A comparison of inoculation techniques for assessment of germplasm susceptibility to Karnal bunt (Tilletia indica) disease of wheat

Three inoculation techniques for Karnal bunt (Tilletia indica) disease of wheat were compared: 1) boot inoculation - injection of inoculum with a hypodermic syringe into the boot; 2) spray inoculation - inoculum sprayed at growth stages between heading and anthesis, and 3) cotton wool inoculation - small pieces of cotton wool saturated in inoculum placed either inside the floret or between the spikelet and rachis. Each inoculation technique was assessed using susceptible cultivars to determine the optimum inoculum concentration, the ideal plant growth stage and the humidity requirements for successful infection.
Boot inoculation did not require high humidity and gave reliable infection with low secondary sporidia concentrations (1000–10 000/ml). The ideal plant growth stages for inoculation were early-boot and mid-boot. Spray inoculation required high secondary sporidia concentrations (50 000/ml) and 48 h of high humidity, but infection was initiated over a range of growth stages throughout heading and anthesis. Cotton wool inoculation gave low levels of infection at growth stages throughout heading and anthesis, even with high secondary sporidia concentrations (100 000/ml).     

A greenhouse assay to screen sunflower for resistance to Alternaria helianthi

A greenhouse assay to screen sunflower for resistance to Alternaria helianthi is described. A comparison of conditions led to the following standard conditions being recommended. The first or second pair of leaves of seedling plants at the V8 growth stage are inoculated using inoculum grown on sunflower leaf extract agar for 5–10 days at an inoculum density of 1–2 spores cm² of leaf tissue. A 48 h dew period should be applied to plants covered by a plastic tent. A dew period temperature of 26/26°C night/day and a post-dew period temperature relative to that experienced under local growing conditions should be applied. Lesions are measured 7 days after inoculation, and mean lesion size per plant is calculated. Mean lesion size of lines being tested is expressed as a proportion of the mean lesion size of a susceptible standard included in each screening experiment.     

Effect of Epidemiological Factors on the Impact of the Fungus Plectosporium tabacinum on False Cleavers ( Galium spurium )

The fungus Plectosporium tabacinum has been evaluated as a bioherbicide for control of false cleavers ( Galium spurium ), but the limiting factors and optimal conditions for successful control using this pathogen are not known. False cleavers mortality and dry weight reductions caused by P. tabacinum were assessed under single or combined factors, including pathogen inoculum concentration, plant growth stage, dew period (duration, frequency, and timing), and dew period temperature. The minimum inoculum concentration required to kill false cleavers seedlings was 1 ×10 7 conidia mL -1 at an application rate of 150 mL m -2 . False cleavers seedlings in the cotyledon or 1-whorl growth stage were the most susceptible. Increasing inoculum concentration increased weed control efficacy on older false cleavers seedlings. When an adequate dew period was provided, 100% mortality and dry weight reduction occurred. The minimum dew period to achieve 100% mortality was 16 h. Delaying the initiation of the dew period by 24 h significantly reduced disease development. Short, repetitive dew periods only improved false cleavers control to a limited extend and the length of initial dew is a critical factor influencing false cleavers mortality due to P. tabacinum . The optimal dew period temperature for disease development was above 15°C.     

Preliminary studies on biological control of the blackpoint complex of wheat in Argentina

Blackpoint is a brownish or black discoloration of wheat kernels and biological control is a complementary strategy to manage the disease. This work evaluated the effect of five strains of Trichoderma harzianum and one strain of T. koningii on the growth of Bipolaris sorokiniana and Alternaria alternata and compared the results of screening tests under controlled conditions and field evaluations on bread and durum wheat ears. Disease incidence, infection percentage and seedling emergence percentage determined in a greenhouse assay were evaluated. Dual cultures showed Trichoderma spp. inhibited significantly the mycelial growth of B. sorokiniana between 36 and 71% and of A. alternata between 41 and 61%. Microscopic examination of B. sorokiniana and A. alternata showed plasmolysis and vacuolization of hyphae of the pathogens in the presence of the antagonists tested. With pre-inoculation of wheat ears at anthesis under field conditions, disease incidence, infection percentage by blotter tests and seedling emergence in the greenhouse did not show significant differences between controls and treatments with Trichoderma spp. This revised version was published online in August 2006 with corrections to the Cover Date.     

Impact of aggressiveness of <Emphasis Type="Italic">Fusarium graminearum</Emphasis> and <Emphasis Type="Italic">F. culmorum</Emphasis> isolates on yield parameters and mycotoxin production in wheat

Plant-associated isolates from Fusarium graminearum and F. culmorum were inoculated on wheat in field experiments in 2007 and 2008 to ascertain their influence on fungal colonization of the ears, as well as mycotoxin contamination (deoxynivalenol, DON; nivalenol, NIV; zearalenone, ZEA) and yield parameters in the mature crop after inoculation with or without irrigation. The isolates were assigned to four different groups of aggressiveness on the basis of pathogenic symptom development and mycotoxin production in vitro. Increased levels of trichothecene-producing Fusarium DNA in the ears indicated a successful inoculation of the plants, which resulted in increased DON content in the wheat kernels in 2007. Dry conditions at anthesis markedly suppressed fungal colonization as well as mycotoxin accumulation. However, due to precipitation during the ripening period, yield and thousand-kernel weight were similar whether or not irrigation was applied at the time of inoculation. The level of aggressiveness among the isolates as determined in vitro was not reflected in the field experiment. The activity of the extracellular invertase in developing ears increased as a plant response to pathogen infection, especially when the plants were irrigated at the time of inoculation. In 2008, the Fusarium inoculation of wheat heads did not cause fungal growth and mycotoxin contamination in the grain, because of the dry weather conditions that occurred over the entire period of anthesis and ripening. The risk of future mycotoxin contamination in grains was discussed based on climate change prognosis.     

Impact of Piriformospora indica on tomato growth and on interaction with fungal and viral pathogens

Piriformospora indica is a root endophytic fungus with plant-promoting properties in numerous plant species and induces resistance against root and shoot pathogens in barley, wheat, and Arabidopsis. A study over several years showed that the endophyte P. indica colonised the roots of the most consumed vegetable crop tomato. P. indica improved the growth of tomato resulting in increased biomass of leaves by up to 20%. Limitation of disease severity caused by Verticillium dahliae by more than 30% was observed on tomato plants colonised by the endophyte. Further experiments were carried out in hydroponic cultures which are commonly used for the indoor production of tomatoes in central Europe. After adaptation of inoculation techniques (inoculum density, plant stage), it was shown that P. indica influences the concentration of Pepino mosaic virus in tomato shoots. The outcome of the interaction seems to be affected by light intensity. Most importantly, the endophyte increases tomato fruit biomass in hydroponic culture concerning fresh weight (up to 100%) and dry matter content (up to 20%). Hence, P. indica represents a suitable growth promoting endophyte for tomato which can be applied in production systems of this important vegetable plant not only in soil, but also in hydroponic cultures.     

Disease spread of non-specialised fungal pathogens from inoculated point sources in intraspecific mixed stands of cereal cultivars

Spread of Septoria nodorum from inoculated point sources was examined in pure stands and mixtures of two spring wheat cultivars Kolibri and Maris Butler. Gradients in disease were observed soon after inoculation; the presence of the more resistant cultivar (Maris Butler) in the mixtures retarded the outward spread of disease compared with the susceptible pure stand (Kolibri). Regression analysis suggested that gradients in incidence were influenced by nearness to the source whereas gradients in severity were not. Spread of disease was also examined in pure stands and mixtures for the host/pathogen combinations, winter wheat (cvs Maris Huntsman and Maris Ranger)/S. nodorum and winter barley (cvs Maris Otter and Hoppel)/Rhynchosporium secalis. In contrast to the spring wheat experiment, no gradients were observed; explanations for the uniform distribution of disease were (a) extensive spread prior to the period of assessment, (b) no physical barrier to dispersal due to the juvenile growth stage of the crop and (c) exhaustion of the point-source.     

The Influence of the Inoculum Source of Rhizoctonia solani on Development of Black Scurf on Potato

Rhizoctonia solani, the causal agent of stem canker and black scurf on potato, survives as sclerotia on tubers, in soil and in plant residues. The objective of the present study was to evaluate the importance of inoculum source on disease development. Disease‐free minitubers and seed tubers contaminated with low levels of R. solani were planted in fumigated or artificially inoculated growth mixture in greenhouse experiments. Black scurf incidence and severity were significantly higher when the inoculum was present in both seed tubers and soil, compared with either of them separately. The severity of disease symptoms on the subterranean parts of the plant also were significantly higher in plots where both seed tubers and soil were contaminated, compared with plots where the inoculum source was either the seed tubers or the soil. Thus, both major sources of inoculum, seed tubers and soil, are important in disease development. However, when both sources are present, black scurf incidence and severity are increased, leading to economical damage to tuber yield and quality. Additional results from field trials support these findings. Disease incidence and severity on daughter tubers were correlated with levels of contamination in seed tubers and soil. When seed tubers and soil were heavily infested, the levels of black scurf incidence and severity on daughter tubers were very high; when seed tuber and soil infestation were very low, black scurf incidence and severity on progeny were also lower. Disease levels were reduced by in‐furrow fungicide treatment, but were less effective when the initial levels of the fungus on the seed tubers and in the soil were high.     

Interaction between zinc nutritional status of cereals and Rhizoctonia root rot severity

An inverse correlation between plant Zn concentration and the severity of Rhizoctonia root rot, described in an earlier paper, was examined in two experiments in a growth chamber. In the first experiment, wheat (Triticum aestivum cv Songlen) was planted in a Zn deficient soil with and without added Zn, and combined factorially with different inoculum densities of Rhizoctonia solani anastomosis group 8. When Zn was added, the percentage of seminal roots infected with R. solani was significantly lower compared to the treatments without added Zn, showing that low Zn potentiated the disease. A subsequent factorial experiment of four inoculum densities and six Zn levels, (0, 0.01, 0.04, 0.1, 0.4 and 6.0 mg Zn kg^–1 soil) was conducted to investigate the Zn effect in more detail. Disease severity was markedly decreased by the higher Zn applications; the disease score dropped sharply between treatments of Zn_0.04 and Zn_0.1, a difference which was reflected in the plant yield response to Zn. For both experiments the Zn concentrations in shoots were significantly different only among Zn treatments, not among the inoculum treatments. This indicated that inoculum density or disease severity did not reduce Zn concentration in the plant. Thus, disease did not exaggerate Zn deficiency, but rather, Zn sufficiency suppressed disease severity. A potentiating link between Zn nutrition and disease severity is thereby established, although this type of experiment did not indicate the mechanism of the Zn effect.     

Effects of ammonium-N on development and infectivity of the take-all fungus

Effects of applications of a mixture of ammonium sulphate and mono-ammonium phosphate and of ammonium nitrate on the incidence of take-all disease of wheat (caused by Gaeumannomyces graminis var. tritici) and on subsequent inoculum levels were studied in field and glasshouse experiments. In a field experiment in Western Australia, on a sandy soil at pH 5·4, nitrogen applications had no detectable effect on disease severity at anthesis, but ammonium sulphate treatment increased the number of propagules of the pathogen in the soil. In a pot experiment, in which seed was sown in the field experiment soils, disease was greater in soil from plots treated with ammonium sulphate and least in soil from the nil-nitrogen plots, reflecting the respective inoculum levels in the field plot. However, treatment of the soils of lower inoculum with ammonium sulphate and ammonium nitrate during this pot experiment decreased disease. A second pot experiment confirmed the effectiveness of ammonium sulphate and ammonium nitrate in reducing take-all at lower inoculum levels, and their ineffectiveness at higher inoculum levels.     

Effects of Leaf Wetness Duration and Inoculum Level on Resistance of Wheat Genotypes to Pyrenophora tritici-repentis

Percent leaf necrosis and lesion length on wheat genotypes increased markedly with increasing duration of leaf wetness (up to 24h or 48 h) following inoculation with Pyrenophora tritici-repentis. A long wetting duration favoured less disease development on resistant (Fink's'), and moderately resistant (Bon/YR/3/F3570/KAL/BB) genotypes than on susceptible Glenlea. No significant difference in percent necrosis was detected among the upper three leaf positions within a genotype. A long wetnessduration had a varying effect on the resistance of wheat genotypes, depending upon the inoculum level. Increasing the inoculum level along with the leaf wetness period increased the per cent leaf necrosis on all three wheat genotypes tested. However, the, ranking of the genotype for resistance did not alter even after prolonged duration of leaf wetness (up to 96 h) and/or high inoculum level (12000 conidia/ml water). Various post-inoculation wet-periods in combination with high conidia concentrations in inoculum should be used in identifying highly resistant germplasm in breeding populations at the seedling stage of the wheats.     

Mycelium of Alternaria alternata as a potential biological control agent for Eupatorium adenophorum

The fungus, Alternaria alternata (Fr.) Keissler Strain 501, has been evaluated as a bioherbicide for control of Eupatorium adenophorum Spreng., but the biology of the pathogen–host interaction and the optimal environmental conditions for disease development and effective weed control are unknown. Disease development of A. alternata Strain 501 mycelia on E. adenophorum was assessed under several factors including pathogen inoculum concentration, plant age, dew period duration, post-dew temperature, storage temperature and duration. The minimum inoculum concentration required to kill E. adenophorum seedlings was 3.2×10⁶ mycelial fragment mL^?1. E. adenophorum seedlings at the four-leaf-pair stage were more susceptible than the older plants, especially those at the older than seven-leaf-pair stage. With a dew period of at least 14 h, 100% mortality occurred. The optimal post-dew temperature for disease development was 18–25°C. Storage at <4°C maintained the infectivity of A. alternata strain 501 mycelia on E. adenophorum longer. Using light and scanning electron microscopy to examine the infection process of A. alternata Strain 501 mycelia, it was shown that the time from initiation to completion of infection with mycelia was much shorter (14 h) than with conidia (72 h). It was further shown that mycelial infection occurred predominately through direct penetration at intercellular junctions, while conidial infection occurred predominately through stomatal penetration. This suggests that mycelia are more suitable as infection propagules for A. alternata strain 501 in a bioherbicide for the control of E. adenophorum.     

Effect of Inoculum Density on Verticillium Wilt Incidence in Commercial Olive Orchards

Verticillium wilt, caused by Verticillium dahliae Kleb., is presently the most destructive disease of olive, particularly in Andalucía (southern Spain). ‘Picual’ and ‘Arbequina’ are the dominant cultivars being planted in Spain. Both cultivars are highly susceptible to the defoliating pathotype of V. dahliae when artificially inoculated by root‐dipping or stem injection. Conversely, ‘Arbequina’ is considered more resistant than ‘Picual’ based on field observations and farmer's experience. In this study, the differential reaction between of cultivars was confirmed by surveys of naturally infested orchards with different inoculum densities of the pathogen. The average percentage of affected olive trees of ‘Picual’ was 60.2%, while only 13.1% of trees of ‘Arbequina’ showed disease symptoms. Overall, the pathogen caused extensive wilting of branches and defoliation on the trees of ‘Picual’, whereas ‘Arbequina’‐infected trees showed chlorotic symptoms and slight defoliation. The relationship between inoculum density and disease incidence fit a logarithmic function for both cultivars. The percentage of affected trees of ‘Arbequina’ per year increased linearly (y = 0.3559x, R² = 0.5652, and P = 0.0195) with the inoculum density in the soil, whereas this relationship was not observed for the ‘Picual’. Planting density had no effect on disease incidence for any of the two cultivars.     

Effects of Leaf Wetness Duration and Inoculum Level on Resistance of Wheat Genotypes to Pyrenophora tritici-repentis

Percent leaf necrosis and lesion length on wheat genotypes increased markedly with increasing duration of leaf wetness (up to 24h or 48 h) following inoculation with Pyrenophora tritici-repentis. A long wetting duration favoured less disease development on resistant (Fink's'), and moderately resistant (Bon/YR/3/F3570//KAL/BB) genotypes than on susceptible Glenlea. No significant difference in per cent necrosis was detected among the upper three leaf positions within a genotype. A long wetness duration had a varying effect on the resistance of wheat genotypes, depending upon the inoculum level. Increasing the inoculum level along with the leaf wetness period increased the per cent leaf necrosis on all three wheat genotypes tested. However, the ranking of the genotype for resistance did not alter even after prolonged duration of leaf wetness (up to 96 h) and/or high inoculum level (12000 conidia/ml water). Various post-inoculation wet-periods in combination with high conidia concentrations in inoculum should be used in identifying highly resistant germplasm in breeding populations at the seedling stage of the wheats.     

Climate change increases risk of fusarium ear blight on wheat in central China

To estimate potential impact of climate change on wheat fusarium ear blight (FEB), simulated weather for the A1B climate change scenario was input into a model for estimating FEB in central China. In this article, a logistic weather‐based regression model for estimating incidence of wheat FEB in central China was developed, using up to 10 years (2001–2010) of disease, anthesis date and weather data available for 10 locations in Anhui and Hubei provinces. In the model, the weather variables were defined with respect to the anthesis date for each location in each year. The model suggested that incidence of FEB is related to number of days of rainfall in a 30‐day period after anthesis and that high temperatures before anthesis increase the incidence of disease. Validation was done to test whether this relationship was satisfied for another five locations in Anhui province with FEB data for 4–5 years but no nearby weather data, using simulated weather data obtained employing the regional climate modelling system PRECIS. How climate change may affect wheat anthesis date and FEB in central China was investigated for period 2020–2050 using wheat growth model Sirius and climate data simulated using PRECIS. The projection suggested that wheat anthesis dates will generally be earlier and FEB incidence will increase substantially for most locations.     

A Simulation Model for Assessing Soybean Rust Epidemics

A soybean rust (causal agent Phakopsora pachyrhizt) simulation model was developed for assessing disease epidemics as a part of pest risk analysis. Equations describing environmental effects on disease components were developed by re-analyzing previous data with a view toward a systems approach. The infection rate was predicted well using dew period and temperature after inoculation as independent variables (R²=0.88, P < 0.0001). The exponential models which used physiological day as an independent variable explained 98% of the variations of latent period and senescence of disease lesions. The simulation model was validated with data from 72 sequential planting experiments during 1980 and 1981 in Taiwan. Time of onset for these epidemics varied from 25—60 days and 50—80 days after planting soybean cultivars TK 5 and G 8587, respectively. The epidemic periods were 75—95 for TK 5 and 100—120 days for G 8587. Variation of epidemics was accurately predicted by the simulator. Predicted disease curves fit well the observed disease curves for the recognized cropping seasons, spring- and autumn-seeded crops. For G 8587, which is very sensitive to photoperiod, the data from spring and autumn gave a better fit compared with data from pre-summer planting. The model underestimated disease epidemics during the winter, probably because the plant growth model failed to reflect the photoperiod rection of soybean. The simulation model was validated with data from other experiments conducted in three cropping seasons in 1979 and 1980. Determination coefficients of the regression between observed and predicted disease severity were significant.     

First Quantitative Study of Rove Beetles in Oklahoma Winter Wheat Fields

Adult rove beetles (Staphylinidae) were sampled every 7–14 days from one winter wheat field located in each of the four major wheat growing regions of Oklahoma during the 1999–2000 and 2000–2001 growing seasons. The number of cereal aphids per tiller, wheat plant growth stage, and wheat tiller density also were estimated. A total of 12 genera representing 13 species of beetles were collected from the field. The density of rove beetles was generally low, ranging from 0.003 beetles per m² in fall to 0.106 beetles per m² in spring. Rove beetle communities differed among seasons. After accounting for the effect of season, there was no statistically significant association between rove beetle community structure and field location, aphid density, wheat plant growth stage, or wheat plant density. Most rove beetle species showed no association with a particular season, however, Aleochara notula Erichson, Lathrobium sp., and Oxypoda sp. were present predominantly in fall, while Bisnius inquitus Erichson was associated with winter. Oxypoda sp. was the most abundant rove beetle in winter wheat fields in spring and was relatively abundant in winter, but was not collected from wheat fields in fall. Tachyporus jocosus Say was present in wheat fields during all seasons. T. jocosus was the most abundant rove beetle species in the winter wheat fields in fall and winter and was the second most abundant species during spring.     

The effect of take-all disease on gas-exchange rates and biomass in two winter wheat lines with different drought response

There have been no studies of the effect of take-all on leaf gas-exchange rates, despite the fact that take-all severely restricts plant water and nutrient uptake, which results in significant biomass and grain yield reduction. Here we describe the effect of inoculation with Gaeumannomyces graminis (Sacc.) var. tritici (Ggt) on carbon assimilation rate (A) and biomass production of wheat plants grown under two water regimes. We show that the impact of Ggt inoculation on plant growth and leaf A may be through reduced photosynthetic capacity of the leaves and not water stress per se. The nature of this reduced photosynthetic capacity remains uncertain but may involve nutrient deficiency and different enzymes produced by the fungus. In each of the 3 years the experiment was conducted, Ggt significantly reduced A, i.e. at anthesis by 18% in 2000, 15% in 2001, and 12% in 2002. In agreement with other field studies, Ggt reduced tiller number and production of all plant components, mostly root dry mass and grain mass per plant. Highly significant negative correlations were found between disease rating and A in all years, showing that at disease ratings equal or higher than 3 (on a scale from 1 to 4) A could practically be zero. While A decreased, intercellular CO₂ concentration increased or did not change, and stomatal conductance was relatively high. In addition, A was more reduced under high than under low soil moisture content. These results support the idea that water stress per se did not contribute to the observed reduction of A. The mechanism of photosynthetic capacity reduction due to the Ggt root-rotting fungus is of interest as it may lead to the molecular mechanisms of plant resistance and ultimately to the development of take-all resistant plants.     

Host and environmental effects on the penetration of oats by Puccinia graminis avenae and Puccinia coronata avenae

The frequency of penetration from appressoria of Puccinia graminis avenae and P. coronata avenae varied among Avena species and between oat cultivars, although both rusts produced susceptible infection type pustules on the cultivars tested. Penetration on cv. Garry was significantly less than that on the Avena species (A. barbata, A.fatua and A. sterilis) studied and penetration of these Avena species was significantly less than on the cvs Algerian and Fulmark. When the rusts were allowed to develop into pustules on seedlings which had been inoculated with fixed amounts of inoculum, there was a direct relationship between number of pustules produced and penetration frequency. The effects of temperature, light and dew period on penetration from appressoria of ‘single race’ and ‘mixed race’ inocula was also studied on these cultivars and species. Penetration by P. graminis avenae was greatest at 30–35 °C and at light intensities of 5625 lux and above, whereas that by P. coronata avenae was greatest at 20 °C and was unaffected by artificial light intensities up to n 250 lux. Maximal penetration by P. graminis avenae and P. coronata avenae was observed after inoculated plants had been exposed to dew periods of 16 and 12 h respectively. Some penetration was observed after a dew period of 8 h. The time taken for each rust to attain maximum penetration varied from 36 to 52 h after inoculation, depending on the environment, and was usually less for P. coronata avenae than for P. graminis avenae.