Susceptible Sorghum Research Articles

Identification of an orthologous clade of peroxidases that respond to feeding by greenbugs (Schizaphis graminum) in C4 grasses.

Knowledge of specific peroxidases that respond to aphid herbivory is limited in C4 grasses, but could provide targets for improving defence against these pests. A sorghum (Sorghum bicolor (L.) Moench) peroxidase (SbPrx-1; Sobic.002G416700) has been previously linked to biotic stress responses, and was the starting point for this study. Genomic analyses indicated that SbPrx-1 was part of a clade of five closely related peroxidase genes occurring within a ~30kb region on chromosome 2 of the sorghum genome. Comparison of this ~30-kb region to syntenic regions in switchgrass (Panicum virgatum L.) and foxtail millet (Setaria italica L.) identified similar related clusters of peroxidases. Infestation of a susceptible sorghum cultivar with greenbugs (Shizaphis graminum Rondani) induced three of the five peroxidases. Greenbug infestation of switchgrass and foxtail millet plants showed similar inductions of peroxidases. SbPrx-1 was also induced in response to aphid herbivory in a greenbug-resistant sorghum line, Cargill 607E. These data indicate that this genomic region of C4 grasses could be valuable as a marker to assess potential insect resistance in C4 grasses.

Identification of novel drought-responsive microRNAs and trans-acting siRNAs from Sorghum bicolor (L.) Moench by high-throughput sequencing analysis.

Small non-coding RNAs (sRNAs) namely microRNAs (miRNAs) and trans-acting small interfering RNAs (tasi-RNAs) play a crucial role in post-transcriptional regulation of gene expression and thus the control plant development and stress responses. In order to identify drought-responsive miRNAs and tasi-RNAs in sorghum, we constructed small RNA libraries from a drought tolerant (M35-1) and susceptible (C43) sorghum genotypes grown under control and drought stress conditions, and sequenced by Illumina Genome Analyzer IIx. Ninety seven conserved and 526 novel miRNAs representing 472 unique miRNA families were identified from sorghum. Ninety-six unique miRNAs were found to be regulated by drought stress, of which 32 were up- and 49 were down-regulated (fold change ≥ 2 or ≤ −2) at least in one genotype, while the remaining 15 miRNAs showed contrasting drought-regulated expression pattern between genotypes. A maximum of 17 and 18 miRNAs was differentially regulated under drought stress condition in the sensitive and tolerant genotypes, respectively. These results suggest that genotype dependent stress responsive regulation of miRNAs may contribute, at least in part, to the differential drought tolerance of sorghum genotypes. We also identified two miR390-directed TAS3 gene homologs and the auxin response factors as tasi-RNA targets. We predicted more than 1300 unique target genes for the novel and conserved miRNAs. These target genes were predicted to be involved in different cellular, metabolic, response to stimulus, biological regulation, and developmental processes. Genome-wide identification of stress-responsive miRNAs, tasi-RNAs and their targets identified in this study will be useful in unraveling the molecular mechanisms underlying drought stress responses and genetic improvement of biomass production and stress tolerance in sorghum.

Marker-assisted introgression improves Striga resistance in an Eritrean Farmer-Preferred Sorghum Variety

The parasitic weed Striga hermonthica hampers the production of sorghum, the most important cereal crop in Eritrea. This weed has a complex mode of infestation that adapts to many hosts and environments, complicating conventional breeding for resistance, which is the only form of crop improvement available to Eritrean breeders, but has failed. This study aimed at improving resistance against this parasite by transferring 5 Striga resistant Quantitative Trait Loci (QTLs) from resistance donor N13 to Striga susceptible Farmer-Preferred Sorghum Variety (FPSV) Hugurtay from Eritrea. The method involved backcrossing using marker-assisted selection (MAS) and evaluation of the best introgressed lines for Striga resistance in artificially infested fields. Foreground selection was performed with up to 11 polymorphic simple sequence repeat (SSR) markers linked to Striga resistance QTLs, while background selection was conducted in the BC3F2 generation with 27 polymorphic unlinked SSR markers to identify the best recovery of the recurrent parent (RP) genetic background. Out of 84 BC3F3 lines, L2P3-B, L1P5-A and L2P5P35 performed best with respect to both grain yield and reduced Striga infestation. These lines were more resistant to Striga than Hugurtay, but less resistant than N13. The three lines yielded twice as much as N13, with Area Under Striga Number Progression Curve (AUSNPC) values on average 18% higher than that of N13 and 38% lower than that of Hugurtay. This suggests that the introgressed QTLs conferred significant Striga resistance and yield advantage to these BC3F3 backcross progenies under Striga pressure. These lines have good potential for future release and demonstrate that when MAS is available to conventional breeders, even in countries with no genotyping facilities, it is a useful tool for enhancement, expediency and precision in crop improvement.

Effect of PEG induced osmotic stress on osmolytes and antioxidative enzymes in sorghum seedlings

The present investigations were undertaken to evaluate the levels of proline and glycine betaine osmolytes and activities of antioxidative enzymes under PEG-6000-induced osmotic stress in drought susceptible, tolerant and stay-green sorghum genotypes. A total of forty-eight drought susceptible, tolerant and stay-green rabi sorghum genotypes were screened to evaluate proline and glycine betaine accumulation under osmotic stress of PEG-6000. Among the tolerant genotypes, RSLG-262 and RSV-1366 accumulated the highest proline content, whereas, RSV-458 and Hadgaon local accumulated the highest glycine betaine content than susceptible genotypes. The high proline accumulating tolerant genotypes, RSLG-262 and RSV-1366 also exhibited correspondingly the highest ∆1_pyrroline-5-carboxylate synthetase activity. The mean activities of antioxidative enzymes such as superoxide dismutase, catalase, peroxidases and relative leaf water content were higher in the leaves of stay-green genotypes, followed by tolerant and susceptible types. Within stay-green genotypes, B-35 and E-36-1 and a drought tolerant genotype RSLG-262 recorded higher antioxidative enzyme activities as compared to other genotypes. The stay-green genotypes, B-35 and E-36-1 and a tolerant genotype RSLG-262 appeared to be the most promising ones.

Effect of PEG Induced Osmotic Stress on Peroxidase and Superoxide Dismutase Isozymes in Sorghum Seedlings

The present investigation was undertaken to understand the effect of PEG-6000– induced osmotic stress on isozymes of superoxide dismutase and peroxidase as well as their activities in four each of drought susceptible, tolerant and stay-green sorghum genotypes at seedling stage. Total number of isozymes of superoxide dismutase was more under stressed than unstressed condition, whereas the band intensity was also higher under stressed condition. Two to four new peroxidase isozymes with very high colour intensity bands were observed in most of the genotypes under stressed condition. The mean activity of superoxide dismutase and peroxidase were higher in the stay-green genotypes, followed by tolerant and susceptible genotypes.

Oxidative enzyme changes in sorghum infested by shoot fly.

This research investigated the role of oxidative enzymes in the defense response of sorghum, Sorghum bicolor (L.) Moench (Poales: Poaceae), to the sorghum shoot fly, Atherigona soccata Rondani (Diptera: Muscidae). Changes in polyphenol oxidase and peroxidase activity and total protein content were observed in resistant and susceptible sorghum genotypes in response to A. soccata feeding. Resistant plants exhibited higher levels of peroxidase and polyphenol oxidase activities and total protein content compared with susceptible plants. Peroxidase and polyphenol oxidase activities and total protein content in the infested resistant and susceptible genotypes were higher when compared with their control plants, respectively. These findings suggest that resistant genotypes may be able to tolerate shoot fly feeding by increasing their peroxidase and polyphenol oxidase activities. Among the enzymes examined, differences in isozyme profiles for peroxidase and polyphenol oxidase were detected between control and infested IS 18551, M35-1, 296B, SSV 84, and DJ 6514 plants. Differences in protein profiles were observed between A. soccata infested and their respective uninfested controls of all the genotypes. In conclusion, this study revealed that these defense enzymes and proteins might attribute to the resistance mechanisms in sorghum plants against A. soccata infestation.

Metabolism of quizalofop and rimsulfuron in herbicide resistant grain sorghum

Studies were conducted to determine if herbicide metabolism is an additional mechanism that could explain the resistance of ACCase- and ALS-resistant grain sorghum to quizalofop and rimsulfuron, respectively. ACCase- and ALS-resistant and -susceptible genetic lines were grown under controlled conditions and treated at the 4-leaf stage with 14C-labeled quizalofop and rimsulfuron. Plants were harvested at 3, 5, and 7d after treatments. In the ACCase metabolism experiment, resistant grain sorghum transformed 88% of quizalofop-ethyl to quizalofop while 91% of the inactive was converted to active form by the susceptible plants 3DAT. By 7DAT, all inactive quizalofop-ethyl was converted to active quizalofop. In the ALS metabolism study, two distinct metabolites were produced from rimsulfuron. Metabolism rate was similar between resistant lines (TX430R and N223R) in all harvest dates except at 7 DAT; however, more rapid metabolism were observed when resistant were compared with the susceptible genotypes (TX430S and N223S). The percentage of recovered rimsulfuron 3DAT corresponded to 80% and 83% in the resistant compared to 87% in the susceptible grain sorghum. At 5DAT, metabolism was near steady in all sorghum plants but by 7DAT, resistant genotypes metabolized 4–12% more than the susceptible sorghum. Metabolism of rimsulfuron in ALS-resistant grain sorghum is an added mechanism that could help elevate the level of rimsulfuron resistance.

Silicon and Fungicide Effects on Anthracnose in Moderately Resistant and Susceptible Sorghum Lines

AbstractThis study aimed to evaluate the effect of silicon (Si) and its interaction with fungicide on the management of sorghum anthracnose. The experiments were carried out in Si‐deficient soil in the 2008/2009 and 2009/2010 growing seasons in a randomized, complete block, split‐split plot design with four replications. Calcium silicate (CS) and lime (L), at the rates of 6 and 5 ton/ha, respectively, were randomly assigned to the main plot. Two sorghum lines, BR‐008 (resistant) and BR‐009 (susceptible), were assigned to the split plots. The split‐split plots corresponded to with or without the fungicide Opera® (epoxiconazole + pyraclostrobin). The residual effect of CS and L from the 2008/2009 growing season was evaluated in the 2009/2010 growing season. For the 2008/2009 growing season, the area under anthracnose progress curve (AUAPC) was reduced by 39 and 42% for lines BR‐008 and BR‐009, respectively, with the application of CS. In the presence of the fungicide, the AUAPC was reduced by 35 and 42% for the CS and L treatments, respectively. Calcium silicate with and without fungicide contributed to decreasing the AUAPC by 44 and 37%, respectively. The fungicide spray decreased the AUAPC by 50 and 39% for lines BR‐008 and BR‐009, respectively. Without fungicide, the AUAPC decreased by 88% for line BR‐008 compared with line BR‐009; however, with fungicide, the reduction reached 90%. The Si leaf tissue concentration significantly increased with the CS application (5.9 g/kg) compared with the L application (0.3 g/kg), regardless of the sorghum line. The yield increased by 0.6 ton/ha with the CS compared to the L application. The fungicide increased yield by 0.48 ton/ha compared with the non‐fungicide spray treatment. The residual effect of CS in the soil increased Si leaf tissue concentration and yield as well as reduced the intensity of anthracnose in the 2009/2010 growing season.

First Report of Sorghum mosaic virus Causing Mosaic in Miscanthus sinensis.

Miscanthus is being evaluated as a bioenergy feedstock because of its potentially significant biomass production, perennial habit, and lack of major diseases and pests. It is also a valuable parent in sugarcane breeding programs as a source of cold tolerance. In May 2010, mosaic symptoms were observed on a clone of Miscanthus sinenesis Anderss. maintained at the USDA, ARS, Sugarcane Research Unit. All plants of the Miscanthus clone in our germplasm collection are from vegetative cuttings of the original infected plant and show mosaic symptoms. Leaves from the ratoon of a single plant tested positive in a reverse transcription-PCR with the Potyvirus Group PCR Test (Agdia, Inc., Elkhart, IN) with two primer sets, Poty-F1/Poty-R1 and Poty-F2/Poty-R2. After sequencing the potyvirus amplicons, a BLAST search in GenBank revealed that these sequences had the highest identities (81 and 69%) with Sorghum mosaic virus (SrMV) at the nucleic acid level and a 72 and 95% similarity at the amino acid level. Extracts from the Miscanthus clone prepared by the indirect extraction buffer (Agdia) containing sodium carbonate also tested positive for potyvirus by indirect ELISA with the 'universal' potyvirus monoclonal antibody, PTY1. To our knowledge, this is the first report of SrMV on Miscanthus. The only other member of the genus Potyvirus reported on Miscanthus is Sugarcane mosaic virus (1,2). Mosaic caused by SrMV could become an economically important disease of Miscanthus if this crop is grown for bioenergy feedstock on large areas. An SrMV-infected Miscanthus crop could pose a threat to established crops of susceptible sugarcane and sorghum since the virus is transmitted in a nonpersistent manner by several aphids, as well as, contributing to geographic shifts of the pathogen.

Genetic Resistance to Acetyl‐Coenzyme A Carboxylase‐Inhibiting Herbicides in Grain Sorghum

ABSTRACTWeed management is one of the most important considerations impacting sorghum [Sorghum bicolor (L.) Moench] production in the United States today. The objectives of this study were to evaluate the level of resistance, type of inheritance, and causal mutation of wild sorghum accessions that are resistant to acetyl‐coenzyme A carboxylase (ACCase)‐inhibiting herbicides compared to susceptible grain sorghum seed parents. Acetyl‐coenzyme A carboxylase‐inhibiting herbicides used in this study were fluazifop‐P and quizalofop‐P from the aryloxyphenoxypropionate (APP) family and clethodim and sethoxydim from the cyclohexanedione (CHD) family. Dose–response studies indicated the level of resistance was very high for APP herbicides but low to nonexistent for CHD herbicides. Estimates of the resistance factors to APP herbicides based on herbicide rate required for 50% growth reduction (GR50) were 54 to 64 for homozygous plants and 9 to 19 for heterozygous plants. Resistance to CHD herbicides was very low with resistance factors ranging from one to about five. Genetic segregation studies indicated a single gene is the cause of resistance to APP herbicides. Polymerase chain reaction (PCR) amplicons from the carboxyl transferase (CT) domain of the chloroplast directed ACCase gene were sequenced and a single point mutation that results in a cysteine replacing tryptophan (conversion from tryptophan at amino acid position 2027 to cysteine [Trp‐2027‐Cys]) was discovered. Given the high level of herbicide resistance and simple inheritance, these sources of APP resistance may provide useful herbicide resistance traits for use in sorghum.

Molecular tagging and validation of microsatellite markers linked to the low germination stimulant gene (lgs) for Striga resistance in sorghum [Sorghum bicolor (L.) Moench

Striga is a devastating parasitic weed in Africa and parts of Asia. Low Striga germination stimulant activity, a well-known resistance mechanism in sorghum, is controlled by a single recessive gene (lgs). Molecular markers linked to the lgs gene can accelerate development of Striga-resistant cultivars. Using a high density linkage map constructed with 367 markers (DArT and SSRs) and an in vitro assay for germination stimulant activity towards Striga asiatica in 354 recombinant inbred lines derived from SRN39 (low stimulant)×Shanqui Red (high stimulant), we precisely tagged and mapped the lgs gene on SBI-05 between two tightly linked microsatellite markers SB3344 and SB3352 at a distance of 0.5 and 1.5cM, respectively. The fine-mapped lgs region was delimited to a 5.8cM interval with the closest three markers SB3344, SB3346 and SB3343 positioned at 0.5, 0.7 and 0.9cM, respectively. We validated tightly linked markers in a set of 23 diverse sorghum accessions, most of which were known to be Striga resistant, by genotyping and phenotyping for germination stimulant activity towards both S. asiatica and S. hermonthica. The markers co-segregated with Striga germination stimulant activity in 21 of the 23 tested lines. The lgs locus similarly affected germination stimulant activity for both Striga species. The identified markers would be useful in marker-assisted selection for introgressing this trait into susceptible sorghum cultivars. Examination of the sorghum genome sequence and comparative analysis with the rice genome suggests some candidate genes in the fine-mapped region (400kb) that may affect strigolactone biosynthesis or exudation. This work should form a foundation for map-based cloning of the lgs gene and aid in elucidation of an exact mechanism for resistance based on low Striga germination stimulant activity.

Yield losses in grain sorghum due to rust infection

Although rust (caused by Puccinia purpurea) is a common disease in Australian grain sorghum crops, particularly late in the growing season (April onwards), its potential to reduce yield has not been quantified. Field trials were conducted in Queensland between 2003 and 2005 to evaluate the effect of sorghum rust on grain yield of two susceptible sorghum hybrids (Tx610 and Pride). Rust was managed from 28–35 days after sowing until physiological maturity by applying oxycarboxin (1 kg active ingredient/100 L of water/ha) every 10 days. When data were combined for the hybrids, yield losses ranged from 13.1% in 2005 to 3.2% in 2003 but differences in yield the between sprayed and unsprayed treatments were statistically significant (P ≤ 0.05) only in 2005. Final area under the disease progress curve (AUDPC) values reflected the yield losses in each year. The higher yield loss in 2005 can be attributed primarily to the early development of the rust epidemic and the higher inoculum levels in spreader plots at the time of planting of the trials.

Disease Severity and Microsclerotium Properties of the Sorghum Sooty Stripe Pathogen, Ramulispora sorghi

Ramulispora sorghi causes sooty stripe of sorghum. Disease severity in irrigated and dryland plots was measured for 25 susceptible sorghum genotypes during the 2007 and 2008 growing seasons using a rating scale based upon percent leaf area infected. Disease severity ratings were approximately 1.4 points higher (P < 0.0001) on the rating scale in the irrigated plots than dryland plots for 2007 and 2008. Sooty stripe lesions were collected from each sorghum genotype in irrigated plots and assessed for mean microsclerotium production within lesions, microsclerotium size, and sporogenic germination, with significant differences apparent between genotypes for microsclerotium size (P = 0.01) and sporogenic germination (P = 0.01). There was no relationship between disease severity and microsclerotium production within leaf lesions, microsclerotium size, or sporogenic germination; however, there was a positive and significant correlation between microsclerotia production within a lesion and microsclerotium size (R2 = 0.19, P < 0.0001). Although microsclerotia from sorghum lesions varied in structural characteristics and their ability to produce spore masses, these qualities were dependent upon the sorghum genotype from which the microsclerotia were derived, because the R. sorghi population was genetically uniform as determined by internal transcribed spacer sequences and random amplified polymorphic DNA polymerase chain reaction.

A highly conserved NB-LRR encoding gene cluster effective against Setosphaeria turcica in sorghum

BackgroundThe fungal pathogen Setosphaeria turcica causes turcicum or northern leaf blight disease on maize, sorghum and related grasses. A prevalent foliar disease found worldwide where the two host crops, maize and sorghum are grown. The aim of the present study was to find genes controlling the host defense response to this devastating plant pathogen. A cDNA-AFLP approach was taken to identify candidate sequences, which functions were further validated via virus induced gene silencing (VIGS), and real-time PCR analysis. Phylogenetic analysis was performed to address evolutionary events.ResultscDNA-AFLP analysis was run on susceptible and resistant sorghum and maize genotypes to identify resistance-related sequences. One CC-NB-LRR encoding gene GRMZM2G005347 was found among the up-regulated maize transcripts after fungal challenge. The new plant resistance gene was designated as St referring to S. turcica. Genome sequence comparison revealed that the CC-NB-LRR encoding St genes are located on chromosome 2 in maize, and on chromosome 5 in sorghum. The six St sorghum genes reside in three pairs in one locus. When the sorghum St genes were silenced via VIGS, the resistance was clearly compromised, an observation that was supported by real-time PCR. Database searches and phylogenetic analysis suggest that the St genes have a common ancestor present before the grass subfamily split 50-70 million years ago. Today, 6 genes are present in sorghum, 9 in rice and foxtail millet, respectively, 3 in maize and 4 in Brachypodium distachyon. The St gene homologs have all highly conserved sequences, and commonly reside as gene pairs in the grass genomes.ConclusionsResistance genes to S. turcica, with a CC-NB-LRR protein domain architecture, have been found in maize and sorghum. VIGS analysis revealed their importance in the surveillance to S. turcica in sorghum. The St genes are highly conserved in sorghum, rice, foxtail millet, maize and Brachypodium, suggesting an essential evolutionary function.

Electrophysiological and Behavioral Responses of Sorghum Shoot Fly, Atherigona soccata, to Sorghum Volatiles

The sorghum shoot fly, Atherigona soccata, is an economically important pest of sorghum in Asia, Mediterranean Europe and Africa. Field observations have suggested that shoot fly susceptible sorghum varieties emit attractive volatiles, but the compounds involved were unknown. The objective of the present study was to identify plant-derived attractants for A. soccata. Headspace samples were collected from the susceptible cultivar 'Swarna,' and when female A. soccata were exposed to the volatiles in an olfactometer bioassay, a strong positive behavioral response was observed. Coupled GC-EAG with female A. soccata revealed eight compounds that elicited an EAG response, which were identified by coupled GC-MS and GC peak enhancement on two GC columns of different polarity as (Z)-3-hexen-1-yl acetate, (-)-α-pinene, (-)-(E)-caryophyllene, methyl salicylate, octanal, decanal, 6-methyl-5-hepten-2-one and nonanal. When an eight-component synthetic blend of the EAG active compounds, at the same concentration (2.64μg) and ratio as in the natural headspace sample, was tested, A. soccata spent more time in the treated region of the olfactometer than controls (P = 0.001). Furthermore, when this synthetic blend and the natural headspace sample were tested in a choice test, the shoot flies did not show any preference for either of the two treatments, demonstrating that the synthetic blend had similar activity to the natural sample. Results are discussed in relation to breeding sorghum varieties less attractive to this pest.

Identification of quantitative trait loci associated with resistance to foliar diseases in sorghum [Sorghum bicolor (L.) Moench

Forage sorghum cultivars grown in India are susceptible to various foliar diseases, of which anthracnose, rust, zonate leaf spot, drechslera leaf blight and target leaf spot cause severe damage. We report here the quantitative trait loci (QTLs) conferring resistance to these foliar diseases. QTL analysis was undertaken using 168 F7 recombinant inbred lines (RILs) of a cross between a female parental line 296B (resistant) and a germplasm accession IS18551 (susceptible). RILs and parents were evaluated in replicated field trials in two environments. A total of twelve QTLs for five foliar diseases on three sorghum linkage groups (SBI-03, SBI-04 and SBI-06) were detected, accounting for 6.9–44.9% phenotypic variance. The morphological marker Plant color (Plcor) was associated with most of the QTL across years and locations. The QTL information generated in this study will aid in the transfer of foliar disease resistance into elite susceptible sorghum breeding lines through marker-assisted selection.

Development of Anthracnose on Grain Sorghum Hybrids Inoculated with Recently Described Pathotypes of Colletotrichum sublineolum Found in Arkansas.

Sorghum anthracnose, caused by Colletotrichum sublineolum, is found in most grain-sorghum-producing areas, including Arkansas. Yield losses can be severe in susceptible sorghum hybrids, suggesting that host resistance will continue to be critical for maintaining high yields. However, genetic control is often unsuccessful because of haplotype and pathotype variability within the C. sublineolum population. The objective of this research was to assess the levels of resistance of selected grain sorghum hybrids to isolates of the recently described pathotypes of C. sublineolum found in Arkansas. Field experiments were conducted in 2005 and 2006 to evaluate the reaction of 10 sorghum hybrids to eight C. sublineolum pathotypes by collecting anthracnose severity ratings at 7, 14, 21, and 28 days after inoculation. Based on these disease severity ratings, the area under the disease progress curve was calculated for each pathotype by hybrid interaction. These data showed that different levels of resistance are available in the current grain sorghum hybrids to different pathotypes. Cargill 888Y was resistant to all pathotypes in both years. However, Asgrow A571, DEKALB DKS53-11, FFR 318, and Pioneer 84G62 were moderately susceptible to susceptible to all pathotypes tested in both years. Pathotype 9 caused the most disease on 7 of the 10 hybrids evaluated in 2005 but, in 2006, pathotypes 12 and 13 surpassed pathotype 9 for disease severity on 9 of the 10 hybrids. More work is needed to assess host resistance and the effect of sorghum anthracnose on yield parameters of grain sorghum hybrids planted in Arkansas.

Cytological investigations on colonization of sorghum roots by the mycoherbicide Fusarium oxysporum f. sp. strigae and its implications for Striga control using a seed treatment delivery system

The application of the potential Striga-mycoherbicide Fusarium oxysporum f. sp. strigae (Foxy 2) by seed coating is an appropriate option for delivering and establishing the biocontrol agent in the rhizosphere, the infection zone of the root parasitic weed Striga hermonthica. Cytological investigations using light and transmission electron microscopy were performed to assess the pattern and extent of colonization of sorghum roots and shoots by Foxy 2, applied as film-coat on seeds. Germination of Foxy 2-treated plants was similar to un-treated plants. During sorghum root development, mycelia of Foxy 2 started from seed coat colonizing root surfaces. The intensity of root colonization increased with time. However, hyphae were not found in the apical zone of the roots. Hyphae growing on the surface of the roots were observed penetrating rhizodermal cells including root hairs, and colonizing the intercellular space and the cells of the cortical parenchyma. Even after four weeks, hyphae were not penetrating the endodermal layer to invade the tissues of the central cylinder. In addition, hyphae of Foxy 2 were also completely absent in the xylem vessels in sorghum shoots even after 11 weeks of sowing, which further proves the non-pathogenicity of the fungus to sorghum. Foxy 2 showed high efficacy in controlling Striga in combination with both tolerant and susceptible sorghum varieties in a root chamber experiment, as reflected by the high percentage of diseased Striga seedlings (95% and 86%, respectively), recorded after 26 days of sowing. The intensity of fungal root colonization, which coincides with the peak of Striga attachments to its host root, will ensure high fungal efficacy and will facilitate the practical use of Foxy 2 against Striga using seed treatment delivery. Indeed, this proof of sorghum non-pathogenicity of the fungus will encourage the acceptance of Foxy 2 by authorities and farmers in Africa.

The effects of cropping history on grain sorghum yields and anthracnose severity in Arkansas

Sorghum anthracnose, caused by Colletotrichum sublineolum , is found in most grain sorghum producing areas in the U.S.A., including Arkansas. High incidences and severities of anthracnose are infrequent and often limited to portions of some Arkansas fields. However, sporadic epidemics have occurred over many hectares and grain yield losses have been severe on susceptible sorghum hybrids suggesting that better management strategies are needed to improve and maintain high sorghum yields. The objective of this research was to evaluate the effect of previous cropping history on grain sorghum yields and on the severity of sorghum anthracnose in several agricultural environments in Arkansas. Replicated field experiments were conducted in 2005 and 2006 to evaluate sorghum following sorghum, sorghum following maize, sorghum following soybeans, and sorghum following rice on grain yields and anthracnose severity for 30 sorghum hybrids in 2005 and ten hybrids in 2006. The results show that the severity of anthracnose ranged from 0% to >23% and planting sorghum in fields one year after maize, soybeans, or rice appeared to significantly reduce the severity of sorghum anthracnose on most of the sorghum hybrids evaluated in both years. Although anthracnose severity varied across years in these experiments, the data show significant yield increases for most of the sorghum hybrids when sorghum was planted after rice, soybeans or maize, but not when sorghum was planted in the same field in which sorghum was grown in the previous year. Grain yields ranged from 0 kg ha −1 to more than 9500 kg ha −1 . These results demonstrate that planting sorghum after sorghum significantly increases anthracnose on most hybrids, that planting sorghum after rice, maize or soybeans can successfully reduce sorghum anthracnose, and that the economic benefits (yields) of these cropping sequences are partially dependent upon the year, the severity of the disease and the hybrid. Other management strategies such as sanitation, elimination of alternative hosts and planting clean, healthy seed or seed treated with an appropriate fungicide, coupled with disease resistance should be helpful and perhaps sufficient to avoid or reduce serious losses from grain sorghum anthracnose.

Infection biology and defence responses in sorghum againstColletotrichum sublineolum

To investigate the infection biology of Colletotrichum sublineolum (isolate CP2126) and defence responses in leaves of resistant (SC146), intermediately resistant (SC326) and susceptible (BTx623) sorghum genotypes. Infection biology and defence responses were studied quantitatively by light microscopy, H(2)O(2) accumulation by DAB staining and HRGP accumulation by immunological methods. Inhibition of conidial germination and appressorium formation may represent prepenetration defence responses on the leaf surface. Inducible defence responses in the resistant genotypes included decreases in formation of appressoria as well as accumulation of H(2)O(2), HRGPs and phytoalexins. Concomitant with these inducible responses, fungal growth was stopped during or just after penetration in genotypes SC146 and SC326. High levels of H(2)O(2) accumulating at late infection stages (5 days after inoculation) in the susceptible genotype BTx623 correlated with necrosis and tissue degeneration. The early accumulation of H(2)O(2) and HRGPs indicates roles in defence whereas the late accumulation in genotype BTx623 correlated with successful pathogenesis. The fact that there is a significant correlation between induced accumulation of H(2)O(2), papilla formation and cell wall cross-linking, as evidenced by HRGP accumulation, and cessation of pathogen growth in resistant genotypes may help exploit host resistance in sorghum.