Figure 1. Low area in a grower’s field near Ogallala, showing wheat infected by soilborne mosaic virus. (April 12) Figure 2. Close-up of wheat soilborne mosaic symptoms. Note the yellow and green islands on the infected leaf. (April 10, UNL Havelock Farm, Lincoln) Figure 3. Breeding nurseries in a section of UNL Havelock Farm, Lincoln, showing stunting of wheat lines infected by wheat soilborne mosaic virus. Note that some lines are highly resistant whereas others are highly susceptible. (April 10, UNL Havelock Farm, Lincoln)

Wheat soilborne mosaic virus showing up in wheat fields

Cause

Wheat soilborne mosaic is caused by a virus belonging to the Furovirus group. The virus survives in the soil in association with resting spores of a primitive fungus called Polymyxa graminis. During cool wet weather late in the fall, resting spores of the fungus germinate by forming zoospores which swim in water and infect root hairs of fall-sown wheat. Following infection, the fungus colonizes root tissues and produces more zoospores and resting spores. The zoospores transmit the virus to plants infected by the fungus. Infection is favored by high soil moisture and soil temperatures ranging from 54^o to 59^oF. Because of the environmental conditions favorable for growth and reproduction of the fungus vector, wheat soilborne mosaic occurs almost exclusively on winter wheat. The virus is spread by factors that disperse soil, such as cultivation, water, and wind.

Symptoms

Symptoms of wheat soilborne mosaic are most prominent on early spring growth. Disease development is optimal at 61^oF and stops above 68^oF. As seasonal temperatures rise, severity of symptoms diminishes. Leaves initially show light green and yellow mosaic patterns. These symptoms become more intense on older leaves (Figure 2). Infected plants often are stunted (Figure 3). Rosetting can occur in highly susceptible varieties. Because the virus is transmitted by a soilborne fungus which does not spread quickly within the field, infected plants are usually localized, especially in areas where water drainage is poor.

Management

There is no cure for wheat soilborne mosaic. When symptoms appear in the spring, it is too late to apply any measures to prevent damage to the crop. Because of the long incubation period (time between infection and symptom expression) of the virus, infections that result in damage occur in the fall. The only effective means of managing the disease is to plant resistant varieties, which are available. A look at breeding nurseries at Havelock Farm in Lincoln reveals that some wheat lines are highly resistant whereas others are highly susceptible to the virus (Figure 3). Sowing late in the fall can somewhat reduce disease because low temperatures reduce activity of the fungus vector. Crop rotation with non-hosts is not effective in managing the disease because the virus and resting spores of its fungus vector remain viable in the soil for a long time. Fumigation can kill the fungus vector, but it is not cost-effective.

Stephen Wegulo
Extension Plant Pathologist, Lincoln
Robert Klein
Cropping Systems Specialist
West Central REC, North Platte

Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.

Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.

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