Sorghum ergot can develop without local Claviceps africana inoculum from nearby infected plants

Batches of glasshouse-grown flowering sorghum plants were placed in circular plots for 24 h at two field sites in southeast Queensland, Australia on 38 occasions in 2003 and 2004, to trap aerial inoculum of Claviceps africana. Plants were located 20–200 m from the centre of the plots. Batches of sorghum plants with secondary conidia of C. africana on inoculated spikelets were placed at the centre of each plot on some dates as a local point source of inoculum. Plants exposed to field inoculum were returned to a glasshouse, incubated at near-100% relative humidity for 48 h and then at ambient relative humidity for another week before counting infected spikelets to estimate pathogen dispersal. Three times as many spikelets became infected when inoculum was present within 200 m of trap plants, but infected spikelets did not decline with increasing distance from local source within the 200 m. Spikelets also became infected on all 10 dates when plants were exposed without a local source of infected plants, indicating that infection can occur from conidia surviving in the atmosphere. In 2005, when trap plants were placed at 14 locations along a 280 km route, infected spikelets diminished with increasing distance from sorghum paddocks and infection was sporadic for distances over 1 km. Multiple regression analysis showed significant influence of moisture related weather variables on inoculum dispersal. Results suggest that sanitation measures can help reduce ergot severity at the local level, but sustainable management will require better understanding of long-distance dispersal of C. africana inoculum.     

Patterns of release of the secondary conidia of Claviceps africana, the sorghum ergot pathogen in Australia

Trials were conducted in southern Queensland, Australia between March and May 2003, 2004 and 2005 to study patterns of hourly and daily release of the secondary conidia of Claviceps africana and their relationships with weather parameters. Conidia were trapped for at least one hour on most (> 90%) days in 2003 and 2004, but only on 55% of days in 2005. Both the highest daily concentration of conidia, and the highest number of hours per day when conidia were trapped, were recorded 1–3 days after rainfall events. Although the pattern of conidial release was different every day, the highest hourly conidial concentrations occurred between 10·00 hours and 17·00 hours on 73% of all days in the three trials. Hours when conidia were trapped were characterized by higher median values of temperature, windspeed and vapour pressure deficit, lower relative humidity, and leaf wetness values of 0%, than hours when no conidia were recorded. The results indicate that fungicides need to be applied to the highly ergot-susceptible male sterile (A-) lines of sorghum in hybrid seed production blocks and breeders' nurseries as soon as possible after rainfall events to minimize ergot severity.     

Predicting light leaf spot (Pyrenopeziza brassicae) risk on winter oilseed rape (Brassica napus) in England and Wales, using survey, weather and crop information

Data from surveys of winter oilseed rape crops in England and Wales in growing seasons with harvests in 1987–99 were used to construct statistical models to predict, in autumn (October), the incidence of light leaf spot caused by Pyrenopeziza brassicae on winter oilseed rape crops the following spring (March/April), at both regional and individual crop scales. Regions (groups of counties) with similar seasonal patterns of incidence (percentage of plants affected) of light leaf spot were defined by using principal coordinates analysis on the survey data. At the regional scale, explanatory variables for the statistical models were regional weather (mean summer temperature and mean monthly winter rainfall) and survey data for regional light leaf spot incidence (percentage of plants with affected pods) in July of the previous season. At the crop scale, further explanatory variables were crop cultivar (light leaf spot resistance rating), sowing date (number of weeks before/after 1 September), autumn fungicide use and light leaf spot incidence in autumn. Risk of severe light leaf spot (> 25% plants affected) in a crop in spring was also predicted, and uncertainty in predictions was assessed. The models were validated using data from spring surveys of winter oilseed rape crops in England and Wales from 2000 to 2003, and reasons for uncertainty in predictions for individual crops are discussed.     

Comparative ecophysiology of grain sorghum and Abutilon theophrasti in monoculture and in mixture

Selection of crop genotypes that are more competitive with weeds for light interception may improve crop yield stability in the presence of weeds. The effects of interference on ecophysiological characteristics of Abutilon theophrasti Medic. and three morphologically diverse grain sorghum hybrids was evaluated to determine the relative tolerance and suppressive ability of the three hybrids and specific traits that may contribute to those differences. A tall hybrid was more tolerant to A. theophrasti interference than two medium stature hybrids. Early leaf area growth of two medium-stature sorghum hybrids was reduced by A. theophrasti interference, whereas early growth of a tall hybrid was unaffected. The height of A. theophrasti was greater than two moderate-stature hybrids but lower than the tall hybrid. Greatest leaf area density (L_D) of the tall sorghum hybrid was above that of A. theophrasti , whereas greatest L_D of medium-stature hybrids was below that of the weed. In monoculture, the partitioning of new biomass to various plant organs was similar among sorghum hybrids, whereas the tall sorghum hybrid partitioned less biomass to leaves and more to stems than medium hybrids in mixture. The results indicate that the three hybrids differ in their susceptibility to A. theophrasti competition. Crop traits that may contribute to greater crop competitiveness include greater maximum height and its growth rate and greater height of maximum leaf area distribution.     

Long-term tillage and crop rotation effects on weed seedbank size and composition

Size and composition of the weed seedbank was assessed after 12 years of application of four tillage systems in two crop rotations. Mouldboard and chisel ploughing at 45 cm, minimum tillage at 15 cm and no tillage were compared in continuous winter wheat and a pigeon bean/winter wheat 2-year rotation. Weed control was based upon post-emergence herbicide application. Weed seedling emergence from soil samples taken at 0–15, 15–30 and 30–45 cm depths was assessed in a non-heated glasshouse for 12 months. The tillage system influenced weed seedbank size and composition to a much greater extent than crop rotation. Total weed seedling density was higher in no tillage, minimum tillage and chisel ploughing plots in the 0–15, 15–30 and 30–45 cm layers respectively. Density in the whole (0–45 cm) layer did not differ significantly among tillage systems. With no tillage, more than 60% of the total seedlings emerged from the surface layer, compared with an average 43% in the other tillage systems. Crop rotation did not influence either weed seedbank size or seedling distribution among soil layers, and only had a small influence on major species abundance. The weed seedbank was dominated (>66%) by Conyza canadensis (L.) Cronq. and Amaranthus retroflexus (L.), which thrived in chisel ploughing and no tillage respectively. Results suggested that crop rotation and substitution of mouldboard ploughing by non-inversion tillage (especially by minimum tillage) would not result in increased weed problems, whereas use of no tillage might increase weed infestations because of higher seedling recruitment from the topsoil.     

利用抗病品种防治高粱丝黑穗病的研究

1979～1982年鉴定出19个免疫高粱品种,2个免疫恢复系,4个免疫和1个高抗不育系,其中Tx622A免疫不育系的抗病性似为显性。用Tx622A免疫不育系组配的杂交种Tx622A×4003、Tx622A×晋辐1、Tx622A×298/4003、Tx622A×锦恢75为免疫杂交种,Tx622A×208为高抗杂交种,Tx622A×白平为抗病杂交种。1981～1983年用以上抗病高产杂交种进行了20多万亩的防治示范,对丝黑穗病的防治效果均极为显著,由92～100％,平均99.0％,增产粮食1000多万斤。     

Field validation of the factors related to clodinafop efficacy on Avena species

Previous research has shown that historical data and interpolated weather information can be used to correlate environmental and other variables with herbicide efficacy. This article examines this possibility using field experiments and on-site weather data. Clodinafop was applied to Avena spp. (wild oat) infestations on 46 occasions, in eight separate experiments, during 2003–2004 across the grain growing regions of New South Wales, Australia. Linear mixed models and covariate analyses were used to determine correlations of agronomic and environmental variables with clodinafop efficacy on Avena spp., as measured by plant mortality, panicle density and seed production. Clodinafop dose, maximum temperature on the day of spraying, spray water volume, the spray water volume by maximum temperature interaction and available soil moisture at spraying were correlated with mortality. Minimum temperatures prior to spraying, previously reported to be correlated with clodinafop efficacy, were not a significant factor in this study. On-site weather data were highly correlated with interpolated, generic weather data and thus there was little impact on which of these meteorological data sets were used to determine the correlations. This suggests that accessible, generic weather data could be used to develop and implement a predictive model for clodinafop efficacy. The relationship between the variables and Avena spp. mortality were generally supported when applied to panicle density and estimated seed production, with some exceptions. The results indicate that it should be possible to incorporate industry and field data to develop a robust predictive model for Avena spp. control with clodinafop.     

Genetic diversity of Claviceps africana on sorghum and Hyparrhenia

RAPD markers were used to survey genetic variability among 140 isolates of Claviceps africana collected from Southern Africa, India, Thailand, Australia and the Americas in 1992–2002. Amplified fragment length polymorphisms were determined for a subset of the isolates. Both markers gave similar results in phenetic analysis of genetic distances between haplotypes of different geographical origin. In the Americas, a single RAPD haplotype was found throughout the various countries. The Eastern lineage consisted of two close haplotypes (one from India, the other from Thailand and Australia). Among five specialized isolates of C. africana from the alternative hosts ( Hyparrhenia spp.), three haplotypes were found. Eleven private alleles distinguished the Hyparrhenia population from that on sorghum. rDNA sequences of sorghum and Hyparrhenia isolates differed in three positions. The African sorghum population of C. africana consisted of 10, mostly closely related haplotypes. Low genotypic diversity ( H _E = 0·0337) and the fact that most of the variation originated from between populations ( G _ST = 0·866) suggested founder effects following recent invasion. In Southern Africa, no significant differentiation was found among six populations. Therefore the data were pooled and tested for prevalence of clonal or sexual reproduction. The presence of the over-represented, widespread RAPD haplotype A; gametic disequilibrium (37% loci detected by exact tests); index of association ( I _A) significantly >0; and the high proportion of compatible loci (in the clone-corrected and total data sets found to be 94 and 99%, respectively) support the hypothesis of clonality as the predominant means of reproduction.     

Relationship between Concentrations of Botrytis Cinerea Conidia in Air, Environmental Conditions, and the Incidence of Grey Mould in Strawberry Flowers and Fruits

Atmospheric concentrations of Botrytis cinerea conidia were monitored for two seasons in a strawberry crop in Moguer (Huelva, southwestern Spain). Concentrations of conidia were estimated using a Burkard volumetric spore sampler. A diurnal pattern of conidial release was observed. Airborne conidial concentration was significantly and positively correlated with the average solar radiation and mean temperature, and negatively with rainfall and relative humidity. Among the weather variables considered, solar radiation showed the most consistent results in the regression analysis, explaining over 40% of airborne conidial concentration variability. Correlation between Botrytis fruit rot incidence and accumulated number of conidia over seven days was significant and positive. Two regression models containing three variables explained over 62 and 52% of the fruit rot incidence variability. A positive but non-significant correlation was established between B. cinerea incidence in flowers and airborne conidial concentration. It was not possible to fit a consistent regression model to relate flower infection incidence to conidial concentration or weather variables.     

Modeling Infection of Strawberry Flowers by Botrytis cinerea Using Field Data

ABSTRACT The incidence of strawberry flower infection by Botrytis cinerea was monitored in unsprayed field plots in three successive years together with meteorological data and numbers of conidia in the air. There were large differences in conidia numbers and weather conditions in the 3 years. Three sets of models were derived to relate inoculum and weather conditions to the incidence of flower infection; by inoculum only, by weather variables only, and by both inoculum and weather variables. All the models fitted the observed incidence satisfactorily. High inoculum led to more infection. Models using weather variables only gave more accurate predictions than models using inoculum only. Models using both weather variables and inoculum gave the best predictions, but the improvement over the models based on weather variables only was small. The relationship between incidence of flower infection and inoculum and weather variables was generally consistent between years. Of the weather variables examined, daytime vapor pressure deficit and nighttime temperature had the greatest effect in determining daily incidence of flower infection. Infection was favored by low day vapor pressure deficit and high night temperature. The accuracy and consistency of the weather-based models suggest they could be explored to assist in management of gray mold.     

Rapid emergence of hybrids between the two subspecies of Ophiostoma novo-ulmi with a high level of pathogenic fitness

During the 1970s Europe was invaded by two subspecies of the Dutch elm disease pathogen Ophiostoma novo-ulmi : subsp . americana from the west and subsp. novo-ulmi from the east. As a result their geographic ranges began to overlap in several areas. Only a weak prezygotic barrier to hybridization exists between the subspecies and in 1980 two hybrids were detected in the Netherlands. A subset of 107 O. novo-ulmi isolates collected in a subspecies overlap zone in Limburg, Netherlands in 1983 was characterized for three phenotypic markers and seven RAPD PCR markers. By phenotype, 33% were shown to be hybrid whereas by RAPD markers 69% were shown to be hybrid. Some isolates shown to be hybrid by phenotype were not revealed to be hybrid by PCR and vice versa. Combining the phenotype and RAPD data the estimated hybrid frequency was ∼78%. The mean growth rate of Limburg hybrid isolates was significantly faster than that of the Limburg subsp. novo-ulmi isolates but not significantly different from Limburg subsp. americana isolates. The Limburg hybrid isolates were just as pathogenic as the parent subspecies on both clonal Ulmus procera and on U.  × Commelin. A subset of 100 isolates collected in another subspecies overlap zone at Orvieto, Italy in 1986 was also assessed with RAPD markers and ∼ 72% were shown to be hybrids. When 20 isolates of a 'pure' subsp. novo-ulmi population in the Baltic Ports area of Poland collected in 1980 were assessed by RAPD markers three isolates exhibited early introgression of subsp. americana DNA. This study therefore demonstrates very rapid emergence of O. novo-ulmi subspecies hybrids and introgressants in Europe in the early 1980s. In terms of two major fitness characters, growth rate and pathogenicity, these early hybrids were as fit as their parent subspecies. It is likely that complex hybrid swarms are now expanding across the continent.     

Effect of chemical and cultural methods of weed control on the yield and growth of maize

Abstract

Traditional sorghum cultivars in the tropics are generally photoperiod sensitive, flowering just as or after the rains cease, so that their grains fill and mature during dry weather. Improved cultivars have been developed which flower and mature earlier in the season, when soil moisture levels are generally more favourable for grain filling, which potentially gives higher grain yields. However, the earlier flowering often results in the exposure of developing grain to wet conditions in which it can deteriorate rapidly. Grain moulds are a major component of the sorghum grain deterioration complex, and have become a widespread problem of improved sorghums in temperate and tropical regions. Many fungi have been isolated from mouldy grain, the most commonly occurring genera being Fusarium and Curvularia, and vary from those such as F. moniliforme Sheldon which are pathogenic on young developing inflorescences, to many saprophytic fungi which develop on the mature grains. The review covers terminology, causal agents, time of infection, predisposing factors, effects on yield and quality, control measures, resistance screening procedures and progress and recommendations for further research particularly in resistance.     

Development of empirical forecasting models for rice blast based on weather factors

Regression equations used as empirical models to predict rice blast caused by Pyricularia grisea on cv. Jinheung at Icheon, South Korea, and on cvs. IR50 and C22 at Cavinti, Philippines, were generated, using weather factors identified by the WINDOW PANE program to be highly correlated with disease. Consecutive days with RH≥80% (CDRH80), number of days with RH≥80% (NDRH80), consecutive days with precipitation, and number of days with precipitation ≥ 84 mm day⁻¹ were important variables predicting blast at Icheon. Total precipitation, precipitation frequency, mean maximum and minimum temperatures, number of days with wind speed above 3.5 m s⁻¹, CDRH80, and NDRH80 were important predictors of blast at Cavinti. The Allen's predicted error sum of squares (PRESS) criterion and a cross-validation procedure were used to evaluate the models using data that were not included in model development. Validation test showed that all models developed for the two sites, except the models predicting maximum lesion number and panicle blast incidence at Icheon, and panicle blast severity on IR50 at Cavinti, predicted blast reasonably well based on low PRESS values and close to zero average prediction errors. These models can be applied in actual rice production systems, but future validation is needed to further improve their predictive ability.     

Epidemics of ray blight on pyrethrum are linked to seed contamination and overwintering inoculum of Phoma ligulicola var. inoxydabilis

Ray blight, caused by Phoma ligulicola var. inoxydabilis, is the most damaging disease of pyrethrum (Tanacetum cinerariifolium) in Australia. Data collected from 72 plots in commercial pyrethrum fields since 2001 to 2009 revealed substantial annual variations in isolation frequency of the pathogen during semidormancy of the crop in autumn and winter. Isolation frequency of the pathogen during this time and subsequent outbreaks of ray blight in spring were similar across the eight production regions where sampling was conducted, and isolation frequency of the pathogen was linearly correlated (r = 0.88; P < 0.0001) with subsequent defoliation severity when plants commenced growth in spring. Isolation frequency and defoliation severity also were correlated with the incidence of seed infested with P. ligulicola var. inoxydabilis (r = 0.71 and 0.44, respectively; P < 0.0001 in both correlations). Highly accurate risk algorithms for the occurrence of severe epidemics of ray blight were constructed using logistic regression. A model based solely on isolation frequency of the pathogen over autumn and winter correctly predicted epidemic development in 92% of fields. Another model utilizing the incidence of infested seed and rain-temperature interactions in early autumn (austral March and April) and late winter (austral June and July) had similar predictive ability (92% accuracy). Path analysis modeling was used to disentangle interrelationships among the explanatory variables used in the second logistic regression model. The analysis indicated that seedborne inoculum of P. ligulicola var. inoxydabilis contributes indirectly to ray blight defoliation severity through directly increasing overwintering frequency of the pathogen. Autumn and fall weather variables were modeled to have indirect effects on defoliation severity through increasing overwintering success of the pathogen but also direct effects on defoliation severity. Collectively, the analyses point to several critical stages in the disease cycle that can be targeted to minimize the probability of regional epidemics of ray blight in this perennial pathosystem.     

Influence of weather factors on sorghum earhead bug Calocoris angustatus Lethierry incidence

Abstract

Weather parameters during the same week and one, two, three or four weeks before were correlated with the population of earhead bug, Calocoris angustatus on different earhead stages of two hybrids, CSH 5 and K‐Tall, in main and ratoon crops. The weather factors in the different weeks contributing to the insect population varied with the stage of earhead, the hybrid and the nature of the crop (main or ratoon). In the main crop, the insect population at pre‐flowering, milky, dough and maturity stage was influenced by weather parameters one week before, the same, two and three weeks before respectively. Morning relative humidity was important in both hybrids at all stages, with a few exceptions. In the ratoon crop, the pre‐flowering and maturity stage populations were influenced by the weather parameters of the same week. The populations at milky and maturity stage were influenced by parameters four weeks and one week before the observations respectively. The factors influencing the population on CSH 5 and K‐Tall varied. In most cases morning relative humidity had a positive influence.     

Models for predicting potential yield loss of wheat caused by stripe rust in the U.S. Pacific Northwest

Climatic variation in the U.S. Pacific Northwest (PNW) affects epidemics of wheat stripe rust caused by Puccinia striiformis f. sp. tritici. Previous models only estimated disease severity at the flowering stage, which may not predict the actual yield loss. To identify weather factors correlated to stripe rust epidemics and develop models for predicting potential yield loss, correlation and regression analyses were conducted using weather parameters and historical yield loss data from 1993 to 2007 for winter wheat and 1995 to 2007 for spring wheat. Among 1,376 weather variables, 54 were correlated to yield loss of winter wheat and 18 to yield loss of spring wheat. Among the seasons, winter temperature variables were more highly correlated to wheat yield loss than the other seasons. The sum of daily temperatures and accumulated negative degree days of February were more highly correlated to winter wheat yield loss than the other monthly winter variables. In addition, the number of winter rainfall days was found correlated with yield loss. Six yield loss models were selected for each of winter and spring wheats based on their better correlation coefficients, time of weather data availability during the crop season, and better performance in validation tests. Compared with previous models, the new system of using a series of the selected models has advantages that should make it more suitable for forecasting and managing stripe rust in the major wheat growing areas in the U.S. PNW, where the weather conditions have become more favorable to stripe rust.     

Effect of temperature and leaf wetness duration on infection of sweet oranges by Asiatic citrus canker

Asiatic citrus canker, caused by Xanthomonas smithii ssp. citri , formerly X. axonopodis pv. citri , is one of the most serious phytosanitary problems in Brazilian citrus crops. Experiments were conducted under controlled conditions to assess the influence of temperature and leaf wetness duration on infection and subsequent symptom development of citrus canker in sweet orange cvs Hamlin, Natal, Pera and Valencia. The quantified variables were incubation period, disease incidence, disease severity, mean lesion density and mean lesion size at temperatures of 12, 15, 20, 25, 30, 35, 40 and 42°C, and leaf wetness durations of 0, 4, 8, 12, 16, 20 and 24 h. Symptoms did not develop at 42°C. A generalized beta function showed a good fit to the temperature data, severity being highest in the range 30–35°C. The relationship between citrus canker severity and leaf wetness duration was explained by a monomolecular model, with the greatest severity occurring at 24 h of leaf wetness, with 4 h of wetness being the minimum duration sufficient to cause 100% incidence at optimal temperatures of 25–35°C. Mean lesion density behaved similarly to disease severity in relation to temperature variation and leaf wetness duration. A combined monomolecular-beta generalized model fitted disease severity, mean lesion density or lesion size as a function of both temperature and duration of leaf wetness. The estimated minimum and maximum temperatures for the occurrence of disease were 12°C and 40°C, respectively.     

Production of conidia by Peronosclerospora sorghi on sorghum crops in Zimbabwe

Factors affecting the production of conidia of Peronosclerospora sorghi , causing sorghum downy mildew (SDM), were investigated during 1993 and 1994 in Zimbabwe. In the field conidia were detected on nights when the minimum temperature was in the range 10–19°C. On 73% of nights when conidia were detected rain had fallen within the previous 72 h and on 64% of nights wind speed was < 2.0 m s⁻¹. The time period over which conidia were detected was 2–9 h. Using incubated leaf material, conidia were produced in the temperature range 10–26°C. Local lesions and systemically infected leaf material produced 2.4–5.7 × 10³ conidia per cm². Under controlled conditions conidia were released from conidiophores for 2.5 h after maturation and were shown to be well adapted to wind dispersal, having a settling velocity of 1.5 × 10⁻⁴ m s⁻¹. Conditions that are suitable for conidia production occur in Zimbabwe and other semi-arid regions of southern Africa during the cropping season.     

Eupatorium Weed Species in Australia

Abstract

High temperature together with high rainfall favour infection and development of Erwinia stalk rot of maize (Erwinia chrysanthemi pv. zeae Victoria, Arboleda and Munoz). An attempt was made to correlate temperature, relative humidity, total rainfall and duration of bright sunshine with disease incidence in six maize cultivars during six crop seasons. Temperature and relative humidity did not fluctuate much during the flowering period at the time of disease appearance. Significant differences in total rainfall and duration of bright sunshine were observed. Longer duration of bright sunshine (8.2 h/day) along with moderate rainfall (81.7 mm) were recorded in high disease incidence years. Using these weather conditions a linear equation was derived to predict disease incidence.