Maize Chlorotic Mottle Virus Research Articles

English

Diagnosis of maize lethal necrosis (MLN)-causing viruses is key in MLN surveillance programs and in testing seed for zero tolerance of Maize chlorotic mottle virus (MCMV) in seed lots. This is crucial for MLN management in farmers’ fields and in commercial seed fields. A customized MCMV detection assay that is specific, sensitive, affordable, and portable is therefore important for this task. Reverse Transcriptase–Recombinase Polymerase Amplification (RT-RPA) meets those conditions earlier described. RPA is a rapid isothermal nucleic acid amplification and detection platform that is based on patented Recombinase Polymerase Amplification (RPA) technology. In this study, a real time endpoint analysis and field deployable RT-RPA diagnostic method for the detection of MCMV was developed. RPA primer sets with their complementary probes were designed, synthesized and tested through a series of primer set evaluations to determine the most efficient primer sets. The primer sets targeted the MCMV genome at position 2765-2948 bp (MCMV_gp2 replicase gene). The parameters evaluated were sensitivity, specificity and reproducibility for the assay with remarkable results. The assay discriminated against other maize infecting viruses hence specific to MCMV. The assay takes only 20 min and its detection limit of 10-4 is well comparable to RT-PCR and other molecular based detection assays. MCMV was also detected directly from leaf saps without the nucleic acid extraction step hence suitable for on-farm testing. RPA is a relatively inexpensive technique that requires minimal instrumentation. This assay is therefore suitable for the detection of MCMV in field surveys, routine MCMV testing for phytosanitary measures and in the seed certification procedures.   Key words: Maize lethal necrosis, maize chlorotic mottle virus, diagnostics, recombinase polymerase amplification.

Maize Lethal Necrosis: Impact and Disease Management

The authors summarise and explore maize lethal necrosis (MLN) global impact, its causal viruses, their vectors, and disease management strategies. MLN is a destructive synergistic virus disease of maize caused by maize chlorotic mottle virus (MCMV) in combination with one of several viruses in the Potyviridae family. MLN causes severe chlorosis, necrosis, and eventual plant death, instigating severe yield losses. The causal viruses of MLN are mostly transmitted between plants by arthropod vectors, but they can also be transmitted at low frequency through seed and soil. Effective MLN management approaches include developing and growing maize with improved tolerance, crop rotation, controlling arthropod vector populations, managing disease reservoirs, and reducing disease transmission and spread through seed and soil. Despite the devastating impact of MLN over the past decade, especially in East Africa, tremendous progress has been achieved towards successful disease management. The rapid development and deployment of MLN tolerant maize in East Africa has proven an extremely effective management strategy, despite continuous maize plantings and the extensive presence of "green bridges". Identifying and stacking additional sources of MLN tolerance is desirable for breeding durable resistance. In the U.S., weed and vector management combined with crop rotation has effectively prevented the spread of MLN beyond a few states in the Midwest. However, it is important to consider the potential impact of climate change and the presence of pesticide/Bt resistant MLN insect vectors, which may facilitate greater vector proliferation and virus spread. Continued vigilance geared towards preventing virus or vector introduction to new locations is desirable, especially in tropical climates where multiple growing seasons occur annually. Fortunately, conditions conducive to an MLN epidemic are complex, requiring the presence of multiple maize infecting viruses and their respective vectors. Thus, several combinations of the disease management strategies discussed above are effective at managing the disease.

The Importance of Agroforestry Systems for Mitigation of Climate Change in Tropical Africa

Plant virus diseases are serious constraints to the productivity and profitability of a wide range of crops. Epidemics of existing plant virus diseases and the emergence of novel virus diseases have become a serious threat to subsistence and commercial agriculture. The knowledge of virus transmission and its survival helps to understand how the disease transmits from infected plant to healthy, where it reserved, and this will lead to identify the most important variables and focus efforts to develop sustainable management strategies. Maize chlorotic mottle virus (MCMV) is transmitted from location to location, and from plant to plant through various mechanisms (mechanically, seed, insect vectors, and soil) and many kinds of wild grass and cultivated crops, maize residue are used as its reservoirs. Different weed species and cultivated plants used as alternate hosts, and soil and seed transmissibility of MCMV are epidemiologically important and contribute to maintaining virus inoculum available in the absence of maize in the field and increase the chances of continuing its survival. Integrated disease management approach, regular field monitoring, assessment of virus symptoms, and rouging-out diseased plants are recommended to prevent further spread by insect vectors. Apart from this, because the disease is still widespread in various countries, intensive MCMV recruitment, combined with integrated disease management, requires ongoing practice in countries where MCMV is prevalent and in those countries that have not yet reported MCMV. Keywords : Integrated management; Insect vector; plant residue; Soil transmission; Zea mays DOI: 10.7176/JNSR/13-5-02 Publication date: March 31 st 2021

Maize chlorotic mottle virus

Maize chlorotic mottle virus

Registration of the maize synthetic population OhMCMV‐1

AbstractMaize lethal necrosis (MLN) is a devastating virus disease of maize, induced by co‐infection by Maize chlorotic mottle virus (MCMV) and one of several potyviruses. The maize synthetic population OhMCMV‐1 (Cycle‐1 [C1]) (Reg. no. GP‐623, PI 699306) was developed and released by the USDA‐ARS Corn, Soybean, and Wheat Quality Research Unit as a germplasm resource for improved MLN, MCMV, and Sugarcane mosaic virus (SCMV) resistance. The population has undergone one complete cycle of S1 recurrent selection for resistance to MCMV. Cycle‐1 S1 lines were significantly more resistant to MLN, MCMV, and SCMV than Cycle‐0 S1 lines. The elite virus resistance present within OhMCMV‐1 (C1) indicates this population has strong utility for developing new breeding lines, hybrids, and varieties and for helping to mitigate the economic impact of MLN.

Occurrence of maize yellow mosaic virus and evidence of co-infection with maize lethal necrosis viruses in Bomet County, Kenya

Maize Lethal Necrosis (MLN) disease is caused by synergistic interaction between maize chlorotic mottle virus (MCMV) and sugarcane mosaic virus (SCMV). However, some East African countries have detected maize infecting polerovirus named maize yellow mosaic virus (MaYMV) also known as maize yellow dwarf virus (MYDV-RMV) co-infecting maize with MLN viruses. Maize yellow dwarf virus occurrence and distribution in different parts of Kenya are not yet elucidated. This study aimed to establish the occurrence of MaYMV in maize and sorghum in Bomet County, MLN hotspot region in Kenya. Maize (n=90) and sorghum (n=19) samples were collected from East and Central sub-counties of Bomet County in 2019/2020. Reverse transcription-polymerase chain reaction (RT-PCR) protocol was developed and optimized for screening the samples using specific primers. Amplicons of 600, 250 and 169 bp were generated for MaYMV, MCMV and SCMV, respectively. The analysis revealed 56% (62/109) of the samples tested positive for MaYMV co-infecting maize with MLN viruses. Sanger sequencing of representative samples confirmed the presence of MaYMV. BLASTN analysis showed 95-100% sequence identity to MaYMV/MYDV-RMV hence confirmed the occurrence of MaYMV infecting maize and sorghum in Bomet County whose impacts is a potential threat to food security. Key words: Occurrence, co-infection, maize lethal necrosis, maize yellow mosaic virus, maize chlorotic mottle virus, sugarcane mosaic virus.

Maize Yellow Mosaic Virus Interacts with Maize Chlorotic Mottle Virus and Sugarcane Mosaic Virus in Mixed Infections, But Does Not Cause Maize Lethal Necrosis.

A maize-infecting polerovirus, variously named maize yellow dwarf virus RMV2 (MYDV RMV2), MYDV-like, and maize yellow mosaic virus (MaYMV), is frequently found in mixed infections in plants also infected with maize chlorotic mottle virus (MCMV) and sugarcane mosaic virus (SCMV), known to synergistically cause maize lethal necrosis (MLN). MaYMV was discovered in deep sequencing studies precipitated by recent MLN emergence and is prevalent at global locations with MLN, but its role in or contribution to disease was not known. We examined how MaYMV impacted disease development in mixed infections with MCMV, SCMV, and both MCMV and SCMV compared with mock-inoculated plants. Results demonstrated that MaYMV symptoms included stunting as well as leaf reddening in single and mixed infections. MaYMV did not recapitulate MLN synergistic disease in double infections in which either MCMV or SCMV was missing (MaYMV + MCMV or MaYMV + SCMV), but did significantly enhance stunting in mixed infections and suppressed titers of both MCMV and SCMV in double infections. Interestingly, MaYMV strongly suppressed the SCMV-induced titer increase of MCMV in triple infections, but MLN symptoms still occurred with the reduced MCMV titer. These data indicate the potential disease impact of this newly discovered ubiquitous maize virus, alone and in the context of MLN.

Maize Chlorotic Mottle Virus: Distribution, Alternative Hosts, Transmission Mechanisms and Management Options

Plant virus diseases are serious constraints to the productivity and profitability of a wide range of crops. Epidemics of existing plant virus diseases and the emergence of novel virus diseases have become a serious threat to subsistence and commercial agriculture. The knowledge of virus transmission and its survival helps to understand how the disease transmits from infected plant to healthy, where it reserved, and this will lead to identify the most important variables and focus efforts to develop sustainable management strategies. Maize chlorotic mottle virus (MCMV) is transmitted from location to location, and from plant to plant through various mechanisms (mechanically, seed, insect vectors, and soil) and many wild types of grass and cultivated crops, MCMV infected maize residue are used as its reservoirs. Different weed species and cultivated plants used as alternate hosts, and soil and seed transmissibility of MCMV are epidemiologically important and contribute to maintaining virus inoculum available in the absence of maize in the field and increase the chances of continuing its survival. Integrated disease management approach, regular field monitoring, assessment of virus symptoms, and rouging-out diseased plants are recommended to prevent further spread by insect vectors. Apart from this, because the disease is still widespread in various countries, intensive MCMV recruitment, combined with integrated disease management, requires ongoing practice in countries where MCMV is prevalent and in those countries that have not yet reported MCMV. Keywords : Integrated management; Insect vector; plant residue; Soil transmission; Zea mays DOI: 10.7176/JBAH/11-19-04 Publication date: October 31 st 2021

Comparison of non-overlapping maize populations of unequal sizes for resistance to maize lethal necrosis

ABSTRACT Contrast between marker-assisted backcross (MABC) and doubled haploid (DH) methods in transferring genes for resistance to maize lethal necrosis (MLN) in maize (Zea mays L.) is not well understood. The MLN is caused by co-infection of maize plant by maize chlorotic mottle virus and sugarcane mosaic virus. Two maize panels consisting of four BC3F2 and six DH populations, separately developed through marker-assisted selection from crosses between susceptible CIMMYT lines and MLN-resistant donor parent (KS23-6), were used in the current study. The two populations were of different population structures with unequal sizes. Experiments were conducted under artificial MLN inoculations for two seasons in 2018. Analyses of variance revealed significant variations among genotypes in both panels (p ≤ 0.001). Levene’s and Welch’s tests found that variances and means of the BC3F2 and DH populations were highly unequal (p ≤ 0.001). The study identified genotypes with reduced MLN infections in both populations; however, lower means for MLN severity and area under disease progress curve (AUDPC) values, and higher heritability estimates were obtained in the DH populations than in the BC3F2 populations. Additionally, the DH populations showed higher relative genetic gains for resistance to MLN compared with the BC3F2 populations. The current study detected superiority of DH over MABC populations for breeding for resistance to MLN. Nevertheless, the results observed in the present study warrant further investigations using the same genetic materials with identical population sizes.

Simultaneous multiplex RT-PCR detection of four viruses associated with maize lethal necrosis disease

Maize lethal necrosis disease (MLND) is a serious disease of worldwide importance. It is caused by the co-infection of maize with maize chlorotic mottle virus (MCMV) and a potyvirus, such as sugarcane mosaic virus (SCMV), that acts synergistically to produce more severe symptoms and production losses. More recently, maize yellow mosaic virus (MaYMV) and maize-associated totivirus (MATV) were found to co-infect with MCMV and SCMV in maize plants. To facilitate the detection of these viruses in co-infected maize, a multiplex RT-PCR assay was developed in this study. The assay used five specific primer pairs and simultaneously amplified these four viruses as well as the elongation factor 1α (EF 1α) gene use as internal control in one tube. The concentration of the primers, annealing temperature, annealing time, extension time and amplification cycles were optimized for the multiplex RT-PCR. The detection limit of the assay was up to 100 pg of total cDNA template. This multiplex RT-PCR assay was shown to be a sensitive and effective tool for the screening of field samples for the presence of these viruses in co-infected maize.

Maize catalases localized in peroxisomes support the replication of maize chlorotic mottle virus

Co-infection of maize chlorotic mottle virus (MCMV) with a virus in the Potyviridae family, such as sugarcane mosaic virus, usually leads to maize lethal necrosis (MLN). Over the past decade, MCMV/MLN has emerged in many countries/regions of the world and resulted in serious yield loss in maize production. Although partial functions of some MCMV-encoded proteins have been identified, the host factors related to MCMV replication are poorly understood. Here, we show that maize peroxisomes can form aggregated bodies in MCMV-infected leaf cells. The dsRNA binding-dependent fluorescence complementation assay indicated that the aggregated peroxisomes in maize served as the major replication site of MCMV. In addition, our results revealed that all the three maize catalases were present mostly in peroxisomes in the presence or absence of MCMV. Furthermore, we determined that inhibition of catalase activity or induction of reactive oxygen species (ROS) in maize protoplasts significantly reduced the accumulation of MCMV RNA. In summary, this research reveals the replication site of MCMV and an important role of maize catalases in supporting virus replication. Our results are conducive to understanding the pathogenesis of MCMV and identifying targets for resistance breeding or gene regulation strategies.

Recombinase Polymerase Amplification Assay for Simultaneous Detection of Maize Chlorotic Mottle Virus and Sugarcane Mosaic Virus in Maize.

Maize chlorotic mottle virus (MCMV) can cause maize lethal necrosis (MLN) when coinfected with potyvirids, such as sugarcane mosaic virus (SCMV), maize dwarf mosaic virus, or wheat streak mosaic virus. MLN is often caused by coinfection of MCMV and SCMV, which has been reported in China and several countries of Africa. In this study, a recombinase polymerase amplification (RPA) assay was established for simultaneous detection of MCMV and SCMV in maize. The RPA assay can be completed within 30 min at 38 °C. The primers for the RPA assay were specific since no crossreaction was detected with other selected viruses that infected maize in China. The detection limit of the RPA method was 102 copies μL–1, which was about 10-fold more sensitive than that of the conventional PCR method. Moreover, the RPA assay can be successfully applied to detect maize samples collected in the field. These results demonstrated that the established RPA assay is a rapid and efficient method to conduct simultaneous detection of MCMV and SCMV, which provides an alternative technology for MLN diagnosis.

Viral synergism and its role in management of maize lethal necrosis disease

Maize lethal necrosis disease (MLND) in Kenya has been reported to be caused by a coinfection between a non-potyvirus Maize chlorotic mottle virus (MCMV) and a potyvirus Sugarcane mosaic virus (SCMV). The control of the disease in Kenya has been a challenge owing to the synergistic interactions that exist between the two viruses. This study, sought to determine the stage of synergism between the two viruses and its role in influencing the severity of the disease. Three maize hybrids were grown in a greenhouse and were mechanically inoculated with MCMV, SCMV and MCMV+SCMV at the vegetative stage, V4-5. The synergism was studied for a period of 90-days and double-antibody sandwich (DAS)-ELISA was used to estimate the viral titer of MCMV and SCMV under individual and co-infection states of maize plants. The results showed that the viral titers of the two viruses in both single and double infection followed a normal curve. Synergistic effect was observed between the 21- and 28-days post-inoculation (dpi). A significant increase in the titers of MCMV was observed at this time in days, while that of SCMV was more or less constant. Also, the study revealed that viral titers of SCMV in both individual and co- infected maize plants remained constant; while the viral titers of MCMV in co- infected maize increased significantly as compared to the individual infections. Furthermore, there was a positive correlation between increased symptom severity and synergism. Based on these results, SCMV plays a major role in the severity and spread of MLN disease in the South-Rift region. Key words: Maize lethal necrosis, maize chlorotic mottle virus, Sugarcane mosaic virus, maize virus diseases.

Detection of Diverse Maize Chlorotic Mottle Virus Isolates in Maize Seed.

Maize chlorotic mottle virus (MCMV) has driven the emergence of maize lethal necrosis worldwide, where it threatens maize production in areas of East Africa, South America, and Asia. It is thought that MCMV transmission through seed may be important for introduction of the virus in new regions. Identification of infested seed lots is critical for preventing the spread of MCMV through seed. Although methods for detecting MCMV in leaf tissue are available, diagnostic methods for its detection in seed lots are lacking. In this study, ELISA, RT-PCR, and RT-qPCR were adapted for detection of MCMV in maize seed. Purified virions of MCMV isolates from Kansas, Mexico, and Kenya were then used to determine the virus detection thresholds for each diagnostic assay. No substantial differences in response were detected among the isolates in any of the three assays. The RT-PCR and a SYBR Green-based RT-qPCR assays were >3,000 times more sensitive than commercial ELISA for MCMV detection. For ELISA using seed extracts, selection of positive and negative controls was critical, most likely because of relatively high backgrounds. Use of seed soak solutions in ELISA detected MCMV with similar sensitivity to seed extracts, produced minimal background, and required substantially less labor. ELISA and RT-PCR were both effective for detecting MCMV in seed lots from Hawaii and Kenya, with ELISA providing a reliable and inexpensive diagnostic assay that could be implemented routinely in seed testing facilities.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Introgression of Maize Lethal Necrosis Resistance Quantitative Trait Loci Into Susceptible Maize Populations and Validation of the Resistance Under Field Conditions in Naivasha, Kenya.

Maize lethal necrosis (MLN), resulting from co-infection by maize chlorotic mottle virus (MCMV) and sugarcane mosaic virus (SCMV) can cause up to 100% yield losses in maize in Africa under serious disease conditions. Maize improvement through conventional backcross (BC) takes many generations but can significantly be shortened when molecular tools are utilized in the breeding process. We used a donor parent (KS23-6) to transfer quantitative trait loci (QTL) for resistance to MLN into nine adapted but MLN susceptible lines. Nurseries were established in Kiboko, Kenya during 2015–2017 seasons and BC3F2 progeny were developed using marker assisted backcrossing (MABC) approach. Six single nucleotide polymorphism (SNP) markers linked to QTL for resistance to MLN were used to genotype 2,400 BC3F2 lines using Kompetitive Allele Specific PCR (KASP) platform. We detected that two of the six QTL had major effects for resistance to MLN under artificial inoculation field conditions in 56 candidate BC3F2 lines. To confirm whether these two QTL are reproducible under different field conditions, the 56 BC3F2 lines including their parents were evaluated in replicated trials for two seasons under artificial MLN inoculations in Naivasha, Kenya in 2018. Strong association of genotype with phenotype was detected. Consequently, 19 superior BC3F2 lines with favorable alleles and showing improved levels of resistance to MLN under artificial field inoculation were identified. These elite lines represent superior genetic resources for improvement of maize hybrids for resistance to MLN. However, 20 BC3F2 lines were fixed for both KASP markers but were susceptible to MLN under field conditions, which could suggest weak linkage between the KASP markers and target genes. The validated two major QTL can be utilized to speed up the breeding process but additional loci need to be identified between the KASP markers and the resistance genes to strengthen the linkage.

Occurrence, Distribution, Economic Importance and Management of Maize Chlorotic Mottle Virus: A Review

Plant virus diseases are serious constraints to the production and productivity of a wide range of crops among which maize chlorotic mottle virus (MCMV) is a serious problem in maize production and productivity. The virus is primarily originated in South America (Peru) and later distributed into many countries of all the continents except Australia and Antarctica. It is transmitted from location to location, and from plant to plant mechanically, through seed, insect vectors, and/or soil. The virus has many host ranges of agricultural crops (Maize, Sorghum, Sugarcane and Finger millet) and wild species (Dinebra retroflexa, Setaria verticillata, Cyperus assimilis, Digitaria ternta and Oplismenus hirtellus). MCMV alone causes 10-15% crop loss and up to 100% loss when it co-infects maize plants with other maize viruses such as maize dwarf mosaic virus, Sugar cane mosaic virus or Wheat streak mosaic virus. No any single disease control measure alone is effective in controlling the MCMV. However, an integrated virus disease management options (the use of disease-resistant crop varieties, proper field sanitation, removal of infection sources, use of virus-free seeds and chemical pesticides to indirectly control insect vectors) play a critical role in controlling the virus. Since the current status of the disease is wide spreading and on increasing trend, intensive MCMV employment, combined with integrated disease management, requires ongoing practice in countries where the virus is prevalent and in those countries that have not yet reported MCMV. Keywords : Integrated management; Insect vector; plant residue; Soil transmission; Zea mays DOI: 10.7176/ALST/87-03 Publication date: April 30 th 2021

Survey for viruses affecting maize along the major grain transport route between Gauteng and KwaZulu-Natal in South Africa

The recent introduction of maize chlorotic mottle virus (MCMV) and spread of the resulting maize lethal necrosis disease (MLN) in East Africa presents a threat to maize production in all Southern African countries. As MLND is the result of the synergistic action of MCMV and various cereal-infecting potyviruses, the existing virus status of maize in KwaZulu-Natal, a province of South Africa bordering Mozambique and Eswatini, was determined. Additional viruses identified in MLND affected plants in East Africa previously were also tested for in this study, including maize streak virus (MSV), Morogoro maize-associated virus (MMaV), maize-associated pteridovirus (MaPV), and two maize-associated totivirus (MATV) variants. No MCMV or potyvirus infections were detected. MSV was identified in 29% of the pools (consisting of five plants each), MaPV was detected in three plants, while only a single plant tested positive for MMaV. The presence of MMaV and MaPV was confirmed with next generation sequencing (NGS) and their complete/near complete genome sequences assembled. MMaV was also further confirmed with additional PCR primer sets. This is the first time MMaV and MaPV have been reported in South Africa. Two distinct MATV variants and two Zea mays chrysovirus 1 strains were also detected using NGS but no further tests were conducted, hence, these findings remain preliminary. Future research is required to determine the incidence and distribution of these new viruses in South Africa, their mode(s) of transmission, and effects on maize yield to determine the possible risk they may pose to the South African maize industry.

A novel pathogenicity determinant hijacks maize catalase 1 to enhance viral multiplication and infection.

Pathogens have evolved various strategies to overcome host immunity for successful infection. Maize chlorotic mottle virus (MCMV) can cause lethal necrosis in maize (Zea mays) when it coinfects with a virus in the Potyviridae family. However, the MCMV pathogenicity determinant remains largely unknown. Here we show that the P31 protein of MCMV is important for viral accumulation and essential for symptom development. Ectopic expression of P31 using foxtail mosaic virus or potato virus X induced necrosis in systemically infected maize or Nicotiana benthamiana leaves. Maize catalases (CATs) were shown to interact with P31 in yeast and in planta. P31 accumulation was elevated through its interaction with ZmCAT1. P31 attenuated the expression of salicylic acid (SA)-responsive pathogenesis-related (PR) genes by inhibiting catalase activity during MCMV infection. In addition, silencing of ZmCATs using a brome mosaic virus-based gene silencing vector facilitated MCMV RNA and coat protein accumulation. This study reveals an important role for MCMV P31 in counteracting host defence and inducing systemic chlorosis and necrosis. Our results have implications for understanding the mechanisms in defence and counter-defence during infection of plants by various pathogens.

Alternate hosts and seed transmission of maize lethal necrosis in Ethiopia

AbstractMaize lethal necrosis (MLN) disease caused by double infection of Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV) has become a major maize production constraint in Ethiopia. A field survey was conducted in areas where MLN infection was reported to identify naturally infected alternate hosts. MCMV host range was also studied by artificially inoculating various weed species in the greenhouse to determine potential alternate hosts. Using the serological method of double‐antibody sandwich enzyme‐linked immunosorbent assay (DAS‐ELISA) test, MCMV was detected in 23 (20.35%), and SCMV was detected in 4 (3.53%) of 113 MLN symptomatic samples collected from the field, while combined infection of both viruses was detected in 11 (9.73%) of the samples. Poaceae family had the highest number of grass species that were alternate hosts for MLN causing viruses. Digitaria sanguinalis, Phalaris paradoxa, Oplismenus hirtellus, Echinocloa colona, Cynodon nlemfuensis, Pennisetum purpureum from Poaceae and Cyperus cyperoids from Cyperaceae family were naturally infected by MCMV. Cyperus rotundus, sugarcane (Saccharum officinarum) and sorghum (Sorghum bicolor) were infected by both MCMV and SCMV under natural field conditions. In addition, seed transmission study was conducted using growing‐on tests to determine the potential of seed transmission and its role in the spread of MLN. Overall mean seed to the seedling transmission rate was 0.073% with a range of 0 to 0.17% among 20 different maize varieties studied. Fourteen maize genotypes had some levels of seed transmission (0.03%–0.017%) for MCMV, whereas, SCMV seed transmission was observed only in a single plant of one genotype, with an overall mean of only 0.003%. Seed transmission rates of the viruses were influenced by the seed lot and maize varieties used. The wide ranges of natural and experimental alternate hosts and seed transmissibility of MLN causing viruses suggest that the viruses play a greater role in MLN epidemiology.

English

Maize is an important staple crop for humans and livestock feed in Zambia. A total of 473 maize germplasms were screened under maize lethal necrosis (MLN) artificial inoculation and disease pressure using an alpha lattice design to identify potential tolerant varieties as sources of resistance.  The analysis of variance (ANOVA) based on the MLN score range of 1-9 showed that the levels of MLN resistance in the maize germplasms were very low, with almost all the hybrids widely grown in Zambia and germplasms drawn from the genebank found to be highly susceptible. However, the pro vitamin A varieties, GV662A and IICZ3085 showed a moderate level of resistance/tolerance, with disease scores of 4.7 and 5.3, respectively. There is need to enhance development of MLN tolerant maize cultivars by the national maize breeding programme and seed companies to avert the possible calamity posed by the threat of the effects of MLN. In addition, intensive awareness creation among various stakeholders in the maize value chain, systematic monitoring and surveillance of MLN, practicing integrated disease management approaches such as avoiding maize monoculture and continuous cultivations of susceptible maize throughout the year, and practicing maize crop rotation with compatible crops especially legumes that do not serve as hosts for maize chlorotic mottle virus (MCMV)/MLN are recommended.   Key words: Maize lethal necrosis, screening, maize germplasm, Zambia.