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Symptoms of MDMV on maize and sorghum are similar. Virus-infected plants are discoloured, exhibit dwarfing, sterility, and sometimes premature death. The severity of symptoms depends primarily on the susceptibility of the genotype being grown, and on the time of infection.
Early infection leads to severe and more pronounced symptoms. Field maize usually shows less severe symptoms than sweetcorn or inbred lines.
Mosaic usually appears first at the base of the youngest leaf still unfolding in the sheath. On one (or both) sides of the main leaf vein, chlorotic dots and streaks appear, which gradually enlarge and spread over the blade of the growing leaf. Streaks and dots can merge into chlorotic stripes or lines, which run along the leaf blade. Normal green colour may remain as streaks, stripes or lines, usually along the leaf veins. Upper leaves which appear after infection became chlorotic and yellowish. Chlorotic-yellow plant apices are a very reliable symptom of MDMV infection of maize plants.
Sweetcorn and maize inbred lines, sometimes also field maize, exhibit mosaic and yellowing of leaves accompanied by a reddish-violet colour. Such discoloration usually appears when infection takes place early in the growing season, when maize seedlings are very young. Reddish-violet discoloration can be dots, streaks, or stripes along the margins, or starting from the leaf tips. Such discoloration is more often on older leaves after an early infection. The leaf tissue showing reddish-violet discoloration may become necrotic. Such symptoms are usually accompanied by severe dwarfing of infected plants. Necrosis overcomes most, or all, of the diseased plant. Such premature necrosis characterises early infected plants of maize inbred lines.
All discoloration occurs on leaves which were growing when infection occurs, or on leaves grown after infection. Leaves which had completed their growth before infection remained symptomless. It is possible to estimate at what state of plant growth the infection occurred.
Growth of maize plants is also affected by infection with MDMV. All internodes on the stalk of infected maize plants, grown after infection, are shorter and sometimes become rosette-like at the apices. The degree of growth reduction also depends on the maize genotype and on the time of infection. Early infection correlates with severe dwarfing under experimental conditions (Genter et al., 1973).
With field maize and when infection is not too early, the height of infected maize plants is decreased by 20-25% (Tosic and Misovic, 1967). When either maize inbred lines or sweetcorn are infected with MDMV they are often short, frequently less than 1 m (Tosic et al., 1990a).
Maize plants infected with MDMV exhibit obvious delayed growth. The mid-silk stage and fertilization may be delayed by 2 weeks or more (Dimitrijevic, 1969; Scott and Nelson, 1972). Ears of maize MDMV-infected plants are not usually ripe at harvest time, but are in transition between the milk and waxy stage, or are just in the waxy stage.
Sterility is a symptom commonly accompanying MDMV infection in maize. Sterility of maize plants infected with the virus occurs due to a delay of the silking and tasseling stages compared with healthy plants (Dimitijevic, 1969; Scott and Nelson, 1972). Lower pollen viability and shorter pollen germ tubes of maize plants infected with MDMV (Mikel et al., 1982) also contribute to the sterility.
Sterility of field maize plants infected with MDMV can often reach 25% or more (Tosic and Misovic, 1967). With maize inbred lines, especially after an early infection, sterility can be much higher, leading to a complete yield loss. Severely infected maize seed crops are therefore usually ploughed under (Tosic et al., 1990a).
Incomplete fertilisation of ears of maize plants is also a symptom of MDMV infection. Due to the symptoms listed (dissimultaneous stages of growth, lower viability of pollen of infected maize) entire rows of grain are incomplete on those ears originating from maize plants infected with MDMV (Tosic and Misovic, 1967).
Some other changes in maize plants infected with MDMV include reduced photosynthesis plus increased respiration (Tu and Ford, 1968), reduced transpiration (Lindsey and Gudauskas, 1975), cell inclusions of pinwheel and scroll types (Krass and Ford, 1969; Moline, 1972; Langenberg and Schroeder, 1973; Edwardson, 1974; Krstic, 1992; Lesemann et al., 1992).
Sorghum reactions to infection with MDMV can be more diverse than those on maize and they depend not only on plant genotype susceptibility but also on climatic conditions. They include discoloration, dwarfing, tillering and necrosis.
Mosaic is a very common symptom on different sorghum genotypes infected with MDMV. The first signs of mosaic can be seen on the base of the youngest leaf, still in the sheath. Chlorotic dots and streaks first appearing later on, merge and spread over the leaf blade forming chlorotic stripes or lines.
Under cool conditions, mosaic on sorghum plants can be accompanied by red stripes. The disease was therefore named sorghum red stripe and the virus sorghum red stripe virus. Red stripes on leaves of sorghum plants infected with MDMV usually became necrotic (Persley et al., 1977; Martin and Hackerott, 1982; Jarjees and Uyemoto, 1983; Mijavec, 1991; Berenji et al., 1996) especially under cool conditions. After such a shock reaction, diseased sorghum plants can recover but the yield of these plants is very reduced and of poor quality.
Dwarfing of sorghum plants infected with MDMV is more pronounced than with infected maize, especially those plants which suffered from the shock reaction.
Tillering is also a common phenomenon which accompanies MDMV infection in sorghum. Plants subjected to leaf necrosis tiller more than those which are healthy or those not subjected to the shock reaction.
Brushes of broom maize plants infected with MDMV are underdeveloped and very often reddish. This results in brushes of lower quality.
Symptoms on Johnsongrass are similar to those on sorghum. Chlorotic streaks of different length and intensity run along the veins of the leaves, and often they turn reddish-violet. The mosaic disappears as the Johnsongrass leaves age. Only the top two or three leaves exhibit sharp mosaic symptoms.
The economic importance, due to damage caused in maize and sorghum production, make control of MDMV mandatory. Since it has a wide host range of ca 250 different plant species and more than 20 aphid vectors, control of MDMV is not easy. Therefore, many measures which can contribute to disease control have to be applied. Integrated MDMV control management should comprise three basic measures: destruction of Johnsongrass plus other wild hosts (the source of MDMV under field conditions); control of aphid vectors; and breeding of resistant maize and sorghum genotypes (Milinko et al., 1979; Gorbunova et al., 1980).
Johnsongrass is usually infected by up to 100% with MDMV (Pop, 1962; Shepherd et al., 1964; Sutic and Tosic, 1966; Reeves et al., 1978). Such a high incidence of infection is due to inoculum accumulation in the perennial Johnsongrass. Therefore, Johnsongrass serves as a permanent source (one of the best) of MDMV under field conditions (Tosic and Simova, 1967; Onazi and Wilde, 1974).
Although nearby sources of MDMV inoculum are the most important (Damsteegt, 1976; Madden et al., 1986), sudden epidemics of MDMV in areas without Johnsongrass was hypothesised by long distance aphid vector movement (Zeyen et al., 1987). Therefore, Johnsongrass should be destroyed wherever possible.
Johnsongrass should be controlled as early as possible after emergence (Vangessel and Coble, 1993). When Johnsongrass is controlled late in the season a higher incidence of MDMV occurs, especially on maize hybrids susceptible to MDMV (Eberwine and Hagood, 1995).
MDMV control by systemic insecticides against aphid vectors is also a big challenge. MDMV is transmitted nonpersistently group of viruses. Thus, viruliferous aphids can inoculate healthy plants before being affected by insecticide. It is suggested that the application of insecticides to source plants of the virus would lead to better results.
Oil application weekly or twice-a-week gave better, but not consistent, results (Ferro et al., 1980; Szatmari-Goodman and Nault, 1983).
Host Plant Resistance
The crucial measures for controlling MDMV is breeding resistance genotypes in maize and sorghum. For this, it is necessary to determine a source of resistance and then to transfer and incorporate the resistance into maize and sorghum genotypes growing in production. Mikel et al. (1984) transferred genetic resistance from Pa405 field maize inbred into sweetcorn.
The resistance against MDMV in maize or in sorghum can be recognised by: (1) the absence of infection or mild symptom expression (Kuhn and Smith, 1977; Fuchs and Bedri, 1993); (2) a lower percentage of plants developing symptoms (Kuch and Smith, 1977; Fuchs and Bedri, 1993); (3) a long incubation period (Kuhn and Smith, 1977; Fuchs and Bedri, 1993); (4) suppressed virus translocation-movement within the infected plant (Jones and Tolin, 1972; Ford and Hill, 1976; Kuhn and Smith, 1977; Law et al., 1987; Fuchs and Bedri, 1993); (5) virus presence restricted to certain leaf areas showing symptoms (stripes, bands etc.) (Jones and Tolin, 1972); and (6) a low virus titre (Fuchs and Bedri, 1993).
Resistance to MDMV should be tested both under greenhouse and field conditions. In some cases, similar results were obtained (Kuhn and Smith, 1977; Kovacs et al., 1993) but different results were obtained in others (Kuhn and Jellum, 1970; Louie et al., 1990; Kovacs et al., 1993; Kovacs et al., 1994). The results concerning resistance to MDMV, and obtained under greenhouse conditions, can be used to predict possible resistance under field conditions (Zuber et al., 1973). Selection of parental lines should be based on the results obtained under both greenhouse and field experiments (Naidu and Josephson, 1976).
Many maize genotypes were shown to possess the resistance gene. Examples of resistant genotypes are Pa405, B68, Ph1EP, 0H7B, Ga209, Oh514, Oh514, Oh07, I11A, W70, Oh28, 38-11, C103, A632, A634, B64, PI536518 and PI536519 (Nault et al., 1971; Findley et al., 1973; Findley et al., 1977; Roane et al., 1983; Mikel et al., 1984; Davis et al., 1988; Roane et al., 1989; Poneleit et al., 1990; Ivanovic, 1991; Kovacs et al., 1994). The resistance of some genotypes can be improved by the incorporation of germplasm from a local population (Ivanovic, 1991).
Zea diploperennis, a relative of maize, is immune to MDMV (Podol'skaya, 1988). The diploid teosinte can serve as a source of resistance for maize. Transfer of resistance to MDMV from the source to maize genotypes for growing purposes, can be achieved by crossing (Josephson and Naidu, 1971; Naidu and Josephson, 1976; Juvik and D'Arcy, 1988). In most cases resistance to MDMV is dominantly inherited (Brewbaker, 1975; Findley et al., 1977; Roane et al., 1983; Heo et al., 1985; Roane et al., 1989; Ivanovic et al., 1992; Kovacs et al., 1993; Kovacs et al., 1994) and is probably controlled by one dominant gene (Josephson and Naidu, 1971; Findley et al., 1977; Roane et al., 1983). In this case, crossing two resistant lines, or one resistant with one susceptible line, usually results in resistant hybrids, while crossing two susceptible lines results in a susceptible hybrid (Giorda and Toler, 1985). After crossing two resistant lines there is no, or very little, segregation in the F1 and F2 generations (Mikel et al., 1984).
In some cases resistance to MDMV is partial (Naidu and Josephson, 1976), and is controlled by few genes, one of which is mandatory (Josephson and Naidu, 1971; Mike et al., 1984). Different types of inheritance of resistance against MDMV with the same inbred line (maize genotype) was also shown to occur. It was dominant under field conditions but intermediate under greenhouse conditions (Kovacs et al., 1994).
In order to transfer and to incorporate resistance against MDMV in maize genotypes, back-crossing, alternative back-crossing and selfing or sib-pollination should be applied as well as crossing (Josephson and Naidu, 1971; Naidu and Josephson, 1976). Selection in F1 and BC1 generations, as well as recurrent selection, should be also be applied (Josephson and Naidu, 1976; Kovacs et al., 1990).
The resistance of inbred lines, their combining abilities (general and specific) and inheritance of resistance should be checked before crossing (Popov and Popova, 1976; Heo et al., 1985). Different maize inbred lines, resistant to MDMV, behave differently when crossing (Heo et al., 1985).
Some results have shown that the gene(s) controlling maize resistance against MDMV is located at chromosome 6, on either the short arm or the proximal region of the long arm (Roane et al., 1989; Louie et al., 1991; Simcox et al., 1993, 1995; Ignjatovic et al., 1995). Transgenic maize plants expressing sugarcane mosaic virus-MDB (former MDMV-B) coat protein were also resistant to MDMV-A (Murry et al., 1993).
The most important control measure is breeding sorghum resistant to MDMV. Reaction of sorghum genotypes to MDMV infection markedly differ (Fazli et al., 1970). Many sorghum genotypes, like RS 621, Tx 414, RS 625, BTx 399 (wheatland) and Tx 398 (Martin) are tolerant (Toler, 1985). A good level of resistance against MDMV exists in sorghum lines NM 960, Mer 75-6, Mer 76-1, Mer 77-2, and Mer 77-7 (Zummo et al., 1981). Many sorghum genotypes, like Tx2536, RTx 430 (Toler et al., 1982), Tx 2726 (Miller and Toler, 1984), RTx 435 (Miller, 1986) and RTx 2858 (Miller and Toler, 1990), are resistant to MDMV.
A high level of resistance (immunity) occurs in some sorghum genotypes derived from a cross of grain sorghum and Johnsongrass (2n=20) x Sorghum roxburghii (2n=20) (Krishnasway et al., 1956); i.e., Krish genotype containing the Krish type of resistance. The Krish resistance was transferred and incorporated into sorghum lines called QL lines, which serve as a good source of resistance (Mijavic, 1991). Among QL lines the most used are QL 11, QL 3-Tx and QL 3-India, which are immune to MDMV (Giorda et al., 1985; Giorda and Toler, 1985; Langham et al., 1985; Mijavec, 1989). Sorghum hybrids with RTx 2858 are highly resistant to MDMV.
Ways of transferring the resistance against MDMV in sorghum are the same as those for maize.
MDMV can also be controlled by some agricultural practices. A lower incidence of disease is correlated with a good tillage system (All et al., 1977), greater plant density (Popov, 1978), early sowing (Scott and Rosenkranz, 1974; Popov, 1979; Forster et al., 1980), polyculture and wide crop rotation (Piper et al., 1996).
On controlling MDMV incidence (and harmfulness) in sorghum, including broomcorn, similar approaches should be effective. The destruction of virus sources, spraying against aphid vectors and agricultural practices can effect MDMV incidence in sorghum as they do in maize.
MDMV causes economic losses in maize, including field maize, sweetcorn and maize inbred lines (seed crop), as well as sorghum, including silage sorghum, grain sorghum and broom maize. Infection on maize plants affects growth stages of development, leaf area, fertility and mature weights, number of seeds and rows, yield (seed yield, total yield), 1000 grain weight, seed quality (viability), and susceptibility of infected plants to other pathogens/diseases.
The extent of pathological changes induced by MDMV mainly depends on cultivar susceptibility, time of infection, water conditions, nutrients etc.
Early infection with MDMV delays all stages of maize development, for ca 2 weeks or more (Dimitrijevic, 1969; Scott and Nelson, 1972; Mikel et al., 1981).
The height of maize plants infected with MDMV is reduced by up to 23% (Tosic and Misovic, 1967; Popov, 1972; Sum et al., 1979; Olsen et al., 1990). In extreme cases, especially with maize inbred lines, the diseased plants are 25-50% of the height of healthy plants (Jansen and Ellet, 1963; Tosic et al., 1990a).
Leaf area reduction in maize plants infected with MDMV is up to 11%, but after early infection and early water stress, the leaf area could be reduced by 33% or more (Olsen et al., 1990). Leaf area reduction, along with a decrease in plant height, have a large influence on crop productivity.
Sterility, which is more frequent with infected maize plants, contributes to the economic impact of MDMV. Reduced growth and development, and changes in some plant development stages (Dimitrijevic, 1969; Scott and Nelson, 1972; Mikel et al., 1981) can lead to sterility in up to 25% of infected plants (Tosic and Misovic, 1967), but in the case of maize inbred lines the sterility can be even higher (Tosic et al., 1990a).
The number of ears per plant, especially of marketable ears of sweetcorn crops, is markedly reduced by MDMV infection (Gregory and Ayers, 1982; Mikel et al., 1982). Fresh weight of ears can be reduced by 30% or more by MDMV infection. (Anzola et al., 1980; Mikel et al., 1981; Antignus, 1987). Ear length reduction due to MDMV infection ranges by up to 25.6% (Tosic and Misovic, 1967; Mikel et al., 1981; Peti, 1983). The diameter of ears of maize plants infected with MDMV is also affected (Mikel et al., 1981). Ear yield can be drastically reduced by MDMV, especially after an early infection. The reduction in ear weight caused by MDMV infection can be nearly 50% (Tosic and Misovic, 1967; Antignus, 1987; Fuchs et al., 1990b; Olsen et al., 1990). Ears of maize plants infected with MDMV are poorly filled and the number of kernels and rows of kernels are reduced (Tosic and Misovic, 1967; Olsen et al., 1990; Kovacs et al., 1994).
Kernels of maize plants infected by MDMV are usually smaller, especially at the basal portion of the ear (Mikel et al., 1981). Consequently, the 1000 grain weight can be reduced by up to 26% (Peti, 1983; Kovacs et al., 1994).
The total yield of maize can be very much affected by MDMV infection. Yield reduction per fertile field maize plant infected with MDMV can be up to 42% (Tosic and Misovic, 1967). However, with maize inbred lines or with sweetcorn, especially after later sowing, the yield can be reduced by 75% or more (Forster et al., 1980).
Germinability of seeds formed on maize plants infected with MDMV may be reduced by nearly 20%, while the length and width of primary roots are smaller by 7.4 and 20.0%, respectively (Stakic and Savic, 1984).
Sorghum is the second main crop which can be affected by MDMV. Symptoms on sorghum plants infected with MDMV can be more severe than those caused by the same virus on maize plants. The symptoms on sorghum depends on cultivar susceptibility, time of infection, and environmental conditions (mainly temperature). Besides mosaic and growth reduction, infected sorghum plants show red stripes and necrosis. Red stripe is a common symptom, characteristic of sorghum plants infected by MDMV. Necrosis usually appears after a low temperature period, when the temperature falls to 15-16°C and below. After a such cool period the leaves of sorghum plants infected with MDMV, and showing mosaic and red stripes symptoms, became necrotic. It appears as a shock reaction, after which plants usually recover but they yield less than healthy plants (Berenji et al., 1996).
Yield of grain sorghum due to MDMV infection can be reduced by 15% (Toler, 1985) or up to 79% (Giorda and Toler, 1986). With broomcorn, depending on the cultivar, yield decrease can range from 32 to 59% (Tosic and Mijavec, 1991).