Yellow Dwarf of Cereals

Agdex#:	110/632
Publication Date:	11/87
Order#:	87-074
Last Reviewed:	11/87
History:
Written by:	Y.C. Paliwal - Plant Research Centre, Ottawa; André Comeau - Agriculture Canada Research Station (Ste-Foy, Quebec)

Table of Contents:

1. Introduction
2. Symptoms And Disease Effects
3. Insect Vectors And Virus Strains
4. Disease Cycle
5. Control Measures

Introduction

Yellow dwarf of barley and wheat and red leaf of oats, collectively referred to as Barley Yellow Dwarf (BYD), are caused by the Barley Yellow Dwarf Virus (BYDV). The "invisible enemy" that can only be seen in a high powered electron microscope, attacks wheat, barley, oats, rye, corn and many grasses including several important forages. BYD was first recognized as a virus disease in 1951 in California. Since then it has been found in most agricultural areas of the world and is the most important virus disease of cereals in Ontario and throughout North America. BYD is present in cereal crops in Ontario every year but the actual incidence varies from one region to another. Localized severe outbreaks of the disease are common but the disease develops into an epidemic every 5-8 years. The last four epidemics of BYD in large sections of Eastern Canada were in 1976, 1982, 1986 and 1987. Losses from BYD occur in both yield and quality of grain and depend on the incidence of infection. In susceptible cultivars of oats and barley, severe strains of the virus can cause 100% loss of grain yield of early infected plants. In a severe outbreak of the disease with 50-60% plants infected in a field about 50% reduction in yield of barley and oats and about 30% of wheat is not uncommon.

Figure 1. Yellow dwarf, caused by an aphid transmitted virus on barley in an experimental planting. Note the yellowing of leaves from tips downward, and the severe stunting of some plants.

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Symptoms And Disease Effects

Symptoms of BYD vary with the plant species, crop cultivar, strain of the virus, age of the plant at the time of infection and environmental conditions. Discoloration of the leaves, stunting and partial or total sterility of the heads/florets are the usual symptoms in cereals. In barley, the first symptoms are a diffuse or blotchy yellowing near the leaf tip; bright yellow discoloration then extends towards the leaf base leaving a strip of green along the side of the midrib. Severely diseased plants may never flower. In wheat, symptoms are generally the same as in barley. Late infections of wheat plants characteristically are manifest as a canary yellow flag leaf. Some cultivars may show bronzing or slight reddening of leaves under cool conditions. Both in barley and wheat, symptomless BYDV infections are common and lead to some loss in yield. BYD symptoms may sometimes be confused with nitrogen or phosphorus deficiency. Mineral deficiencies are manifest uniformly throughout the field whereas BYDV-infected plants may be scattered or occur in small pockets. In oats, leaves turn yellowish brown or pale orange but a striking red or purplish discoloration is characteristic in cool conditions. Rye and triticale often do not show symptoms although yield reduction occurs. Infection of young plants in cool conditions causes yellowish blotches on leaves. Leaves of BYDV-infected plants, especially oats and barley, are often stiffer, shorter and more erect than those on healthy plants and water soaked areas may be present on the leaf blades. In corn, the leaves show yellowing or purpling which may be generalized or may occur as stripes along the midrib. Symptomless infection of plants is also quite common and may be accompanied by yield loss in some cultivars.

Figure 2. Red leaf of oats caused by the barley yellow dwarf virus.

Plants infected at the seedling stage suffer the most severe disease and may die without flowering. Generally, the older the plants at the time of infection the less damaging is the disease. Susceptibility of plants and disease severity on the great variety of cereal cultivars available today differ considerably. Also, different strains of the virus produce symptoms of differing severity on a given cereal cultivar. Symptoms are most severe under cooler (16-200C), sunny conditions. In oats and barley, BYD is easily confused with symptoms of Aster Yellows, a leafhopper disseminated disease caused by a mycoplasma. Fall infection with BYDV reduces the ability of barley and wheat to tolerate subfreezing temperature and ice encasement during the winter. BYDV infected plants are more susceptible to attack by certain fungal diseases. Leaf diseases caused by fungi may proceed so rapidly on infected plants that BYDV symptoms may be masked. All parts of the plant are invaded by the virus. Internally in the plants, the virus concentrates in the food translocating vessels (phloem cells) and severely affects their functioning. Development of the root system is retarded maldrig plants more susceptible to drought and root diseases.

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Insect Vectors and Virus Strains

BYDV is entirely dependent on aphids for its transmission to healthy plants and dissemination within a field or over long distances. Although as many as 15 different species of aphids are known to transmit the virus in North America, important aphid vectors in Ontario are cherry oat aphid, corn leaf aphid and English grain aphid. The greenbug and the rose grass aphid may also play some role in dissemination of the virus in some years.

At least five strains of BYDV occur on cereals and grasses in Ontario. They are classified according to the aphid species that transmit them efficiently. The 'non-specific' strains are transmitted by all the major grain aphid species but more effectively by the cherry oat and English grain aphids. Such strains have been more prevalent in Ontario in recent years and generally cause the most severe disease. There are strains transmitted specifically and efficiently by each of the above mentioned grain aphids and the greenbug. The relative prevalence of these strains varies considerably from one year to another.

Disease Cycle

Since transmission through aphid vectors is the only way of disease spread, aphids are the most important link between various components of the disease cycle. Aphids that once become infective remain so for most of their life. Most of the initial infection of cereal crops in Ontario results from infective aphids that are blown with south, southwesterly winds from the United States in the spring, early summer and fall. Year round continuity of BYDV and aphids on cereals in southern States allows the virus to build up on cereals in central and northern States which are planted earlier than in Ontario. Primary infections in Ontario crops from showers of migrant aphids appear as discrete, circular patches of infected plants. If conditions are favorable for aphid development virus then spreads further from these patches within the field and to other fields. Stands of perennial grasses in Ontario carry low levels of BYDV infection. None of the aphid vectors are known to live through the winter as adults except that in southwestern Ontario some adults may survive in some years under good snow cover. However, infective aphids do not pass on the virus to their progeny. Adults developing from the overwintered eggs of cherry oat aphid (the only one overwintering as eggs), or various early migrant aphids may transfer some virus from grasses to cereals in the spring and early summer. Also, overwintered fall infections in winter wheat can serve as sources of infection for the overlapping spring cereals in the same vicinity if aphids are abundant. BYDV has been found in corn in Ontario at several locations. Overlap of maturing corn that has some green tissue and aphids with the early seeded winter wheat in the same vicinity may cause transfer of the virus from corn to wheat. Epidemiological research by Agriculture Canada showed that the BYDV disease cycle involving movement of virus from spring cereals to both corn and grasses, from these hosts to winter cereals and from winter cereals back to spring cereals may play only a limited role in outbreaks of the disease witnessed in Ontario. Infective aphids wind blown from the south, southwest are the main source of the virus. Although most migrant populations of grain aphids arriving in Ontario contain a significant proportion of infective aphids, occasionally, a migrant population may be virus-free or have a very low proportion of infective aphids. Drought in cereal growing areas of United States often results in greater numbers of aphids reaching Canada. In the normal season too, ripening U.S. crops always release aphids into the winds that are directed predominantly to north, northeast during spring and fall.

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Control Measures

Virus-infected plants cannot be cured by application of pesticides as the fungal and bacterial disease-affected plant can be. Therefore, nothing can be done to save the plants already infected with BYDV. Three approaches can be taken for preventive control measures: (1) to avoid or minimize the primary infection; (2) to arrest the spread of the disease within and to outside the field; and (3) to tolerate the infection of plants but sustain little or no yield loss.

If cereals are seeded early, plants face BYDV infection at an advanced developmental stage so that less damage will result due to virus infection. Since aphids become more abundant and active in early summer, and younger plants are preferred by aphids over older plants, late sown crops will have higher incidence of the disease. Late seeding of winter cereals allows plants to escape infection. But early seeding in spring or late seeding in fall is not always possible and may affect yield and/or winter survival in some regions. The effectiveness in reducing BYD incidence and the cost effectiveness of insecticide applications to control aphids vary considerably with the circumstances. Insecticide sprays to avoid or minimize initial BYDV infections in the spring-early summer from wind-borne aphids must be timed to immediately follow major aphid arrivals. This requires an elaborate early warning system which has so far not been developed in North America due to lack of sufficient U.S.-Canada cooperative research on this subject. Control of secondary spread of the disease within the crop and to the nearby fields is best achieved by insecticide sprays. The more cost effective and most dependable preventive measure is to use BYDV tolerant cultivars. No cereals are known anywhere to be immune to BYDV. Tolerant cultivars can live in a compromise with the virus in that they suffer much less or no yield loss, despite being infected, compared to susceptible cultivars. Cultivars can therefore be graded as tolerant, moderately tolerant, intermediate and susceptible according to their level of BYD tolerance as judged by their yield under BYDV infection. Cereal cultivars with at least moderate tolerance to the disease are now available or will be available in the near future. A classification of current cereal cultivars according to their level of BYDV tolerance is given in Table 1. Local Crop Specialists should be consulted for additional advice and up to date information.

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