Natural variation in GmGBP1 promoter affects photoperiod control of flowering time and maturity in soybean

The present study screened for polymorphisms in coding and non‐coding regions of the GmGBP1 gene in 278 soybean accessions with variable maturity and growth habit characteristics under natural field conditions in three different latitudes in China. The results showed that the promoter region was highly diversified compared with the coding sequence of GmGBP1. Five polymorphisms and four haplotypes were closely related to soybean flowering time and maturity through association and linkage disequilibrium analyses. Varieties with the polymorphisms SNP_‐796G, SNP_‐770G, SNP_‐307T, InDel_‐242normal, SNP_353A, or haplotypes Hap‐3 and Hap‐4 showed earlier flowering time and maturity in different environments. The shorter growth period might be largely due to higher GmGBP1 expression levels in soybean that were caused by the TCT‐motif with SNP_‐796G in the promoter. In contrast, the lower expression level of GmGBP1 in soybean caused by RNAi interference of GmGBP1 resulted in a longer growth period under different day lengths. Furthermore, the gene interference of GmGBP1 also caused a reduction in photoperiod response sensitivity (PRS) before flowering in soybean. RNA‐seq analysis on GmGBP1 underexpression in soybean showed that 94 and 30 predicted genes were significantly upregulated and downregulated, respectively. Of these, the diurnal photoperiod‐specific expression pattern of three significant flowering time genes GmFT2a, GmFT5a, and GmFULc also showed constantly lower mRNA levels in GmGBP1‐i soybean than in wild type, especially under short day conditions. Together, the results showed that GmGBP1 functioned as a positive regulator upstream of GmFT2a and GmFT5a to activate the expression of GmFULc to promote flowering on short days.     

Linear relationship between Gaeumannomyces graminis var. tritici (Ggt) genotypic frequencies and disease severity on wheat roots in the field

In order to investigate potential links existing between Gaeumannomyces graminis var. tritici (Ggt) population structure and disease development during polyetic take-all epidemics in sequences of Ggt host cereals, seven epidemics in fields with different cropping histories were monitored during the seasons 2001/2002 (two fields), 2002/2003 (two fields) and 2003/2004 (three fields). Take-all incidence and severity were measured at stem elongation and Ggt populations were characterized. The 73 isolates collected in the two fields in 2001/2002 were distributed into two multilocus genotypes, G1 and G2 according to amplified fragment length polymorphism analysis. A monolocus molecular marker amplified by F-12 random amplification polymorphism DNA primer sizing between 1.9 and 2.0 kb that gave strictly the same distinction between the two multilocus genotypes was further applied to measure G1/G2 frequencies among Ggt populations in all fields (266 isolates). The ratios of G1 to G2 differed between fields with different cropping histories. A linear relationship between G2 frequency among Ggt populations and disease severity at stem elongation was measured during the three cropping seasons. When take-all decline was observed, G2 frequencies were low in first wheat crops, highest in short-term sequences and intermediate in longer sequences of consecutive crops of Ggt host cereals. This pattern could be the result of population selection by environmental conditions, in particular by microbial antagonism during the parasitic phase of the fungus. In order to better understand take-all epidemic dynamics, the distinction between these two genotypes could be a basis to develop models that link approaches of quantitative epidemiology and advances in population genetics of Ggt.     

Performances of improved cassava (Manihot esculenta Crantz) cultivars against root rot disease and yield in cassava-maize intercropping systems under natural infection

This study examined the performances of 21 cassava cultivars in two cropping seasons on the field against root rot disease and the yield in cassava-maize intercrop. Data were collected on number of root/plant, weight of root (t/ha) and disease severity (DS) on rotted roots at 12 and 16 months after planting (MAP), respectively. There were significant (P ≤ 0.05) differences for DS at 12 and 16 MAP in both seasons with cultivar TMS 97/JW2 having the least DS score. TMS 97/JW2 was resistant to the root rot pathogen, while eleven other cultivars were moderately resistant to the disease at 16 MAP. There was no consistency in the roots weight for the cultivars over the two cropping seasons but higher roots weight was recorded at 16 MAP than 12 MAP with different cultivars having highest roots weight at these periods. Intercropping maize with cassava does not have any management potential on root rot development.     

Distribution and association of factors influencing bean common bacterial blight (Xanthomonas axonopodis pv. phaseoli) epidemics in Southern Ethiopia

Abstract

Field surveys were conducted to determine the spatiotemporal distribution and the association of disease intensity with cropping systems and other environmental factors in Southern Ethiopia during 2016 and 2017. In both years, a total of 190 bean fields were surveyed in five districts, and common bacterial blight (CBB) was 100% prevalent. But disease incidence and severity varied among districts and between cropping years. Bean planting in Arbaminch suffered from 31% to 38%, 6–13%, 12–17% and 8–12% higher CBB severity than bean cultivation in Burjdi, Mihirab Abaya, Demba Gofa and Konso districts, respectively. The associations of disease parameters with independent factors were assessed using the logistic regression analyses. District, cropping year, altitude, land preparation, cropping system, fertiliser application, planting date, growth stage and weed infestation were associated with both disease incidence and severity with variable levels of significance. Common bean genotype was also significantly (P?<?.005) associated with disease severity in a multiple variable model. Concerning incidence, variable associations were demonstrated by the model. These results indicate that CBB is severe and highly prevalent in Southern Ethiopia. Therefore, efforts should be geared towards crop residue management, optimal tillage and fertiliser application, weeding, clean seed source and early planting approach to manage the disease.     

中国大豆品种生态区划的修正Ⅱ.各区范围及主要品种类型

根据我国各地256份代表性大豆地方品种在南京分期播种,延长或缩短光照长度各处理条件下的生育期表现,结合供试材料来源地的地理与气候条件,播种季节类型,熟期组类型以及光温反应特性等因素,将我国大豆品种生态区划分为：北方一熟制春作大豆品种生态区（Ⅰ）,黄淮海二熟制春夏作大豆品种生态区（Ⅱ）,长江中下游二熟制春夏作大豆品种生态区（Ⅲ）,中南多熟制春夏秋作大豆品种生态区（Ⅳ）,西南高原二熟制春夏作大豆品种生态区（Ⅴ）,华南热带多熟制四季大豆品种生态区（Ⅵ）等六大区,10亚区,并阐述各区的一般生态条件及代表生态类型。     

Spatial distribution and association of banana (Musa spp.) Fusarium wilt (Fusarium oxysporum f.sp. cubense) epidemics with biophysical factors in southwestern Ethiopia

Abstract

Field surveys were conducted in 2014 and 2015 to determine the distribution and association of wilt incidence with biophysical factors. Spatio-temporal analysis of wilt epidemics in the farms revealed a wide distribution of the disease. Cropping year, district, cultivar, crop density, planting date, weeding, sucker and bud management, source of moisture and planting materials, presence/absence of wilt in neighboring farms, disease management practices, farmers awareness towards wilt diseases, planting date and cropping system were significantly associated with disease incidence in a multiple-variable model. A wilt incidence of ≤30% had high probability of association to good weed, sucker and male bud management practices, high level of farmers awareness towards wilt, low population density, disease-free suckers, rainfed production, absence of wilt in neighboring farms and good disease management schemes. Findings of this study showed Fusarium wilt is a major constraint and need for on-farm trainings to concerned bodies about wilt management schemes.     

Effects of light leaf spot (Pyrenopeziza brassicae) on yield of winter oilseed rape (Brassica napus)

The relationship between development of light leaf spot and yield loss in winter oilseed rape was analysed, initially using data from three experiments at sites near Aberdeen in Scotland in the seasons 1991/92, 1992/93 and 1993/94, respectively. Over the three seasons, single-point models relating yield to light leaf spot incidence (% plants with leaves with light leaf spot) at GS 3.3 (flower buds visible) generally accounted for more of the variance than single-point models at earlier or later growth stages. Only in 1992/93, when a severe light leaf spot epidemic developed on leaves early in the season, did the single-point model for disease severity on leaves at GS 3.5/4.0 account for more of the variance than that for disease incidence at GS 3.3. In 1991/92 and 1992/3, when reasonably severe epidemics developed on stems, the single-point model for light leaf spot incidence (stems) at GS 6.3 accounted for as much of the variance. Two-point (disease severity at GS 3.3 and GS 4.0) and AUDPC models (disease incidence/severity) accounted for more of the variance than the single-point model based on disease incidence at GS 3.3 in 1992/93 but not in the other two seasons. Therefore, a simple model using the light leaf spot incidence at GS 3.3 (x) as the explanatory variable was selected as a predictive model to estimate % yield loss (y_r): y_r= 0.32x– 0.57. This model fitted all three data sets from Scotland, When data sets from Rothamsted, Rosemaund and Thurloxton in England were used to test it, this single-point predictive model generally fitted the data well, except when yield loss was clearly not related to occurrence of light leaf spot. However, the regression lines relating observed yield loss to light leaf spot incidence at GS 3.3 often had smaller slopes than the line produce, by the model based on Scottish data.     

Yield loss of faba bean (Vicia faba) due to chocolate spot (Botrytis fabae) in sole and mixed cropping systems in Ethiopia

Two field experiments were conducted during 2004 and 2005 cropping seasons at Adet Agricultural Research Center, Ethiopia to assess yield losses caused by chocolate spot(Botrytis fabae) of faba bean in sole and mixed cropping systems using two cultivars. Cropping systems were sole faba bean (FB), faba bean mixed with field pea (FB: FP), barley (FB: BA) and maize (FB: MA). Mancozeb was sprayed at the rate of 2.5 kg a.i/ha at 7-, 14- and 21-day interval to generate different levels of chocolate spot disease in all the four cropping systems, and unsprayed control was also included. The treatments were arranged in a randomised complete block design (RCBD) with four replications. FB: MA mixed cropping significantly reduced disease severity and the area under disease progress curve (AUDPC) and increased faba bean grain yield. The highest faba bean grain yield among the three mixed croppings under different spray schedules was obtained from FB: MA mixed cropping in both 2004 and 2005 (2.56 and 3.74 t/ha, respectively) cropping seasons. There were highly significant yield differences (P < 0.05) among the spray intervals of mancozeb in both seasons. The highest grain yield (4.9 t/h) was recorded from the 7-day spray interval in 2005. The unsprayed faba bean had a lower grain yield (1.9 t/ha in 2004 and 2.3 t/ha in 2005) compared to the sprayed plots. The highest relative yield loss (67.5%) was calculated in 2005 from FB: FP mixed cropping in unsprayed plots. The relative yield losses in the unsprayed plots were in the range of 35.8–41.5% in 2004 and 52.6–67.5% in the 2005 cropping season. Severity and AUDPC were inversely correlated with faba bean grain yield. Significant differences were recorded in the 100-seed weight and days to maturity (DM). The unsprayed plots had shorter DM ranging from 126 to 128.5 day (except FB: MA mixed cropping) in 2004 and 122–123.9 days in 2005. In the sprayed plots DM was relatively longer than the unsprayed plots. A higher seed weight was recorded in the sole FB (56 g) and FB: MA (55 g) mixed cropping, and the lowest value of 100-seed weight was recorded from FB: FP (53 g) mixed cropping. The productivity of the mixed cropping evaluated by land equivalent ratio (LER) exceeded that of sole cropping. Faba bean grain yield was highly influenced by the severity of chocolate spot. The disease affects the DM, forcing early maturing of the plants.     

Weather and fireblight in England

Agriculture in tropical countries produces many of the world's most valuable crops, but productivity is limited by a multiplicity of different diseases, many of which have been inadequately studied. Certain characteristics of tropical climates have a fundamental influence on the incidence and severity of many of these diseases. The main seasonal variations which control crop growth and disease epidemiology depend on differences in moisture availability rather than daylength and temperature differences. Marked dry seasons hinder the survival of many pathogens outside hosts unless they can produce drought-resistant spores. In many tropical areas continuous cropping throughout the year is possible, enabling disease epidemics to continue without interruption over long periods. Much agriculture in the tropics is at fairly high altitudes enabling temperate crops to be grown, but low temperatures may present special disease hazards. Violent storms are a feature of many tropical climates and have been implicated in the spread of many bacterial diseases. Drought is a factor which influences the severity of many root diseases. The various effects which moisture, as rain or dew, temperature and cropping cycle can have are illustrated by discussion of sigatoka disease of bananas (Mycosphaerella musicola Leach), South American leaf blight of rubber (Microcyclus ulei (R. Henn.) Arx) and coffee berry disease (Colletotrichum coffeanum Noack).     

The Intensity of a Bean Fusarium Root Rot Epidemic is Dependent on Planting Strategies

Restricting Fusarium root rot (FRR) epidemics and improving productivity using fewer chemicals is a major concern in bean‐growing regions. The main purpose of this research was to identify the planting strategies associated with FRR development and seed production in bean crops. A 2‐year study of 122 commercial bean farms in four major producing regions in Zanjan, Iran, was performed to characterize the associations of farming indicators with FRR and productivity. Linear mixed‐effect models, contingency tables and correspondence analyses were used to describe the variables relationships. Low disease and high productivity were linked to herbicide and manure applications, fungicidal treatment of seeds, manual sowing and sprinkler irrigation. Furrow irrigation, mechanical sowing, planting on raised seedbeds, the lack of fertilizer and herbicide use accounted for high disease and low seed production. The results of the study provide further evidence of factors contributions to the wider FRR spread over bean‐cropping systems. Overall, this suggests that the selected composition of planting strategies can improve bean production and reduce FRR intensity.     

Influence of age structure of plant populations on damping-off epidemics

Summary Germination synchrony may facilitate damping-off epidemics by creating a high density of uniformly susceptible individuals. We tested the hypothesis that synchronous germination causes increased seed and seedling mortality from damping-off in two legume species attacked by the fungal pathogen, Pythium aphanidermatum. Glycine max exhibited rapid, synchronous germination compared to its progenitor, G. soja, and suffered greater mortality from both pre-and postemergent damping-off in controlled environment experiments. However, when mixed-aged populations of G. max were created experimentally by staggering planting times, a significant increase in damping-off mortality occurred. In G. soja, which typically has mixed-aged populations due to asynchronous germination, experimental populations with an even-aged distribution also suffered increased damping-off mortality. Hence, the relationship between population age structure and damping-off mortality was species-specific. We propose that species differences in the duration of individual seedling susceptibility to disbase interact with population age structure to control the cutcome of damping-off epidemics.     

Modelling temperature,photoperiod and vernalization responses of Brunonia australis (Goodeniaceae) and Calandrinia sp. (Portulacaceae) to predict flowering time

Background and Aims

Crop models for herbaceous ornamental species typically include functions for temperature and photoperiod responses, but very few incorporate vernalization, which is a requirement of many traditional crops. This study investigated the development of floriculture crop models, which describe temperature responses, plus photoperiod or vernalization requirements, using Australian native ephemerals Brunonia australis and Calandrinia sp.

Methods

A novel approach involved the use of a field crop modelling tool, DEVEL2. This optimization program estimates the parameters of selected functions within the development rate models using an iterative process that minimizes sum of squares residual between estimated and observed days for the phenological event. Parameter profiling and jack-knifing are included in DEVEL2 to remove bias from parameter estimates and introduce rigour into the parameter selection process.

Key Results

Development rate of B. australis from planting to first visible floral bud (VFB) was predicted using a multiplicative approach with a curvilinear function to describe temperature responses and a broken linear function to explain photoperiod responses. A similar model was used to describe the development rate of Calandrinia sp., except the photoperiod function was replaced with an exponential vernalization function, which explained a facultative cold requirement and included a coefficient for determining the vernalization ceiling temperature. Temperature was the main environmental factor influencing development rate for VFB to anthesis of both species and was predicted using a linear model.

Conclusions

The phenology models for B. australis and Calandrinia sp. described development rate from planting to VFB and from VFB to anthesis in response to temperature and photoperiod or vernalization and may assist modelling efforts of other herbaceous ornamental plants. In addition to crop management, the vernalization function could be used to identify plant communities most at risk from predicted increases in temperature due to global warming.     

Determination of anti-microbial properties of Ocimum viride concentrations on Rhizopus stolonifer infection and germination of soybean (Glycine max L. Merill)

Investigation was conducted to determine the anti-microbial properties of different concentrations of Ocimum viride and germination of soybean. Analysis of variance recorded a highly significant difference on the percentage occurrence of different fungal diseases at 5% probability level. Ocimum viride at 0.04 ppm had the highest mycelia growth inhibition 77.0%, while 0.00 ppm (control) recorded the lowest mycelia inhibition 1.00% of Rhizopus stolonifer. Investigation revealed that spot recorded the highest percentage occurrence 50%, scab 34%, mould 43% and blight 31.0% recording the lowest. Also the microorganisms isolated from diseased soybean were Rhizopus stolonifer, Fusarium oxysporum, Aspergillus species although the inhibition was very high on Rhizopus stolonifer, followed by Aspergillus sp, then mould when Fusarium sp was low. Also, Ocimun viride extract was observed to possess a high anti-microbial properties as it inhibited the growth of all the micro-organisms identified to be associated with the soybean sees. Result showed that seeds treated with crude extract of 0.04ppm had the highest percentage germination irrespective of the days of germination, followed by 0.03 ppm, then 0.02 ppm, when 0.00 ppm had the lowest. However, 0.01 and 0.02 ppm, respectively, recorded similar percentage germination 64%; 64% at 4 days after planting. Days of planting were highly significant on the percentage germination at 5% probability level, as germination of the soybean seeds increased with days of planting and highest percentage germination of 98% was recorded on the 6th day of planting while no germination was recorded from the 1st till the 4th day of planting.     

光温耦合的中国温带地区旱柳花期时空格局模拟

建立基于温度和光周期驱动的旱柳花期物候模型,旨在寻找影响旱柳花期时空变化的主要气象因子,揭示调控植物开花时间的生态机制,还可为改善柳絮造成的环境污染和花粉过敏等人类健康问题提供参考信息和依据。利用中国气象局农业气象观测网提供的中国温带地区1982-2011年49个站点的旱柳开花始期、盛期和末期观测资料及平行的逐日气象数据,分别对6种模型（简单积温模型、温度三基点模型、八时段温度模型、简单积温-日长模型、温度三基点-日长模型和八时段温度-日长模型）进行了参数率定和假设检验,根据外部检验结果,从中选出针对旱柳3个花期的最优物候模型,进而利用连续地理气象数据和最优物候模型重建了1982-2011年旱柳开花始期、盛期、末期和花期长度的时空变化特征。结果表明：光温耦合的物候模型对旱柳花期的模拟效果和外推效果优于仅基于温度的模型。旱柳开花始期和盛期最优模型均为八时段温度-日长模型,末期为温度三基点-日长模型,说明光周期和温度可能是影响旱柳花期开始、繁盛和结束时间的主要气象因子。同时,优选出的物候模型能够较准确地对不同年份和不同地区的旱柳花期进行模拟及预测。重建的1982-2011年旱柳平均开花始期、盛期和末期日期分别为4月24日、4月28日和5月3日,平均花期长度为9 d,始期、盛期和末期出现日期呈现出从海拔低到高、从南向北、从西向东逐渐推迟的空间格局。1982-2011年旱柳开花始期、盛期和末期在大部分地区呈提前趋势,呈显著提前趋势的面积分别占总面积的49.78%、50.01%和53.40%,花期长度变化差异不显著。     

Efficacy of intercropping and harvest time modification for reducing field infestation of maize by Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae)

Most management practices of Sitophilus zeamais Motschulsky, a field-to-post-harvest insect pest of cereals, have focused on post harvest control methods. This experiment was designed to investigate the potential of cropping system and modification of time of harvest to control S. zeamais. Intercropping and harvest time modification had significant (P < 0.05) effect on the number of S. zeamais emerging 42 days post-harvest. For the early harvest (15 weeks after planting (WAP)), the mean number of S. zeamais recorded from a maize monoculture (7.39) was significantly (P < 0.05) higher than the mean numbers of weevils emerging from a maize–soybean intercrop (2.31), but not significantly higher than the number recorded in maize–groundnut (3.87) intercrop. For the late harvest (18 WAP), the mean number of emerged adult observed in the maize–soybean intercrop (6.13) was significantly lower than the mean number of adult emerging from the monocrop maize (13.24). Maize–groundnut intercrop did not significantly reduce field infestation of S. zeamais compared with monocrop maize. Percentage weight loss observed in early harvested maize was significantly (P < 0.0001) lower than what was observed in late-harvested maize. Percentage weight loss was highest in stored maize harvested from monocrop maize plots for the early harvest, whereas intercropping maize with soybean reduced percentage weight loss when harvest was delayed.     

Winter oilseed production for biofuel in the US Corn Belt: opportunities and limitations

Interest from the US commercial aviation industry and commitments established by the US Navy and Air Force to use renewable fuels has spurred interest in identifying and developing crops for renewable aviation fuel. Concern regarding greenhouse gas emissions associated with land‐use change and shifting land grown for food to feedstock production for fuel has encouraged the concept of intensifying current prominent cropping systems through various double cropping strategies. Camelina (Camelina sativa L.) and field pennycress (Thlaspi arvense L.) are two winter oilseed crops that could potentially be integrated into the corn (Zea mays L.)–soybean [(Glycine max (L.) Merr.] cropping system, which is the prominent cropping system in the US Corn Belt. In addition to providing a feedstock for renewable aviation fuel production, integrating these crops into corn–soybean cropping systems could also potentially provide a range of ecosystem services. Some of these include soil protection from wind and water erosion, soil organic C (SOC) sequestration, water quality improvement through nitrate reduction, and a food source for pollinators. However, integration of these crops into corn–soybean cropping systems also carries possible limitations, such as potential yield reductions of the subsequent soybean crop. This review identifies and discusses some of the key benefits and constraints of integrating camelina or field pennycress into corn–soybean cropping systems and identifies generalized areas for potential adoption in the US Corn Belt.     

Soybean Yield as Related to Rates of 1,3-Dichloropropene Applied at Planting for Management of Root-Knot Disease

1,3-Dichloropropene (1,3-D) at rates of 17.2 to 51.6 liters/ha applied 3 days preplant or at planting significantly (P < 0.05) reduced the amount of galling on roots of soybean grown in sites infested with Meloidogyne incognita or M. arenaria. Populations of M. incognita second-stage juveniles at harvest were significantly (P < 0.05) reduced by all treatments. Only the 51.6-liters/ ha treatments and a 3-day preplant 34.4-liters/ha application significantly reduced at-harvest juvenile infestations of M. arenaria. Equations (P < 0.001) relating soybean yield and 1,3-D dosage indicated soybean phytotoxicity at the upper range of the nematicide rates. The maximum yield response was predicted at 40 liters/ha applied 3 days preplant at both infestation sites. Maximum yield response was predicted with 30 liters/ha applied at planting to M. incognita-infested soil and from 25 liters/ha applied at planting to M. arenaria-infested soil. Application of economic factors suggested that management of M. incognita may be cost effective with at-plant treatments of low rates of 1,3-D. Yield responses of M. arenaria-infected soybean exposed to similar treatments were insufficient to justify their use at prevailing prices.     

How efficiently do corn‐ and soybean‐based cropping systems use water? A systems modeling analysis

Agricultural systems are being challenged to decrease water use and increase production while climate becomes more variable and the world's population grows. Low water use efficiency is traditionally characterized by high water use relative to low grain production and usually occurs under dry conditions. However, when a cropping system fails to take advantage of available water during wet conditions, this is also an inefficiency and is often detrimental to the environment. Here, we provide a systems‐level definition of water use efficiency (sWUE) that addresses both production and environmental quality goals through incorporating all major system water losses (evapotranspiration, drainage, and runoff). We extensively calibrated and tested the Agricultural Production Systems sIMulator (APSIM) using 6 years of continuous crop and soil measurements in corn‐ and soybean‐based cropping systems in central Iowa, USA. We then used the model to determine water use, loss, and grain production in each system and calculated sWUE in years that experienced drought, flood, or historically average precipitation. Systems water use efficiency was found to be greatest during years with average precipitation. Simulation analysis using 28 years of historical precipitation data, plus the same dataset with ± 15% variation in daily precipitation, showed that in this region, 430 mm of seasonal (planting to harvesting) rainfall resulted in the optimum sWUE for corn, and 317 mm for soybean. Above these precipitation levels, the corn and soybean yields did not increase further, but the water loss from the system via runoff and drainage increased substantially, leading to a high likelihood of soil, nutrient, and pesticide movement from the field to waterways. As the Midwestern United States is predicted to experience more frequent drought and flood, inefficiency of cropping systems water use will also increase. This work provides a framework to concurrently evaluate production and environmental performance of cropping systems.     

Effects of Temperature and Moisture Variables on Brown Rust Epidemics in Sugarcane

Epidemics of brown rust in sugarcane, caused by Puccinia melanocephala, vary in severity between seasons. Natural epidemics were studied to determine the effects of temperature and moisture variables on epidemic onset, severity and decline. Variables were monitored with disease severity in two cultivars, each grown at a different location in Louisiana. Maximum daily temperature was the variable most correlated with seasonal epidemic development and decline. Disease severity was high during 2009 and low during 2010. This contrast allowed evaluation of the effects of conducive and limiting environmental conditions on severity. Lower severity resulted from a combination of unfavourable temperature and leaf wetness conditions that delayed onset then reduced the rate of disease increase. An accumulation of 23–25 days with leaf wetness periods of at least 7 h after the daily minimum temperature exceeded 17°C preceded the onset of disease on young leaves in both severe and mild epidemics. Severe epidemics in both cultivars declined once maximum ambient daily temperature was 32°C or higher. Low and high limiting temperatures determined the initiation and decline of an epidemic, respectively, under Louisiana climatic conditions. The availability of leaf wetness was then an important determinant of disease severity during the epidemic.     

Effect of Cercospora zeae-maydis infested maize residue on progress and spread of grey leaf spot of maize in central Uganda

The spread of grey leaf spot caused by Cercospora zeae‐maydis from infested maize residue on the soil surface and progress over time were studied under tropical conditions at two locations in central Uganda, Kabanyolo and Namulonge, using a susceptible local cultivar, LP16. Infested maize residue collected the previous season was used as the inoculum source in varying amounts; 80% coverage, 40% coverage and 0% soil coverage were used to simulate no tillage, minimum tillage and maximum tillage with complete burial of residues. In all the seasons, disease spread was adequately described by a power law model, whereas disease progress over time was equally well represented by both Gompertz and logistic models. The time factor was highly significant (P < 0.05) for disease development, indicating both increases in disease intensity over time and change in gradients with time. The amount of infested residue on the soil surface in the maize planting was highly significant (P < 0.05) for slope (b) and intercept (a) of the linearised disease gradient. The gradient parameter (b) based on the average for the four directions from the residue source ranged from ?0.96 to ?0.08 in the second cropping season of 1999, from ?2.16 to ?0.01 and from ?0.87 to ?0.03 in the first and second seasons of 2000, respectively. The distance from the infested residue significantly affected foliar disease, but the direction from the residue inoculum source did not, nor was the distance by direction interaction significant. However, interactions between amount of residue and distance were significant. Percentage leaf area affected by grey leaf spot, the areas under disease progress curves and disease intercepts decreased with distance at both locations, but rate of disease increase (r) was generally constant with distance from residue source at Kabanyolo, but not at Namulonge. The relationship between disease severity and amount of residue cover appeared to be affected by the level of background inoculum. At Kabanyolo, where little background inoculum was present, differences were detected in disease severity parameters between the no‐residue plots and residue plots, whereas at Namulonge, where high background levels existed, there was no effect of residue on disease.