Incidence and geographical distribution of downy mildew on maize caused by Peronosclerospora sorghi in Nigeria

Abstract

Incidence and geographical distribution of downy mildew (Peronosclerospora sorghi) on maize (Zea mays) were determined in a systematic survey of 181 maize fields in the major maize‐growing regions and ecological zones of Nigeria. Downy mildew was observed in the forest and Guinea savanna ecologies, but no disease was detected in other ecologies. Highest levels of disease incidence (63%) were observed in the forest zone, and progressively less disease incidence was found at the southern and northern Guinea savanna zones. Spatial pattern analysis indicated an aggregation and randomness of infected plants in the northern Guinea savanna and the forest zone, respectively. Disease incidence was positively correlated with the presence of downy mildew infected sorghum plants and relative amount of sporulation on infected sorghum (Sorghum bicolor) but negatively correlated with plant age and cropping pattern. Relative sporulation on maize plants was negatively correlated with age of maize plants. Higher levels of disease incidence were observed when the previous crop was either maize or sorghum than when previous crops were cassava, okra, tomatoes, or rice.     

Quantification of within‐season focal spread of wheat take‐all in relation to pathogen genotype and host spatial distribution

Within season gradient of wheat take‐all was measured in field experiments according to line or random sowing host spatial distribution, and two Gaeumannomyces graminis var. tritici (Ggt) isolates (G1i and G2i), representative of the G1 and G2 genotype groups in terms of aggressiveness. Root disease incidence and severity were assessed at six dates from early March to late June on plants located at regular distances from the inoculum sources. Simple models relating disease intensity at different levels of hierarchy fitted observed data well, and indicated a strong disease aggregation both within and among plants. Disease severity on source plants placed nearby the inoculum source increased over time, ranging from 5 to 46% at the first assessment, and from 55 to 98% at the last assessment, being in general larger for G2i than G1i. In line‐sown plots, disease progressed steadily along the line but did not extend beyond 20 cm, seldom reaching the neighbour line. Disease rarely reached 25 cm in the direct‐seeded crop stands. These results indicate that Ggt intensifies but does not spread to a large extent during a cropping season. Distance from the source, pathogen genotype and assessment date had a significant effect on disease severity according to mixed model analyses, disease spread being larger for G2i than G1i. However, no significant effect of host spatial distribution could be detected. Yield loss within 20 cm of the source plant ranged between 20 and 40%, and was not significantly affected by pathogen genotype or host spatial distribution.     

Weed interference in maize, cowpea and maize/cowpea intercrop in a subhumid tropical environment. II. Early growth and nutrient content of crops and weeds

Early growth and nutrient content of crops and weeds from weed-free and weedy no-tillage maize (Zea mays L, cv. TZB), cowpea (Vigna unguiculata (L) Walp. cv. VITA-5) and maize/cowpea intercrop at populations of 40000, 50000 and 30000 + 40000 plants ha^?1 grown on a loamysand Oxic Ustropept in a subhumid tropical location were monitored in the early and late 1979 cropping seasons. In the first 6 weeks of growth in the early season, cropping pattern had no effect on weed growth; weeds did not suppress crop growth significantly until 5–6 weeks after sowing and total crop dry weights were not affected by cropping pattern. Three weeks after sowing, weeds from weedy crop plots had taken up two to four times as much nutrient (N, P, K, Ca + Mg) as was taken up by corresponding weed-free crops. In the late season, weed dry weight 6 weeks after sowing was depressed in the intercrop compared to monocultures and dry-matter production of the intercrop was higher than those of monocultures. The resource use index (RUI), defined as the amount of an environmental resource used by a weed-free crop divided by the combined amount of the same resource used by the corresponding weedy crop and the associated weeds, increased with age of crop and was higher for the intercrop than the monocultures only in the late season.     

四种中国检疫性玉米病毒的研究进展

玉米是我国重要的粮食作物、饲料和工业原材料。随着我国粮食贸易量增加,病原物入侵的风险也随之加大。该文对4种主要的检疫性玉米病毒——玉米褪绿斑驳病毒 Maize chlorotic mottle machlomovirus、玉米矮花叶病毒 Maize dwarf mosaic potyvirus、小麦线条花叶病毒 Wheat streakmosaic tritimovirus和玉米褪绿矮缩病毒 Maize chlorotic dwarf waikavirus的生物学特性及其危害与分布情况进行归纳总结,同时对这些病毒致病与寄主抗病的相关机制以及检测方法的研究进展进行介绍,并针对检疫性玉米病毒的防控,在快速检测体系的建立与抗病毒种质的创制方面提出展望。     

Aspects of the growth and development of Cyperus rotundus under arable crop canopies: implications for integrated control

Cyperus rotundus L. is one of the most difficult weeds to control worldwide. In Zimbabwe, it is an important weed in arable areas with high crop production potential. The effect of maize, cotton and groundnut canopies on the suppression of C. rotundus was investigated under field conditions over two growing seasons. Similarly, C. rotundus growth and development under a constant maize population planted in isometric and rectangular stands were also studied over one growing season. The capacity of C. rotundus to spread in the short term and its regenerative capacity were curtailed under maize compared with groundnut and cotton canopies. The same was also true under isometric maize stands compared with rectangular maize stands. Significant interactions between crop canopy and time of sampling for C. rotundus shoot counts, leaf area, rhizome length, tuber counts and tuber dry weight suggest the effectiveness of the maize canopy in suppressing the spread and regenerative capacity of C. rotundus early in the season. The implications of these results for the integrated control of C. rotundus are discussed.     

Consequences of narrow crop row spacing and delayed Echinochloa colona and Trianthema portulacastrum emergence for weed growth and crop yield loss in maize

Echinochloa colona and Trianthema portulacastrum are weeds of maize that cause significant yield losses in the Indo‐Gangetic Plains. Field experiments were conducted in 2009 and 2010 to determine the influence of row spacing (15, 25 and 35 cm) and emergence time of E. colona and T. portulacastrum (0, 15, 25, 35, 45 and 55 days after maize emergence; DAME) on weed growth and productivity of maize. A season‐long weed‐free treatment and a weedy control were also used to estimate maize yield and weed seed production. Crop row spacing as well as weed emergence time had a significant influence on plant height, shoot biomass and seed production of both weed species and grain yield of maize in both years. Delay in emergence of weeds resulted in less plant height, shoot biomass and seed production. However, increase in productivity of maize was observed by delay in weed emergence. Likewise, growth of both weed species was less in narrow row spacing (15 cm) of maize, as compared with wider rows (25 and 35 cm). Maximum seed production of both weeds was observed in weedy control plots, where there was no competition with maize crop and weeds were in rows 35 cm apart. Nevertheless, maximum plant height, shoot biomass and seed production of both weed species were observed in 35 cm rows, when weeds emerged simultaneously with maize. Both weed species produced only 3–5 seeds per plant, when they were emerged at 55 DAME in crop rows spaced at 15 cm. Infestation of both weeds at every stage of crop led to significant crop yield loss in maize. Our results suggested that narrow row spacing and delay in weed emergence led to reduced weed growth and seed production and enhanced maize grain yield and therefore could be significant constituents of integrated weed management strategies in maize.     

利用小RNA高通量测序技术检测玉米病毒

正玉米是我国重要的粮食作物,种植范围日趋增大,病害的发生对玉米造成极大为害,病毒病对玉米稳产高产已构成严重威胁。近年来,安徽、山东和辽宁玉米主要种植区病毒病危害较重。为了检测发病玉米的病毒种类,本研究利用小RNA高通量测序技术鉴定玉米病毒,明确种类,以期为制定抗病毒策略提供理论依据。据不完全统计,世界上有40多种玉米病毒病(http://en.wikipedia.org),在我国发生并报道的有5种,分别为玉米粗缩病、玉米矮花叶病、玉米条纹矮缩病、玉米红叶病和玉     

Weed interference in maize, cowpea and maize/cowpea intercrop in a subhumid tropical environment. I. Influence of cropping season

Field experiments were conducted in the early and late cropping seasons of 1979 on a loamysand Oxic Ustropept in a subhumid environment in Nigeria, using 40000, 50000 and 30000 + 40000 plants ha^?1 of maize (Zea mays L. cv. TZB), cowpea (Vigna unguiculata (L.) Walp. cv. VITA-5) and maize/cowpea intercrop respectively. These indicated that weed interference effects on crops under no-tillage depended on cropping season, cropping pattern and crop species. In the early and late seasons respectively, thirty-five and twenty-nine different weed species were recorded and weed dry weights of approximately 10·4 and 5·7 t ha^?1 from the plots kept weedy throughout the season reduced corresponding food energy yields by 60 and 82%. Except for the intercrop, which in the early season showed significant yield reduction when exposed to 4 weeks’ weed interference after sowing, all cropping patterns needed more than 4 weeks’ interference to show significant yield reductions, regardless of cropping season. In the early season, weed interference accounted more for the yield reductions in monocultures than it did for those in the intercrop, but in the late season all cropping patterns were equally sensitive to weed association. Maize, which performed much better in the early season, showed greater yield reductions than cowpea under early weed interference but less under full-season interference irrespective of cropping pattern and season. Cowpea seed quality was more reduced by intercropping than by weed interference in the early season but neither of these factors affected seed quality significantly in the late season.     

The occurrence and effects of barley yellow dwarf virus in maize in SW England

During 1975 and 1976 barley yellow dwarf virus was isolated, using the aphid Rhopalosiphum padi (L.) from five maize cultivars in the Exeter area. The isolates produced interveinal flecking, often accompanied by leaf reddening, in the maize cultivar Anjou 210 and several other susceptible cultivars. ln 1976 visual assessment of six commercial maize plantings indicated natural infections ranging from 11 to 14% of plants, whilst infection in a disease observation plot ranged from 0.8 to 27.5%. Population counts for the three aphid vectors Sitobion avenae (F.), Metopolophium dirhodum (W.) and Rhopalosiphum padi (L.), associated with the crop, corresponded closely with the Rothamsted Insect Survey results from Starcross, Devon.
The maize cultivars Anjou 210 and De Kalb 202 were grown in experimental plots with individual rows containing 0, 25, 50, or 100% barley yellow dwarf-infected plants. Infection caused severe stunting of Anjou 210 but had less effect on De Kalb 202. Dry matter yield losses of 50% for Anjou 210 and 27% for De Kalb 202 were recorded; there were also significant changes (p<0.05) in dry matter content, metabolisable energy, digestible crude protein and water soluble carbohydrate.     

Viral inclusions in monocotyledons infected by maize streak and related geminiviruses

Isolates of maize streak virus (MSV) were examined by thin-section electron microscopy in plants, assessed for characteristic features of infection and compared with other related geminiviruses infecting monocotyledons from Africa, islands in the Indian Ocean, and the Pacific Island of Vanuatu. Arrays of virus particles, often crystalline, were most often seen in the nucleus. The morphology of the nuclear crystalline arrays was characteristic of certain isolates or groups of isolates (strains). Infected nuclei could be seen in cells from the phloem parenchyma, vascular bundle sheath and mesophyll tissue, and also in epidermal guard cells of plants infected with the maize strain of MSV. The particle arrays varied in morphology from regular rows of virions forming distinctive blocks, to randomly arranged aggregates in certain areas of the nucleus. We consistently failed to find viral crystalline arrays associated with infection of panicum streak virus (PSV) and sugar cane streak virus (SSV) isolates either in these hosts or in maize. Occasionally arrays of MSV particles were found outside the nuclear envelope in physiologically active cells. Accumulations or sheets of MSV particles were seen lining the walls of some phloem companion cells. Crystalline aggregates of particles were frequently observed in the cell vacuole, after lysis of the nuclear membrane of dead cells which made up the chlorotic lesions, the typical symptom of virus infection. Virus preparations from all hosts contained typical geminate particles regardless of the morphology of the virion arrays. The effect on chloroplasts appeared to vary between isolates and this is discussed in relation to lesion colour. The arrangement of virions in the nucleus as a taxonomic character is diagnostic for MSV. Inclusions with crystalline structure found in sieve elements of infected plants were not immunogold labelled when thin sections were probed using antiserum to the virus particles.     

Host and virus effects on reversion in cassava affected by cassava brown streak disease

The phenomenon of virus‐infected plants naturally recovering health is known as ‘reversion’, and is a type of resistance mechanism exploited in some crop plants for disease control. Various parameters were investigated that affect reversion from cassava brown streak disease (CBSD) in three cassava varieties (Albert, Kaleso and Kiroba) that differ in levels of resistance to the disease. Cassava plants were inoculated by grafting with two virus species (Ugandan cassava brown streak virus, UCBSV and Cassava brown streak virus, CBSV) that cause CBSD, and the plants grown from them were subsequently assessed for reversion. The rate of reversion depended on the cassava variety, virus species, and the length and position of the stem cuttings used. A significantly high proportion of progenies were virus‐free (reverted) for the resistant variety Kaleso (64·1% for UCBSV and 54·9% of CBSV), compared to the tolerant variety Kiroba (56·7 and 45·5%) and the susceptible control Albert (38·9 and 35·1%). The highest number of virus‐free plants was generated from short 10 cm long cuttings (e.g. 60·1% for Kaleso for CBSV) compared to 20 cm long stem cuttings (e.g. 21·4% for Albert). Cuttings taken from upper stems of diseased plants produced most virus‐free progenies compared to middle and lower parts. More than 50% virus‐free plants were obtained in the resistant and tolerant varieties. This is a highly valuable finding and could be exploited for developing strategies to control the current CBSD epidemic in eastern and central Africa.     

Damage by pod‐sucking bugs on cowpea when intercropped with maize

Abstract

Agroecosystems in the tropics include growing two or more crops in diverse polycultural patterns. In Kenya, cowpea Vigna unguiculata (Walp.) is grown in combination with cereals such as maize and sorghum, perennial legumes such as pigeon peas and root crops such as cassava and sweet potatoes. In the present investigations, field experiments were conducted to study the incidence and damage caused by Clavigralla tomentoscollis Germ, and C. shadabi after flowering, on cowpea intercropped with maize. It was evident from the present studies that C. tomentoscollis and C. shadabi were more prevalent on cowpea during the first season rather than during the second season. Both species had a preference for feeding on cowpea intercropped with maize than when it was grown in pure stands. Consequently more pod damage was recorded on cowpea planted with maize. It was therefore concluded that this agricultural system, which is commonly practised by small‐scale farmers in Kenya, is undesirable from the point of view of bug damage.     

Horizontal spread of beet necrotic yellow vein virus in soil

Horizontal dispersal of beet necrotic yellow vein virus (BNYVV) by means of viruliferous zoospores ofPolymyxa betae was studied in greenhouse experiments. BNYVV was not detected in roots of sugar beet plants grown in silver sand for 4 weeks at a root-free distance of 5 cm from eitherP. betae- and BNYVV-infected plants or BNYVV-infested soil. Spread of BNYVV from inoculum sources in the field was studied in the absence and presence of tillage practices. Active dispersal in combination with root growth from and towards point sources of inoculum contributed only little to horizontal dispersal of viruliferous inoculum and spread of disease during the season, as determined for one soil type, two different years and in the absence of tillage and tread. In the second beet crop after application of inoculum to whole field plots, more BNYVV-infected plants were detected at 2 m than at 8 m distance from the infested plots in the tillage direction. In the third year, disease incidence at 8 m was high and equivalent to that at 2 m.     

Competition between maize and Datura stramonium in an irrigated field under semi-arid conditions

Crop growth of maize ( Zea mays L.) and Datura stramonium L. in monoculture and competition was studied over 4 years in a flood irrigated field in Zaragoza (Spain). Plant density was 8.33 m^–2 for maize and 16.66 m^–2 (1994 and 1995) and 8.33 m^–2 (1996 and 1997) for D. stramonium . Maize yield was decreased by 14–63% when competing with the weed. Yield reduction increased as the time between crop and weed emergence decreased. The development of leaf area per plant during the exponential growth phase was faster in maize primarily because the leaf area of maize seedlings at emergence time was greater than that of the weed. The faster growth of maize in leaf area and height reduced the photosynthetically active radiation received by the weed. Datura stramonium had a lower radiation use efficiency (RUE) than maize. Competition from the weed slightly decreased the maximum leaf area index (LAI) of the crop, and leaf senescence of maize was accelerated. The weed competed with the crop late in the season reducing crop growth rate, grain number per ear and grain weight. Competitive ability of D. stramonium for light was mainly due to its growth habit, with the leaves concentrated in the upper part of the canopy (more than 75% of LAI in the upper 25% of its height), its higher light extinction coefficient (0.89) and its indeterminate growth habit. The N plant content of maize was not influenced by the presence of the weed. The weed had a higher N plant content than the crop throughout the season and took up more N in monoculture.     

玉米弯孢菌叶斑病传播梯度模型

 根据植物病害流行学原理,采用人工接种方法,在田间造成玉米弯孢菌叶斑病不同的发病梯度,分析连续2年玉米弯孢菌叶斑病传播动态。利用SPSS统计软件构建了此病害的传播梯度模型,结果表明指数模型是沈阳地区玉米弯孢菌叶斑病传播梯度的最佳模型。接种2个月,掖单13玉米弯孢菌叶斑病传播梯度最佳模型是x=9.606×EXP (-0.2829×d),海试16最佳模型是x=7.154×EXP (-0.2351×d)(x:病情指数,d:距菌源中心的距离)。预测玉米弯孢菌叶斑病在2个月最远传播距离为28 m;传播速度为0.4~0.5 m/d。     

Does soil nitrogen affect early competitive traits of annual weeds in comparison with maize?

Soil nitrogen (N) is considered an important driver of crop‐weed interactions, yet the mechanisms involved have been only partially explored, especially with respect to early‐season growth, when competitive hierarchies are formed. This study characterises the effects of different N levels on biomass accumulation and plant morphology for maize (Zea mays), and four important weed species (Amaranthus retroflexus, Abutilon theophrasti, Setaria faberi, and Chenopodium album). Under glasshouse conditions, plants were grown in separate pots and irrigated with nutrient solution at four N concentrations (0.2, 0.5, 2, 5 μm L⁻¹) until 57 days after emergence. Except for S. faberi, which was unresponsive to N, the relative biomass growth rates (RGR) of maize and the broad‐leaved weeds were positively and similarly affected by increasing nitrogen. At all N levels, maize had a height advantage by virtue of its larger seed size, which conferred early growth benefits independent of RGR. At low N, biomass growth was instrumental to S. faberi’s improved competitive position, whereas height development per unit biomass improved the competitive position of A. theophrasti, C. album and A. retroflexus. The approach presented could be applied to other crop‐weed systems to evaluate environmental impacts on competitive outcomes.     

Cucumber mosaic virus infection of chickpea stands: temporal and spatial patterns of spread and yield‐limiting potential

Patterns of spread and yield losses were examined when migrant aphid vectors transmitted Cucumber mosaic virus (CMV) within chickpea (Cicer arietinum) plots. When numbers of chickpea plant infection foci were varied to provide initial infection incidences of 0·3–2%, rate of virus spread and its final incidence increased in proportion to initial virus incidence and pathogen progress curves reflected a polycyclic pattern of spread. Chickpea seed yields decreased by 44–45% when CMV incidence reached 61–74% at final assessment in plots with simulated 1–2% initial incidence. In chickpea plots with or without introduced lupin plant infection foci, cumulative spatial data for diseased plants were assessed using spatial analysis by distance indices (sadie ). When CMV spread within a plot, infection was concentrated in large patch clusters and was consistent with comprehensive localized spread around primary infection foci. When introduced infection foci were absent, there was more diffuse spread with many isolated clusters. In individual plants that developed CMV symptoms at different growth stages, shoot and pod dry weight were reduced by 60–65% and 77–79% (early infection) and 44 and 66% (late infection), respectively. Seed yield losses were 78–80% (early) and 65–67% (late), and reduction in 50‐seed weight was 20–25% regardless of time of infection, so seed number was reduced more by early than late infection. Infection also reduced seed quality as, in addition to smaller seed size, seed coats were discoloured and malformed: the proportions of malformed seeds were 9–11% (early), 3–6% (late) and 0·5% (healthy).     

Cicadulina species and maize streak virus in Ethiopia

Abstract

A preliminary survey was made in the most important maize‐growing regions of Ethiopia to record the species of Cicadulina present, their distribution and abundance. Five species of Cicadulina were found, including two not previously reported from Ethiopia. The maize strain of maize streak geminivirus (MSV) occurred predominantly between 450 and 1800 m above sea level, which coincides with the altitudinal range of C. mbila implicating this species as the principal vector of MSV in Ethiopia. The maize strain isolate was indistinguishable from others from elsewhere in Africa.     

The Distribution and Spread of Sorghum Downy Mildew in Sorghum and Maize Fields in Nigeria and Zimbabwe

Sorghum downy mildew (Peronosclerospora sorghi, SDM) is a damaging disease of sorghum and maize crops in Africa. Runs analysis was used to study the distribution of systemically infected sorghum and maize plants in Nigeria and Zimbabwe. The temporal and spatial development of local lesions of SDM on sorghum in Zimbabwe was investigated by assessing the local lesion symptoms caused by conidia in plots with a single point source of inoculum. With ordinary runs analysis, there was evidence of clustering of disease in some fields in the humid areas of Nigeria and the semi-arid areas of Nigeria and Zimbabwe. Clustering was found in two of the eight runs analyses performed on maize in the humid south of Nigeria, and in only one of the eight runs in Zimbabwe, which was interpreted as a predominance of random infection at the time of assessment and at the spatial scales assessed. Symptoms of local lesions of SDM developed rapidly across plots from an introduced point source of infection. After 9 days-exposure to the source of inoculum, the incidence of diseased leaves was 1.2%, and after 50 days it was 74.5%. A disease gradient which initially developed flattened as the plot became uniformly diseased. The predominant wind direction was NNE, and most rapid spread of disease was towards the SSW and WSW. In conclusion, local lesions can spread rapidly in sorghum crops, suggesting that they may be an important source of conidial inoculum for further local and systemic infections during the growing season.     

Velvetbean (Mucuna spp.) suppresses speargrass (Imperata cylindrica (L.) Raeuschel) and increases maize yield

Speargrass, Imperata cylindrica (L.) Raeuschel, is a serious weed, threatening crop productivity in smallholder farms in West Africa. Since the use of more effective practices such as deep tillage and chemical control is beyond the means of resource-poor farmers who carry out most agricultural activities in this region, low-input alternative technology needs to be developed. Field studies were conducted during the 1993/1994 and 1995/1996 growing seasons to investigate the influence of three velvetbean accessions and two levels of fertilizer on the control of speargrass during the year of cover crop planting and one year later. The velvetbean accessions in 1993 were: Mucuna cochinchinensis and M. pruriens var. utilis. In 1995, M. pruriens var. IRZ was included in the study. M. cochinchinensis in 1993 and M. pruriens var. IRZ in 1995 had the highest ground cover rating early in the growing seasons whereas M. pruriens var. utilis had the lowest ground cover rating in both years. Inorganic fertilizer at30 kg ha- 1 each of N, P and K increased velvetbean ground cover by 2-22%, with M. cochinchinensis (14-22%) and M. pruriens var. IRZ (5-15%) showing the highest response in 1993 and 1995, respectively. M. pruriens var. utilis showed the least response in both years. After one growing season M. pruriens var. utilis, M. cochinchinensis, and M. pruriens var. IRZ reduced speargrass shoot density by 50, 76, and 68%, and shoot dry matter by 72, 92, and 79%, respectively. Fertilizer reduced speargrass growth in velvetbean plots, while the opposite occurred in plots without velvetbean. Velvetbean residue effectively suppressed speargrass until the beginning of the subsequent cropping season. Maize grown 1 year after velvetbean required 50% less weeding than plots without velvetbean. Maize shade reduced speargrass shoot growth by 30-80% but regrowth of the weed occurred 4 weeks before maize harvest. Maize grain yield was higher in plots previously seeded to velvetbean than in plots without velvetbean. Speargrass shoot density and dry matter were negatively correlated with maize grain yield (r =_-0.42 and r =_-0.32, respectively, P < 0.01). Although velvetbean may effectively reduce speargrass during the year of establishment and the subsequent cropping phase it has a limited effect on rhizomes and, as such, does not provide a long term control.