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Yellow Wheat - Think Virus!

As Keith Reid (OMAFRA Soil Fertility Specialist) points out yellowing of wheat can be caused by a number of factors including wet soils, low pH, nutrient deficiencies and of course my favourite "disease" or in this case, those pesky viruses!

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

The symptoms, life cycle and field pattern of wheat soilborne mosaic virus (SBMV) and wheat spindle streak mosaic virus (SSMV) are very similar. It is not uncommon to find that many plants are infected with both viruses since they share a common vector - 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. Plants will recover. Yield losses range from less than 5% to 10% for wheat spindle streak mosaic to 10-40% for soilborne mosaic virus.

Symptoms from 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 (>30%) 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 Mosaic Virus (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.

Related Links

• Yellow Wheat Part II - Causes and Corrections