Wheat Viruses - What to Look For!

Although the general wheat disease pressure is low throughout most fields things could change rather quickly and therefore, keep scouting those wheat fields. One group of diseases that are often overlooked and have been increasing are the viruses. They are a fascinating group of pathogens and although they are really small they can take a significant punch out of yields. For example, Arend Smid (Emeritus University of Guelph Ridgetown Campus) observed in 2006, a 20 to 35% yield loss for seven varieties infected with viruses in his Ridgetown plots (Figure 1). Tolerant varieties in these same plots (Figure 2) had either little visual disease symptoms and/or recovered from the infection sufficiently resulting in minimal yield loss (<5%).

Figure 1. Wheat Plots Infected by Virus (Arend Smid, University of Guelph)
Centre of Picture - light green is susceptible

 

Figure 2. Wheat Virus Damage (Arend Smid, University of Guelph)
Left side of picture - susceptible; Right side of picture - tolerant

Viruses hijack the plants own metabolic machinery and turn the plant's engine into a virus-producing machine. There are 500 to 600 plant pathogenic viruses that can cause considerable plant damage in many crops. Once new virus particles are produced they need to find a way to other plants. Spread or transmission of can occur through various means. These include a direct transfer from an infected plant to a non-infected plant through direct plant-plant contact (such as roots), mechanical injury, seed infection, and various vectors. These vectors include a) Insects such as leafhoppers, aphids, thrips and mites, b) Fungi and c) Nematodes.

Three of the most common wheat virus diseases in Ontario are Soilborne Wheat Mosaic Virus (SBWMV), Wheat Spindle Streak Mosaic Virus (WSSMV) and Barley Yellow Dwarf (BYD). Table 1 compares the three viruses.

The symptoms, life cycle and field pattern of wheat soilborne mosaic virus (SBWMV) and wheat spindle streak mosaic virus (WSSMV) are very similar. It is not uncommon to find that many plants are infected with both viruses since they share a common vector. The common link is a soilborne fungus called Polymyxa graminis.

The fungus produces zoospores (swimming spores) which invade root hairs and epidermal cells of young plants during periods of high soil moisture or in low, wet areas of the field. The fungus can remain in the soil for at least 8 years. Temperature also plays an important role in these diseases. As the temperature increases the visual symptoms and damage decreases allowing the plants time to recover. Yield losses often range from less than 5% to 10% for wheat spindle streak mosaic to 10-40% for soilborne mosaic virus.

The primary symptoms of Barley Yellow Dwarf Virus are stunted plants that have yellowing, reddening or purpling leaf tips. BYD is often confused with spindle streak mosaic virus, soilborne mosaic virus, nutrient deficiency or environmental causes. Unlike SSMV and SBMV, Barley Yellow Dwarf is transmitted by aphids. Several species of aphids have been identified as vectors for BYD, these include the greenbug, the corn leaf aphid, the English grain aphid and the oat birdcherry aphid.

BYD is usually found in patches 1 to 2 metres in diameter but can occur uniformly throughout the field if aphid populations are also uniform throughout the field. Yield losses are very dependent on when the crop was infected. Generally, losses are greater when infection occurs in the fall rather than the spring.

Identifying viral pathogens is very difficult and require accurate serological tests. Best to send samples to a diagnostic lab such as the University of Guelph's Pest Diagnostic Clinic in Guelph which have such capabilities. Some of the serological tests that are available at the Pest Diagnostic Clinic include Barley Yellow Dwarf Virus (BYDV), Soil-Borne Wheat Mosaic Virus (SBWMV), Wheat Spindle Streak
MosaicVirus (WSSMV), Wheat Streak Mosaic Virus (WSMV) and many others. They can test for one virus or screen for a group of viruses. Sample turnaround time varies between 24 hrs and two weeks depending on the viruses you wish to test for. The cost varies with the number of viruses tested and the number of samples submitted. For more information, call the Pest Diagnostic Clinic at (519) 767-6256 to receive a sample submission form and fee schedule.

Table 1. Comparison of BYD, SBWMV and SSWMV

Virus	Transmission	Major Symptoms	Additional Hosts Besides Wheat
Barley Yellow Dwarf	Aphids	General chlorosis, reddening, purpling, stunting	Barley, oats, corn, sorghum, millet, grasses
Soilborne Wheat Mosaic	Soilborne fungus (Polymyxa graminis)	Yellow-green mosaic, stunting, rosetting	Rye, barley, grasses, sorghum
Wheat Spindle Streak Mosaic	Soilborne fungus (Polymyxa graminis)	Green-yellow mosaic, streaks, spindles	Rye, barley

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