First Report of Ramulispora sorghicola in the United States Causing Oval Leaf Spot on Johnsongrass and Sorghum in Texas.

Oval leaf spot (OLS) caused by Ramulispora sorghicola Harris was observed on grain sorghum, Sorghum bicolor (L.) Moench, and johnsongrass, S. halepense (L.) Pers., near Beeville, TX during August 2002. Symptoms were first observed on several sorghum lines and hybrids in a field nursery including a bulk planting of the line ATx623. Highest incidence of OLS occurred in rows adjacent to johnsongrass with symptoms of OLS. Average lesion size (mm) was 1.3 × 2.8 with a range from 0.5 to 2.5 × 1.5 to 5.0. Lesions had a straw-colored sunken center and on red- and purple-pigmented sorghums, lesion borders were highly pigmented. Cone-shaped conidial masses and superficial sclerotia (subglobose, black, 80 to 190 µm in diameter × 50 to 70 µm high, with spiny setae) were sometimes present or readily produced on lesions within 48 to 72 h after placement in humidity chambers. Conidia were branched, filiform, tapered, and 1.1 to 2.4 × 20 to 75 µm. The pathogen, R. sorghicola, was isolated from conidia and sclerotia. A water suspension of culturally derived conidia of R. sorghicola (3 × 104 conidia per ml) was spray inoculated (5:30 p.m., October 11, 2002) onto four or more upper leaves per plant of six grain sorghum plants (ATx623) and approximately nine johnsongrass plants (three tillers each of three plants) at a Corpus Christi field location where OLS was absent. Three grain sorghum and one johnsongrass plant were sprayed with a water control. Cloudy, wet, and cool conditions after inoculation and increasingly cooler nights probably delayed symptom expression until 3 to 4 weeks after inoculation. Typical lesions were observed simultaneously on both hosts with symptoms restricted to inoculated plants. Lesions from both hosts were placed onto water agar at 25°C for 24 h, and the pathogen was reisolated from field-produced conidia of rehydrated conidial masses. Through 2004, OLS was observed on sorghum hosts in 29 counties from central Texas to the Lower Rio Grande Valley. During the growing season, OLS was predominantly absent in grain and forage sorghum fields and absent or often difficult to detect in johnsongrass. In all 3 years, OLS was most common after the normal growing season from August through December with occurrence primarily on johnsongrass but also on late-planted and feral S. bicolor hosts, especially when proximal to symptomatic johnsongrass. Presence and incidence of OLS was highly variable between and within stands of johnsongrass with incidence ranging from a few to most plants. Incidence in forage or grain sorghum fields was highest at field borders adjacent to johnsongrass with OLS. Disease severity was low except on johnsongrass at a few locations. The pathogen appears to pose low economic risk to any sorghum host in Texas at any time of the year although highly susceptible lines and hybrids should be identified and possibly avoided. The previous most proximal report of R. sorghicola in the Western Hemisphere was in Honduras (1). The widespread distribution of OLS across southern Texas and its pattern of occurrence in johnsongrass suggest that the pathogen may have been unobserved in Texas for several years. Presence of OLS near the Rio Grande indicates probable occurrence in johnsongrass at least in some areas along this river in northeastern Mexico. Reference: (1) G. C. Wall et al. Trop. Pest Manag. 35:57, 1989.

Mapping QTLs associated with drought resistance in sorghum (Sorghum bicolor L. Moench).

Drought is a major abiotic stress factor limiting crop production. Identification of genetic factors involved in plant responses to drought stress will provide a solid foundation to improve drought resistance. Sorghum is well adapted to hot dry environments and regarded as a model for studying drought resistance among the grasses. Significant progress in genome mapping of this crop has also been made. In sorghum, rapid premature leaf death generally occurs when water is limited during the grain filling period. Premature leaf senescence, in turn, leads to charcoal rot, stalk lodging, and significant yield loss. More than 80% of commercial sorghum hybrids in the United States are grown under non-irrigated conditions and although most of them have pre-flowering drought resistance, many do not have any significant post-flowering drought resistance. Stay-green is one form of drought resistance mechanism, which gives sorghum resistance to premature senescence under soil moisture stress during the post-flowering period. Quantitative trait locus (QTL) studies with recombinant inbred lines (RILs) and near-isogenic lines (NILs) identified several genomic regions associated with resistance to pre-flowering and post-flowering drought stress. We have identified four genomic regions associated with the stay-green trait using a RIL population developed from B35 x Tx7000. These four major stay-green QTLs were consistently identified in all field trials and accounted for 53.5% of the phenotypic variance. We review the progress in mapping stay-green QTLs as a component of drought resistance in sorghum. The molecular genetic dissection of the QTLs affecting stay-green will provide further opportunities to elucidate the underlying physiological mechanisms involved in drought resistance in sorghum and other grasses.

Molecular mapping of QTLs conferring stay-green in grain sorghum (Sorghum bicolor L. Moench).

Drought resistance is of enormous importance in crop production. The identification of genetic factors involved in plant response to drought stress provides a strong foundation for improving drought tolerance. Stay-green is a drought resistance trait in sorghum (Sorghum bicolor L. Moench) that gives plants resistance to premature senescence under severe soil moisture stress during the post-flowering stage. The objective of this study was to map quantitative trait loci (QTLs) that control the stay-green and chlorophyll content in sorghum. By using a restriction fragment length polymorphism (RFLP) map, developed from a recombinant inbred line (RIL) population, we identified four stay-green QTLs, located on three linkage groups. The QTLs (Stg1 and Stg2) are on linkage group A, with the other two, Stg3 and Stg4, on linkage groups D and J, respectively. Two stay-green QTLs, Stg1 and Stg2, explaining 13-20% and 20-30% of the phenotypic variability, respectively, were consistently identified in all trials at different locations in two years. Three QTLs for chlorophyll content (Chl1, Chl2, and Chl3), explaining 25-30% of the phenotypic variability were also identified under post-flowering drought stress. All coincided with the three stay-green QTL regions (Stg1, Stg2, and Stg3) accounting for 46% of the phenotypic variation. The Stg1 and Stg2 regions also contain the genes for key photosynthetic enzymes, heat shock proteins, and an abscisic acid (ABA) responsive gene. Such spatial arrangement shows that linkage group A is important for drought- and heat-stress tolerance and yield production in sorghum. High-resolution mapping and cloning of the consistent stay-green QTLs may help to develop drought-resistant hybrids and to understand the mechanism of drought-induced senescence in plants.

Mapping of post-flowering drought resistance traits in grain sorghum: association between QTLs influencing premature senescence and maturity.

The identification of genetic factors underlying the complex responses of plants to drought stress provides a solid basis for improving drought resistance. The staygreen character in sorghum (Sorghum bicolor L. Moench) is a post-flowering drought resistance trait, which makes plants resistant to premature senescence under drought stress during the grainfilling stage. The objective of this study was to identify quantitative trait loci (QTLs) that control premature senescence and maturity traits, and to investigate their association under post-flowering drought stress in grain sorghum. A genetic linkage map was developed using a set of recombinant inbred lines (RILs) obtained from the cross B35 x Tx430, which were scored for 142 restriction fragment length polymorphism (RFLP) markers. The RILs and their parental lines were evaluated for post-flowering drought resistance and maturity in four environments. Simple interval mapping identified seven stay-green QTLs and two maturity QTLs. Three major stay-green QTLs (SGA, SGD and SGG) contributed to 42% of the phenotypic variability (LOD 9.0) and four minor QTLs (SGB, SGI. 1, SGI.2, and SGJ) significantly contributed to an additional 25% of the phenotypic variability in stay-green ratings. One maturity QTL (DFB) alone contributed to 40% of the phenotypic variability (LOD 10.0), while the second QTL (DFG) significantly contributed to an additional 17% of the phenotypic variability (LOD 4.9). Composite interval mapping confirmed the above results with an additional analysis of the QTL x Environment interaction. With heritability estimates of 0.72 for stay-green and 0.90 for maturity, the identified QTLs explained about 90% and 63% of genetic variability for stay-green and maturity traits, respectively. Although stay-green ratings were significantly correlated (r = 0.22, P< or =0.05) with maturity, six of the seven stay-green QTLs were independent of the QTLs influencing maturity. Similarly, one maturity QTL (DFB) was independent of the staygreen QTLs. One stay-green QTL (SGG), however, mapped in the vicinity of a maturity QTL (DFG), and all markers in the vicinity of the independent maturity QTL (DFB) were significantly (P< or =0.1) correlated with stay-green ratings, confounding the phenotyping of stay-green. The molecular genetic analysis of the QTLs influencing stay-green and maturity, together with the association between these two inversely related traits, provides a basis for further study of the underlying physiological mechanisms and demonstrates the possibility of improving drought resistance in plants by pyramiding the favorable QTLs.

Genetics of nonsenescence and charcoal rot resistance in sorghum.

Nonsenescence is a delayed leaf and plant death resistance mechanism in sorghum that circumvents the detrimental effects of reduced soil moisture combined with high temperatures during post-anthesis growth. This drought-tolerance mechanism is often equated with charcoal rot resistance, a widespread root and stalk disease of great destructive potential. Therefore, the inheritance of charcoal rot resistance was investigated directly, by exposure of sorghum to Macrophomina phaseolina, the causal organism, and indirectly, by determination of the inheritance of nonsenescence. Sorghum families derived from diallel crosses between two nonsenescent, resistant inbreds (B35, SC599-11E) and two senescent, susceptible inbreds (BTx378, BTx623) were evaluated in 1989 at College Station and at Lubbock, Texas, under controlled and field conditions. We determined that nonsenescence was regulated by dominant and recessive epistatic interactions between two nonsenescence-inducing loci and a third locus with modifying effects. The same conclusion was reached for charcoal rot resistance. The presence of different genetic mechanisms within SC599-11E for nonsenescence and charcoal rot resistance verifies that these two forms of resistance are not different manifestations of a single trait, i.e., they are not to be equated with each other. We conclude that nonsenescence alone cannot account for, and should not be used as the sole breeding criterion for, resistance to charcoal rot in sorghum.

Genetic variation for gas exchange rates in grain sorghum.

Carbon assimilation rate (A) and stomatal conductance (g) are highly correlated. However, the slope of the A versus g relationship differs among species and environments resulting in differences in gas exchange efficiency which should reflect water use efficiency. The objective of this research was to determine the genetic variation for A and g in grain sorghum (Sorghum bicolor [L.] Moench.). Field experiments were conducted using 30 sorghum hybrids with four water supply treatments. A, g, and leaf water potential (Psi(w)) of individual leaves were monitored every 15 to 20 days. Significant genetic variation existed among the hybrids for A and g. Plant age and water supply also affected A and g as expected. When A was regressed on g for each hybrid, large and significant differences existed among the slopes, implying differences in intrinsic gas exchange efficiency. The regression analysis of A and g versus Psi(w) suggested that A was more sensitive than g to increasing water stress. Genetic differences in the rate of change in A as water stress increased were observed. Regression analysis was used to evaluate the individual hybrid response relative to other hybrids. Twofold difference in slopes existed for A. These results provide evidence for genetic variation in gas exchange rates which might directly contribute to whole plant water use efficiency and productivity.