Literatura académica sobre el tema "Pasture plants. Signalgrass"

The mineralization rate of ruminant manure may influence the fertilization management of pastures. This study aimed to evaluate feces decomposition of heifers grazing signalgrass (Brachiaria decumbens Stapf.) fertilized or not with N, or intercropped with legumes in the dry forest region. Two experiments were conducted; the first one was a CRD that evaluated the evolution of CO2 from a mixture of soil and feces (10:1) during 22 days of incubation in a hermetically sealed bucket with a solution of NaOH 0.5 mol L-1. The second one was a RCBD that evaluated the in situ decomposition of feces in nylon bags in time periods 4, 8, 16, 32, 64, 128 and 256 days after incubation above ground. The single negative exponential mathematical model was adequate (P ≤ 0.0001) to quantify the CO2 evolution of the mixture of soil and feces, indicating that 78% of CO2 was released at the beginning of the incubation, especially for the feces collected in the signalgrass pastures intercropped with Gliricidia sepium (Jacq.) Kunth ex Walp. (gliricídia). After the first 5 days, CO2 evolution was more stable. Remaining biomass in the litterbag along decomposition fitted the single negative exponential model (P < 0.001). Greater relative decomposition rate (k) of bovine fecal biomass occurred for the N-fertilized signalgrass treatment (k = 0.0031 g g-1 day-1) and a lesser rate for the treatment intercropped with Mimosa caesalpiniifolia Benth. (sabiá) (k = 0.0018 g g-1 day-1). Nitrogen fertilization in signalgrass pasture favored the decomposition of bovine feces at the end of 256 days of incubation.

The use of herbicides to control grass in Medicago sativa (alfalfa) pastures is still incipient. Therefore, the objective of this study was to evaluate the efficiency of fluazifop-p-butyl in the control of Brachiaria decumbens (signalgrass) in alfalfa. Thus, randomized block design was used, with seven doses of fluazifop-p-butyl (0, 25, 50, 100, 200, 300, 400 g ha-1), and four replications. Herbicide application was performed when the plants had about 20 cm height. Chlorophyll fluorescence, control of signalgrass and alfalfa toxicity were evaluated at 7, 15 e 30 days after application (DAA) and, at 45 DAA and 45 days after cut (DAC), both species were cut and tiller density, as well as branches and dry matter of forage species, were determined. Fluazifop-p-butyl does not affect the integrity of the photosynthetic apparatus of alfalfa plants, due to high tolerance to this mechanism of action presented by dicotyledonous species. However, signalgrass had physiological variables negatively affected by the herbicide, indicating the presence of physiological stress, even at the lowest doses of the product. The dose of 50 g ha-1 of fluazifop-p-butyl is effective in controlling signalgrass, without causing physiological and growth damage in alfalfa plants.

Johnsongrass (Sorghum halepense (L.) Pers.), broadleaf signalgrass (Brachiaria platyphylla (L.) Beauv.), and yellow foxtail (Setaria glauca L.) are common volunteer grasses in bermudagrass (Cynodon dactylon (L.) Pers.) pastures in the southeastern United States. Johnsongrass and broadleaf signalgrass are potential forages whereas yellow foxtail is a noxious weed. In 1999 and subsequent years, necrosis and dieback of leaves, stems, and roots, stunting, and plant death were observed on all three species in bermudagrass pastures in north Mississippi (3). Symptoms on johnsongrass and yellow foxtail were most severe where bermudagrass exhibited severe symptoms of infection caused by dematiaceous hyphomycetes (2,3); symptoms on broadleaf signalgrass often occurred independently. Symptomatic leaf tissues from 15 to 33 plants of each species and stem and root tissues from 4 to 14 plants of johnsongrass and yellow foxtail were surface disinfested, plated on water agar, and examined for sporulation after 5 to 10 days (2,3). Pathogens were identified by specific morphological features of spores and sporulation as on bermudagrass (3), and axenic cultures were established by spore transfers to cornmeal agar. Bipolaris cynodontis (Marig.) Shoemaker, Curvularia lunata (Wakk.) Boedijn, C. geniculata (Tracy & Earle) Boedijn, and Exserohilum rostratum (Drechs.) Leonard & Suggs were isolated from symptomatic leaves of all three grasses and frequently also observed on stems and roots. B. stenospila (Drechs.) Shoemaker was observed only on broadleaf signalgrass (19 of 33 plants) and B. spicifera (Banier) Subr. on johnsongrass and yellow foxtail. Species most frequent on leaves (58 to 100%) were B. spicifera, C. lunata, and E. rostratum on johnsongrass and yellow foxtail and B. cynodontis, B. stenospila, and E. rostratum on broadleaf signalgrass. The three grasses were grown from seed in potting mix in the greenhouse (one plant per 375-cm3 container), and five replicates 31 to 60 days old were inoculated with a mixture of three isolates of each pathogen observed on them in two experiments. Conidia produced from infested wheat and oat grain were atomized onto foliage (1.2 to 4 × 104 conidia per ml, 20 ml per plant) as described (2). All pathogens incited similar necrotic lesions and streaks on the three grasses after 12 to 15 days, and B. stenospila also caused extensive golden yellow chlorosis on broadleaf signalgrass. All pathogens caused significant (P = 0.05) necrosis (means = 5 to 35% of foliage necrotic based on visual estimates, controls = 1 to 3%), and all were reisolated and grown in pure culture by spore transfers to cornmeal agar from surface-disinfested, symptomatic leaf tissue of each grass. When bermudagrass grown from seed was inoculated at similar spore concentrations, isolates of E. rostratum, B. cynodontis, and B. spicifera from two or all three grasses caused symptoms as severe as did isolates from bermudagrass. Results document new North American or worldwide records of occurrence and pathogenicity for B. cynodontis, C. geniculata, and C. lunata on all three grasses, B. stenospila and E. rostratum on broadleaf signalgrass, and B. spicifera on johnsongrass and yellow foxtail (1). These volunteer grasses, bermudagrass, and the six fungi all appear to represent large, interacting complexes of multiple hosts and potentially cross-infecting pathogens. Reference: (1) D. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory. Online publication. USDA, ARS, 2005. (2) R. Pratt, Agron. J. 92:512, 2000. (3) R. Pratt. Phytopathology 95:1183, 2005.

The fungus Rhizoctonia solani anastomosis group (AG)-1 IA emerged in the early 1990s as an important pathogen causing foliar blight and collar rot on pastures of the genus Urochloa (signalgrass) in South America. We tested the hypothesis that this pathogen emerged following a host shift or jump as a result of geographical overlapping of host species. The genetic structure of host and regional populations of R. solani AG-1 IA infecting signalgrass, rice, and soybean in Colombia and Brazil was analyzed using nine microsatellite loci in 350 isolates to measure population differentiation and infer the pathogen reproductive system. Phylogeographical analyses based on the microsatellite loci and on three DNA sequence loci were used to infer historical migration patterns and test hypotheses about the origin of the current pathogen populations. Cross pathogenicity assays were conducted to measure the degree of host specialization in populations sampled from different hosts. The combined analyses indicate that the pathogen populations currently infecting Urochloa in Colombia and Brazil most likely originated from a population that originally infected rice. R. solani AG-1 IA populations infecting Urochloa exhibit a mixed reproductive system including both sexual reproduction and long-distance dispersal of adapted clones, most likely on infected seed. The pathogen population on Urochloa has a genetic structure consistent with a high evolutionary potential and showed evidence for host specialization.

The fungus Rhizoctonia oryzae-sativae is an important pathogen that causes the aggregated sheath spot disease on rice. In this study, we investigated the genetic structure of rice-adapted populations of R. oryzae-sativae sampled from traditional rice-cropping areas from the Paraíba Valley, São Paulo, Brazil, and from Meta, in the Colombian Llanos, in South America. We used five microsatellite loci to measure population differentiation and infer the pathogen’s reproductive system. Gene flow was detected among the three populations of R. oryzae-sativae from lowland rice in Brazil and Colombia. In contrast, a lack of gene flow was observed between the lowland and the upland rice populations of the pathogen. Evidence of sexual reproduction including low clonality, Hardy-Weinberg equilibrium within loci and gametic equilibrium between loci, indicated the predominance of a mixed reproductive system in all populations. In addition, we assessed the adaptive potential of the Brazilian populations of R. oryzae-sativae to emerge as a pathogen to Urochloa spp. (signalgrass) based on greenhouse aggressiveness assays. The Brazilian populations of R. oryzae-sativae were probably only incipiently adapted as a pathogen to Urochloa spp. Comparison between RST and QST showed the predominance of diversifying selection in the divergence between the two populations of R. oryzae-sativae from Brazil.