Evaluation of polyclonal-antibody-based immunoassays for detection of Sclerotinia sclerotiorum on canola petals, and prediction of stem rot

The potential for using polyclonal-antibody-based immunoassays for detection of Sclerotinia sclerotiorum on canola petals as part of a disease prediction model was investigated. A commercial ELISA kit designed for Sclerotinia homoeocarpa was evaluated for specificity to S. sclerotiorum in comparison to other Sclerotinia spp., and known phyllosphere fungi. This polyclonal-antibody-based kit cross-reacted with antigens from other Sclerotinia spp., and fungi, and absorbance values obtained from S. sclerotiorum-infested canola petals were poorly correlated with percentages of infested petals as determined by plating on semi-selective medium, except when petals were incubated at high humidity for 24 h at 20 degrees C-22 degrees C prior to ELISA evaluation. Additional polyclonal antibodies were prepared from mycelial and semi-purified cell wall antigens, and these antibodies were more specific to S. sclerotiorum than the ELISA kit. However, absorbance values obtained from S. sclerotiorum-infested canola petals were poorly correlated with percentages of infested petals as determined by plating on semi-selective medium. The results are discussed in relation to the use of polyclonal-antibody-based immunoassays for the prediction of epidemics or crop risk from sclerotinia stem rot of canola.     

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.     

Expression of anti-sclerotinia scFv in transgenic Brassica napus enhances tolerance against stem rot

Canola is an important agricultural crop imparting a significant contribution to global oilseed production. As such, optimizing yield and quality is of paramount importance and canola production can be significantly affected by sclerotinia stem rot. The utility of recombinant antibody technology in plant protection has been explored by many researchers and shows promise for the generation of new lines of agriculturally significant crops with greater resistance to diseases. The objective of the current study was to generate recombinant pathogen specific antibody (scFv)-expressing transgenic Brassica napus plants with increased tolerance to the phytopathogenic fungus, Sclerotinia sclerotiorum. Transgenic canola (B. napus) lines expressing S. sclerotiorum-specific scFv antibody showed a significant level of tolerance towards S. sclerotiorum as compared to their non-transformed counterparts. Both incidence and progression of S. sclerotiorum-induced disease symptoms were reduced in plants expressing the recombinant scFv.     

Microsatellite markers used for genome-wide association mapping of partial resistance to <Emphasis Type="Italic">Sclerotinia sclerotiorum</Emphasis> in a world collection of <Emphasis Type="Italic">Brassica napus</Emphasis>

The fungal pathogen Sclerotinia sclerotiorum causes stem rot of oilseed rape (Brassica napus) worldwide. In preparation for genome-wide association mapping (GWAM) of sclerotinia resistance in B. napus, 152 accessions from diverse geographical regions were screened with a single Canadian isolate, #321. Plants were inoculated by attaching mycelium plugs to the main stem at full flower. Lesion lengths measured 7, 14 and 21 days after inoculation were used to calculate the area under the disease progress curve (AUDPC). Depth of penetration was noted and used to calculate percent soft and collapsed lesions (% s + c). The two disease traits were highly correlated (r = 0.93). Partially resistant accessions (AUDPC <7 and % s + c <2) were identified primarily from South Korea and Japan with a few from Pakistan, China and Europe. Genotyping of accessions with 84 simple sequence repeat markers provided 690 polymorphic loci for GWAM. The general linear model in TASSEL best fitted the data when adjusted for population structure (STRUCTURE), GLM + Q. After correction for positive false discovery rate, 34 loci were significantly associated with both disease traits of which 21 alleles contributed to resistance, while the remaining enhanced susceptibility. The phenotypic variation explained by the loci ranged from 6 to 25 %. Five loci mapped to published quantitative trait loci conferring sclerotinia resistance in Chinese lines.     

Canola quality affects second (<Emphasis Type="Italic">Brevicoryne brassicae</Emphasis>) and third (<Emphasis Type="Italic">Diaeretiella rapae</Emphasis>) trophic levels

Biological control agents can be used as a complementary control measure that can be combined with resistant host plants to control pests. In this study, the effects of different canola cultivars (Karaj-1, Karaj-2, Karaj-3, Licord, Okapi, Opera, RGS₀₀₃, Sarigol, Talaye and Zarfam) on the performance and life table parameters of the cabbage aphid, Brevicoryne brassicae, and its parasitoid, Diaeretiella rapae, were determined under laboratory conditions. Total fecundity of the cabbage aphid differed with cultivar, with the highest value (59.41 nymphs per female) of this parameter observed on Opera and the lowest (1.67) observed on RGS₀₀₃. The highest and lowest intrinsic rates of increase (r) of the cabbage aphid were observed on Opera (0.331 day^?1) and RGS₀₀₃ (? 0.242 day^?1) cultivars, respectively, suggesting these to be the most susceptible and most resistant cultivars to this pest. However, because the aphid did not settle and feed well on RGS₀₀₃, it was not possible to determine demographic parameters for its parasitoid. Consequently, the Okapi cultivar, which was the most resistant cultivar to the cabbage aphid after RGS₀₀₃, was used in this study to assess the parasitoid wasp. The parasitoid’s intrinsic rate of increase (r) varied from 0.426 day^?1 on the susceptible cultivar (Opera) to 0.341 day^?1 on the resistant canola cultivar Okapi. Aphid performance decreased 93% on the resistant canola cultivar, while parasitoid performance decreased only 20% on the resistant cultivar compared to more susceptible cultivar.     

Inhibition of Polygalacturonase Activity Produced by Leptosphaeria maculans with Stem Extracts of Canola, and the Relationship of Inhibition to Resistance

Leptosphaeria maculans , the causal agent of blackleg of canola, produces polygalacturonases during infection. Stem extracts of spring and winter canola cultivars contained a water-soluble inhibitor of the polygalacturonase activity of L. maculans . The polygalacturonase inhibitor material had different characteristics dependent upon the cultivar. Some canola cultivars had a polygalacturonase inhibitory compound(s) which was heat liable, low molecular weight and required divalent cations, and other cultivars had a heat stable, low molecular weight compound(s). The cultivar Maluka had a unique polygalacturonase inhibitory compound(s) that was heat labile, low molecular weight and did not need divalent cations. The level of the polygalacturonase inhibitory activity in the stem extracts was significantly related to the resistance of the cultivars to L. maculans as measured by the rate of lesion elongation, but was less related to the rate of stem girdling. The significant correlation between levels of polygalacturonase inhibitor activity and stem resistance in canola cultivars indicates that polygalacturonase inhibitors may be involved in the resistance of stems to blackleg. The two quantitative measures of stem resistance, rate of lesion elongation and rate of stem girdling, were significantly correlated to cotyledon resistance and to each other.     

A cysteine-rich antimicrobial peptide from Pinus monticola (PmAMP1) confers resistance to multiple fungal pathogens in canola (Brassica napus)

Canola (Brassica napus), an agriculturally important oilseed crop, can be significantly affected by diseases such as sclerotinia stem rot, blackleg, and alternaria black spot resulting in significant loss of crop productivity and quality. Cysteine-rich antimicrobial peptides isolated from plants have emerged as a potential resource for protection of plants against phytopathogens. Here we report the significance of an antimicrobial peptide, PmAMP1, isolated from western white pine (Pinus monticola), in providing canola with resistance against multiple phytopathogenic fungi. The cDNA encoding PmAMP1 was successfully incorporated into the genome of B. napus, and it's in planta expression conferred greater protection against Alternaria brassicae, Leptosphaeria maculans and Sclerotinia sclerotiorum. In vitro experiments with proteins extracted from transgenic canola expressing Pm-AMP1 demonstrated its inhibitory activity by reducing growth of fungal hyphae. In addition, the in vitro synthesized peptide also inhibited the growth of the fungi. These results demonstrate that generating transgenic crops expressing PmAMP1 may be an effective and versatile method to protect susceptible crops against multiple phytopathogens.     

Effect of mulching with cereal straw and row spacing on spread of bean yellow mosaic potyvirus into narrow-leafed lupins (Lupinus angustifolius)

Two field experiments examined the effect of straw spread on the soil surface on the incidence of bean yellow mosaic potyvirus (BYMV) in plots of narrow-leafed lupin (Lupinus angustifolius) sown at narrow (17.5 cm) vs wide (35 cm) row spacing and low (25–30 kg/ha) vs medium (50–60 kg/ha) seeding rates. Virus ingress was by vector aphids flying from adjacent pastures dominated by subterranean clover. In Expt 1, in which BYMV infection was extensive, straw greatly decreased the rate and amount of virus spread regardless of row spacing or plant density, decreasing infection more than 70% by the final assessment date. This effect of straw was attributed to decreased landing rates of incoming vector alates. In the plots without added straw, narrow row spacing decreased BYMV % infection by 38% by the last assessment date. Sowing at the medium seeding rate also decreased infection. The effect of wide row spacing seemed due to delayed canopy closure between rows which is likely to have increased the landing of aphids while the effect of medium seeding rate was attributed partly to the dilution effect of greater plant numbers and partly to the effects of partial canopy development in decreasing landing rates. In Expt 2, in which the incidence of BYMV infection was low, added straw again decreased BYMV spread, but by only 25–27% at final assessment; there were no effects of row spacing or seeding rate. In both experiments, an additional “reference” treatment was included which had a high (90–100 kg/ha) seeding rate, narrow rows and no straw. The dense canopy it developed also decreased BYMV incidence but less than in the plots with added straw in Expt 1. In Expt 1, adding straw and the resulting decrease in plants killed by BYMV, were associated with an overall increase in lupin grain yield of 20%. The greater plant densities resulting from the medium seeding rate also increased grain yield but row spacing did not affect it significantly. These results indicate that retaining stubble on the soil surface at seeding will assist in management of BYMV infection in lupin crops but that wide row spacing in the absence of retained stubble is undesirable.     

Candidate gene association mapping of Sclerotinia stalk rot resistance in sunflower (Helianthus annuus L.) uncovers the importance of COI1 homologs

Key message

Functional markers for Sclerotinia basal stalk rot resistance in sunflower were obtained using gene-level information from the model species Arabidopsis thaliana.

Abstract

Sclerotinia stalk rot, caused by Sclerotinia sclerotiorum, is one of the most destructive diseases of sunflower (Helianthus annuus L.) worldwide. Markers for genes controlling resistance to S. sclerotiorum will enable efficient marker-assisted selection (MAS). We sequenced eight candidate genes homologous to Arabidopsis thaliana defense genes known to be associated with Sclerotinia disease resistance in a sunflower association mapping population evaluated for Sclerotinia stalk rot resistance. The total candidate gene sequence regions covered a concatenated length of 3,791 bp per individual. A total of 187 polymorphic sites were detected for all candidate gene sequences, 149 of which were single nucleotide polymorphisms (SNPs) and 38 were insertions/deletions. Eight SNPs in the coding regions led to changes in amino acid codons. Linkage disequilibrium decay throughout the candidate gene regions declined on average to an r ² = 0.2 for genetic intervals of 120 bp, but extended up to 350 bp with r ² = 0.1. A general linear model with modification to account for population structure was found the best fitting model for this population and was used for association mapping. Both HaCOI1-1 and HaCOI1-2 were found to be strongly associated with Sclerotinia stalk rot resistance and explained 7.4 % of phenotypic variation in this population. These SNP markers associated with Sclerotinia stalk rot resistance can potentially be applied to the selection of favorable genotypes, which will significantly improve the efficiency of MAS during the development of stalk rot resistant cultivars.     

Prokaryotic expression and protein function of Brassica napus PGIP2 and its genetic transformation

Sclerotinia rot is a fungal disease caused by Sclerotinia sclerotiorum (Lib.) de Bary, which has severely reduced rapeseed production worldwide. Polygalacturonase-inhibiting proteins (PIGPs) inhibit the activity of polygalacturonases, which are secreted during fungal infection in plants. This study investigated the function of the polygalacturonase-inhibitor gene 2 (PGIP2) in sclerotinia rot resistance. The PGIP2 was successfully expressed in a prokaryotic system, and recombinant PGIP2 protein, purified after enterokinase treatment to remove tag peptide, inhibited S. sclerotiorum PG activity in vitro. PGIP2 was overexpressed in the susceptible Brassica napus cultivar 98c40 via Agrobacterium-mediated transformation. After inoculation with S. sclerotiorum mycelia, the transgenic rapeseed demonstrated greatly reduced leaf damage, as compared with their non-transgenic plants. Therefore, the PGIP2 encodes a functional protein and would be a candidate gene for enhancing Sclerotinia rot resistance.     

Economic impact of Hessian fly (Diptera: Cecidomyiidae) on spring wheat in Oregon and additive yield losses with Fusarium crown rot and lesion nematode

Damage caused by Hessian fly, Mayetiola destructor (Say), was quantified in spring wheat, Triticum aestivum L., trials near Pendleton and Moro, OR, during 2001 and 2002. Five field experiments were established to examine genetic resistance to Fusarium crown rot, Fusarium pseudograminearum (O'Donnell & Aoki), and economic damage by lesion nematode, Pratylenchus neglectus (Rensch, 1924) (Filipjev Schuurmanns & Stekhoven, 1941) and Pratylenchus thornei (Sher & Allen, 1941). Hessian fly became the dominant factor affecting grain yield in four experiments. Genotypes carrying the H3-resistance gene had grain yields 66 and 68% higher than susceptible genotypes in cultivar trials during 2001 and 2002, respectively. Yield reductions were detected when Hessian fly infestation rates exceeded 50% plants during 2001 and 15% plants (8% tillers) during 2002. In two trials during 2001, in-furrow application of aldicarb (Temik) at planting improved yields of four Hessian fly-susceptible cultivars by 72 and 144% (up to 1,959 kg/ha) and yields of one Hessian fly-resistant cultivar by 2 and 3%. Resistant cultivars and aldicarb improved grain quality as much as two market grades during 2001. The value of increased grain production with Hessian fly-resistant cultivars in four field experiments ranged from dollar 112 to dollar 252/ha, excluding price incentives for improved market quality. Yield reduction due to combined damage from Hessian fly and either Fusarium crown rot or lesion nematode was additive. This report seems to be the first quantitative yield loss estimate for Hessian fly in spring wheat in the semiarid environment of the inland Pacific Northwest.     

Parasitism capacity and searching efficiency of Diaeretiella rapae parasitizing Brevicoryne brassicae on susceptible and resistant canola cultivars

Different cultivars of aplant species can affect the foraging and efficiency of natural enemies, both directly through physical and biochemical properties or indirectly through the herbivore's diet. In this study, the parasitism capacity and functional response of Diaeretiella rapae McIntosh were determined on the cabbage aphid, Brevicoryne brassicae (L.) reared on susceptible (Opera) and resistant (Okapi) canola cultivars under laboratory conditions at 25?±?1?°C, 60?±?5% RH and a16:8?h L:D photoperiod. The parasitoid exhibited Type II and Type III functional responses on the resistant and susceptible cultivars, respectively. The estimated value of searching efficiency (a) was 0.1637?±?0.1095?h^?1 on the resistant cultivar whereas its value was dependent on host density on the susceptible cultivar. The handling times (T_h) on the susceptible and resistant canola cultivars were 0.108?±?0.040 and 0.320?±?0.048?h, respectively. The net parasitism rate (C₀) of the parasitoid wasp varied from 128.09 hosts per parasitoid lifetime on the susceptible to 71.01hosts on the resistant canola cultivar. The transformation rate from host population to parasitoid offspring (Qp) was equal to 1 on both cultivars (C₀?=?R₀). The finite parasitism rate (ω) on the susceptible cultivar (0.819 hosts per parasitoid per day) was significantly higher than that on the resistant one (0.578 hosts per parasitoid per day). In conclusion, canola cultivars affected the performance of D. rapae in controlled small-scale laboratory experiments and compared with the susceptible cultivar, the resistant one had anadverseeffect on the efficiency of the parasitoid.     

Eco-friendly approaches for the management of fenugreek root rot

To evolve eco-friendly management of fenugreek root rot caused by Rhizoctonia solani, a field trial was conducted during Kharif 2002 and Rabi seasons of 2002–2003 and 2003–2004. Experiments were conducted with eight treatments and three replications in RBD using the variety CO-2. The pooled analysis of the three season data showed that seed treatment with Trichoderma viride at 4g/kg of seed + soil application of Trichoderma viride at 5 kg/ha + soil application of neem cake at 150 kg/ha (T₃) recorded a percent disease index (PDI) of 23.1 versus 65.5 PDI in the control which accounted for a disease reduction of 64.7%. It was on par with seed treatment with Trichoderma viride at 4g/kg of seed + soil application of T. viride at 5 kg/ha (T₂) which reduced the disease incidence by 62.3% (24.7 PDI). The chemical treatment used for comparison, i.e. seed treatment with carbendazim + soil drenching at 0.1% + soil application of neem cake at 150 kg/ha recorded the lowest PDI of 16.8 with 74.4% disease reduction. Among the various treatments T₃ gave a seed yield of 572.7 kg/ha followed by T₂ (555.7 kg/ha). Treatment T₇ recorded the highest yield of 578.7 kg/ha. In the control plot the recorded yield was only 359.3 kg/ha. Though T₃ was more effective at reducing the disease incidence than T₂, the C:B ratio was higher (1:9.1) in respect of T₂ than T₃, which gave a C:B ratio of only 1:3.9. Hence, seed treatment with T. viride at 4g/kg + soil application of T. viride at 5kg/ha is a cost effective, eco-friendly management strategy for fenugreek root rot.     

Fertilization effects with P on accumulated leaf area duration,biomass and yield of three cultivars of maize in Toluca,Mexico

The effect of six phosphorus levels (0, 40, 80, 120, 160 and 200 kg/ha) on the duration of cumulative leaf area, biomass and agronomic yield was determined in the maize cultivars: Amarillo Almoloya, Cacahuacintle and Condor in 2010 and 2011. Such cultivars were sown in the Cerrillo Piedras Blancas Mexico. A completely randomized complete block design with factorial arrangement was utilized. High phosphorus levels (120, 160 and 200 kg/ha) positively affected the duration of cumulative leaf area; greatest values were obtained in Cacahuacintle. A greater duration of accumulated leaf area contributes to determine high values of biomass accumulation and grain yield in this cultivar. Leaf area duration appeared to be a useful tool for evaluating different genotypes in a given environment.     

The effect of nitrogen and growth regulators on stem and root characteristics associated with lodging in two cultivars of winter wheat

The effects of nitrogen and plant growth regulators (stem shorteners)on root and shoot characteristics associated with lodging resistancewere investigated in two winter wheat (Triticum aestivum L.)cultivars of contrasting lodging resistance: the susceptibleGalahad and the resistant Hereward. The morphology and mechanicalstrength of the stems and anchorage systems grown at two levelsof nitrogen and with or without growth regulators were measuredand related to the incidence of lodging recorded in a fieldtrial. In both cultivars high levels of nitrogen increased theheight of the stem, thereby increasing the ‘self-weight’moment transmitted into the ground and weakened both the stemsand the anchorage coronal roots. As a result, the anchoragestrength was also reduced, plants failing in the root systemin simulated lodging tests. Growth regulators, in contrast,had little effect on the bending strength of the shoots androot systems, but reduced plant height so that the over turningmoments generated by the weight of the shoot were less. Therewere also differences between cultivars: Galahad plants hadweaker anchorage due to the smaller number and lower strengthof the coronal roots. The morphological and mechanical measureswere used to calculate a safety factor against both stem androot lodging. Five factors were found to influence the safetyfactors, these were: cultivar type, the type of lodging, therate of nitrogen and growth regulator application, and time,being lowest in Galahad plants at high levels of nitrogen andwithout growth regulators and at grain filling when the earswere heaviest. This was consistent with the observed patternof lodging: root lodging occurred at grain filling and onlyin Galahad which had been treated with high nitrogen rates,most strongly in plants without growth regulators. Key words: Lodging, safety factors, anchorage, ‘self-weight’ moment     

The plant-associated Bacillus amyloliquefaciens strains MEP2 18 and ARP2 3 capable of producing the cyclic lipopeptides iturin or surfactin and fengycin are effective in biocontrol of sclerotinia stem rot disease

Aims: This work was conducted to identify the antifungal compounds produced by two previously isolated Bacillus sp. strains: ARP₂3 and MEP₂18. Both strains were subjected to further analysis to determine their taxonomic position and to identify the compounds responsible for their antifungal activity as well as to evaluate the efficiency of these strains to control sclerotinia stem rot in soybean. Methods and Results: The antifungal compounds were isolated by acid precipitation of cell‐free supernatants, purified by RP‐HPLC and then tested for antagonistic activity against Sclerotinia sclerotiorum. Mass spectra from RP‐HPLC eluted fractions showed the presence of surfactin C₁₅, fengycins A (C₁₆–C₁₇) and B (C₁₆) isoforms in supernatants from strain ARP₂3 cultures, whereas the major lipopeptide produced by strain MEP₂18 was iturin A C₁₅. Alterations in mycelial morphology and sclerotial germination were observed in the presence of lipopeptides‐containing supernatants from Bacillus strains cultures. Foliar application of Bacillus amyloliquefaciens strains on soybean plants prior to S. sclerotiorum infection resulted in significant protection against sclerotinia stem rot compared with noninoculated plants or plants inoculated with a nonlipopeptide‐producing B. subtilis strain. Conclusions: Both strains, renamed as B. amyloliquefaciens ARP₂3 and MEP₂18, were able to produce antifungal compounds belonging to the cyclic lipopeptide family. Our data suggest that the foliar application of lipopeptide‐producing B. amyloliquefaciens strains could be a promising strategy for the management of sclerotinia stem rot in soybean. Significance and Impact of the Study: Sclerotinia stem rot was ranked as one of the most severe soybean disease in Argentina and worldwide. The results of this study showed the potential of B. amyloliquefaciens strains ARP₂3 and MEP₂18 to control plant diseases caused by S. sclerotiorum.     

Wyoming Big Sagebrush Density: Effects of Seeding Rates and Grass Competition

The mining industry commonly seeds shrubs and grasses concurrently on coal‐mined lands of northeastern Wyoming, but ecological interactions between seeded shrubs and grasses are not well documented. Artemisia tridentata Nutt. ssp. wyomingensis (Beetle and Young) (Wyoming big sagebrush) is the dominant pre‐mining shrub on many Wyoming mine sites. Despite past failures to establish Wyoming big sagebrush, the Wyoming Department of Environmental Quality, Land Quality Division's rules and regulations require establishment of 1 shrub per m² on 20% of post‐mined land in Wyoming. A study was established at the Belle Ayr Coal Mine south of Gillette, Wyoming to evaluate the effects of sagebrush seeding rates and grass competition on Wyoming big sagebrush seedling density. Three sagebrush seeding rates (1, 2, and 4 kg pure live seed [pls]/ha; 350, 700, and 1,400 pls/m², respectively) and seven cool‐season perennial grass mixture seeding rates (0, 2, 4, 6, 8, 10, and 14 kg pls/ha; 0, 187, 374, 561, 750, 935, and 1,309 pls/m², respectively) were applied during winter 1998–1999. Pascopyrum smithii (Rydb.) A. Love (western wheatgrass), Elymus lanceolatus (Scribner & J.G. Smith) Gould (thickspike wheatgrass), and Elymus trachycaulus (Link) Gould ex Shinners (slender wheatgrass) comprised the grass seed mix (equal seed numbers of each species). Sagebrush seedling density differed among sagebrush seeding rates but not among grass seeding rates. On all sampling dates in 1999 and 2000, sagebrush seedling density differed among sagebrush rates and was greatest at the 4 kg pls/ha sagebrush seeding rate. All sagebrush seeding rates provided densities of at least 1 shrub per m² after two growing seasons. Grass density and production in 2000 suggest that adequate grass production (75 g/m²) was achieved by seeding at 6 to 8 kg pls/ha. Within these grass seeding rates, four or more sagebrush seedlings per m² were attained when sagebrush was seeded at 2 to 4 kg pls/ha. Use of these seeding rate combinations in mine reclamation can achieve Wyoming big sagebrush standards and reduce reseeding costs.     

Effect of plant genotype and nitrogen fertilizer on symbiotic nitrogen fixation by soybean cultivars

Summary Isotopic as well as non-isotopic methods were used to assess symbiotic nitrogen fixation within eight soybean [Glycine max (L.) Merr.] cultivars grown at 20 and 100 kg N/ha levels of nitrogen fertilizer under field conditions.The¹⁵N methodology revealed large differences between soybean cultivars in their abilities to support nitrogen fixation. In almost all cases, the application of 100 kg N/ha resulted in lower N₂ fixed in soybean than at 20 kg N/ha in the first year of the study. However, N₂ fixed in one cultivar, Dunadja, was not significantly affected by the higher rate of N fertilizer application. These results were confirmed by measurements of acetylene reduction activity, nodule dry weight and N₂ fixed as measured by the difference method. Further proof of differences in N₂ fixed within soybean cultivars and the ability of Dunadja to fix similar amounts of N₂ at 20 and 100 kg N/ha was obtained during a second year experiment. Dunadja yield was affected by N fertilizer and produced larger yield at 100 kg N/ha than at 20 kg N/ha. This type of cultivar could be particularly useful in situations where soil N levels are high or where there is need to apply high amounts of N fertilizer.The present study reveals the great variability between legume germplasms in the ability to fix N₂ at different inorganic N levels, and also the potential that exists in breeding for nitrogen fixation associative traits. The¹⁵N methodology offers a unique tool to evaluate germplasms directly in the field for their N₂ fixation abilities at different N fertilizer levels.     

Mutual shading and the photosynthetic capacity of exposed leaves of field grown soybeans

Light-saturated photosynthetic rates at air levels of carbon dioxide were measured about weekly in upper canopy leaves of two soybean cultivars grown at stand densities of 40 and 100 plants per square meter. Early in the season, when leaf area indices differed between stand densities, plants of both cultivars grown at high stand density had photosynthetic rates which averaged 23% lower than plants at low stand density. Later in the season, when there were no differences in leaf area index between stand densities, there were no differences in photosynthetic rates in the cultivar Kent, but rate differences of about 14% persisted in the cultivar Williams. In Williams mainstem leaves emerged into full sunlight later in their development at high than at low stand density. In both cultivars the oldest fully exposed leaves were photosynthetically immature for much of the season, as higher rates could be achieved by lower leaves which were shaded in situ. The results identify shading of young developing leaves and photosynthetic immaturity of fully exposed leaves as factors limiting canopy photosynthesis in soybeans, and indicate cultivar differences in how much high stand density reduces photosynthetic capacity.     

Defoliation of Brassica napus increases severity of blackleg caused by Leptosphaeria maculans: implications for dual-purpose cropping

Canola (Brassica napus) crops for grazing and grain (dual-purpose) production provide an economic break-crop alternative for dual-purpose cereals in Australian mixed farming systems. Infection by Leptosphaeria maculans is the most prevalent disease in Australian canola crops with airborne inoculum released throughout the autumn and winter when crops are grazed. Glasshouse and field experiments were conducted to investigate the effect of mechanical defoliation (simulated grazing) on disease severity at plant maturity. In glasshouse experiments, stem canker severity increased from 4% to 24% in severely defoliated plants, but light defoliation had no effect compared with undefoliated control plants. Disease severity was increased with defoliation in all field experiments. Defoliation increased crown canker severity from 22.6% to 39.3% at Wagga Wagga and from 3.0% to 7.1% at Canberra and lodging from 9.6% to 11.9% at Naracoorte in the same set of cultivars assessed at each site. The increase in disease severity with defoliation was less in canola lines with moderate to high levels of stem canker resistance. Plants defoliated before stem elongation tended to develop less disease than those defoliated during the reproductive phase of plant growth. These findings suggest that the impact of grazing on L. maculans infection of canola crops can be minimised by sowing cultivars with a high level of stem canker resistance and grazing during the vegetative stage of plant growth prior to stem elongation. Further research is required to determine whether these management strategies are applicable in canola crops defoliated by grazing animals.