain low for most areas there are fields in which disease levels are increasing and as Pat Lynch, Cargill Agronomist noted some areas north of London need to be monitored closely. However what some are calling disease in fact could be confused with physiological fleck. What is Physiological fleck and how can it be distinguished from tan spot, Septoria and leaf rust.

Physiological fleck symptoms start as small yellow (chlorotic) spots (1-3mm) on the upper leaves and as they develop a dark brown center forms in these spots. The yellow flecking can be confused with early leaf rust development while in the later stages the production of a dark brown lesion resembles early tan spot and Septoria infection.

Which leaves depend on the plant's growth stage when the stress occurred? In most cases, the flag and second leaf often show the damage but this year the mid to lower canopy could be showing flecking due to the variability in growth stages in many fields this year.

Weather conditions are the key to physiological flecking. The extended cloudy and rainy conditions we have experienced much too often this spring in conjunction with the few bright sunny periods are perfect for physiological fleck (yes we have had a few but not enough!). In its simplest term the damage is due to sunburning of the leaf surface from UV_A nd UV_B.

There are differences in variety susceptibility so make note of the variety and compare it to any other wheat varieties you may have on your farm. Since the damage is due to a stress applied across the field, the symptoms on a susceptible variety is often uniform across the field which is very different to what we often see with diseases which are patchy or in localized areas. In previous years symptoms often decreased or disappeared once the weather warmed up. In addition yield losses were not observed from physiological flecking injury. Although this is good news, let us remember that the more stresses whether it is environmental or as a result of disease on a plant the increased likelihood that their combined impact will affect development and yield, so keep scouting that wheat!

Tan spot is probably the most obvious disease this year but again at very low levels. The disease in most cases is of minor economic importance but can be confused with Septoria leaf spot and misdiagnosis could result in unnecessary applications of foliar fungicides. Barley and oats are much more tolerant to tan spot than wheat where most varieties are susceptible to the disease.

Tan spot begins on the lower leaves as small, tan-brown flecks that enlarge into oval- or lens-shaped tan lesions (5-15 mm) with a small, dark brown centre. A bright yellow zone or halo surrounds the tan lesion. The lesion is best viewed when the leaf is held to the sun. The weather conditions (prolonged, cool, cloudy, humid/wet) weather have been ideal for tan spot basically from the get-go this spring.

Leaf rust is beginning to develop but the good news is we are getting late in the season. The bad news as mentioned earlier is we have different growth stages in many fields and theses late maturing wheat plants or fields could become a problem. Leaf rust affects the leaf blades and sheath, whereas stem rust can be found on leaves, sheaths, stem and heads.

The disease begins as small, yellow-brown spots (pustules) that contain orange-to-orange-brown spores. In most cases, infection is found on the upper surfaces of the leaves and leaf sheath. In severe cases, leaves turn yellow and brown (necrotic). In spring grains, late-planted fields are most likely to show the disease, whereas late-maturing winter wheat may be slightly more at risk. Since leaf rust usually appears on the upper two leaves first, it is important when scouting for rust to check the second leaf from the top prior to head emergence, and the flag leaf during head emergence for signs of disease. Use foliar fungicide treatments when the flag leaf has 5-10 pustules or 1% of the flag leaf area is affected (during head emergence to the end of flowering) and the weather forecast predicts rainy, wet weather.