Barley yellow dwarf luteoviruses and other virus diseases
M. Henry, R.T. Plumb

Wheat is a natural host for many viruses. Brunt et al. (1996) reported 55 viruses to which Triticum aestivum is susceptible, and Wiese (1987) described around 30 viruses that naturally infect wheat. This chapter will not describe in detail all wheat viruses, but will give an overview of the most important ones and those that could become a problem in tropical wheat-growing areas. Infection by wheat viruses produces symptoms such as foliar chlorosis (mottle, mosaic, streaks and yellowing), necrosis, stunting and rosetting. Some of the symptoms, especially foliar discoloration, can easily be confused with nutritional and abiotic disorders. Furthermore, it is common to find several viruses infecting the same plant. This multiple infection makes identification complex, but advances in technology (serology and molecular techniques) have made identification, characterization and classification of viruses easier. However, new viruses are still being described and others are awaiting identification.

Table 21.1 lists viruses that have been described as naturally infecting wheat. Of these, only a few cause extensive damage and have an economic effect on wheat production. These will be described in more detail. Throughout this chapter, the common name and standard acronym that appear in Brunt et al. (1996) have been used. Others viruses not mentioned in this chapter have been found occasionally on wheat. For more complete information, refer to Brunt et al. (1996) and Wiese (1987).

BARLEY YELLOW DWARF LUTEOVIRUSES

Barley yellow dwarf is the most widely distributed and the most economically important virus disease of wheat. It has been extensively studied, and excellent reviews have been published (D'Arcy and Burnett, 1995; Miller and Rasochová, 1997). Barley yellow dwarf was first reported by Oswald and Houston (1953). It is caused by a group of luteoviruses called barley yellow dwarf luteoviruses (BYDVs). Barley yellow dwarf luteoviruses are not mechanically transmissible, nor through the seed, but are transmitted by aphids in a persistent, circulative but non-propagative manner. Aphids acquire and transmit BY-DVs while feeding on the phloem sieve tube elements of host plants. The different viruses that cause barley yellow dwarf disease (BYD) were originally characterized by Rochow (1969) and Rochow and Muller (1971), who identified five strains from New York in the United States based on their transmission phenotypes in an experimental system. The nomenclature used to identify the five strains of BYDV reflected the species of aphid that most efficiently transmitted that strain and were not intended to imply absolute specificity (Power and Gray, 1995). The strains and their principal vectors are RPV (Rhopalosiphum padi), RMV (R. maidis), MAV (Sitobion avenae), SGV (Schizaphis graminum) and PAV (R. padi, S. avenae and others). This nomenclature system has been adopted by all BYD researchers. The five strains are also distinguishable serologically. However, aphid-transmission properties do not always correlate with serotypes. The transmission phenotypes are based on the experimental system, and the five New York BYDV strains are not representative of all strains studied. For example, Metopolophium dirhodum (not included in Rochow's experimental system) has been reported to be an efficient vector of MAV (Gildow and Rochow, 1983) and PAV. This species is also used as the principal vector for MAV-Mexico in BYDV screening trials at the International Maize and Wheat Improvement Center (CIMMYT).

TABLE 21.1
Viruses naturally infecting wheat

The International Committee on the Taxonomy of Viruses (ITCV) has divided the BYDVs into two viruses, BYDV (PAV and MAV) and cereal yellow dwarf polero-virus, CYDV (RPV) (D'Arcy et al., 1999), according to the previous grouping based on cytopathology (Gill and Chong, 1979), serology and nucleic acid sequences (Martin and D'Arcy, 1990).

Barley yellow dwarf luteoviruses are restricted to the Poaceae (Gramineae). Cultivated hosts include all the major cereal crops: barley, maize, oat, rice, rye and wheat, as well as many annual and perennial cultivated and wild grasses. BYD has been reported from over 50 countries (Lister and Ranieri, 1995). Symptoms caused by BYDVs differ with the host species and cultivar, the age and the physiological condition of the host plant at the time of infection, the strain and the environmental conditions; and they can be easily confused with nutritional and abiotic disorders. In wheat, one symptom includes leaf discoloration from tip to base and from margin to centre, in shades of yellow and sometimes red. Plants are usually stunted, with a decrease in tiller number and biomass and a weak root system (Plate 51). Suppressed heading, sterility and failure to fill grains occur in the most severe cases. In the field, symptoms usually appear as yellow or red patches of stunted plants. In aeroponic culture, the root system of BYDV-infected seedlings was initially more severely affected than the shoot; stunting was observed four days after infection in roots and only after 18 days in shoots (Hoffman and Kolb, 1997). In general, PAV causes severe symptoms, MAV moderately severe and RPV, RMV and SGV produce mild symptoms. However, Chay et al. (1996) reported PAV isolates ranging from mild to very severe; an RPV isolate producing corkscrew symptoms was isolated in Mexico and in Ecuador, MAV is known to be severe. Zhang et al. (1983) reported the strain GPV, DAV and GPDAV from China. It seems that some of the Chinese strains have a serological relation to the US isolates (MAV and PAV) but that they differ slightly in their aphid-transmission patterns.

Several strains of BYDV can frequently coexist in the same plant. The resulting symptoms can be more severe when the strains are from different groups, but when from the same group, they may result in the anamelioration of symptoms through the mechanism of cross-protection (Haber, 1995). Mixed infections can also result in an alteration of the transmission pattern through transcapsidation (Rochow, 1982).

Losses in wheat due to BYD can be very serious but differ with the BYDV strains, the growth stage at infection, the wheat varieties and the environmental conditions. Pike (1990) and Lister and Ranieri (1995) compiled data on yield losses from different countries. Losses of 47 percent and 26 percent have been reported after artificial inoculation with PAV in Kenya (Wangai, 1990) and Mexico (Burnett and Mezzalama, 1992). Losses of around 11 to 12 percent due to natural infection have been reported in Morocco (El Yamani and Hill, 1990) and in Chile (Ramirez et al., 1992). In Australia, Banks et al. (1995a) reported yield losses of about 2.2 tonnes/ha in a susceptible wheat and losses of about 1.1 tonnes/ha in tolerant varieties.

The epidemiology of BYD is influenced by the strains involved, the aphid vectors present in the area, the crop rotation, environmental conditions (temperature and rainfall), the time of sowing and the timing of aphid flights. Barley yellow dwarf alternates from reservoir hosts (grasses, maize, other cereals and volunteer plants) to small grain cereals. The relative importance of primary infection and secondary spread differs from region to region and year to year.

The critical time for BYD control is at an early growth stage (Plumb and Johnstone, 1995). The need for aphid control can either be prophylactic or based on a forecasting system, such as the ones described in Europe by Plumb et al. (1986) and Gillet et al. (1990). The most commonly used aphicides have been organophosphates or synthetic pyrethroids. Imidacloprid, an insecticidal seed treatment, decreased BYD infection in certain conditions (Gray et al., 1996; McKirdy and Jones, 1996) and is now quite widely used in Europe. Success with biological control has been reported from South America, where S. avenae and M. dirhodum were controlled through the introduction of Coccinellid predators and Aphelinid and Aphidiid parasites (Zuniga, 1990) and in New Zealand with the introduction of Aphidius rhopalosiphi (Farrell and Stufkens, 1990). As in most areas, natural enemies limit aphid populations, and it is important to integrate chemical and natural control methods. Adequate cultural practices can be used to limit infection with BYDV. Incorporating resistance or tolerance (Cooper and Jones, 1983) to BYDVs or their vectors is one of the most promising approaches to control. Most of the screening for field 'resistance' to BYD has actually been directed to the identification of tolerance. In wheat, sources of tolerance have been reported by several researchers (Burnett et al., 1995). Tolerance in the variety Anza (Qualset et al., 1984) has been associated with the presence of the gene Bdv1, a partially dominant, partially effective gene that induces slow yellowing (Singh et al., 1993). However, other genes are involved in tolerance to BYD (Henry, unpublished data). The winter wheat germplasm Elmo and Caldwell were released as tolerant to BYDV (Ohm et al., 1981; Patterson et al., 1982). Comeau et al. (1998) selected a durum wheat line with good tolerance to BYD.

A decrease in virus multiplication has been reported from several wheat relatives, such as Aegilops, Elymus, Elytrigia, Hordeum, Leymus and Thinopyrum (Agropyron) (Xu et al., 1994; Sharma et al., 1984; Larkin et al., 1990; Makkouk et al., 1994). Recently, much effort has been directed toward incorporating these alien-derived resistances into wheat. Thinopyrum intermedium has been widely used to produce resistant introgressed material, such as the TC lines (Banks et al., 1995b), Zhong 4 (Xin et al., 1991) and Zhong 5-derived lines (Larkin et al., 1995a), and most recently L1-derived Ym lines (Xin et al., 2001). The 42 chromosome winter wheat line P29 and spring wheats TC5, TC6 and TC9, as well as the genetic stock Z1, Z2 and Z6 with alien-derived resistance, have been registered (Sharma et al., 1997; Banks and Larkin, 1995; Larkin et al., 1995b). The amphiploid derived from the wheat x Agrotricum cross was reported to be immune to BYDV (Chen et al., 1998). In TC14, the alien segment is located on 7DL (Hohmann et al., 1996). This line has been incorporated into the CIMMYT breeding programme (Ayala et al., 2001).

WHEAT STREAK MOSAIC RYMOVIRUS

Wheat streak mosaic rymovirus (WSMV) is a serious pathogen of wheat, particularly in the Great Plains region of the United States. It has been reported from several parts of North America, Jordan, Romania, former Yugoslavia, the Russian Federation, Iran (Brakke, 1971a) and most recently in Mexico (Sánchez Sánchez et al., 2001). It is persistently transmitted by the wheat curl mite, an Eryophid mite, formally identified as Aceria tulipae (Slykhuis, 1955) and recently named Aceria tosichella (Amrine and Stasny, 1994). Wheat streak mosaic disease (WSM) is estimated to decrease annual wheat yields by about 2 percent/year in the Great Plains region of the United States (McNeil et al., 1996). Symptoms differ with wheat cultivar, strain of the virus, time of infection and environmental conditions. Even if winter wheat is infected in the autumn, it rarely shows symptoms until spring. Infected plants are stunted, with mottled green- and yellow-streaked leaves (Plate 52). Symptoms range from mild mosaic to severe chlorosis resulting in the death of tillers, reduced seed set and shrivelled kernels.

The mite relies on wind to move from plant to plant or field to field. Wheat streak mosaic is controlled primarily by breaking the life cycle of the mite vector through elimination of volunteer wheat and grasses and delaying planting. Because corn is a host for both the mite and the virus, it is recommended that wheat not be planted next to corn that has not yet matured. The insecticidal seed-coating imidacloprid has recently been shown to be ineffective against the wheat curl mite and WSMV (Harvey et al., 1998).

The genetic control of WSMV can be achieved either by selecting for resistance to the vector or developing lines resistant to the virus. No resistance to WSMV has been found in wheat, but resistance exists in wheat relatives, such as T. intermedium (Sharma et al., 1984). Wheat-T. ponticum (OK65C77-6, OK65C93-8) and wheat-T. intermedium (KS93WGRC27) lines have been registered as resistant to WSMV (Sebesta et al., 1995; Gill et al., 1995). In KS93WGRC27, the gene Wsm1 confers resistance to WSMV. Friebe et al. (1996) reported resistance different from Wsm1 in lines 6957 and 6961 derived from T. intermedium. Genes conferring resistance to wheat curl mite colonization have been transferred from Ae. tauschii, Cmc1, and from Ag. elongatum, Cmc2. A resistance breaking strain of wheat curl mite has been recently reported (Harvey et al., 1995).

SOIL-BORNE WHEAT MOSAIC FUROVIRUS AND OTHER FUROVIRUSES

Soil-borne wheat mosaic furovirus (SBW-MV) occurs in China (Chen, 1993), Japan, Italy, France (Lapierre et al., 1985), Germany, Brazil (Caetano et al., 1977) and the United States. In China, soil-borne cereal viruses (including SBWMV and wheat spindle streak mosaic bymovirus) became a serious agricultural problem in the 1970s when susceptible cultivars were grown continuously on a large scale. Annual cereal yield losses were estimated at 400 000 tonnes (Chen and Wilson, 1995).

Soil-borne wheat mosaic furovirus is naturally transmitted by the fungus Polymyxa graminis (Chen and Wilson, 1995) and mechanically transmitted to wheat, barley, rye, Bromus and Chenopodium. Polymyxa graminis, is an obligate parasite and is wide-spread in temperate (United States, New Zealand, Europe, China and Japan) and tropical (West Africa, India and South America) regions. The virus is carried inside zoospores and persists inside the fungal resting spore for many years (Chen and Wilson, 1995). The best conditions for disease transmission (as for vector development) are high soil moisture, humid weather, soil temperature around 15° to 18°C and slightly neutral to alkaline soil. The optimum temperatures for symptom development are 15° to 20°C. Symptoms range from mild green to prominent yellow leaf mosaics. The new unfolding leaves show mottles and streaks. Stunting can range from moderate to severe, and certain strains of the virus can cause rosetting of plants. The symptoms are more prominent in early spring growth and rarely appear in autumn (Plate 53).

Control measures include rotation, delayed sowing, increasing fertilizer application and the use of resistant cultivars. Application of chemical sterilants and fungicides can help reduce inoculum but are not practical for large-scale field control. Chen and Wilson (1995) indicate that resistance is controlled by a single locus and that it is dominant. In Italy, the most widely grown bread wheat cultivars are resistant to SBWMV. The durum wheat cultivar Ares showed resistance to SBWMV in northern Italy (Vallega et al., 1999). The hard winter wheat germplasm, KS92WGRC21 and KS92WGR22, have been registered as resistant to SBWMV (Cox et al., 1994). A potential source of resistance to SBWMV has been reported from the wheat-T. intermedium addition line L2 (Rumjaun et al., 1996). Recently, Aubian wheat mosaic virus, a soil-borne wheat tubular virus which is serologically distinct from other soil-borne wheat viruses, was described in France (Hariri et al., 2001). Another tubular virus, which has been described in England (Clover et al., 1999), has similarities to the Aubian wheat mosaic virus. In China, Ye et al. (1999) reported the presence of a new furovirus, Chinese wheat mosaic virus, which presents difference in serology and nucleotide sequence to SBWMV.

WHEAT SPINDLE STREAK MOSAIC BYMOVIRUS AND WHEAT YELLOW MOSAIC BYMOVIRUS

Wheat spindle streak mosaic bymovirus (WSSMV) and wheat yellow mosaic bymovirus (WYMV), before considered to be strains of the same virus, were shown to be distinct species through genome analysis (Namba et al., 1998). A multiplex RT-PCR assay permits the discrimination of these two viruses (Clover and Henry, 1999). Nucleotide and amino acid sequence comparison demonstrated that the European and North American isolates were extremely similar and were WSSMV, while the Chinese isolates were close to the Japanese isolates and were WYMV. Wheat spindle streak mosaic bymovirus was first reported from Canada (Slykhuis, 1976). Wheat spindle streak mosaic bymovirus or WYMV have been reported from the United States, France, Germany, India, Italy, Zambia, China, Japan, the Democratic People's Republic of Korea and the Republic of Korea (Clover and Henry, 1999). Wheat spindle streak mosaic bymovirus and WYMV are economically important wherever they occur.

They are both sap-transmitted to wheat and naturally transmitted through the soil by the fungus Puccinia graminis. Since WSSMV and WYMV are transmitted by P. graminis, much of what has been said previously for SBWMV applies equally to WSSMV. However, WSS-MV is usually more uniformly distributed in fields than SBWMV. Symptoms develop only in spring when soil temperatures are between 5°C and 15°C. The virus is usually detected earlier in roots than in leaves. The optimum temperature for the development of P. graminis in wheat roots is 15° to 22°C, for transmission of the virus 15°C and for virus development 10°C (Slykhuis and Barr, 1978), perhaps the lowest optimal temperature range of any plant virus disease. Wheat spindle streak mosaic bymovirus and WYMV cause similar symptoms on susceptible plants. Symptoms vary seasonally and disappear soon after the temperature exceeds 20°C. In early spring, symptoms appear as yellow-green mottling, dashes and streaks (Plate 54). The discontinuous streaks are parallel to the vein and form spindles. In cool temperatures, symptoms are more severe, showing necrosis in the chlorotic spindle. Infected plants can be mildly stunted and with few tillers. Autumn infections are most common, and spring infection results in few symptoms.

Control of both viruses is difficult. Slykhuis (1973) reported that the incidence of WSSMV was suppressed by pre-seeding application of urea (NH₄O₃), or poultry manure, each supplying nitrogen (N) at 500 to 1 800 kg N/ha, but not by cattle manure supplying up to 1 000 kg N/ha. However, these measures cannot be recommended for practical disease control. The only practicable way to reduce yield losses is through resistance to WSSMV and WYMV or their vector. Resistance to WSSMV is seen mainly as a tendency to avoid infection (Bergstrom and Sorrells, 1997). Some cultivars are resistant to WSSMV, and the resistance appears to be dominant and highly heritable. The cultivars KS92WGR21 and KS92WGR22 have been released as highly resistant to WSSMV (Cox et al., 1994).

BROME MOSAIC BROMOVIRUS

Brome mosaic bromovirus (BMV) was first reported to cause economic damage in wheat in South Africa and to mimic BYDV infection (von Wechmar and Rybicki, 1985). It has been reported to naturally infect wheat in the former Soviet Union, former Yugoslavia, Hungary (Pocsai et al., 1991) and Brazil (Caetano et al., 1990), as well as in Canada (Haber and Hamilton, 1989). Some wheat cultivars are symptomless carriers of the disease. Symptoms on leaves are yellow or white spots and streaks that turn into a yellow mosaic pattern. Infected plants can be slightly stunted and produce shrivelled grains. It is transmitted mechanically to cereals and grasses.

WHEAT DWARF GEMINIVIRUS

Wheat dwarf geminivirus (WDV) is an economically important disease on spring and winter wheat in France, Sweden and Eastern Europe. The disease also affects rye and barley. Infected plants develop vein distortions on the underside of leaves and develop fine green to yellow spots. Plants are very stunted, especially if they were infected as seedlings. The main vector is the leafhopper Psammotettix alienus. Recent outbreaks have been reported from Sweden (Lindsten and Lindsten, 1999).

BARLEY STRIPE MOSAIC HORDEIVIRUS

Barley stripe mosaic hordeivirus (BSMV) occurs in North America, Europe, Japan, Australia, the former Soviet Union and China (Wiese, 1987), but is not an economically important disease of wheat. It is mentioned here because it is transmitted through the seed and pollen. Barley stripe mosaic hordeivirus causes yellow to white mottling, spotting and streaking on leaves, severe mosaic, dwarfing, excessive tillering and necrosis. Plants grown from BSMV-infected seeds may show symptoms as early as the second or third leaf stage.

REOVIRUSES

Wheat is a host for several reoviruses: rice dwarf phytoreovirus (RDV), oat sterile dwarf fijivirus (OSDV), maize rough dwarf fijivirus (MRDV) and rice black-streaked dwarf fiji-virus (RBSDV) (Conti, 1987). Nariño dwarf virus (enanismo, possible oryzavirus) has been associated with strong symptoms in Colombia (Uyeda and Milne, 1995). Another reovirus (Mal del Rio Cuarto) has been reported to cause extensive damage to wheat and maize in Argentina (Rodriguez Pardina et al., 1998). Symptoms are deformed leaves, spikes and spikelets, shortened internodes, leaves with serrated borders and sterile spikelets. Mal del Rio Cuarto was classified as a strain of maize rough dwarf fijivirus (Brunt et al., 1996), but recent molecular data (Rodriguez Pardina et al., 1998) do not support this. Reoviruses are exclusively transmitted by leafhoppers (phytoreoviruses) and planthoppers (fijiviruses), which propagate in the vector and the plant.

RHABDOVIRUSES

There are several rhabdoviruses that infect wheat, but none of them are important economically. They are transmitted in a propagative manner by the planthoppers. Recent reports mentioned the occurrence of American wheat striate mosaic in the United States (Seifers et al., 1995) and barley yellow striate mosaic rhabdovirus in Turkey (Makkouk et al., 1996).

NEWLY DESCRIBED WHEAT VIRUSES

Cereal flame chlorosis was first observed in Canada in 1985 and has only been reported in this country (Haber et al., 1990). It is soil-borne and associated with specific double-stranded RNAs. Its spread has been associated with a trend towards more intensive cropping and the presence of certain zoosporic fungi (Haber et al., 1991). Disease symptoms include leaf chlorosis in a flame-like pattern, severe stunting and sterility.

High plains disease has been recently described as associated with WSMV in wheat and maize in the United States and to cause the aggravation of WSMV symptoms. It is transmitted by the wheat curl mite (A. tosichella) and has been associated with the presence of a 32kDa nucleoprotein (Jensen et al., 1996).

Indian peanut clump pecluvirus, highly infectious on graminaceous plants (Doucet et al., 1999), has been reported on wheat in Africa and India (Delfosse et al., 1995). First classified as furovirus, both peanut clump virus and Indian peanut clump pecluvirus belong now to the new group of pecluviruses (Torrance and Mayo, 1997).

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