Breeding for Disease Resistance in Maize

Maize is a staple food in Zambia and contributes immensely to food security for smallholder farmers. Disease outbreaks such as Maize Lethal Necrosis Disease (MLND) can be a key constraint to maize production. This disease is caused by synergistic co-infection with Maize Chlorotic Mottle Virus (MCMV) and any virus from the family Potyviridae, particularly, Sugarcane Mosaic Virus (SCMV), Maize Dwarf Mosaic Virus (MDMV) or Wheat Streak Mosaic Virus (WSMV). In 2011, an outbreak of MLND affecting almost all of the currently grown commercial varieties posed a challenge to maize production in Kenya and it has since been reported in DR Congo, Ethiopia, Kenya, Rwanda, Tanzania, and Uganda causing yield losses of up to 100%. Despite MLND having been reported in some neighboring countries, there is no information on the status of the disease in Zambia. Additionally, there is a lot of grain and seed trade between Zambia and other countries among which MLND has been reported. The aim of this study was to establish: (a) the status of MCMV; (b) agricultural practices used by farmers and (c) insect vectors associated with MLND. A survey was conducted in nine (9) provinces of Zambia during 2014/2015 and 2015/ 2016 cropping seasons. Farmers’ maize fields were sampled at every five to ten-kilometer interval and tested using rapid diagnostic kits capable of detecting MCMV. Four hundred and nineteen samples collected all tested negative for MCMV. Zambian Agricultural Research Institute (ZARI), with all stakeholders in the maize value chain should continue implementing measures aimed at preventing the introduction of MLND in Zambia. Key words: Survey, MLND, Losses, food security

The distribution, incidence and severity of maize lethal necrosis (MLN) disease in major maize growing agro-ecological zones (AEZ) of Uganda was determined following field surveys carried out in 16 major maize growing districts from 5 AEZ over three consecutive seasons. A total of 604 maize fields were visited and MLN disease status visually assessed and 3,624 maize leaf samples collected for identification and confirmation of the MLN causal viruses by Double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and Reverse Transcription Polymerase Chain Reaction (RT-PCR). MLN disease was not widely distributed at an epidemic proportion, with only 36 (5%) of the 604 farms surveyed over three seasons confirmed to have the disease. The MLN incidence and severity was significantly (P < 0.05) higher in the Eastern AEZ during the three seasons. The main MLN-causing viruses detected using DAS-ELISA were Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV). MCMV was the most prevalent MLN causing virus driving the epidemic in Uganda. The three major districts where MLN disease has been found in all three seasons surveyed are Bulambuli, Tororo and Busia which are hotspots for MLN disease. Strategies to control spread of MLN disease should focus on high risk AEZs and hotspot districts.

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

Maize streak virus (MSV) is responsible for streak disease of maize and poses a serious threat to maize production in sub-Saharan Africa. Polygenic resistance to MSV has become an essential requirement in modern maize cultivars to mitigate yield losses. Many single nucleotide polymorphism (SNP) markers linked to putative MSV resistance loci have been identified for use in forward breeding. This study aimed to validate, using the high-throughput kompetitive allele specific PCR (KASP) assay, the diagnostic ability of the three SNP markers linked to the loci for the Msv1 resistance trait in 151 early generations inbred lines with diverse genetic backgrounds, together with nine MSV-resistant elite lines and a susceptible check (cv. Pool-16). The phenotypic responses were determined by MSV inoculation using viruliferous leafhoppers (Cicadulina triangular) under screenhouse conditions. Based on an established MSV disease rating system, the maize lines were categorized into resistant, moderately resistant, susceptible, and highly susceptible. The three SNPs associated with MSV resistance were detected in 133 lines, which were categorized as resistant (54), moderately resistant (76), and susceptible (1). The 18 early generation lines without these SNPs were classified as moderately resistant (10), susceptible (5), and highly susceptible (3). This study confirms the strong association of SNPs with MSV resistance and their usefulness for forward breeding in maize while emphasizing the need to identify additional markers to screen lines for MSV resistance without any ambiguity.

Maize streak virus (MSV), which causes maize streak disease (MSD), is the major viral pathogenic constraint on maize production in Africa. Type member of the Mastrevirus genus in the family Geminiviridae, MSV has a 2.7 kb, single-stranded circular DNA genome encoding a coat protein, movement protein, and the two replication-associated proteins Rep and RepA. While we have previously developed MSV-resistant transgenic maize lines constitutively expressing “dominant negative mutant” versions of the MSV Rep, the only transgenes we could use were those that caused no developmental defects during the regeneration of plants in tissue culture. A better transgene expression system would be an inducible one, where resistance-conferring transgenes are expressed only in MSV-infected cells. However, most known inducible transgene expression systems are hampered by background or “leaky” expression in the absence of the inducer. Here we describe an adaptation of the recently developed INPACT system to express MSV-derived resistance genes in cell culture. Split gene cassette constructs (SGCs) were developed containing three different transgenes in combination with three different promoter sequences. In each SGC, the transgene was split such that it would be translatable only in the presence of an infecting MSV’s replication associated protein. We used a quantitative real-time PCR assay to show that one of these SGCs (pSPLITrepIII-Rb-Ubi) inducibly inhibits MSV replication as efficiently as does a constitutively expressed transgene that has previously proven effective in protecting transgenic maize from MSV. In addition, in our cell-culture based assay pSPLITrep III-Rb-Ubi inhibited replication of diverse MSV strains, and even, albeit to a lesser extent, of a different mastrevirus species. The application of this new technology to MSV resistance in maize could allow a better, more acceptable product.

Maize lethal necrosis (MLN) disease emerged as a threat to maize, which is an important food security crop in the majority of Sub-Saharan Africa countries. The study was carried out to determine the transmission of Maize chlorotic mottle virus and Sugarcane mosaic virus through seeds harvested from maize varieties; H614 and WE1101 with varying MLN disease severity and evaluate the effect on germination, emergence, disease intensity, virus titre and yields. Field and greenhouse evaluations were conducted over two cropping cycles during the 2016 short rain and 2017 long rain seasons. Maize chlorotic mottle virus was detected in plants from seeds of symptomatic mother plants. Seeds from MLN-infected plants resulted in a significant effect on the plant growth, disease intensity, virus titre and yields. The study underscores the importance of seed testing for viruses causing MLN disease as an integral part of maize seed certification for effective management of the disease.

Maize lethal necrosis (MLN) has emerged as a serious threat to food security in sub-Saharan Africa. MLN is caused by coinfection with two viruses, Maize chlorotic mottle virus and a potyvirus, often Sugarcane mosaic virus. To better understand the dynamics of MLN and to provide insight into disease management, we modeled the spread of the viruses causing MLN within and between growing seasons. The model allows for transmission via vectors, soil, and seed, as well as exogenous sources of infection. Following model parameterization, we predict how management affects disease prevalence and crop performance over multiple seasons. Resource-rich farmers with large holdings can achieve good control by combining clean seed and insect control. However, crop rotation is often required to effect full control. Resource-poor farmers with smaller holdings must rely on rotation and roguing, and achieve more limited control. For both types of farmer, unless management is synchronized over large areas, exogenous sources of infection can thwart control. As well as providing practical guidance, our modeling framework is potentially informative for other cropping systems in which coinfection has devastating effects. Our work also emphasizes how mathematical modeling can inform management of an emerging disease even when epidemiological information remains scanty. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Multiple disease resistance (MDR) in maize has attracted increasing attention. However, the interplay between cell death and metabolite changes and their contributions to MDR remains elusive in maize. In this study, we identified a mutant named as lesion mimic 30 (les30) that showed 'suicidal' lesion formation in the absence of disease and had enhanced resistance to the fungal pathogen Curvularia lunata. Using map-based cloning, we identified the causal gene encoding pheophorbide a oxidase (PAO), which is known to be involved in chlorophyll degradation and MDR, and is encoded by LETHAL LEAF SPOT1 (LLS1). LLS1 was found to be induced by both biotic and abiotic stresses. Transcriptomics analysis showed that genes involved in defense responses and secondary metabolite biosynthesis were mildly activated in leaves of the les30 mutant without lesions, whilst they were strongly activated in leaves with lesions. In addition, in les30 leaves with lesions, there was overaccumulation of defense-associated phytohormones including jasmonic acid and salicylic acid, and of phytoalexins including phenylpropanoids, lignin, and flavonoids, suggesting that their biosynthesis was activated in a lesion-dependent manner. Taken together, our study implies the existence of an interactive amplification loop of interrupted chlorophyll degradation, cell death, expression of defense-related genes, and metabolite changes that results in suicidal lesion formation and MDR, and this has the potential to be exploited by genetic manipulation to improve maize disease resistance.

Sugarcane mosaic disease caused by sugarcane mosaic virus (SCMV), Johnsongrass mosaic virus (JGMV), maize dwarf mosaic virus (MDMV) and sorghum mosaic Virus (SrMV) is an economically important viral disease of sugarcane worldwide. Field survey was conducted to assess the presence of the viruses involve in mosaic disease of sugarcane in Makarfi Local Government Areas of Kaduna State (Northern Guinea Savannah), Nigeria. A range of symptoms were observed on the  infected land races from the pale green stripes to yellow chlorotic stripes on a dark green background. The purple land race (“Bakarkwama”) was highly susceptible followed by green land race (“Bahausa”) and the least infected was the white land race (“fararkwama”). 63 symptomatic and asymptomatic sugarcane leaves and stem juice extract from 14 villages of Makarfi L.G.A. were screened for the four viruses using DAS and TAS ELISA methods. SCMV, MDMV and SrMV were detected from both symptomatic and asymptomatic sugarcane leaf samples whereas JGMV was not detected in the locations sampled. SCMV isolate has the highest incidence (83%) from all the locations followed by SrMV (10%) and MDMV (5%) isolates. Mixed infections of the three viruses were also detected in some samples. This is the first report of identification of virus isolates inducing sugarcane disease in one of the major sugarcane producing areas of the Northern Guinea Savannah of Nigeria.   Key words: Sugarcane, SCMV, MDMV, SrMV, TAS-ELISA, DAS-ELISA.

Maize dwarf mosaic (MDM) is one of the most important virus diseases of maize worldwide. Caused by the potyviruses maize dwarf mosaic virus (MDMV) or sugarcane mosaic virus (SCMV), MDM can cause up to 90% yield loss in susceptible hybrids. One of the most effective management strategies for MDM is growing potyvirus-resistant corn varieties. However, yield impacts associated with MDM and the corresponding efficacy of genetic resistance present in modern United States commercial hybrid lines is uncharacterized. In this study, we evaluated the disease response of 78 commercial hybrids to MDMV and SCMV and quantified yield losses associated with infection over multiple trials. We determined that while 97% of the hybrids tested were resistant to MDMV, 100% were susceptible to SCMV, with mean disease incidence per line averaging between 45 and 78% across six trial years. Despite only one hybrid displaying visible mosaic symptoms when inoculated with MDMV, MDMV reduced average yields by approximately 5% across all hybrids compared with the mock-inoculated treatment. The yield impact of SCMV was more severe, reducing average yields by 10% across replicated experiments. These results indicate that while most commercial hybrids are resistant to MDMV, possibly due to the presence of the major Scmv1 resistance locus on chromosome 6, additional potyvirus resistance genes are needed to manage SCMV-induced MDM. Pyramiding resistance loci, such as Scmv2 on chromosome 3 or Scmv3 on chromosome 10 in addition to Scmv1, could be an effective strategy to mitigate the yield impact of MDM disease.

Maize Lethal Necrosis (MLN) and Maize Streak Virus (MSV) are among maize diseases which affect productivity in Tanzania and Africa at large. These diseases can be detected early for timely interventions and minimal losses. Machine learning (ML) has emerged as a powerful tool for automated diseases detection, offering several advantages over traditional methods. This article presents the updated dataset of 9356 imagery maize leaves to assist researchers in developing technological solutions for addressing crop diseases. The high-resolution imagery data presented in this dataset were captured using smartphone cameras in farm fields which were not selected in the previously published dataset. Also, data collection was taken in the range of three months from November 2022 to January 2023 to incorporate farming season not covered in the previously published dataset. The presented dataset can be used by researchers in the field of Artificial Intelligence (AI) to develop ML solutions and eliminate the need of manual inspection and reduce human bias. Developing ML solutions require large amount of data therefore, the updated and previously published datasets can be combined to accommodate diverse and wider applicability.

Maize Lethal Necrosis Disease (MLND) develops from synergistic co-infection by Sugarcane mosaic virus and Maize chlorotic mottle virus. The disease causes symptoms ranging from leaf tissue mottling and malformed ears to premature plant death. Information on the diversity and transmission of those viruses are limited in Ethiopia. No attempts were made to characterize and identify SCMV and MCMV strains of Oromia and Benishangul-Gumuz regions. The aim of this research was to study MLND responsible viruses via serological and molecular techniques. A total of 174 symptomatic maize leaf samples were collected. Using Double Antibody Sandwich-ELISA (DAS-ELISA), 43 samples showed strong reaction for MCMV antibody and only 14 samples were clearly positive for SCMV. Reverse transcriptase polymerase chain reaction (RT-PCR) technique revealed the presence of MCMV and SCMV in most samples. Analysis of the genetic diversity of MCMV and SCMV by sequencing the coat protein of all isolates shows strong homology with previous East African isolates deposited in the GenBank. The phylogenetic analysis indicated that the virus isolates are highly similar with each other. In conclusion, this study unequivocally confirmed the association of MCMV and SCMV with MLND in the investigated areas and for the first time provided information on the genetic variability by sequence analysis of several Ethiopian samples. Since the risk posed by MLND in the investigated area is high, there is an urgent need for developing integrated management options for controlling the disease.

‘Defying’ democratization and environmental protection in Kenya: The case of Karura Forest reserve in Nairobi

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.

Maize is a major staple food in Sub-Saharan Africa (SSA). Vitamin A deficiency index is high in Africa and could be reduced through the consumption of provitamin-A-enriched maize. However, foliar diseases such as maize streak virus, northern corn leaf blight and common rust constrain maize production in SSA. The cultivation of host-resistant varieties is the most effective approach to mitigate their effects. Therefore, maize synthetics improved for PVA carotenoids, their selection cycles and crosses as well as a commercial disease-resistant check were assessed for resistance to maize streak virus, northern corn leaf blight and common rust at hotspots in Nigeria. The foliar diseases’ effects on the agronomic performance and carotenoid content of the maize genotypes were assessed. The Genotypes differed for most agronomic traits and foliar disease resistance. Stepwise regression revealed that, although the agronomic traits determined 93% of the grain yield, each foliar disease had effect on the yield. A unit increase in maize streak virus score increased plant aspect and husk cover scores by 0.6 and 0.4, respectively, whereas an increase in common rust score decreased plant height by 16.2 cm and increased plant aspect score by 0.7. Maize streak virus and common rust decreased genotypic variability for lutein by 36.7 and 18.7%, respectively, while northern corn leaf blight decreased genotypic variability for provitamin A by 27.1%. Most of the genotypes exhibited moderate susceptibility to northern corn leaf blight. However, three selection cycles and three crosses exhibited high tolerance to maize streak virus and moderate tolerance to common rust, thus can serve as sources of PVA-enriched, maize streak virus and common rust tolerant lines.