Why is My Wheat Yellow?

Nov 14, 2024

By Alyssa Collins and Paul D. Esker et.al

As small grains emerge and establish in autumn, we hope to see lush green growth across fields of wheat, barley, and forages like triticale. But what if we also start to see areas of yellowing, stunting, or sparse growth in our fields? Proper diagnosis is key to solving the problem, so here we will discuss some of the possibilities for the cause of these issues.

Virus diseases

The most common viral disease of these crops in Pennsylvania and the one that most growers are familiar with remains Barley Yellow Dwarf Virus (BYDV), which is transmitted by feeding by aphids. Many different grass-feeding aphid species can transmit BYDV to wheat, barley, maize, oat, and ryegrasses (among others). Aphids can colonize emerging small grains in autumn and transmit BYDV to plants; therefore, it is really important to scout for aphids in fall when their feeding can transmit BYDV. An accepted economic threshold for aphids in fall is 25 aphids per foot of row. If populations in a field are above this amount, there should be an economic benefit to treating the field.

After overwintering, cereals that were infected in the fall display bright yellow or red discoloration and stunting, usually in patches in the field that reflected by aphids in the fall display bright yellow or red discoloration and stunting, usually in patches in the field that reflects where the pests fed (Figure 2). Preventing this disease depends upon eliminating volunteer cereal plants on your farm and delaying planting. This reduces the opportunity for the pathogen to survive and be spread by aphids to your freshly planted crop.

Other viruses, Diseases of Wheat: Wheat Viruses (PDF, Purdue Extension), may also be found in small grains in our region (Figure 1). Soilborne wheat mosaic and wheat spindle streak mosaic (SBWMV, WSSMV) can also initiate severe stunting, yellowing, and tip dieback. These viruses are transmitted by a fungus-like soil organism that can survive for several years in affected fields, making management of these diseases challenging. Some varieties resistant to these viruses are available, so consider this in your cropping plans.

Diagnosing these viral diseases requires specialized laboratory testing, so work with your Extension Educator to identify the most appropriate options.

Fungal diseases

A struggling stand, stunting, yellowing, or failure to tiller can all be indicators of Pythium root rot.  This is caused by a fungus-like pathogen that infects the roots and crown of the plant.  This disease is favored by cold, wet weather periods that can stress plants in poor drainage situations.  Symptoms may not be apparent until many weeks after the infection occurs.  Seed treatments are available but do not protect beyond the first few weeks after planting.  Other strategies to prevent Pythium are to avoid poorly drained fields and manage residue so that fields do not lie wet and cold.

Septoria tritici blotch is a common fungal leaf spot in our region that tends to appear earlier than other fungal leaf diseases in our region. It may be seen as elongated yellow to brown spots on leaves as early as tillering when conditions are mild to cool and wet. Fungicides are effective against this and other fungal leaf spots.

Cephalosporium stripe is caused by a fungus that lives in the soil, and infected plants will begin to show symptoms during spring green-up as a mottled discoloration. As the plant moves into elongation, yellow stripes will appear along the lowest leaves' mid-vein (Figure 3). This disease can result in depressed growth and dead seed heads. Since this fungus survives in wheat residue and is favored by areas with poor drainage, rotation, and tolerant variety selection is recommended for fields with low-lying areas.

Bacterial diseases

A bacterium called Clavibacter michiganense subsp. tessellarius can cause bacterial mosaic of wheat, a relatively uncommon disease characterized by diffuse yellow flecking of the leaves. This bacterium persists in crop residue but may also be seed-borne. To avoid this disease, starting with certified seed is recommended.

Physiological causes

Not everything that causes symptoms on small grains and forage grasses may result from disease.  Environmental factors at certain times in the plant's development may also induce undesirable growth.  Sub-freezing temperatures are not always required for this.  When crops experience extreme temperature swings, damage can occur, especially if the plant has experienced a long period of mild temperatures and is no longer tolerant of cold weather.  

Plants unprotected by snow cover are also more vulnerable to winter desiccation, which can result in crown injury and limited tillering. To determine if environmental factors are the cause of a poor stand, note the weather since the crop was planted and note the varieties and growth stages affected, along with the areas of the field impacted.

If you need help in diagnosing your problematic small grain stands, work with your Extension Educator to determine whether you should send samples to the Penn State Plant Disease Clinic.

Source : psu.edu
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