Abiotic (Non-Infectious) Symptoms in Wheat (06/10/21)
The extreme differences in temperature, low relative humidity, and water stress has led to unusual leaf spots on wheat. This year several reports of wheat with bizarre and unusual leaf lesions have been received and can be misdiagnosed as a disease. This report will review some of the key diagnostic features of abiotic (non-infectious) disorders.
The extreme differences in nighttime and daytime temperatures has led to several reports of color banding in wheat. Symptoms of color banding include yellowing and/or purpling of leaves that have recently emerged (Figure 1). These first leaves are sensitive to soil temperature and injury can commonly occur. We usually observe this abiotic disorder early in the growing season during frost events. This year we have observed color banding on late emerging wheat that has been exposed to temperature extremes. Heat canker (leaf tissue death) is another abiotic disorder that is commonly observed on wheat emerging during hot days. Regardless, wheat will recover from this injury and it should not impact crop development.
Brown Irregular Spots
Brown to chocolate colored lesions have been reported from several fields (Figure 2). The brown spots have been observed on the natural fold of the leaf or confined to certain leaves. These lesions can be confused with fungal leaf spots due to the color and appearance on a leaf. Fungal leaf spot lesions (ie: tan spot or Septoria) will often have a definitive margin, have an ellipsoid shape, will not be confined to a certain leaf, and will be found in fields near wheat residue experiencing several days of prolonged leaf wetness. The brown abiotic lesions are likely a physiological response to the environment and will not affect crop development.
Crop injury from a chemical application has also been observed this year. Chemical injury can occur if pesticides are applied during hot days or can result from an off-target application (Figure 3). Lesions caused by chemicals may be irregular, necrotic, have a bleached center (Figure 3) or may have a distinct “halo” surrounding the lesion. Necrotic lesions can sometimes be caused by oil adjuvants or oil-based herbicide formulations, but the injury is usually associated with a contact herbicide. Lesions with a distinct halo and bleached center are typical of paraquat and group 14 herbicides. Chemical injury will often follow a field spray pattern (ie: field wide or boom width) and be found on plant tissue available at the time of application. Finally, chemical injury will suddenly appear after an application has been made.
Extension Plant Pathology, Cereal Crops
Extension Weed Specialist