Maize Yield Loss Research Articles

Fine Mapping of RppP25, a Southern Rust Resistance Gene in Maize

Southern rust (Puccinia polysora Underw.) is a major disease that can cause severe yield losses in maize (Zea mays L.). In our previous study, a major gene RppP25 that confers resistance to southern rust was identified in inbred line P25. Here, we report the fine mapping and candidate gene analysis of RppP25 from the near-isogenic line F939, which harbors RppP25 in the genetic background of the susceptible inbred line F349. The inheritance of resistance to southern rust was investigated in the BC1 F1 and BC3 F1 populations, which were derived from a cross between F939 and F349 (as the recurrent parent). The 1:1 segregation ratio of resistance to susceptible plants in these two populations indicated that the resistance is controlled by a single dominant gene. Ten markers, including three simple sequence repeat (SSR) markers and seven insertion/deletion (InDel) markers, were developed in the RppP25 region. RppP25 was delimited to an interval between P091 and M271, with an estimated length of 40 kb based on the physical map of B73. In this region, a candidate gene was identified that was predicted to encode a putative nucleotide-binding site leucine-rich repeat (NBS-LRR) protein. Two co-segregated markers will aid in pyramiding diverse southern rust resistance alleles into elite materials, and thereby improve southern rust resistance worldwide.

Enhanced virus resistance in transgenic maize expressing a dsRNA-specific endoribonuclease gene from E. coli.

Maize rough dwarf disease (MRDD), caused by several Fijiviruses in the family Reoviridae, is a global disease that is responsible for substantial yield losses in maize. Although some maize germplasm have low levels of polygenic resistance to MRDD, highly resistant cultivated varieties are not available for agronomic field production in China. In this work, we have generated transgenic maize lines that constitutively express rnc70, a mutant E. coli dsRNA-specific endoribonuclease gene. Transgenic lines were propagated and screened under field conditions for 12 generations. During three years of evaluations, two transgenic lines and their progeny were challenged with Rice black-streaked dwarf virus (RBSDV), the causal agent of MRDD in China, and these plants exhibited reduced levels of disease severity. In two normal years of MRDD abundance, both lines were more resistant than non-transgenic plants. Even in the most serious MRDD year, six out of seven progeny from one line were resistant, whereas non-transgenic plants were highly susceptible. Molecular approaches in the T12 generation revealed that the rnc70 transgene was integrated and expressed stably in transgenic lines. Under artificial conditions permitting heavy virus inoculation, the T12 progeny of two highly resistant lines had a reduced incidence of MRDD and accumulation of RBSDV in infected plants. In addition, we confirmed that the RNC70 protein could bind directly to RBSDV dsRNA in vitro. Overall, our data show that RNC70-mediated resistance in transgenic maize can provide efficient protection against dsRNA virus infection.

Determination of the critical soil mineral nitrogen concentration for maximizing maize grain yield

A critical soil mineral nitrogen concentration (Nmin) for guiding fertilizer application and maximizing maize grain yield is needed. A three-year field experiment with three N regimes, unfertilized (N0), optimized N management (Opt.) and conventional N practice (Con.) was performed in maize. The mean soil Nmin in 0–60 cm soil profile for N0, Opt. and Con. treatments was 2.0, 6.7 and 8.9 mg kg–1 at V8–VT growth stages and 2.2, 6.1 and 11.2 mg kg–1 on average over the whole growth season, respectively. Correspondingly, the soil N supplying capacity (soil Nmin content + fertilizer N) of the three N treatments was smaller, identical or greater than the plant N accumulation at different growth stages. The Opt. treatment had significantly higher N use efficiency, N recovery efficiency and N partial factor productivity compared with the Con. treatment, while it did not cause maize yield loss. Compared with the insensitivity of the critical shoot N dilution curve to excessive N application, soil Nmin showed strong response to all treatments. We propose a minimum of soil Nmin of 6.1 mg kg–1 at the sowing–V8, 6.7 mg kg–1 at the V8–VT, and 5.5 mg kg–1 at the VT–R6 growing stages with an average of about 6 mg kg–1 of soil Nmin in the 0–60 soil depth for maximizing maize yield and N use efficiency in northern China. To maintain this critical Nmin value over the whole growth period, N topdressing at V8 and V12 stages was recommended.

Vulnerability evaluation and regionalization of drought disaster risk of maize in Northwestern Liaoning Province

本文基于IPCC(Intergovernmental Panel on Climate Change )报告中对气候变化背景下脆弱性的定义, 从暴露程度、敏感性、自身恢复能力和环境适应能力4个方面建立了干旱灾害脆弱性评价模型。以辽西北地区的玉米干旱灾害作为研究对象, 根据干旱灾害脆弱评价模型选取了气象因子、玉米生理因子以及经济社会因子等17项指标, 运用熵权法、加权综合评价法计算得到了辽西北地区玉米干旱脆弱性指数。为了验证和检验模型的适用性, 选取了辽西北1999年、2000年、2001年和2006年4个典型干旱年份, 将玉米干旱脆弱性指数划分为5个等级, 借助GIS技术绘制了玉米干旱脆弱性区划图。结果表明: 辽西北玉米干旱脆弱性强的区域主要集中在西部地区的阜新、朝阳、葫芦岛一带。重度以上脆弱性区域范围比例表现为2006年>1999年>2001年>2000年的规律; 其中, 2006年脆弱性最强, 影响面积最广, 造成的损失也最严重, 与实际灾情变化规律一致。分析2006年的玉米干旱脆弱性, 多数区域是由玉米生长季的降水异常引起的。通过对4个典型干旱年份的玉米干旱脆弱性指数与玉米减产率进行回归分析, 发现二者之间基本吻合, 通过了α=0.05的显著性F检验, 说明利用该模型对玉米干旱脆弱性的评价与区划是合理的, 可以用来评价和预测玉米干旱脆弱性、干旱灾害风险以及因干旱造成的玉米产量损失。研究结果可为当地农业干旱灾害风险评估和预警提供依据。

Dissecting Maize Productivity: Ideotypes Associated with Grain Yield under Drought Stress and Well‐watered Conditions

To increase maize (Zea mays L.) yields in drought-prone environments and offset predicted maize yield losses under future climates, the development of improved breeding pipelines using a multi-disciplinary approach is essential. Elucidating key growth processes will provide opportunities to improve drought breeding progress through the identification of key phenotypic traits, ideotypes, and donors. In this study, we tested a large set of tropical and subtropical maize inbreds and single cross hybrids under reproductive stage drought stress and well-watered conditions. Patterns of biomass production, senescence, and plant water status were measured throughout the crop cycle. Under drought stress, early biomass production prior to anthesis was important for inbred yield, while delayed senescence was important for hybrid yield. Under well-watered conditions, the ability to maintain a high biomass throughout the growing cycle was crucial for inbred yield, while a stay-green pattern was important for hybrid yield. While new quantitative phenotyping tools such as spectral reflectance (Normalized Difference Vegetation Index, NDVI) allowed for the characterization of growth and senescence patterns as well as yield, qualitative measurements of canopy senescence were also found to be associated with grain yield.

Radiation and water balance properties of cadmium-polluted maize in a wet year

The effect of spraying maize weekly with a 0.5 M concentration of Cd was studied in Hungary at two water supply levels. Cd reduced the size of the leaf area and the annual water loss. The intensity of evapotranspiration (mm water/unit leaf area) of the polluted crops grew. The higher net radiation of polluted maize resulted in an increase in sensible heat and a warmer canopy. The warmer the canopy, the higher the stress level of the contaminated crops was. While Cd caused a 28% significant decline in plant dry matter production in the rainfed plots, only a declining tendency amounting to a few percent was recorded in the ET treatment.Cd of atmospheric origin was only detected in the leaves; it was not accumulated in the grain. It was concluded from the results that the yield losses in grain maize grown on Cd-polluted areas could be mitigated by irrigation. The production of silage maize, however, is not recommended in regions affected by Cd pollution, especially in areas where irrigation is required, since supplementary water supplies promote the accumulation of Cd in the vegetative organs.

Maize–grain legume intercropping is an attractive option for ecological intensification that reduces climatic risk for smallholder farmers in central Mozambique

Many farmers in central Mozambique intercrop maize with grain legumes as a means to improve food security and income. The objective of this study was to understand the farming system, and to evaluate the suitability of maize–legume intercropping to alleviate the biophysical and socio-economic constraints faced by smallholder farmers in Ruaca and Vunduzi villages, central Mozambique. To achieve this we characterised the farming systems and measured grain yields, rainfall infiltration, economic returns and acceptability of maize–legume intercrops under different N and P application rates. Two intercropping strategies were tested: (a) an additive design of within-row intercropping in which legume was intercropped with alternating hills of maize within the same row; maize plant population was the same as sole crop maize, and (b) a substitutive design with distinct alternating rows of maize and legume (local practice). Fertiliser treatments imposed on all treatments were: (i) no fertiliser, (ii) 20kgPha−1, (iii) 20kgPha−1+30kgNha−1, and (iv) 20kgPha−1+60kgNha−1. Intercrops were relatively more productive than the corresponding sole crops; land equivalent ratios (LER) for within-row intercropping ranged between 1.1 and 2.4, and between 1.0 and 1.9 for distinct-row intercropping. Average maize yield penalty for intercropping maize and pigeonpea in the within-row was small (8%) compared with 50% in the distinct-row design; average (season×fertiliser) sole maize yield was 3.2tha−1. Intercropping maize and cowpea in within-row led to maize yield loss of only 6%, whereas distinct-row intercropping reduced maize yield by 25% from 2.1tha−1 of sole maize (season×fertiliser). Cowpea yield was less affected by intercropping: sole cowpea had an average yield of 0.9tha−1, distinct-row intercropping (0.8tha−1) and the within-row intercropping yielded 0.9tha−1. Legumes were comparatively less affected by the long dry spells which were prevalent during the study period. Response to N and P fertiliser was weak due to poor rainfall distribution. In the third season, maize in rotation with pigeonpea and without N fertiliser application yielded 5.6tha−1, eight times more than continuous maize which was severely infested by striga (Striga asiatica) and yielded only 0.7tha−1. Rainfall infiltration increased from 6mmh−1 to 22mmh−1 with long-term maize–legume intercropping due to a combination of good quality biomass production which provided mulch combined with no tillage. Intercropping maize and pigeonpea was profitable with a rate of return of at least 343% over sole maize cropping. Farmers preferred the within-row maize–legume intercropping with an acceptability score of 84% because of good yields for both maize and legume. Intercropping increased the labour required for weeding by 36% compared with the sole crops. Farmers in Ruaca faced labour constraints due to extensification thus maize–pigeonpea intercropping may improve productivity and help reduce the area cultivated. In Vunduzi, land limitation was a major problem and intensification through legumes is amongst the few feasible options to increase both production and productivity. The late maturity of pigeonpea means that free-grazing of cattle has to be delayed, which allows farmers to retain crop residues in the fields as mulch if they choose to; this allows the use of no-tillage practises. We conclude that maize–legume intercropping has potential to: (a) reduce the risk of crop failure, (b) improve productivity and income, and (c) increase food security in vulnerable production systems, and is a feasible entry point to ecological intensification.

Mechanisms of Yield Loss in Maize Caused by Weed Competition

The physiological process underlying grain yield (GY) loss in maize as a result of weed competition is not understood clearly. We designed an experiment to test the hypotheses that early season stress caused by the presence of neighboring weeds will increase plant-to-plant variability (PPV) of individual plant dry matter (PDM) within the population. This increase in PPV will reduce GY through a reduction in harvest index (HI). Field experiments were conducted in 2008, 2009, and 2010. A glyphosate-resistant maize hybrid was cropped at a density of 7 plants m−2. As a model weed, winter wheat was seeded at the same time as maize and controlled with glyphosate at the 3rd or 10th to 12th leaf-tip stage of maize. Weed competition early in the development of maize decreased PDM and GY. This reduction in PDM, which occurred early in the development of maize, was attributed initially to a delay in rate of leaf appearance. Reductions in PDM were accompanied by an increase in PPV of PDM. This increase in PPV, however, did not reduce HI and did not contribute to the GY reductions created by weed competition, as hypothesized. As weed control was delayed, a reduction in fraction of photosynthetically active radiation (fIPAR) accounted for a further reduction in PDM and notably, a reduction in DMA from 17th leaf-tip stage through to maturity. The rapid loss of PDM and the subsequent inability to accumulate dry matter during maturation accounted for a rapid decline in kernel number (KN) and kernel weight (KW).

On the nature of temporary yield loss in maize following canola

Background and Aims Soy is currently the most important biodiesel feedstock crop in the U.S., but canola is an attractive alternative because of its potential for greater oil yields. Nevertheless, factors other than oil yield must be considered in the choice of biodiesel feedstock crop. For example, soy is mycorrhizal and canola is not. We examined the consequences of soy and canola crops to subsequently-grown maize, which is mycorrhizal. We hypothesized that canola would reduce mycorrhization, P uptake and yield of subsequently-grown maize when compared to soy, and that winter cover cropping with wheat (mycorrhizal) would ameliorate canola’s negative impacts.

Rice Influence on Grain Yield of Maize in Maize/Rice Mixture in the Northern Guinea Savanna Zone of Ghana

Intercropping maize (Zea mays) and rice (Oryza sativa L.) may ensure efficient and effective use of available resources, but little is known about its effect on the output of the staple crops. A field experiment was conducted to determine the effect of intercropping of maize in maize/rice mixture in northern Ghana in 2007 and 2008 using randomized complete block design with five treatments and four replications. Treatments were sole rice drilled (SRD), sole rice broadcast (SRB), sole maize (SM), rice drilled in maize/rice intercrop (MRD), and rice broadcast in maize/rice intercrop (MRB). Fertilizer was applied at planting at the rate of 60-30-30 kg/ha as N, P2O5, and K2O. Additional 30 kg N/ha as ammonium sulfate was applied five weeks after planting. Grain yields from both monocrops were significantly higher than those from the intercrops. The trend was similar for straw weight and 100 seed weight. In 2007, maize yield losses caused by the inclusion of rice in the systems were 21% for MRD and 46% for MRB. In 2008, the reduction in maize yield was 27% for MRD and 43% MRB, suggesting that maize yield reduction in maize/rice mixture was higher when rice was broadcast than when it was drilled. The land equivalent ratio was more than one in both years, indicating yield advantage from intercropping maize and rice over growing the two crops in pure stands. We recommend that to maintain high maize yields in maize/rice intercropping system, it is better to drill than to broadcast the rice.

Identification of a major quantitative trait locus for resistance to maize rough dwarf virus in a Chinese maize inbred line X178 using a linkage map based on 514 gene-derived single nucleotide polymorphisms

Maize rough dwarf disease (MRDD) is a worldwide viral disease and causes significant yield losses in maize (Zea mays L.) production. In this study, we mapped and characterized quantitative trait loci (QTL) conferring resistance to MRDD using 89 F8 recombinant inbred lines derived from a cross between X178 (resistant parent) and B73 (susceptible). The population was evaluated for MRDD resistance in Baoding, Hebei Province, China (a hot spot of MRDD incidence) under natural infection in 2008 and 2009 and artificial inoculation in 2010. Genotypic variances for disease severity index (DSI) were highly significant in the population. Heritability estimates for DSI evaluation were 0.472 and 0.467 in 2008 and 2009, respectively. The linkage map was constructed using 514 gene-derived single nucleotide polymorphisms (SNPs) and 72 simple sequence repeat markers, spanning a genetic distance of 1,059.72 cM with an average interval of 1.8 cM between adjacent markers. Multiple-QTL model mapping detected a major QTL for MRDD resistance on chromosome 8, explaining 24.6–37.3% of the phenotypic variation across three environments. In 2010, an additional QTL was detected on chromosome 10, explaining 15.8% of the phenotypic variation. The major QTL on chromosome 8 and the SNP markers (SNP31, SNP548, and SNP284) co-located with the QTL peak have potential for further functional genomic analysis and use in molecular marker-assisted selection for MRDD resistance in maize.

Genetic analysis of resistance to nematodes in inbred maize (Zea mays L.) and maize hybrids

Nematodes cause substantial grain yield loss in susceptible maize (Zea mays L.) cultivars. This study was conducted to estimate general combining ability (GCA), specific combining ability (SCA) and genetic effects associated with nematode resistance in maize. The 30 F1 hybrids generated from a 6 × 6 diallel and two local checks were evaluated in 2009 at three sites in Uganda. A split plot design was used with nematode treatments serving as whole plots and the hybrids as subplots but arranged in an 8 × 4 spatially adjusted alpha-lattice design. The experiment was replicated three times. Results showed GCA to be important for the reduction of P. zeae and Meloidogyne spp. densities and increase of root mass, with a contribution of 72 to 93% of the phenotypic variance. Inbreds MP709 and CML206 had the highest GCA for Pratylenchus zeae resistance, whereas for grain yield, it was CML444, CML312 and CML395 that were outstanding. The SCA influenced plant height and grain yield under nematode infestation, contributing 43 and 58% of the phenotypic variance, respectively. Observed reciprocal differences due to maternal effects also played a role in influencing the grain yield under nematode infestation. Overdominance genetic effects explained the non-additive variance recorded for the plant height, grain yield, number of root lesions, and P. zeae and Meloidogyne spp. densities under nematode infestation. The parents MP709, CML206, 5057, and CML444 contributed most of the dominant genes for the P. zeae resistance in all their crosses. The parent CML444 contributed most of the dominant genes for improved grain yield in all of its crosses. The high GCA effects among some parents support their utility in breeding of widely adapted nematode-resistant cultivars. The dominant genes and SCA effects would favour pedigree and various sib tests to improve grain yield under nematode pressure.

Genetic analysis and genotype × environment (G × E) for grey leaf spot disease resistance in elite African maize (Zea mays L.) germplasm

Maize grey leaf spot (GLS) disease remains an important foliar disease in sub-Saharan Africa accounting for more than 25% yield losses in maize. Information on inheritance of GLS resistance of germplasm adapted to African environments is required in new sources being identified. Therefore, hybrids generated from a 10 × 10 half-diallel mating of tropical advanced maize inbred lines were evaluated in six environments to determine combining ability, genotype × environment interaction (G × E) and the impact of GLS disease on grain yield. General combining ability effects were highly significant and accounted for 72 and 68% of the variation for GLS resistance and grain yield, respectively. Significant specific combining ability effects associated with reduced disease levels were observed in some hybrids when one parent was resistant, and these may be exploited in developing single cross maize hybrids. Regression analysis showed a 260–320 kg ha−1 decrease in maize grain yield per each increase in GLS disease severity score, and significant associations (r = −0.31 to −0.60) were observed between grain yield and GLS severity scores. This showed the potential of GLS disease to reduce yield in susceptible varieties grown under favourable disease conditions, without control measures. Genotype and genotype × environment biplots and correlation analysis indicated that the significant G × E observed was not due to changes in hybrid ranking, implying absence of a significant crossover interaction. Therefore, predominance of additive gene effects imply that breeding progress for GLS disease resistance would be made through selection and this could be achieved at a few hot-spot sites, such as Baynesfield and Cedara locations in South Africa, and still deploy the resistant germplasm to other environments in which they are adapted.

Evaluation of maize core collection for drought tolerance

The maize genebank (GBmaize) preserves nearly 4,000 accessions for conservation and use. The use is however restricted because the accessions do not perform as well as the elite genotypes. This problem can be reduced by pre-breeding, i.e., by extending the information on germplasm and introgressing useful alleles. Since irregular rainfall distribution and drought induce maize yield losses, drought tolerance is a main breeding target. In this study, the GBmaize accessions were evaluated for drought tolerance. Environmental factors, genotypes and the respective interactions influence the phenotypic expression. There was however no interaction genotype - irrigation level, so no accessions with different performance under the two water regimes could be identified. The performance of the following accessions was promising for a number of traits: SP154, BA166, MG099, CE002, SE025, BA154, BA194, BA085, MG076, PR053, Roxo Macapá, SE016, and AL018.

Impacts of climate change as a function of global mean temperature: maize productivity and water use in China

Projections of future climate change are plagued with uncertainties from global climate models and emission scenarios, causing difficulties for impact assessments and for planners taking decisions on adaptation measure. Here, we developed an approach to deal with the uncertainties and to project the changes of maize productivity and water use in China using a process-based crop model, against a global mean temperature (GMT) increase scale relative to 1961-1990 values. From 20 climate scenarios output from the Intergovernmental Panel on Climate Change Data Distribution Centre, we adopted the median values of projected changes in monthly mean climate variables for representative stations and driven the CERES-Maize model to simulate maize production under baseline and future climate scenarios. Adaptation options such as automatic planting, automatic application of irrigation and fertilization were considered, although cultivars were assumed constant over the baseline and future. After assessing representative stations across China, we projected changes in maize yield, growing period, evapotranspiration, and irrigation-water use for GMT changes of 1A degrees C, 2A degrees C, and 3A degrees C, respectively. Results indicated that median values of projected decreases in the yields of irrigated maize without (with) consideration of CO(2)-fertilization effects ranged from 1.4% to 10.9% (1.6% to 7.8%), 9.8% to 21.7% (10.2% to 16.4%), and 4.3% to 32.1% (3.9% to 26.6%) for GMT changes of 1A degrees C, 2A degrees C, and 3A degrees C, respectively. Median values of projected changes in irrigation-water use without (with) consideration of CO(2)-fertilization effects ranged from -1.3% to 2.5% (-18.8% to 0.0%), -43.6% to 2.4% (-56.1% to -18.9%), and -19.6% to 2.2% (-50.6% to -34.3%), which were ascribed to rising CO(2) concentration, increased precipitation, as well as reduced growing period with GMT increasing. For rainfed maize, median values of projected changes in yields without (with) consideration of CO(2)-fertilization effects ranged from -22.2% to -1.0% (-10.8% to 0.7%), -27.6% to -7.9% (-18.1% to -5.6%), and -33.7% to -4.6% (-25.9% to -1.6%). Approximate comparisons showed that projected maize yield losses were larger than previous estimates, particularly for rainfed maize. Our study presents an approach to project maize productivity and water use with GMT increases using process-based crop models and multiple climate scenarios. The resultant impact function is fundamental for identifying which climate change level is dangerous for food security.

Potential of seeds of Psoralea corylifolia L. for the management of phytopathogenic spp.

Aqueous and solvent extracts of seeds of P. corylifolia were evaluated for antifungal activity by poisoned food technique against eight important phytopathogenic species of Fusarium commonly associated with maize seeds. Antifungal activity was observed in both aqueous and solvent extracts. Petroleum ether extract showed highly significant activity against all the Fusarium species. F. graminearum was highly susceptible, while F. lateritium was least susceptible. The antifungal activity increased with increasing concentration of the extract. The minimal inhibitory concentration (MIC) value of the aqueous extract for F. graminearum was 15% and for F. equiseti, F. moniliforme, F. semitectum and F. solani it was 40%. Total inhibition was not observed in the case of F. lareritium, F. oxysporum and F. proliferatum. The results of the study are of immense value in the management of seed borne phytopathogenic species of Fusarium known to cause significant yield loss in maize.

Interference between maize and Xanthium strumarium or Datura stramonium

Karimmojeni H, Rahimian Mashhadi H, Alizadeh HM, Cousens RD & Beheshtian Mesgaran M (2010). Interference between maize and Xanthium strumarium or Datura stramonium. Weed Research50, 253–261.Summary Xanthium strumarium and Datura stramonium are two of the most important weeds of maize in Iran. Their growth was compared in a field study conducted over 2 years. Relationships were derived between weed density and crop yield, yield components and various growth parameters. Yield, grain number ear−1 and grain weight were affected more by X. strumarium than by D. stramonium. Xanthium strumarium followed a pattern of growth resembling that of the maize, whereas D. stramonium continued growing for a longer period, mainly affecting maize during the grain‐filling period. A large portion of the loss in maize yield caused by X. strumarium could be explained by a reduction in grain number ear−1, whereas a greater portion of the maize yield loss from D. stramonium was explained by a reduction in grain weight. Weed biomass was greater for X. strumarium than for D. stramonium, whereas seed production was higher for D. stramonium. It is concluded that although X. strumarium should be given the highest priority for control in maize on the basis of its superior competitive ability, control of D. stramonium is also essential because of both its considerable competitiveness in the current crop and its long‐term effects resulting from its greater seed production.

Relationship of Genetic Diversity of Inbred Lines with Different Reactions to Striga hermonthica (Del.) Benth and the Performance of Their Crosses

Striga hermonthica causes significant yield loss in maize (Zea mays L.) and other cereals. Limited studies have been conducted to determine the relationship between marker‐based genetic distance among maize inbred lines expressing varying levels of field resistance to S. hermonthica and the reaction patterns of the resulting crosses to S. hermonthica. Forty‐five diallel crosses of 10 parental lines were evaluated with and without S. hermonthica infestation at two locations each in Benin Republic and Nigeria for 3 yr. Canonical discriminant analysis using nine traits separated the crosses into three distinct groups in each country depending on doses of resistance obtained from their parental lines. Hybrids formed from two resistant parents had the highest levels of resistance, while those formed from two susceptible parents exhibited the lowest levels of resistance to S. hermonthica Crosses that involved one resistant line as a parent showed intermediate levels of field resistance to S. hermonthica Assessment of genetic divergence among the 10 parental lines using 18 amplified fragment length polymorphism (AFLP) primer combinations generated a total of 1176 polymorphic AFLP fragments. The genetic distance (GD) estimates among all pairs of parental lines varied from 0.44 to 0.93, with an average of 0.63 ±0.023. The correlation between the AFLP‐based GD estimates of parental lines and the means observed in diallel crosses under S. hermonthica infestation were not significant for grain yield and other traits. Some hybrids formed from inbred lines with GD estimates both below and above 0.50 exhibited good performance under S. hermonthica infestation. The observed broad range of genetic divergence detected with AFLP markers indicates the presence of a significant reservoir of diversity among resistant lines that can be exploited in breeding.

Maize streak virus: an old and complex ‘emerging’ pathogen

Maize streak virus (MSV; Genus Mastrevirus, Family Geminiviridae) occurs throughout Africa, where it causes what is probably the most serious viral crop disease on the continent. It is obligately transmitted by as many as six leafhopper species in the Genus Cicadulina, but mainly by C. mbila Naudé and C. storeyi. In addition to maize, it can infect over 80 other species in the Family Poaceae. Whereas 11 strains of MSV are currently known, only the MSV-A strain is known to cause economically significant streak disease in maize. Severe maize streak disease (MSD) manifests as pronounced, continuous parallel chlorotic streaks on leaves, with severe stunting of the affected plant and, usuallly, a failure to produce complete cobs or seed. Natural resistance to MSV in maize, and/or maize infections caused by non-maize-adapted MSV strains, can result in narrow, interrupted streaks and no obvious yield losses. MSV epidemiology is primarily governed by environmental influences on its vector species, resulting in erratic epidemics every 3-10 years. Even in epidemic years, disease incidences can vary from a few infected plants per field, with little associated yield loss, to 100% infection rates and complete yield loss. The only virus species known to cause MSD is MSV, the type member of the Genus Mastrevirus in the Family Geminiviridae. In addition to the MSV-A strain, which causes the most severe form of streak disease in maize, 10 other MSV strains (MSV-B to MSV-K) are known to infect barley, wheat, oats, rye, sugarcane, millet and many wild, mostly annual, grass species. Seven other mastrevirus species, many with host and geographical ranges partially overlapping those of MSV, appear to infect primarily perennial grasses. MSV and all related grass mastreviruses have single-component, circular, single-stranded DNA genomes of approximately 2700 bases, encapsidated in 22 x 38-nm geminate particles comprising two incomplete T = 1 icosahedra, with 22 pentameric capsomers composed of a single 32-kDa capsid protein. Particles are generally stable in buffers of pH 4-8. In infected maize plants, streak disease initially manifests as minute, pale, circular spots on the lowest exposed portion of the youngest leaves. The only leaves that develop symptoms are those formed after infection, with older leaves remaining healthy. As the disease progresses, newer leaves emerge containing streaks up to several millimetres in length along the leaf veins, with primary veins being less affected than secondary or tertiary veins. The streaks are often fused laterally, appearing as narrow, broken, chlorotic stripes, which may extend over the entire length of severely affected leaves. Lesion colour generally varies from white to yellow, with some virus strains causing red pigmentation on maize leaves and abnormal shoot and flower bunching in grasses. Reduced photosynthesis and increased respiration usually lead to a reduction in leaf length and plant height; thus, maize plants infected at an early stage become severely stunted, producing undersized, misshapen cobs or giving no yield at all. Yield loss in susceptible maize is directly related to the time of infection: infected seedlings produce no yield or are killed, whereas plants infected at later times are proportionately less affected. Disease avoidance can be practised by only planting maize during the early season when viral inoculum loads are lowest. Leafhopper vectors can also be controlled with insecticides such as carbofuran. However, the development and use of streak-resistant cultivars is probably the most effective and economically viable means of preventing streak epidemics. Naturally occurring tolerance to MSV (meaning that, although plants become systemically infected, they do not suffer serious yield losses) has been found, which has primarily been attributed to a single gene, msv-1. However, other MSV resistance genes also exist and improved resistance has been achieved by concentrating these within individual maize genotypes. Whereas true MSV immunity (meaning that plants cannot be symptomatically infected by the virus) has been achieved in lines that include multiple small-effect resistance genes together with msv-1, it has proven difficult to transfer this immunity into commercial maize genotypes. An alternative resistance strategy using genetic engineering is currently being investigated in South Africa. http://www.mcb.uct.ac.za/MSV/mastrevirus.htm; http://www.danforthcenter.org/iltab/geminiviridae/geminiaccess/mastrevirus/Mastrevirus.htm.

Estimating irrigation use and effects on maize yield during the 2003 heatwave in France

The decline in maize yield and production during the 2003 heatwave and associated drought in France was only partly minimized by irrigation. National 2003 maize yield loss equalled ∼1.5 t ha −1 compared to the 2000–2006 average. Spatially distributed maize irrigated area percentages were calculated earlier ( Wriedt et al., 2009a) and correlate negatively to the 2003 yield anomaly between 44.5° and 48° latitude. The percentages are used to weigh irrigated and rainfed simulations with the EPIC crop growth model that runs on a 10 by 10 km grid with relevant land use, terrain, soil and management information. Maize was not irrigated in one simulation while other simulations allowed for daily, weekly and biweekly irrigation with a maximum application of 60 mm day −1. The model reasonably reproduces regionally reported yields from 1999 to 2003. In regions with maize area irrigation percentages >20% yield loss in 2003 was reduced by ∼53% relative to regions with maize irrigation percentages <20%. Similarly, simulated yield loss was compensated by irrigation by ∼25% with biweekly and by ∼42% with weekly irrigation in these regions. Even though yield loss was lower in regions with higher maize irrigation percentages; yield loss was still very considerable. Modelling suggests that regional drought mitigation increased with increasing maize irrigation percentages between 0 and 40%. At higher irrigation percentages the compensating effect of irrigation was small. Although the current irrigation infrastructure is sufficient under normal meteorological conditions, areas without irrigation infrastructure experienced high irrigation requirements during the extreme conditions in 2003. Since increasing the irrigation frequency from two weeks to one week had a significant impact on maize yield in 2003, but not in 2002, the most appropriate difference in irrigation rate is provided by the difference between the biweekly rate in 2002 (484 mm year −1) and the weekly rate in 2003 (743 mm year −1) which equals 259 mm year −1. This corresponds to an increase in irrigation water use of ∼1761 million m 3 compared to 2002 (∼0.68 million ha of irrigated maize). Adapting to increased frequency of droughts under further climate change will require robust water allocation policies.