Papers by Christopher Little
Crop Protection, Oct 1, 2019
Globally, stalk rots are among the highest priority grain sorghum (Sorghum bicolor (L.) Moench) d... more Globally, stalk rots are among the highest priority grain sorghum (Sorghum bicolor (L.) Moench) diseases. This study examined the variation of leaf chlorophyll index and photosystem II quantum yield (Fv/Fm ratio) of sorghum genotypes after inoculation with two stalk rot fungi. In 2014, 2015, two resistant and two susceptible sorghum genotypes were evaluated in the field. Fusarium thapsinum (FT) and Macrophomina phaseolina (MP) are fungal pathogens that cause Fusarium stalk rot and charcoal rot, respectively. Plant stalks were inoculated with FT, MP, or phosphate-buffered saline (control). Flag leaves were measured for chlorophyll index (using soil and plant development meter: SPAD) and Fv/Fm ratio at three developmental stages (soft and hard dough, and physiological maturity). Both pathogens significantly reduced Fv/Fm ratio (P < 0.016) of the susceptible genotypes across environments and developmental growth stages. Neither pathogen affected the Fv/Fm ratio of the resistant genotypes. FT and MP inoculations significantly reduced chlorophyll index compared to the mockinoculated control (P < 0.0001) across genotypes and developmental stages. For the susceptible genotypes, there was a significant and positive correlation between mean chlorophyll index and Fv/Fm ratio across developmental stages (r ¼ þ0.67, P ¼ 0.018) while the relationship for resistant genotypes was significant and negative (r ¼ À 0.59, P ¼ 0.043). The ability of certain sorghum genotypes to retain higher photosynthetic competence during development regardless of their foliar chlorophyll decline under disease pressure appeared to be a possible mechanism of physiological resistance against stalk rot diseases.
Plant Breeding, Oct 22, 2019
Production of renewable and eco-friendly fuels has become a critical need due to the possibility ... more Production of renewable and eco-friendly fuels has become a critical need due to the possibility of diminishing fossil fuel supplies (Shafiee & Topal, 2009) and their combustion-related global warming and environmental pollution (Román-Leshkov, Barrett, Liu, & Dumesic, 2007). Consequently, many countries have examined various renewable energy production systems to replace fossil fuels (Demirbas, 2005). Ethanol produced from biomass is a contemporary form of bioenergy (also known as advanced biofuels) and could become a sustainable transportation fuel in the future, as well as a fuel oxygenate that can replace gasoline (Wang, 2000). Brazil and the United States are the major global ethanol producers and account for about 62% of world production (Kim & Dale, 2004). The primary feedstock for ethanol in Brazil is sugarcane, while corn grain functions this role in the United States (Kim & Dale, 2004). Each year, about 40 and 29% of the domestic maize and grain sorghum crops in the United States are processed into liquid ethanol, respectively (Goettemoeller & Goettemoeller, 2007). However, continued use of these crops for bioethanol production could threaten global food supplies and result in food market instability (Hahn-Hägerdal, Galbe, Gorwa-Grauslund, Lidén, & Zacchi, 2006). Therefore, discovering appropriate alternative sources for bioethanol generation is essential. Ethanol can be produced from any sugar or starch crop. Materials such as agricultural residues (e.g. corn stover, crop straw, sugarcane bagasse) and herbaceous crops (e.g. alfalfa, switchgrass) compose lignocellulosic biomass, which is another potential resource of ethanol (Wyman, 1996). Sweet sorghum is an attractive feedstock as it produces stalks with higher amounts of sugar
Crop Science, Oct 6, 2016
G enetic variation for stalk rot resistance in sorghum is well documented (Dodman et al., 1992). ... more G enetic variation for stalk rot resistance in sorghum is well documented (Dodman et al., 1992). However, because of the low-throughput nature of the existing disease phenotyping protocols, not many programs have picked up stalk rot resistance as a principal breeding target. Much of the progress achieved in addressing this problem and associated lodging has been through indirect selection for postflowering drought tolerance expressed as the stay-green trait. Stay-green, also known as nonsenescence is the ability of a plant to maintain active green leaf area and continue to photosynthesize under limited moisture during crop maturation (Rosenow and Clark, 1981). Stay-green genotypes have also been reported to exhibit resistance to charcoal rot caused by Macrophomina phaseolina (Rosenow, 1984; Borrell et al., 2000b) and lodging (Henzell et al., 1984). As a result, the trait has attracted significant attention over the last decades, and almost all breeding lines released in the recent past and many commercial hybrids have the stay-green trait incorporated into them. Quantitative trait loci associated with the stay-green phenotype in sorghum have been mapped in many populations including the prominent Tx642 Tx7000 RIL population (
Phytopathology, Aug 1, 2018
The cell-wall-degrading enzymes (CWDE) secreted by necrotrophs are important virulence factors. A... more The cell-wall-degrading enzymes (CWDE) secreted by necrotrophs are important virulence factors. Although not unequivocally demonstrated, it has been suggested that necrotrophs induce hosts to cooperate in disease development through manipulation of host CWDE. The necrotrophic fungus Macrophomina phaseolina causes charcoal rot disease in Sorghum bicolor. An RNA-seq experiment was conducted to investigate the behavior of sorghum CWDE-encoding genes after M. phaseolina inoculation. Results revealed M. phaseolina's ability to significantly upregulate pectin methylesterase-, polygalacturonase-, cellulase-, endoglucanase-, and glycosyl hydrolaseencoding genes in a charcoal rot-susceptible sorghum genotype (Tx7000) but not in a resistant genotype (SC599). For functional validation, crude enzyme mixtures were extracted from M. phaseolina-and mock-inoculated charcoal-rot-resistant (SC599 and SC35) and-susceptible (Tx7000 and BTx3042) sorghum genotype stalks. A gel diffusion assay (pectin substrate) revealed significantly increased pectin methylesterase activity in M. phaseolina-inoculated Tx7000 and BTx3042. Polygalacturonase activity was determined using a ruthenium red absorbance assay (535 nm). Significantly increased polygalacturonase activity was observed in two susceptible genotypes after M. phaseolina inoculation. The activity of cellulose-degrading enzymes was determined using a 2-cyanoacetamide fluorimetric assay (excitation and emission maxima at 331 and 383 nm, respectively). The assay revealed significantly increased cellulose-degrading enzyme activity in M. phaseolina-inoculated Tx7000 and BTx3042. These findings revealed M. phaseolina's ability to promote charcoal rot susceptibility in grain sorghum through induced host CWDE.
A survey was conducted in 2014 to determine the prevalence of fungal diseases on sorghum across d... more A survey was conducted in 2014 to determine the prevalence of fungal diseases on sorghum across different agroecological zones in Tanzania and Uganda. In Tanzania, 37 sorghum fields were sampled in Dodoma and Singida Provinces, representing the central drier areas, and in Simiyu, Shinyanga, Mwanza, and Mara Provinces, representing the lake-zone region. In Uganda, 134 fields were sampled across four agroecological zones of Teso, Western, Northern, and West Nile. Farmers were purposively selected, and at least 30 plants per field along two diagonal transects were visually assessed for disease. Ten and 14 diseases were identified in Tanzania and Uganda, respectively. Among the major diseases identified, those more prevalent in Tanzania than in Uganda were leaf blight caused by Setosphaeria turcica (P = 0.0031) and rust caused by Puccinia purpurea (P < 0.0001). Major diseases more prevalent in Uganda than in Tanzania were anthracnose caused by Colletotrichum sublineola (P = 0.0207) and zonate leaf spot caused by Gloeocercospora sorghi (P = 0.0312). We report for the first time the occurrence of ladder leaf spot caused by Cercospora fusimaculans in Uganda and confirm its occurrence in Tanzania. This is the first comprehensive sorghum disease survey report in over 15 years in both countries. Sorghum (Sorghum bicolor L.) is an important crop utilized for food and brewing alcohol in Tanzania and Uganda. In Tanzania, the crop is mainly grown in Dodoma, Singida, Shinyanga, Mwanza, and Mara Provinces, which fall in the central, western, and lakezone regions of the country (Monyo et al. 2004; Rowhani et al. 2011). Improved varieties such as Lulu, Pato, Macia, Serena, and Tegemeo are liked by farmers, who cite early maturity and drought tolerance as important traits (Monyo et al. 2004); however, landraces are still planted (Table 1). Interestingly, over the past few years, the net change of area under production and the annual production have both increased (by 4.6 and 1.6%, respectively); however, yield gains over the same period have reduced by 2.9% (FAO 2015), probably owing to increased biotic and abiotic stresses. Between 2010 and 2013, the total production of sorghum in Tanzania ranged from 798,000 metric tons to a high of 838,000 metric tons in 2012 (FAO 2015). In Uganda, sorghum, the third most important cereal food crop, is grown across five agroecological zones: Teso, Western, Northern, West Nile, and the South West highlands (Ebiyau et al. 2005). Both local (i.e., Akindi and Dura) and improved varieties (i.e., Seso1 and Gadam) are also grown in Uganda (Table 1). Recent crop production statistics indicate that production of sorghum in Uganda peaked in 2011 at 437,000 metric tons but dropped to 300,000 metric tons in 2013. Productivity also dropped from 1.2 metric tons/ ha in 2011 to 854 kg/ha in 2013. Importantly, unlike in Tanzania, TABLE 1 Varieties and landraces of sorghum identified in the field during the survey in Tanzania and Uganda Country, agroecological zone Varieties or landraces
Plant Disease, 2011
Ramulispora sorghi causes sooty stripe of sorghum. Disease severity in irrigated and dryland plot... more 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 ...
European Journal of Plant Pathology, 2012
Amplified fragment length polymorphism (AFLP) based genetic diversity was analyzed for 232 Collet... more Amplified fragment length polymorphism (AFLP) based genetic diversity was analyzed for 232 Colletotrichum sublineolum isolates collected between 2002 and 2004 from three geographically distinct regions of Texas, and from Arkansas, Georgia, and Puerto Rico. Results revealed significant levels of polymorphism (59%) among the isolates. Even so, genetic similarity between isolates was high, ranging from 0.78 to 1.00. Clustering of similar isolates did not correlate with either geographic origin or year of collection. Pathotypes of 20 of the isolates were determined using 14 sorghum lines previously used in Brazil and the United States and 4 from Sudan. Seventeen new pathotypes were established from the 18 isolates that gave uniform and consistent reactions on all host differentials over 2 years of greenhouse testing. Differentials BTx378 and QL3 were resistant to all isolates while BTx623 and TAM428 were universally susceptible both years. Each of these lines had shown differential responses in prior studies indicating that the pathogen population has sufficient diversity to adapt rapidly to changes in resistant host lines deployed. When the 2step pathotype classification scheme was used, the 18 isolates examined in this study were placed in four
SpringerPlus, 2013
Sorghum [Sorghum bicolor (L.) Moench] grain yield is severely affected by abiotic and biotic stre... more Sorghum [Sorghum bicolor (L.) Moench] grain yield is severely affected by abiotic and biotic stresses during postflowering stages, which has been aggravated by climate change. New parental lines having genes for various biotic and abiotic stress tolerances have the potential to mitigate this negative effect. Field studies were conducted under irrigated and dryland conditions with 128 exotic germplasm and 12 adapted lines to evaluate and identify potential sources for post-flowering drought tolerance and stalk and charcoal rot tolerances. The various physiological and disease related traits were recorded under irrigated and dryland conditions. Under dryland conditions, chlorophyll content (SPAD), grain yield and HI were decreased by 9, 44 and 16%, respectively, compared to irrigated conditions. Genotype RTx7000 and PI475432 had higher leaf temperature and grain yield, however, genotype PI570895 had lower leaf temperature and higher grain yield under dryland conditions. Increased grain yield and optimum leaf temperature was observed in PI510898, IS1212 and PI533946 compared to BTx642 (B35). However, IS14290, IS12945 and IS1219 had decreased grain yield and optimum leaf temperature under dryland conditions. Under irrigated conditions, stalk and charcoal rot disease severity was higher than under dryland conditions. Genotypes IS30562 and 1790E R had tolerance to both stalk rot and charcoal rot respectively and IS12706 was the most susceptible to both diseases. PI510898 showed combined tolerance to drought and Fusarium stalk rot under dryland conditions. The genotypes identified in this study are potential sources of drought and disease tolerance and will be used to develop better adaptable parental lines followed by high yielding hybrids.
Plant Health Progress, 2011
A recent outbreak of sorghum downy mildew (SDM) in Texas has led to the discovery of both metalax... more A recent outbreak of sorghum downy mildew (SDM) in Texas has led to the discovery of both metalaxyl fungicide resistance and a new pathotype, P6, in the causal organism Peronosclerospora sorghi. New and alternate sources of host plant resistance are needed for successful management of SDM. To identify sources of resistance, a total of 333 (242 minicore lines representing diverse germplasm from India, 67 commercial hybrids from Kansas, and 24 elite breeding lines from Texas) were inoculated in the greenhouse. Using an established sandwich inoculation technique, artificial inoculation of test lines with P. sorghi conidia, resulting in < 10% infection, were scored as “resistant.” Fifty-two minicore and 20 accessions from Kansas exhibited ≤ 10% infection and were selected as resistant. Out of 52 resistant minicore accessions, 28 were photoinsensitive. Eleven of 20 commercial hybrids from Kansas showed zero percent infection. Thirteen of 24 elite breeding lines from Texas were also re...
Phytoparasitica
ABSTRACT Stalk rots are major fungal diseases of sorghum [Sorghum bicolor (L.) Moench] worldwide ... more ABSTRACT Stalk rots are major fungal diseases of sorghum [Sorghum bicolor (L.) Moench] worldwide and cause significant economic loss. Conventionally, the length of stem lesions, produced by Fusarium thapsinum (FT; Fusarium stalk rot) and Macrophomina phaseolina (MP; charcoal rot), are measured to assess the degree of plant resistance. Genotypes with shorter lesion length (LL) are more resistant and expected to exhibit improved yield compared to susceptible genotypes. However, recent reports reveal inconsistent correlations between yield and LL, demonstrating the inadequacy of LL to predict yield under disease pressure. In this study, a new resistance-tolerance index (Index RT ) was used to rank 36 advanced sorghum male sterility maintainers (B-lines). Index RT was formulated in such a way that a lower index value indicates increased disease resistance and reduced yield loss (i.e., greater tolerance) and vice-versa after infection. When ranked by LL, ARCH11035B, -11025B and -11011B were the best performing lines against Fusarium stalk rot, whereas the same lines ranked 1, 3, and 9, respectively, using Index RT . Similarly, ARCH11018B, -11010B and -11014B had the lowest LLs respectively against charcoal rot, whereas the same lines were ranked 1, 4, and 30, respectively, based upon Index RT . The LL- or Index RT -dependent ranking differences of certain lines such as FT-inoculated ARCH11011B and MP-inoculated ARCH11014B indicated the effectiveness of deploying Index RT for better evaluating sorghum lines against stalk rot diseases. There was no significant correlation between LL and Index RT , revealing the independence of the two ranking systems. It is anticipated that this novel stalk rot screening procedure could be deployed by sorghum breeders for improved selection of parental lines.
HortScience
Fungi are major biotic constraints for optimum production and quality of glasshouse plants. When ... more Fungi are major biotic constraints for optimum production and quality of glasshouse plants. When plants are infested with sooty mold (Capnodium spp.) or infected with pathogens, the reflected wavelengths of the electromagnetic spectrum are altered. Spectroradiometric measurements and color infrared (CIR) images of control, honeydew-coated, and sooty mold-infested saplings and individual leaves from trifoliate orange (Poncirus trifoliata), sour orange (Citrus aurantium), ‘Valencia’ orange (C. sinensis), and ‘Bo’ tree (Ficus religiosa) were obtained. Grapefruit saplings and individual leaves infected with Mycosphaerella citri (greasy spot) were imaged under glasshouse conditions. Similarly, muskmelon foliage showing low and high levels of powdery mildew (Sphaerotheca fuliginea) disease severity were analyzed. When examining individual leaves, all fungal biotic stressors generally resulted in variable spectral reflectance data, especially in the blue (450 nm) and green (550 nm) wavelen...
Phytopathology, Jan 21, 2018
The cell wall degrading enzymes (CWDEs) secreted by necrotrophs are important virulence factors. ... more The cell wall degrading enzymes (CWDEs) secreted by necrotrophs are important virulence factors. Although not unequivocally demonstrated, it has been suggested that necrotrophs induce hosts to cooperate in disease development through manipulation of host CWDEs. The necrotrophic fungus, Macrophomina phaseolina (Tassi) Goid. (MP), causes charcoal rot disease in Sorghum bicolor (L.) Moench. An RNA-seq experiment was conducted to investigate the behavior of sorghum CWDEs encoding genes after MP inoculation. Results revealed MP's ability to significantly up-regulate pectin methylesterase-, polygalacturonase-, cellulase-, endoglucanase-, and glycosyl hydrolase-encoding genes in a charcoal rot susceptible sorghum genotype (Tx7000), but not in a resistant genotype (SC599). For functional validation, crude enzymes mixtures (CEM) were extracted from MP- and mock-inoculated charcoal rot resistant (SC599, SC35) and susceptible (Tx7000, BTx3042) sorghum genotype stalks. A gel diffusion assay...
Physiological and Molecular Plant Pathology
Grain mold of Sorghum bicolor is one of the leading constraints for the production of optimum qua... more Grain mold of Sorghum bicolor is one of the leading constraints for the production of optimum quality sorghum grain worldwide. Differences in mold levels among different varieties grown in the same environment imply that genes play a role in controlling mold severity. In order to determine if genes that function in active defense responses also affect molding, panicles of resistant and susceptible cultivars were inoculated at anthesis with conidial suspensions of Fusarium thapsinum and Curvularia lunata, the fungi most often found in naturally infected grain. RNA was extracted from the immature floral tissues at various times following inoculation. Levels of mRNA for four known defense-response genes, phenylalanine ammonia lyase (PAL1-1), chalcone synthase (CHS2G), b-1,3-glucanase (GLUC2-1) and chitinase (CHIT25-1) were examined by hybridization to PCR generated clones of the respective genes. Expression of each gene increased rapidly following inoculation with either fungus. Although differences were seen in response to the two pathogens, the general pattern was similar in resistant and susceptible cultivars. The results imply that factors other than the level or timing of active defense responses account for the cultivar differences seen when the plants are challenged at the time of flowering.
Foliar diseases and stalk rots are among the most damaging diseases of sorghum in terms of lost p... more Foliar diseases and stalk rots are among the most damaging diseases of sorghum in terms of lost production potential, thus commanding considerable research time and expenditure. This review will focus on anthracnose, a fungal disease that causes both foliar symptoms and stalk rots along with the stalk rots caused by Fusarium spp. and Macrophomina phaseolina. Although the downy mildews are caused by oomycetes rather than true fungi, recent outbreaks have revealed resistance to previously effective chemical seed treatments and the evolution of new pathogenic races, once again pointing out the need for continuous vigilance. Sorghum diseases are described with respect to the causal organism or organisms, infection process, global distribution, pathogen variability and effects on grain production. In addition, screening methods for identifying resistant cultivars and the genetic basis for host resistance including molecular tags for resistance genes are described where possible along with prospects for future advances in more stable disease control.
Plant Health Instructor, 2012
What is a fungus? A fungus is a eukaryote that digests food externally and absorbs nutrients dire... more What is a fungus? A fungus is a eukaryote that digests food externally and absorbs nutrients directly through its cell walls. Most fungi reproduce by spores and have a body (thallus) composed of microscopic tubular cells called hyphae. Fungi are heterotrophs and, like animals, obtain their carbon and energy from other organisms. Some fungi obtain their nutrients from a living host (plant or animal) and are called biotrophs; others obtain their nutrients from dead plants or animals and are called saprotrophs (saprophytes, saprobes). Some fungi infect a living host, but kill host cells in order to obtain their nutrients; these are called necrotrophs. Fungi were once considered to be primitive members of the plant kingdom, just slightly more advanced than bacteria. We now know that fungi are not primitive at all. In fact, recent taxonomic treatments such as the Tree of Life Project show that fungi and animals both belong to the group Opisthokonta (Fig. 1). Fungi may not be our next of kin, but they are more closely related to animals than they are to plants. We also recognize that organisms traditionally studied as "fungi" belong to three very different unrelated groups: the true fungi in Kingdom Fungi (Eumycota), the Oomycetes, and the slime molds (Fig. 1). Figure 1 Let's briefly consider the major groups in Kingdom Fungi-they will be described in greater detail later. Open most introductory mycology books and you'll see that there are four main groups (phyla) of true fungi-Ascomycota, Basidiomycota, Chytridiomycota and Zygomycota (e. g., Alexopoulos et al. 1996; Webster and Weber 2007). Recent studies have provided support for the recognition of additional phyla, such as Glomeromycota, a group of fungi once placed in Zygomycota that form an association with the roots of most plants (Fig. 2). A group of parasitic organisms called Microsporidia that live inside the cells of animals are also now considered to belong in the fungal kingdom (Fig. 2). Hibbett et al. (2007) published a comprehensive classification of the Kingdom Fungi, the result of collaboration among many fungal taxonomists. This classification is used in the Dictionary of the Fungi (Kirk et al. 2008) and other fungal references and databases. However, the classification system will undergo additional changes as scientists use new methods to study the
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Papers by Christopher Little