Cui Yu, Zhang Ai-hong, Ren Ai-jun, and Miao Hong-qin

1College of Humanities & Social Science, University of Science and Technology of China, Hefei 230026, China

2Institute of Plant Protection, Hebei Academy of Agriculture and Forest Sciences/IPM Center of Hebei Province/Key Laboratory of IPM on Crops in Northern Region of North China, Ministry of Agriculture, Baoding 071000, Hebei, China

Types of Maize Virus Diseases and Progress in Virus Identification Techniques in China

Cui Yu1, Zhang Ai-hong2, Ren Ai-jun1, and Miao Hong-qin2

1College of Humanities & Social Science, University of Science and Technology of China, Hefei 230026, China

2Institute of Plant Protection, Hebei Academy of Agriculture and Forest Sciences/IPM Center of Hebei Province/Key Laboratory of IPM on Crops in Northern Region of North China, Ministry of Agriculture, Baoding 071000, Hebei, China

There are a total of more than 40 reported maize viral diseases worldwide. Five of them have reportedly occurred in China. They are maize rough dwarf disease, maize dwarf mosaic disease, maize streak dwarf disease, maize crimson leaf disease, maize wallaby ear disease and corn lethal necrosis disease. This paper reviewed their occurrence and distribution as well as virus identification techniques in order to provide a basis for virus identification and diagnosis in corn production.

maize virus disease, virus species, identification technique

Introduction

Over 40 kinds of maize virus diseases have been reported worldwide. Six of them have reportedly occurred in China, namely maize rough dwarf (MRDD), maize dwarf mosaic (MDMD), maize streak dwarf (MSDD), maize crimson leaf (MCLD), maize wallaby ear (MWED) and corn lethal necrosis (CLND) (Zhang et al., 2001; Ma et al., 1999; Peng et al., 1979; Wu et al., 1984; Qing et al., 2005). In recent years, several outbreaks of one of them, MRDD, have caused a severe yield loss in northern China. Moreover, field diagnosis of some virus diseases is rather complicated due to occasional co-infection of different viruses. This paper outlined the occurrence and distribution of the six maize virus diseases mentioned above as well as virus identification techniques in order to provide a basis for accurate virus identification and diagnosis in corn production.

Occurrence, Distribution and Damage of Maize Virus Diseases

Maize rough dwarf disease (MRDD)

First reported in Israel, MRDD is an explosive, prevalent and destructive virus disease. In China, it was first found in a suburb of Baoding City, Hebei Province in the early 1960s (Chen et al., 1986); in 1970s and 1990s, it became pandemic in northern, northeastern, northwestern, southwestern and central regions. In recent years, several outbreaks were recorded in the main corn planting areas, such as Hebei, Shandong, Shanxi, Jiangsu Provinces and caused severe yield losses (Zhang et al., 2001; Chen et al., 1986). The smaller the plant was, the severer disease it showed, and the greater the loss was. From the coleoptile stage to the 9-leaf stage, the infection-induced yield loss was from 100% to 64.2%; at the 10-12-leaf stage, it was 29.4%; at the 13-leaf stage and beyond, no symptomswere shown at all (Chen et al., 1986).

Maize dwarf mosaic disease (MDMD)

First found in the United States in 1965, MDMD made its first appearance in Xinxiang and Anyang in central China's Henan Province in 1968. Then it rapidly spread throughout most corn planting areas and resulted in different degrees of yield losses. To evaluate disease severity and yield loss, a set of criteria on a scale from 1 to 4 was established with 1 representing half of the leaves above the ear with mosaic symptoms and a yield loss of 24.1%, 2 representing leaves above the ear with obvious mosaic and a yield loss of 44.6%, 3 representing half of the plant leaves with severe mosaic and a yield loss of 67%, and 4 being the whole plant leaves with severe chlorotic mosaic and a yield loss of 88.9% (Ma et al., 1999).

Maize streak dwarf disease (MSDD)

In the early 1970's, MSDD was discovered in Dunhuang in China's Gansu Province, although a disease with similar symptoms had occurred in Kashi in Xinjiang Uyghur Autonomous Region as early as 1960. From the end of the 1960's to the early 1970's, the disease was pandemic mainly in Hexi Corridor of Gansu as well as in Xinjiang and the Province of Liaoning. Yield losses were greater during the early stages of the infection (Peng et al., 1979).

Maize crimson leaf disease (MCLD)

MCLD was first reported in Shihezi in 1979. It subsequently spread to China's northern, northwestern, northeastern, eastern and southwestern regions (Wu et al., 1984).

Maize wallaby ear disease (MWED)

MWED was first found in Queensland, Australia. In China, it first occurred in Sichuan Province's Nanchong prefecture in 1988, resulting in devastating damage to maize production. Since then, this disease has spread successively to Guizhou Province's Dafang County and Chongqing's Bishan County (Qing et al., 2005).

Corn lethal necrosis disease (CLND)

CLND is the result of a synergistic interaction between maize chlorotic mottle virus (MCMV) and maize dwarf mosaic virus (MDMV) or wheat stripe dwarf virus (WSDV) and usually occurs in Argentina, Mexico, Peru, the USA (e.g., Kansas, Nebraska and Hawaii), and Kenya. Under natural conditions, MCMV could cause a yield loss of 10%-15%, while in experimental conditions, the yield loss could rise up to 59%. In 2009, MCMV was first reported to have been found in China's Yunnan Province (Xie et al., 2011) and then it was also detected from imported maize seeds at ports in Fujian, Heilongjiang Province and Qingdao City (Gong et al., 2010).

Identification Techniques for Viruses

Identification methods for plant viruses include traditional biological assays, electron microscopy (EM) observation, serological analysis and molecular biological identification. Biological assays are based on virus infectiousness and observation of symptomatic host plants; EM observation depends on virus particle morphology; serological analysis is based on virus outer capsid protein and molecular biological identification is based on virus nucleic acid.

Biological detection of maize viruses

Through biological detection, symptoms of host plants can be observed after artificial inoculation or natural infection. The advantage of this method is that virusgenerated symptoms can be visually observed and the virus source can be obtained and saved for further study. Its disadvantage is that it needs a relatively long time to carry on the experiment and occupies more space in terms of greenhouses and other places.

Pathogen of MRDD

In China, the pathogen of MRDD was identified as maize rough dwarf virus (MRDV) before, but now it is corroborated that the disease is caused by rice black-streaked dwarf virus (RBSDV) (Zhang et al., 2001), a member of the genus Fijivirus within the family Reoviridae.

The characteristic symptoms of MRDD are pronounced stunting of the plants, shortening of the internodes, darkening of the leaves, and white waxy swellings and further developed enations along the veins on the underside of the leaf blades and on the leaf sheaths and ears. The diseased plants often bear multiple small malformed ears, but with few or no kernels.

RBSDV is propagatively transmitted by Laodelphax striatellus in a persistent manner, not by seeds and mechanical means. Apart from maize, RBSDV can also infect other Gramineae crops, such as wheat, oat, barley, rye, millet, sorghum and 41 tested grasses. The symptoms on all hosts typically include stunting, dark greening, enations and excess tillering. In artificial inoculation, maize is an ideal diagnosis host and wheat serves as the insect rearing and virus culturing host. Viruliferous Laodelphax striatella and overwintering infected wheat plants act as the inoculum reservoir for MRDD. The shortest virus acquisition time is 4 h and the circulative period of the virus inside the planthoppers varies with temperature, ranging from 8-35 days. Once acquiring the virus, Lodelphax striatella can transmit the virus in its lifetime intermittently, but not pass to the progeny via eggs.

Pathogen of MDMD

Maize dwarf mosaic virus (MDMV) and sugarcane mosaic virus (SCMV) can both cause MDMD symptoms. Both viruses belong to the genus Potyvirus in family Potyviridae. SCMV is the main causal agent of MDMD in China (Jiang and Zhou, 2002).

Typical symptoms of MDMD on maize are dwarfed plants and mosaic, green and yellow stripes on leaves. In the early stage of infection, interveinal chlorotic spots and stripes appear on the base part of new leaves, forming intermittent yellow stripes along the veins, but the veins remain green. Then yellow stripes develop into wider chlorotic stripes of different lengths and quickly spread to the whole leaf. Severely infected plants turn yellow and wither prematurely.

In artificial inoculation, maize and sorghum serve as differential hosts of MDMV. The symptoms on sorghum are mosaic in the early stage of infection and purple or violet brown streaks on leaves and withering of the whole plant as symptom develops.

SCMV can be transmitted by seeds, mechanical inoculation and aphids in a non-persistent manner. Its field dissemination mainly depends on the occurrence and migration of aphids (Ma et al., 1999; Jiang and Zhou, 2002). Twenty-three species of aphids were reported as vectors abroad, among which Rhopalosiphummaidis, Rhopalosiphumpadi, Myzuspersicae, Dactynotusambrosiae, Aphis gossypii, Schizaphisgraminum and Hysteroneurasetariae are the main vectors in China. The virus acquisition time of the aphids is 10-30 s and the virus-carrying time is 30-240 min. A few aphids, however, could carry the virus for more than 19 h (Jiang and Zhou, 2002).

Pathogen of MSDD

Maize streak dwarf virus (MSDV) is the pathogen of maize streak dwarf disease and possibly belongs to Nucleorhabdovirus: Rhabdoviridae in Taxonomy.

The typical symptoms of MSDD is chlorotic streaking of the leaves from the base to tip with brown necrotic spots appearing on streaks and veins rapidly withering into red necrotic streaks. Brown necrotic spots also appear on sheaths, subtending leaves and stems. The piths, cobs, and small stems of the staminate flowers can also be infected. The internodes are shortened and the plant is dwarfed (Peng et al., 1979).

So far the artificial inoculation of maize streak dwarf virus (MSDV) has not yet been reported. MSDV can be transmitted only by Laodelphax striatellus, not by soil, seed or mechanical inoculation. The virus acquisition feeding period is at least 8 h and the shortest circulative period is 5 days. The host range includes crops, such as maize, wheat, barley and millet and grasses like oat grass and setaria (Peng et al., 1979).

Pathogen of MCLD

Maize crimson leaf disease is caused by the barley yellow dwarf virus (BYDV), which belongs to the Luteovirus genus of the Luteoviridae family. Based ondifferent aphid vectors, BYDV could be divided into different strains - GPV, GAV, PAGV, DAV and RMV in China, and MAV, PAV, SGV, RPV and RMV in the United State. GPV is the main strain in China (Zhou et al. 1987).

Infected plants often show symptoms in the late growing stage, starting from the fourth or fifth leaf at the base and proceeding gradually to the upper part. The main symptoms are a yellowing or reddening of leaves from tip to base along the edge, an upright posture of thickened stiff leaves, and premature withering in severely affected parts from tip to base along the edge. Infected young plants are dwarfed with thin stems and narrow leaves. Plants that are infected later are not dwarfed obviously. The latent period varies from different symptoms. On the average, it is 20.9-25 days for a reddening on leaves and 13.4-18.6 days for leaf streaking (Wu et al., 1984).

In artificial inoculation, naked oats Huabei No. 2 is an ideal host for maize crimson leaf disease. Diseased leaves show chlorotic mottle on tips or margins, and then turn purple. The sheaths turn purple as well. Generally stripes do not appear on the blades. The host range of BYDV covers more than 100 monocotyledons like barley, oats, wheat, rice, maize, rye, naked oat and graminaceous grasses widely distributed in the lawn, meadow and steppe. The most effective vectors of the diseases are Rhopalosiphum prunifoliae and Rhopalosiphum maidis, followed by Schizaphis graminum and Macrosiphum avenae (Zhou et al., 1987).

Pathogen of MWED

The field diagnosis of MWED is mainly based on symptoms in corn fields. The infected corn plants are obviously dwarfed and the internodes are shortened. The leaves have become smaller, thicker and involute. White, flat, waxy galls appear on the back of the leaves along the veins. On seriously infected plants, the galls on the leaves could be in a criss-cross pattern and the leaves have shrunk transversely; roots which are short and easy to break; and the ears and tassels are deformed with less or no pollen and grains (Qing et al., 2005).

MWED is caused by maize wallaby ear virus (MWEV) can be transmitted through leafhoppers and grafting, not by seeds, soil, dodder and mechanical inoculation. Its insect vectors are Cicadulabipunctella, Balclutapunctata and Nirvanapallida. The virus transmission and acquisition periods are both 7 days. MWEV can pass to the progeny via eggs. Gramineous crops like maize, rice, barley, wheat, sorghum and millet are susceptible hosts of MWEV, while digitariasanguinalis, arthraxonhispidus, bothriochloaischaem, etc. could be used as intermediate hosts (Li, 2004).

Pathogen of CLND

Maize chlorotic mottle virus (MCMV) belongs to the Machlomovirus genus of the Tombusviridae family. Affected corn plants show such symptoms as chlorotic mottling of leaves, shortening of internodes, stunting, delayed (or no) heading, and premature death. Necrosis of young leaves tends to lead to a 'dead heart', and plant stunting. Severely affected plants produce few or no seeds.

MCMV is spread mainly by insects, seeds and mechanical transmission. It can be vectored by six chrysomelid beetle species mainly including the cereal leaf beetle, two flea beetles species, and three corn rootworm species (Jiang et al., 1992). Thrips are also main vectors (Jiang et al., 1992). Seed-borne MCMV is responsible for long distance spreading. Yet the incidence of seed transmission does not relate to the quality of seeds. It was recently reported that two out of 600 planted maize seeds tested positive for MCMV, which was much higher than the previously reported incidence of 17 out of 42 000 (Gong et al., 2010).

Electron microscopic (EM) detection

EM detection is based on different morphologies of virus particles. This technique has reached such a level of development that it is possible to make a dynamic study of in vivo localization of viruses and in vivo viral infection and replication, as well as visually observing the subunits of biological macromolecules. It can be especially applied to some unknown virusesand materials difficult to purify. Therefore, it plays a significant and irreplaceable role in virological detection and practical production.

Pathogen causing MRDD

MRDV and RBSDV are both structurally complex spherical icosahedrons with isometric particles. They have double capsids and the outer capsid is quite easily removed. The intact spherical particles have a diameter of 70-75 nm with spikes as tall as 11 nm, called A spikes. Subviral particles enclosed in a protein coat are 65 nm in diameter and have their own spikes, called B spikes. Core particles with a diameter of about 45 nm may have a third protein coat.

Pathogen causing MDMD

The purified pathogen of MDMD is a slightly curved, linear virion without a membrane. With a size of 430-750 nm×13-15 nm, it can be seen in both virus preparation and crude extract. Pinwheel inclusion bodies can be found in host cells (Jiang et al., 2002).

Pathogen causing MSDD

MSDV is rod-shaped. In an ultrathin section, its virions, 43-64 nm×150-220nm, can be seen to be distributed between nuclear membranes and among the membranes of the endoplasmic reticulum in the cytoplasm. A purified virion is 78-80 nm×200-250 nm (Peng et al., 1979).

Pathogen causing MCLD

The earliest identification of the pathogen that causes the maize crimson leaf disease was carried out through electron microscopic observation. Each of the virions is an icosahedral structure with all the faces being of uniform size. The average diameter is 23.66±1.2 nm and most of them are between 21 nm and 26.25 nm in diameter.

Pathogen causing MWED

Enveloped spherical virions with a diameter of about 85 nm have been observed in ultrathin sections of a viruliferous leafhoppers salivary gland and infected maize tissue. They are arranged in a tubular shape and the electron dense core is about 50 nm in diameter.

Through an electron microscope, suspected virions have been observed in the ultrathin section a leafhopper's of intestinal tissue, but not in the ultrathin sections of diseased roots, leaves and enations and of the salivary glands and malpighian tubules of leafhoppers (Li, 2004).

Pathogen causing CLND

Virus particles with a diameter of 30 nm have been found in infected leaf samples by transmission electronic microscope (TEM). Purified virus preparations also contain abundant unenveloped virions with a diameter of 30 nm (Gong et al., 2010).

Serological detection methods

Serological detection is based on virus coat protein. Serum antibodies can combine specifically with homologous antigens in vitro to detect the antigens. Serological methods include a dot immunoassay (DIA), direct tissue blot immunoassay (DTBIA), and enzymelinked immunosorbent assay (ELISA) and so on. The successful detection of plant viruses using ELISA was first reported by Clark (1977). ELISA has significant advantages, such as high sensitivity, strong specificity, safety, and the ability to observe the results quickly and easily. It is one of the most commonly used methods for detection of plant viruses.

Pathogen causing MRDD

To date, no whole-virion antisera have been reported. However, there is a report of the successful detection of RBSDV from wheat leaves using monoclonal antibodies made from a RBSDV P10 fragment (Wang et al., 2006). ID-ELISA has been used to determine the best conditions for the use of p10 monoclonal antibodies to detect the density of RBSDV in maize plants. It has also been used to report on differences in accumulation of RBSDV in maize plants with different resistances.

Pathogen causing MDMD

Antisera with high specificity and a titer of 1 : 2 048 have been obtained from immunized rabbits (Chen et al., 1992). ELISA and dot-ELISA have been used to detect viruses in seeds, aphids and pollen. The use of DTBIA to detect SCMV infection and movement in maize inbred lines with different resistances has alsobeen reported.

Pathogen causing MSDD

Identification of MSDV relies mainly on EM. No serological identification has been reported yet.

Pathogen causing MCLV

With the development of virus purification technology, high titer antisera against GAV strain of BYDV have been prepared. ELISA, immunosorbent electron microscopy (ISEM), and immune colloidal gold technique have been applied successively in diagnosis and classification of BYDV.

Pathogen causing MWED

Specific antisera against MWEV have been prepared and a quick and sensitive double antibody sandwich enzyme-linked immunosorbent assay DAS-ELISA has been established. This detection method is quite effective in detecting MWEV in maize plants that show no or very mild symptoms, but is not effective in detecting the virus carried by insect vectors (Li, 2004).

MCMV serology

There is now a commercial kit for serological detection of MCMV, which can test samples in batches quickly and sensitively. In one test, two out of 302 tested maize plants showed a positive reaction to antibodies against MCMV. In another test, MCMV was detected in maize leaf samples with lethal necrosis symptoms using DAS-ELISA (Gong et al., 2010; Xie et al., 2011).

Molecular biological identification

Molecular biological identification, which is based on viral nucleic acid, has brought a new leap forward for virus detection, making it possible to detect viruses at the ultra trace or microgram level. There are now also many mature methods, such as the molecular hybridization probe technique that uses a certain sequence of nucleotide as a probe to detect the base sequence complemented with the sample. The polymerase chain reaction (PCR) technique, one of the most rapidly developing and popular technologies of molecular biology in recent years, is a method for rapid amplification of specific genes or DNA sequences in vitro. Molecular biological identification is widely used in biomedical science, since it has a strong amplifying ability, and can be used together with other molecular biological methods (e.g. nucleic acid hybridization) and immunological methods (e.g. ELISA) so as to greatly enhance its sensitivity and specificity.

Pathogen causing MRDD

RBSDV was the cause of MRDD outbreak in Hebei, Hubei and Zhejiang Provinces, China. Four of the 10 genome segments corresponded to RBSDV, sharing with it a relatively high genome sequence homology. Sequencing and comparative analysis indicated that Chinese RBSDV isolates were very similar to each other (having 94.0%-99.0% nucleotide sequence homology and 96.3%-100% amino acids homology) and were more homologous to Japanese RBSDV isolates (92.2%-95.5%) than to Italian ones (76.6%-88.4 %) (Zhang et al., 2001).

Pathogen causing MDMD

The genome of a Spanish isolate of MDMV is made up of 9 414 nucleotides (nts) and that of a Spanish isolate of SCMV including the poly (A) tail consists of 9 596 nts. The ORF of SCMV genome can encode 3 064 amino acids, and the 5' and 3' UTRs are 149 and 255 nts long, respectively. The complete sequence of the genomic RNA of a MDMV isolate from Beijing has a length of 9.6 kb and its encoded polyprotein contains 3 063 aa, sharing 69% genome sequence homology and 74% polyprotein homology with the Bulgarian isolate of MDMV (MDMV-Bg). The full-length nucleotide sequence of a Henan isolate (not counting the poly A tail) is comprised of 9 596 nucleotides with its encoded polyprotein containing 3 603 aa. The entire genomic sequence of this isolate shares a 94.2%-98.3% identity with a Hangzhou isolate of sugarcane mosaic virus at the nucleotide and amino acid levels, respectively, but only a 69.1% identity with a Bulgarian isolate of MDMV (MDMVBg) at the nucleotide level (Liu et al., 2003).

Pathogen causing MCLD

Biotin-labeled BYDV cDNA hybridization probescan be used to detect BYDV with 1 ng sensitivity. The use of PCR to detect single aphids carrying BYDV can shorten the detection time to 6 h and reduce the sample size to at least 1/20. The coat protein gene of GPV strain is composed of 603 nucleotides, encoding 201 amino acids with a molecular weight of 22 218 ku. Sequence homology analysis of coat protein genes shows that GPV strain shares high homology with RPV strain with nucleotide and amino acid homology being 83.7% and 77.5%, respectively, while PAV and MAV strains share low homology with nucleotide and amino acid homology being 56.9% and 53.2%, respectively. The coat protein gene of PAV strain consists of 600 nucleotides, encoding 199 amino acids with at most 74.5% homology to typical isolates from other strains of BYDV (Cheng and Zhou, 1986).

Pathogen causing CLND

Maize chlorotic mottle virus (MCMV) consists of a 4 437 nt single-stranded RNA surrounded by 25.1 ku CP subunits. Sequence similarity to the CPs of MCMV and panucum mosaic virus, tobacco necrosis virus, and southern bean mosaic virus suggest that MCMV is a T=3 icosahedral virion with 180 copies of its CP in the viral shell (Lommel et al., 1991).

The complete nucleotide sequence (4 436 nt) of a Yunnan isolate of MCMV has been determined; it shares 97% nucleotide sequence identity with previously reported MCMV isolates (Xie et al., 2011).

Pathogen causing MSDV and MWED

So far, nothing has been reported about the molecular biological identification of maize streak dwarf virus and maize wallaby ear virus.

Comparison of virus identification methods

In identification of plant viruses, how to choose among the four methods mentioned above should depend on the type of virus to be detected, the availability of the equipment required, and the level of technical mastery achieved. Biological identification is intuitive and can obtain corresponding inoculum sources, while artificial inoculation takes a longer time and sometimes the symptoms can be complex, especially in the field. The EM technique can directly observe the morphology of virions and plays an irreplaceable role in virus identification, but it requires certain technical skills and expensive equipment. The serological technique is most commonly used due to its easy operation, short detection time, high sensitivity, strong specificity and mass rapid detection. ELISA is one of the most commonly used serological methods, yet its sensitivity and accuracy can be greatly affected by the antibody titer and specificity. With its high sensitivity, specificity and accuracy, genetic engineering technology has brought a new leap forward in virus detection and identification. However, some advanced molecular biological methods need special instruments and therefore cannot satisfy the needs of most laboratories.

Importance of Identification of Maize Virus Species

The establishment of a fast virus detection system with high sensitivity and high throughput is a prerequisite for viral disease prevention and IPM as well as an important link in prediction of plant virus disease, quarantine of imported and exported seeds and control of plant virus disease hazards.

Significant quarantine virus disease threatens maize production in China

Maize is an important crop in China, which is the world's second largest production area after the United States. But many viruses and quarantine viruses are emerging to threaten maize production. In 2010, MCMV was detected in the seeds imported from the United States and soon after; in 2011, the virus was found in fields. And a mixed infection between MCMV and maize dwarf mosaic virus, sugarcane mosaic virus or wheat streak mosaic virus can cause more than 90% yield loss. Therefore, it is of vital significance to detect it timely and discover new viruses to prevent artificial diffusion.

Requirement for identifying research and breeding objectives

Six kinds of maize virus diseases have been found and reported in China and each kind of virus disease has its place of origin and affected areas. True ascertainment of local maize virus types is necessary to identify research and breeding objectives. In recent years, scholars have ascertained that it is rice black-streaked dwarf virus that caused MRDD in China, rather than MRDV as previously reported. This finding has helped set clear goals for transgenic resistance cultivation and MRDD-resistant seed breeding.

Requirement for assuring stable and high yields of maize

There are not yet efficient, economical measures for the prevention and control of maize virus disease. Sterilized land has appeared due to MRDD in Hebei, Shandong, Jiangsu and other places since 2005. To ensure high and stable maize yields, it is necessary to identify the main types of virus diseases, guide production and disease control in an appropriate and timely manner, rationalize the distribution of different strains or crops, with and strengthen preventive agricultural measures.

Requirement for developing domestic and international economy

Maize virus diseases are diverse and widespread in the globe. With the development of world agricultural trade and the increased international exchange of scientific research, the probability of artificial virus spread raises, especially some kinds of viruses that are usually transmitted through seeds and pests. And the changing global climate can also offer a new habitable environment for viruses. With domestic agricultural and forestry production expanding, new crop varieties increasing year by year, and large-scale transportation and circulation of seeds and agricultural products increasing steadily plant protection and quarantine departments are presented with new challenges.

IPM Strategies for Prevention and Control of Maize Virus Diseases

Strengthen quarantine inspection and prevent seed-borne virus intrusion

It is significant to prevent the use of seeds from affected areas, strengthen seed quarantine and improve seed treatment research. Isolated planting and the eradication of the primary infection source are necessary to prevent seed-borne virus spread. Strict disease surveillance should be conducted on growing maize, burying or burning suspected diseased plants. In a current experiment, MCMV antiserum has been obtained from the United States that PAS-ELISA can be adopted in the growing season for mass rapid detection in nurseries and other key areas.

Agricultural measures

With a clear understanding of the regularity of virus disease occurrence and virus vectors as well as their relationship to cropping systems and the climate, an effort should be made to reduce the damage of virus disease by eliminating the source of infection, exercising proper field plant and soil management, cutting off the paths of virus transmission, adjusting sowing time appropriately, and strengthening the management and cultivation of strong seedlings as well as eradicating early-stage infected plants.

Prevent and control virus vectors

The main vectors of maize virus diseases are Laodelphax striatellus, aphids, leafhoppers, beetles, and thrips. Systemic insecticide can be used at maize sowing time for seed coating and seed dressing. Pesticides need to be sprayed on the weeds before and after planting to control insect vectors.

Boost breeding and utilization of diseaseresistant varieties

The promotion and application of disease-resistant varieties is the most fundamental method of con-trolling maize virus diseases. Maize virus diseases have worsened in recent years, becoming an important factor affecting maize yield. It is an important task for maize research to select highly resistant varieties from major existing ones while boosting innovation in resistant germplasm resources, provide accurate virus identification, and efficiently conduct breeding for virus disease resistance.

Integrated pest management (IPM)

Maize virus diseases are mainly dealt with an integrated control strategy that combines agricultural and chemical control. In order to control the occurrence and damage of maize virus diseases and secure high and stable maize yields. First of all, disease resistant or tolerant varieties should be used and then various measures can be taken, such as adjusting sowing time in a timely manner to avoid the virus transmission peak, removing weeds and diseased plants to reduce virus and pest sources, strengthening field management to keep seedlings growing strong, and enhancing the control of insect vectors by chemical means.

Chen X Z, Liu X Y, Yang M C. 1986. Studies on the occurrence of maize rough dwarf virus disease and integrated program control. Acta Agriculturae Boreali-Sinica, 1(2): 90-97.

Cheng Z M, Zhou G H. 1986. Purification and serological studies on the GPV strain of wheat yellow dwarf virus. Chinese Journal of Virology, 2(3): 275-277.

Gong H Y, Zhang Y J, Zhang Z Y, et al. 2010. Detection and identification of maize chlorotic mottle virus from imported maize seeds. Acta Phytopathological Sinica, 40(4): 426-429.

Jiang J X, Zhou X P. 2002. Maize dwarf mosaic disease in different regions of China is caused by sugarcane mosaic virus. Archives of Virology, 147(12): 2437-2443.

Jiang X Q, Meinke L J, Wright R J, et al. 1992. Campbell. maize chlorotic mottle virus in Hawaiian-grown maize: vector relations, host range and associated viruses. Crop Protection, 11(3): 248-254.

Li X Y. 2004. Pathogen identification and detection system of maize wallaby ear virus. Southwest Agricultural University, Sichuan.

Liu X, Wang X, Zhao Y, et al. 2003. Complete nucleotide sequence of a potyvirus causing maize dwarf mosaic disease in central China. Acta Virologica, 47(4): 223-227.

Lommel S A, Kendall T L, Siu N F, et al. 1991. Characterization of maize chlorotic mottle virus. Phytopathology, 81(8): 819-823.

Ma Z H, Li H F, Zhou G H. 1999. Research progress and existing problems of maize dwarf mosaic virus. Plant Protection, 2: 33-35.

Peng J M, Chen J Y, Shen C Y, et al. 1979. Nature of the pathogen of maize streak dwarf disease of Tuenhuang, Gansu Province. Acta Biochemical et Biophysica Sinica, 11(2): 187-188.

Qing L, Niu Y B, Liu Y H, et al. 2005. Research advances in maize wallaby ear disease. Plant Protection, 31(4): 14-18.

Wu E F, Wang M Q. 1984. Identification diagnosis of maize crimson leaf disease and its chemical control. Chinese Journal of Nature, 7(2): 83-86.

Xie L, Zhang J Z, Wang Q A, et al. 2011. Characterization of Maize chlorotic mottle virus associated with corn lethal necrosis disease in China. Journal of Phytopathology, 159(3): 191-193.

Wang Z H, Fang S G, Yu J L, et al. 2006. Development of an ID-ELISA for the detection of rice black-streaked dwarf virus in plants. Journal of Virological Methods, 134(1-2): 61-65.

Zhang H M, Chen J P, Adams M J. 2001. Sequence analysis shows that a dwarfing disease on rice, maize and wheat in China is caused by rice black-streaked dwarf virus (RBSDV). Eur J Plant Pathol, 107: 563-567.

Zhou G H, Zhang S X, Qian Y T. 1987. Identification and application of four strains of wheat yellow dwarf virus. Scientia Agricultura Sinica, 20(4): 7-12

Q943

A

1006-8104(2014)-01-0075-09

Received 3 May 2013

Supported by the Finance Department of Hebei Province (A2012120104)

Cui Yu (1987-), female, Master, engaged in the research of translation of scientific English. E-mail: cuiyutina@126.com