Field Crop Disease Update 2019: Why is My Wheat Turing Yellow? Accurate Diagnosis of Potential Viruses Causing Disease in Wheat.

Wheat has greened-up and is actively growing across Indiana. Our southern field plots in Vincennes were at Feekes 8 (flag leaf emerged) earlier this week, while in West Lafayette plots are at Feekes 5 (leaf sheath strongly erect). There have been some concerns about fields with yellow, stunted patches of plants. A variety of factors can contribute to these symptoms including several wheat virus pathogens as well as soil pH, fertility and environmental factors. It is important to observe the whole field to determine the special distribution of symptoms – are they uniform, spotty or patterned in appearance? Are they in a low, wet area or are they along a field edge where insect vectors are moving off overwintering hosts.

There are four common viruses that cause disease in Indiana wheat. They include five strains of barley yellow dwarf virus (BYDV), wheat spindle streak mosaic virus (WSSMV), soilborne wheat mosaic virus (SBWMV), and wheat streak mosaic virus (WSMV). The Purdue Plant and Pest Diagnostic Lab (PPDL) received their first set of wheat samples this week and confirmed the presence of WSSMV.

Wheat Virus Identification

You can guess at wheat virus identification based on symptoms in the field however lab testing is essential for accurate diagnosis. Typically virus infected plants appear in uneven patches of yellow to light green areas in a field, this can easily be confused with environmental/site issues including low pH and related nutritional deficiencies.

Symptoms of Wheat spindle streak mosaic virus (WSSMV) and Soilborne wheat mosaic virus (SBWMV) are visible in the spring and are  generally uniform across a section of the field. A yellow/green mottled, mosaic pattern will be visible on the leaves. The plants may be stunted with leaf tip dieback and lower leaves may have a reddish appearance. Plants infected by WSSMV or SBWMV may have fewer tillers, stems and heads with fewer kernels. Infection occurrs in the fall. The viruses are transmitted to the wheat root by a soilborne fungus vector, Polymyxa graminis, and can survive in the soil for up to five years.

 

Figure 1. Wheat infected by wheat spindle streak mosaic virus (WSSMV). Typical display of spindle-like chlorotic lesions of WSSMV on foliage and mosaic pattern. Yellow, mottled, mosaic patterns can also be a symptom of SBWMV infection.

Figure 1. Wheat infected by wheat spindle streak mosaic virus (WSSMV). Typical display of spindle-like chlorotic lesions of WSSMV on foliage and mosaic pattern. Yellow, mottled, mosaic patterns can also be a symptom of SBWMV infection.

 

Symptoms caused by Wheat streak mosaic virus (WSMV) on wheat growing in Indiana may include yellowing, stunting and curling of leaves due to feeding by the wheat curl mite,  the primary vector of WSMV.  Time of infection and environmental conditions will determine severity of disease ; warm dry conditions favor WSMV due to an increase in the wheat curl mite population. Wheat curl mites can transmit the virus for up to a week after feeding on an infected plant and generally move in the wind from one plant to another in a field.  Early colonization of the wheat by the wheat curl mite can lead to a more pronounced WSMV symptom development, premature plant death and greater yield loss.

Barley yellow dwarf virus (BYDV) has many different strains that can infect more than 150 different grass species, including wheat, oats, barley, rice and corn. These viruses are vectored by aphids. Once an aphid acquires the virus by feeding on an infected host it can transmit the virus to a new host for two to three weeks. It will take two to three weeks for symptoms to appear in wheat after the initial infection while fall infections may not appear until the following spring. Symptoms of BYD in wheat include stunted tillers and roots and discolored foliage. A reddish discoloration usually starts at the tip of the flag leaf and moves downward; eventually in wheat the leaves may appear yellow, red or purple in color. BYDV infection can lead to reduced tillering, poor flowering and kernel sterility or failure to fill.

 

Figure 2. A yellow patch of wheat in a field due to wheat spindle streak mosaic virus (WSSMV).

Figure 2. A yellow patch of wheat in a field due to wheat spindle streak mosaic virus (WSSMV).

Management Options for Wheat Viruses

There are no control options to reduce viral symptoms in currently infected plants, however accurate diagnosis of wheat viruses is important for future disease management plans. Healthy plants can better tolerate infection so it is important to manage other foliar diseases in wheat and maintain adequate moisture and nutrients.

Many wheat varieties are available that have partial resistances to one or more of these wheat viruses and/or their vectors. Understanding the risk of field to a specific virus can aid in variety selection down the road, as planting a less susceptible variety is the first line of defense in viral disease management.

Removal of volunteer wheat or other grasses late in the season may also help reduced the “green bridge” for the vectors or virus survival into the next season.

If you suspect viral infection a sample can be submitted to the Purdue Plant and Pest Diagnostic Lab (PPDL) (https://ag.purdue.edu/btny/ppdl/Pages/Submit-A-Sample.aspx). The PPDL can test for the presence of ten wheat viruses. They include five strains of Barley Yellow Dwarf Virus (BYDV; strains PAV, MAV, RMV, SGV, and RPV); Wheat Spindle Streak Mosaic Virus (WSSMV); Soilborne Wheat Mosaic Virus (SBWMV); Wheat Streak Mosaic Virus (WSMV); High Plains Virus (HPV) and Triticum Mosaic Virus (TriMV). The cost of the wheat virus screen is $50, in addition to the usual sample handling fee of $11 for in-state samples and $22 for out-of-state samples.

For more information see Purdue Extension publication “Diseases of Wheat: Wheat Viruses” https://extension.purdue.edu/extmedia/BP/BP-146-W.pdf

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