Susceptible Wheat Plants Research Articles

The F-box protein encoding genes of the leaf-rust fungi Puccinia triticina: genome-wide identification, characterization and expression dynamics during pathogenesis.

The F-box proteins in fungi perform diverse functions including regulation of cell cycle, circadian clock, development, signal transduction and nutrient sensing. Genome-wide analysis revealed 10 F-box genes in Puccinia triticina, the causal organism for the leaf rust disease in wheat and were characterized using in silico approaches for revealing phylogenetic relationships, gene structures, gene ontology, protein properties, sequence analysis and gene expression studies. Domain analysis predicted functional domains like WD40 and LRR at C-terminus along with the obvious presence of F-box motif in N-terminus. MSA showed amino acid replacements, which might be due to nucleotide substitution during replication. Phylogenetic analysis revealed the F-box proteins with similar domains to be clustered together while some sequences were spread out in different clades, which might be due to functional diversity. The clustering of Puccinia triticina GG705409 with Triticum aestivum TaAFB4/TaAFB5 in a single clade suggested the possibilities of horizontal gene transfer during the coevolution of P. triticina and wheat. Gene ontological annotation categorized them into three classes and were functionally involved in protein degradation through the protein ubiquitination pathway. Protein-protein interaction network revealed F-box proteins to interact with other components of the SCF complex involved in protein ubiquitination. Relative expression analysis of five F-box genes in a time course experiment denoted their involvement in leaf rust susceptible wheat plants. This study provides information on structure elucidation of F-box proteins of a basidiomycetes plant pathogenic fungi and their role during pathogenesis.

The Different Metabolic Responses of Resistant and Susceptible Wheats to Fusarium graminearum Inoculation

Fusarium head blight (FHB) is a serious wheat disease caused by Fusarium graminearum (Fg) Schwabe. FHB can cause huge loss in wheat yield. In addition, trichothecene mycotoxins produced by Fg are harmful to the environment and humans. In our previous study, we obtained two mutants TPS1− and TPS2−. Neither of these mutants could synthesize trehalose, and they produced fewer mycotoxins. To understand the complex interaction between Fg and wheat, we systematically analyzed the metabolic responses of FHB-susceptible and -resistant wheat to ddH2O, the TPS− mutants and wild type (WT) using NMR combined with multivariate analysis. More than 40 metabolites were identified in wheat extracts including sugars, amino acids, organic acids, choline metabolites and other metabolites. When infected by Fg, FHB-resistant and -susceptible wheat plants showed different metabolic responses. For FHB-resistant wheat, there were clear metabolic differences between inoculation with mutants (TPS1−/TPS2−) and with ddH2O/WT. For the susceptible wheat, there were obvious metabolic differences between inoculation with mutant (TPS1−/TPS2−) and inoculation with ddH2O; however, there were no significant metabolic differences between inoculation with TPS− mutants and with WT. Specifically, compared with ddH2O, resistant wheat increased the levels of Phe, p-hydroxy cinnamic acid (p-HCA), and chlorogenic acid in response to TPS− mutants; however, susceptible wheat did not. Shikimate-mediated secondary metabolism was activated in the FHB-resistant wheat to inhibit the growth of Fg and reduce the production of mycotoxins. These results can be helpful for the development of FHB-resistant wheat varieties, although the molecular relationship between the trehalose biosynthetic pathway in Fg and shikimate-mediated secondary metabolism in wheat remains to be further studied.

Biology and molecular interactions of Parastagonospora nodorum blotch of wheat.

Parastagonospora nodorum is one of the important necrotrophic pathogens of wheat which causes severe economical loss to crop yield. So far, a number of effectors of Parastagonospora nodorum origin and their target interacting genes on the host plant have been characterized. Since targeting effector-sensitive gene carefully can be helpful in breeding for resistance. Therefore, constant efforts are required to further characterize the effectors, their interacting genes, and underlying biochemical pathways. Furthermore, to develop effective counter-strategies against emerging diseases, continuous efforts are required to determine the qualitative resistance that demands to screen of diverse genotypes for host resistance. Stagonospora nodorum blotch also refers to as Stagonospora glume blotch and leaf is caused by Parastagonospora nodorum. The pathogen deploys necrotrophic effectors for the establishment and development on wheat plants. The necrotrophic effectors and their interaction with host receptors lead to the establishment of infection on leaves and extensive lesions formation which either results in host cell death or suppression/activation of host defence mechanisms. The wheat Stagonospora nodorum interaction involves a set of nine host gene-necrotrophic effector interactions. Out of these, Snn1-SnTox1, Tsn1-SnToxA and Snn-SnTox3 are one of the most studied interaction, due to its role in the suppression of reactive oxygen species production, regulating the cytokinin content through ethylene-dependent wayduring initial infection stage. Further, although the molecular basis is not fully unveiled, these effectors regulate the redox state and influence the ethylene biosynthesis in infected wheat plants. Here, we have discussed the biology of the wheat pathogen Parastagonospora nodorum, role of its necrotrophic effectors and their interacting sensitivity genes on the redox state, how they hijack the resistance mechanisms, hormonal regulated immunity and other signalling pathways in susceptible wheat plants. The information generated from effectors and their corresponding sensitivity genes and other biological processes could be utilized effectively for disease management strategies.

Disease Severity, Resistance Analysis, and Expression Profiling of Pathogenesis-Related Protein Genes after the Inoculation of Fusarium equiseti in Wheat

Wheat (Triticum aestivum) is an important cereal crop, grown throughout the temperate and in some tropical and sub-tropical zones, at higher elevations. Several biotic and abiotic factors influence the production of wheat. In the present study, two wheat varieties have been subjected to disease severity and resistance analysis against Fusarium equiseti. Disease severity analysis revealed Shafaq-2006 to be more resistant than Sahar-2006. Both varieties were further subjected to the expression analysis of six important defense-related genes by RT-PCR and quantitative real-time PCR. This analysis revealed that PR-1, TLP, Chitinase, and β-1,3-glucanase genes were highly expressed in Shafaq-2006 and possibly play a significant role in its defense mechanism. In addition, biochemical and physiochemical parameters were also studied to further explore the difference between resistant and susceptible varieties. With total proline and protein contents, sugar and chlorophyll contents also increased significantly in resistant variety. Likewise, higher relative water content, total plant length, and the high root–shoot ratio was observed in resistant plants, compared to susceptible wheat plants. These increases in chemical and physiological parameters might be related to the activation of the defense mechanism due to the higher expression of PR genes in the resistant wheat variety. These genes can further be employed for cloning into wheat and other transgenic crops to develop resistance against F. equiseti.

Characterization of histological changes at the tillering stage (Z21) in resistant and susceptible wheat plants infected by Tilletia controversa K\xfchn

BackgroundDwarf bunt, which is caused by Tilletia controversa Kühn, is a soilborne and seedborne disease that occurs worldwide and can lead to 70% or even total losses of wheat crops. However, very little information is available about the histological changes that occur in dwarf bunt-resistant and dwarf bunt-susceptible wheat plants at the tillering stage (Z21). In this study, we used scanning electron microscopy and transmission electron microscopy to characterize the histological changes at this stage in resistant and susceptible wheat cultivars infected by T. controversa.ResultsUsing scanning electron microscopy, the root, stem, and leaf structures of resistant and susceptible cultivars were examined after T. controversa infection. The root epidermal and vascular bundles were more severely damaged in the susceptible T. controversa-infected plants than in the resistant plants. The stem cell and longitudinal sections were much more extensively affected in susceptible plants than in resistant plants after pathogen infection. However, slightly deformed mesophyll cells were observed in the leaves of susceptible plants. With transmission electron microscopy, we found that the cortical bundle cells and the cell contents and nuclei in the roots were more severely affected in the susceptible plants than in the resistant plants; in the stems and leaves, the nuclei, chloroplasts, and mesophyll cells changed significantly in the susceptible plants after fungal infection. Moreover, we found that infected susceptible and resistant plants were affected much more severely at the tillering stage (Z21) than at the seedling growth stage (Z13).ConclusionHistological changes in the wheat roots, stems and leaves were much more severe in T. controversa-infected susceptible plants than in infected resistant plants at the tillering stage (Z21).

Differential localization of Hessian fly candidate effectors in resistant and susceptible wheat plants.

Hessian fly Mayetiola destructor is a notorious pest of wheat. Previous studies suggest that Hessian fly uses effector‐based mechanisms to attack wheat plants during parasitism, but no direct evidence has been reported to support this postulation. Here, we produced recombinant proteins for five Family‐1 candidate effectors and antibodies. Indirect immunostaining and western blots were carried out to examine the localization of Hessian fly Family‐1 proteins in plant and insect tissues. Confocal images revealed that Family‐1 putative effectors were exclusively produced in the basal region of larval salivary glands, which are directly linked to the mandibles’ ducts for effector injection. The five Family‐1 proteins were detected in infested host plants on western blots. Indirect immunostaining of sectioned host tissues around the feeding site revealed strikingly different localization patterns between resistant and susceptible plants. In susceptible plants, the Family‐1 proteins penetrated from the feeding cell into deep tissues, indicative of movement between cells during nutritive cell formation. In contrast, the Hessian fly proteins were primarily limited to the initially attacked cells in resistant plants. The limitation of effectors’ spread in resistant plants was likely due to wall strengthening and rapid hypersensitive cell death. Cell death was found in Nicotiana benthamiana in association with hypersensitive reaction triggered by the Family‐1 effector SSGP‐1A2. Our finding represents a significant progress in visualizing insect effectors in host tissues and mechanisms of plant resistance and susceptibility to gall midge pests.

Wheat-associated microbiota and their correlation with stripe rust reaction.

This study was aimed at revealing the composition of microbiota in leaves, roots and rhizosphere soil of wheat plants that are resistant or susceptible to stripe rust, one of the most widely destructive leaf diseases in wheat production. A total of 36 wheat plants that showed resistant or susceptible reactions to stripe rust were sampled. Three compartments of each plant including leaves, roots and rhizosphere soil were used for whole-genomic DNA extraction and the DNA samples were subjected to high-throughput 16S rRNA gene sequencing. A total of 2885 operational taxonomic units (OTUs) were revealed from the sequencing, and they mainly distributed in the phylum of Proteobacteria. Twenty-nine OTUs formed the core microbiota of wheat plants. The differences between above- and below-ground environments could explain most of the dissimilarity of wheat-associated microbial communities. Therefore, those microbes that were able to adapt to the above-ground (leaf) environment might be more important resources for biocontrol agents against stripe rust, and they could be from genera Hymenobacter, Flavobacterium, Chitinophage, Flavisolibacter, Niastella, Mucilaginibacter, Pedobacter, Aquincola, Massilia, Citrobacter, Cronobacter, Ewingella, Acinetobacter and Pseudomonas. No matter the microbial taxa were significantly selected by the resistant or susceptible wheat plants, they contained the members with plant growth promoting (PGP) features and could be used as potential biocontrol agents to reduce stripe rust damage. The core microbiota associated with wheat plants and microbial taxa that were significantly correlated with reactions to stripe rust were identified in this study. Few studies had been done on the microbiota associated with wheat so far. Our study will not only provide fundamental knowledge about the composition of wheat-associated microbiota but also reveal the microbial taxa that have the potential to be integrated into the strategy of stripe rust management.

Host-induced silencing of essential genes in Puccinia triticina through transgenic expression of RNAi sequences reduces severity of leaf rust infection in wheat.

SummaryLeaf rust, caused by the pathogenic fungus Puccinia triticina (Pt), is one of the most serious biotic threats to sustainable wheat production worldwide. This obligate biotrophic pathogen is prevalent worldwide and is known for rapid adaptive evolution to overcome resistant wheat varieties. Novel disease control approaches are therefore required to minimize the yield losses caused by Pt. Having shown previously the potential of host‐delivered RNA interference (HD‐RNAi) in functional screening of Pt genes involved in pathogenesis, we here evaluated the use of this technology in transgenic wheat plants as a method to achieve protection against wheat leaf rust (WLR) infection. Stable expression of hairpin RNAi constructs with sequence homology to Pt MAP‐kinase (PtMAPK1) or a cyclophilin (PtCYC1) encoding gene in susceptible wheat plants showed efficient silencing of the corresponding genes in the interacting fungus resulting in disease resistance throughout the T2 generation. Inhibition of Pt proliferation in transgenic lines by in planta‐induced RNAi was associated with significant reduction in target fungal transcript abundance and reduced fungal biomass accumulation in highly resistant plants. Disease protection was correlated with the presence of siRNA molecules specific to targeted fungal genes in the transgenic lines harbouring the complementary HD‐RNAi construct. This work demonstrates that generating transgenic wheat plants expressing RNAi‐inducing transgenes to silence essential genes in rust fungi can provide effective disease resistance, thus opening an alternative way for developing rust‐resistant crops.

Genome-wide identification and characterization of NB-ARC resistant genes in wheat (Triticum aestivum L.) and their expression during leaf rust infection

NB-ARC domain-containing resistance genes from the wheat genome were identified, characterized and localized on chromosome arms that displayed differential yet positive response during incompatible and compatible leaf rust interactions. Wheat (Triticum aestivum L.) is an important cereal crop; however, its production is affected severely by numerous diseases including rusts. An efficient, cost-effective and ecologically viable approach to control pathogens is through host resistance. In wheat, high numbers of resistance loci are present but only few have been identified and cloned. A comprehensive analysis of the NB-ARC-containing genes in complete wheat genome was accomplished in this study. Complete NB-ARC encoding genes were mined from the Ensembl Plants database to predict 604 NB-ARC containing sequences using the HMM approach. Genome-wide analysis of orthologous clusters in the NB-ARC-containing sequences of wheat and other members of the Poaceae family revealed maximum homology with Oryza sativa indica and Brachypodium distachyon. The identification of overlap between orthologous clusters enabled the elucidation of the function and evolution of resistance proteins. The distributions of the NB-ARC domain-containing sequences were found to be balanced among the three wheat sub-genomes. Wheat chromosome arms 4AL and 7BL had the most NB-ARC domain-containing contigs. The spatio-temporal expression profiling studies exemplified the positive role of these genes in resistant and susceptible wheat plants during incompatible and compatible interaction in response to the leaf rust pathogen Puccinia triticina. Two NB-ARC domain-containing sequences were modelled in silico, cloned and sequenced to analyze their fine structures. The data obtained in this study will augment isolation, characterization and application NB-ARC resistance genes in marker-assisted selection based breeding programs for improving rust resistance in wheat.

Genome-wide identification, cloning and characterization of SNARE genes in bread wheat (Triticum aestivum L.) and their response to leaf rust

SNAREs (soluble N-ethylmaleimide sensitive factor adaptor protein receptors) are small polypeptides characterized by a particular domain called the SNARE motif. Compared with the genome of other eukaryotes, monocotyledonous and dicotyledonous plants have more SNAREs indicating their important roles in higher plant species. Higher plants have the capability to form SNARE complexes that are important in determining the precise process of vesicle fusion for intracellular trafficking pathways. SNAREs have been reported to be engaged in the delivery of cell wall precursors to the newly formed cell plate during cytokinesis. The role of SNARE genes in response to plant-pathogen interaction is still not well understood. We found 35 SNARE genes in the wheat genome using a Hidden Markov Model. In this study with combined usage of in silico and molecular cloning technologies, we identified and characterized three SNARE genes (SNARE3, SNARE5 and SNARE6). The deduced amino acid sequences of these SNARE genes contained two characteristic conserved domains – a SNARE motif and a transmembrane domain, and they showed a high degree of homology with other eukaryotic SNARE genes. Phylogenetic analysis and three dimensional structures built with the help of Modeller software confirmed the presence of SNARE motifs in the proteins. The spatio-temporal expression profiling studies exemplify the positive role of SNARE transcripts have in resistant and susceptible wheat plants during incompatible and compatible interaction respectively, in response to Puccinia triticina induced leaf-rust infection. Taken together, our study suggests a role for SNARE genes in vesicle mediated resistance to leaf rust in wheat.

Differential expression analysis of defense-related genes responsive to Tilletia indica infection in wheat

Karnal bunt caused by Tilletia indica is a quarantine disease responsible for qualitative and quantitative losses in wheat. The present study aimed to determine the expression of defense-related genes involved in early stages of infection in response to T. indica by using the differential display reverse transcriptase-PCR method (DDRT-PCR) in susceptible (WL 711) and resistant (HD 29) cultivars of wheat. The DDRT-PCR banding profiles generated by the random primers in combination with oligo-dT primers of resistant and susceptible wheat plants were evaluated at 0 h, 12 h, 24 h, and 48 h after inoculation. Out of 9 sets of primers, 7 sets of primers produced remarkable induced transcripts. The highest up-regulated transcripts were observed in the resistant cultivar (HD 29), whereas the down-regulated transcripts were found in the susceptible (WL 711) cultivar. Several sets of primers were designed from the defense-related gene sequences available in the GenBank database, and of these, 4 primer sets amplified the desired specific bands. Based on the putative functions, 3 defense-related genes, namely puroindoline protein PINB (AT_-_ KB2), \Beta-1,4-glucanase (AT_-_ KB3), and chitinase (AT_-_ KB7), and 1 housekeeping gene, actin (AT_-_ KB1), were characterized.

Transcript profiles of two wheat lipid transfer protein-encoding genes are altered during attack by Hessian fly larvae

A sequence encoding a putative type-1 lipid transfer protein from wheat (Triticum aestivum L. em Thell) was identified through 'GeneCalling', an mRNA profiling technology. The mRNA for the Hfr-LTP (Hessian fly-responsive lipid transfer protein) gene decreased in abundance (196-fold) in susceptible wheat plants over the first eight days of attack by virulent Hessian fly larvae (Mayetiola destructor Say). Hfr-LTP encodes a putative protein containing eight cysteine residues that are conserved among plant LTPs and are responsible for correct protein folding through formation of disulfide bridges. Twelve hydrophobic amino acids in addition to arginine, glycine, proline, serine, threonine and tyrosine, plus an LTP signature sequence were present in conserved positions. A highly conserved signal peptide sequence was also present. Although attack by one virulent larva was sufficient to cause a decrease in Hfr-LTP mRNA abundance, higher infestation levels led to near silencing of the gene. Hfr-LTP transcript levels were not affected by other biotic factors (feeding by bird cherry-oat aphid, Rhopalosiphum padi L., and fall armyworm larvae, Spodoptera frugiperda Smith) or abiotic factors tested (mechanical wounding or treatment with abscisic acid, methyl jasmonate, or salicylic acid). Comparison to a previously described Hessian fly-responsive wheat LTP gene, TaLTP3, confirmed an initial increase in TaLTP3 mRNA in resistant plants. However, when quantified through eight days after egg hatch, responsiveness to infestation level and a marked decrease in susceptible plant TaLTP3 mRNA abundance were detected, as was seen for Hfr-LTP. Possible functions of LTP gene products in wheat-Hessian fly interactions are discussed.

Proximate Cues for Reduced Oviposition by Hessian Fly on Wheat Plants Attacked by Conspecific Larvae

Abstract In a previous study, we showed that female Hessian fly, Mayetiola destructor (Say), oviposits significantly fewer eggs on susceptible wheat plants attacked by conspecific larvae. Here we tested whether proximate cues for reduced egglaying on larval-occupied plants are associated with larvae per se or plant-based cues related to larval attack. We used a novel method, i.e., R gene–defended wheat Triticum aestivum L. genotypes, to decouple Hessian fly larvae from their effects on the plant. On R gene–defended plants, Hessian fly larvae survive for up to 5 d but do not grow. A comparison of egglaying patterns on four near-isogenic wheat genotypes, a susceptible control ‘Newton’ and three R gene wheat genotypes expressing the H6, H9, or H13 gene, showed that the presence of live larvae on plants was not the cause of reduced egglaying. Growth deficits in the youngest leaves, a well-known plant response to Hessian fly larval attack, also were examined as a possible cause of reduced oviposition but showe...

Ascosporic and conidial inoculum ofGibberella zeaeplay different roles in Fusarium head blight and crown rot of wheat in Australia and the USA

Air-borne spores of the Fusarium head blight (FHB) pathogen were trapped at field sites in both Australia and the USA using susceptible wheat plants to establish whether ascospores or conidia were the dominant FHB inoculum. Although both ascospores and conidia were recovered from flowering spikes during a 28-day period between anthesis and grain filling, ascospores were the dominant spore type at Fargo, North Dakota in the USA. Conversely in Australia, conidia were the dominant spore type trapped during 16 and 21-day periods of sampling over 2 years at two sites in northern New South Wales. The effectiveness of Gibberella zeae ascospores and conidia as inoculum for FHB and crown rot (CR) was compared in plant infection assays using 10 isolates each from the USA and Australia. Overall, ascospores were less effective in causing FHB and CR than conidia, although the difference in FHB severity between the two spore types was small. Ascospores of isolates from the USA caused significantly less severe CR on three North American cultivars and some isolates with highly aggressive conidial inoculum were weakly aggressive with ascosporic inoculum. In contrast, both spore types of the Australian isolates caused severe CR and there was no significant difference between the spore types for CR severity on three North American cultivars. Studies such as these on pathogen biology and epidemiology are essential to examine whether the information gained in one geographical location can be effectively used to manage the pathogen in another location.

Salicylic acid is involved in resistance responses in the Russian wheat aphid-wheat interaction

Summary The effect of Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), infestation on the salicylic acid (SA) content and catalase (EC 1.11.1.6) activity were studied in resistant (cv. Tugela DN) and near-isogenic susceptible (cv. Tugela) wheat ( Triticum aestivum L.). The aim was to gain information on the involvement of SA in the resistance responses in wheat against the RWA. RWA infestation selectively induced SA accumulation and peroxidase (EC 1.11.1.7) activity in the resistant wheat. Catalase activity was, however, inhibited in both resistant and susceptible wheat plants. Intercellularly applied SA confirmed the possible inhibitory effect of SA on catalase and stimulating effect on defense responses as indicated by peroxidase used as a marker. Results suggest the involvement of SA in the resistance of wheat against the RWA. A SA inhibitable catalase is also present in wheat; indications are that it may not be involved in SA-mediated defence responses.

Neural Network Classification of Tan Spot and Stagonospora Blotch Infection Periods in a Wheat Field Environment

ABSTRACT Tan spot and Stagonospora blotch of hard red spring wheat served as a model system for evaluating disease forecasts by artificial neural networks. Pathogen infection periods on susceptible wheat plants were measured in the field from 1993 to 1998, and incidence data were merged with 24-h summaries of accumulated growing degree days, temperature, relative humidity, precipitation, and leaf wetness duration. The resulting data set of 202 discrete periods was randomly assigned to 10 modeldevelopment or -validation (n = 50) data sets. Backpropagation neural networks, general regression neural networks, logistic regression, and parametric and nonparametric methods of discriminant analysis were chosen for comparison. Mean validation classification of tan spot incidence was between 71% for logistic regression and 76% for backpropagation models. No significant difference was found between methods of modeling tan spot infection periods. Mean validation prediction accuracy of Stagonospora blotch incidence was 86 and 81% for backpropagation and logistic regression, respectively. Prediction accuracies of other modeling methods were </=78% and were significantly different (P = 0.01) from backpropagation, but not logistic regression, results. The best backpropagation models of tan spot and Stagonospora blotch incidences correctly classified 82 and 84% of validation cases, respectively. High classification accuracy and consistently good performance demonstrate the applicability of neural network technology to plant disease forecasting.

Effects of wheat volunteers and blackgrass in set-aside following a winter wheat crop on soil infectivity and soil conduciveness to take-all

Two experiments were carried out in France in which disease indices were used to evaluate the effects of wheat volunteers and blackgrass (Alopecurus myosuroides) on soil infectivity and soil conduciveness to take-all caused by Gaeumannomyces graminis var. tritici. Soil infectivity was evaluated by measuring the disease index on susceptible wheat plants grown on soil samples collected from the field. Soil conduciveness to the disease was obtained by measuring disease indices on plants grown on soil samples to which different amounts of take-all fungus inoculum were added. One experiment (Expt. 1) was carried out using soils from farmers' fields (two fields in 1994 and two in 1995); soil infectivity and soil conduciveness were evaluated for three experimental situations: bare soil, soil with wheat volunteers and soil with blackgrass plants. In 1994 the soil infectivity was zero in bare soil, high with the wheat cover, and intermediate with the blackgrass cover. In 1995 the soil infectivity was uniformly low for all three conditions. Soils bearing wheat were less conducive than bare soil, soils bearing blackgrass and bare soils were similarly conducive. A second experiment (Expt. 2) carried out in 1995 compared the soil infectivity and soil conduciveness to take-all of soils planted with wheat or blackgrass in set-aside land after periods of wheat monoculture of 0–6 yr. The soil infectivity was low for all treatments. The soil was more conducive after blackgrass than after wheat. In both cases, the soil conduciveness was less when the monoculture had continued for more than 4 yr. The decline was less after blackgrass than after wheat. Thus, whenever set-aside is set up during the increase phase of the disease in fields with cereal successions, abundant wheat volunteers might hinder the expected positive effect of a break in cereal successions on take-all development. The presence of blackgrass in a set-aside field, with significant soil infectivity and high soil conduciveness, might increase the risks of take-all development in a wheat crop following set-aside.

RESPONSE OF TRITICALE TO THE HESSIAN FLY (DIPTERA: CECIDOMYIIDAE)

Hessian fly infestation on triticale (X triticosecale Wittmack) has not been previously reported. Severe infestations on wheat in Georgia prompted the evaluation of triticale germplasm in the greenhouse to known biotypes of Hessian fly and in the field to natural populations. Susceptible triticale plants showed the same characteristics as susceptible wheat plants. ‘Beagle 82’, ‘Wytch’, and ‘Marta’, as well as several experimental lines, were highly resistant to biotype D Hessian fly. ‘M2A-Beagle’ and ‘Great Northern’ were the only entries highly resistant to biotype E, the predominant biotype population in the Southeast. The total number of flies per 30-cm row averaged 58 and 283 at Experiment and Plains, respectively. Morrison had the lowest percentage of infested tillers. ‘Jenkins’ and ‘Council’ were the most severely infested cultivars. Crosses have been made to determine the genetics of Hessian fly resistance identified in these studies.