Browsing by Subject "Rhizoctonia solani"
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Item Improving upon the management of soilborne diseases of peanut in West Texas(2012-05) Thiessen, Lindsey; Woodward, Jason E.; Ong, Kevin; Sharma, JyotsnaPeanut (Arachis hypogaea L.) is susceptible to numerous soilborne pathogens in the High Plains of West Texas including Botrytis cinerea Pers.:Fr., Pythium spp., Rhizoctonia solani Kühn AG-4, Sclerotinia minor Jagger and Sclerotinia sclerotiorum (Lib.) de Bary, Sclerotium rolfsii Sacc., and Verticillium dahliae Kleb. Because these pathogens may cause similar symptoms in peanut fields, accurate diagnosis is important for proper management to mitigate damages and losses caused by these pathogens. Fungicides are the primary management option for this region, and current application regimes are based on models developed in the Southeastern United States. This region has an arid climate and peanut development is generally later than in other areas of the United States. These factors impact initial fungicide application timing and may affect efficacy. The objectives of this research were to i) develop a disease diagnostic guide for peanut producers of this region, ii) evaluate fungicide application timing for pod rot and iii) quantify the residual activity of fungicides used in the management of pod rot of peanut. Large plot fungicide trials were conducted in the growing seasons of 2010 and 2011 to study the effects of delaying initial applications of fungicides, azoxystrobin, mefenoxam, and flutolanil on disease development. Initial applications were made 68 or 82 days after planting (DAP) with subsequent applications using a banded spray application. In 2010, the percentage of pods affected and percent damaged kernels were reduced by early applications of mefenoxam and azoxystrobin. The flutolanil treatment in 2010 showed the greatest improvement in yield compared to the control. The 2011 trial was greatly affected by drought, and no appreciable differences were seen for any of the parameters evaluated. In 2011, a fungicide bioassay was conducted to determine the residual activity of the fungicides azoxystrobin and flutolanil + propiconazole on the foliage and pods of the peanut cultivars GA09B and McCloud. Broadcast applications of both fungicides were made three times in the growing season using a backpack sprayer system. Plants were destructively sampled prior to fungicide application and 1, 3, 5, 7, 14, 21 and 28 days after application. A detached leaflet assay was completed using Sclerotium rolfsii as a quantitative measure. Fungicides did not show significant differences between one another, with both fungicides reducing activity of S. rolfsii as compared to the non-treated control. The residual activity of azoxystrobin and flutolanil + propiconazole ranged from 7 to 28 days. Additional testing is required to obtain a more accurate determination of residual activity of these fungicides. Despite poor environmental conditions for 2011, the studies conducted show that application timing may differ from current recommendations. Further study is needed to improve current fungicide application regimes and improve the management of soilborne disease in this region.Item Single Plant Selection as a Screening Method for Resistance to Rhizoctonia solani and Pythium ultimum in Cotton(2014-04-15) Jones, Whitney MUpland cotton, Gossypium hirsutum L., is grown extensively in the southern United States with an annual farmgate value of $6 billion and an annual national economic impact of over $120 billion. Damage due to biotic pests, including what is known as the cotton seedling disease complex (CSDC), contribute to these losses. Two particular CSDC pathogens, Rhizoctonia solani and Pythium ultimum, are the most significant soilborne pathogens of cotton in the United States. A program for R. solani and P. ultimum resistant cotton germplasm was established at Texas A&M University AgriLife Research. Five germplasm families selected for elevated levels of condensed tannins were evaluated for resistance to R. solani and P. ultimum. Two generations of single plant selections resulted in three generations, C_(0) (original families or Cycle 0) C_(1), selected from the C_(0) family, and C_(2), selected from the C_(1) generation. C_(1) and C_(2) were putative resistant families after one or two generation(s) of selection, respectively. Individual plants from the three generations within five families were challenged with either or both R. solani or P. ultimum to evaluate the progress of single plant selection for resistance. A susceptible cultivar for R. solani- and P. ultimum-resistance respectively, were included. Different R. solani and P. ultimum families from each generation of selection were evaluated at three inoculation levels with four replications per family. Differences in level of resistance between each generation were evaluated by comparing disease level in a randomized complete block. Cross-resistance was evaluated, i.e., C_(2) families originally screened under R. solani pressure were inoculated and screened for P. ultimum resistance and vice versa. Individual plant selection (IPS) in an artificial environment may be a useful and important tool in developing seedling disease-resistant cotton germplasm. Furthermore, it can be concluded that the family evaluated is of importance to determine the amount of progress made in terms of disease resistance with IPS. Individual plant selection when challenged with appropriate levels R. solani and P. ultimum appears to be an effective tool for selection of germplasm resistant to these seedling disease causing pathogens.Item Soil microbial response to glyphosate-base cotton pest management systems(2009-05-15) Lancaster, Sarah ReneeCurrently, 74% of cotton acres in the United States are planted with glyphosatetolerant varieties. The average glyphosate-tolerant cotton crop is treated with glyphosate 2.1 times each year in addition to other herbicides, insecticides, and fungicides. The primary objectives of this research were to: 1) describe the influence of glyphosate and pesticides commonly applied at or near the time of cotton planting on soil microbial activity and biomass; 2) study the effect of glyphosate on fluometuron degradation; 3) evaluate the response of Rhizoctonia solani to glyphosate and fluometuron; 4) study changes in glyphosate metabolism that occur as a result of repeated glyphosate applications; and 5) define shifts in the soil microbial community. Additionally, methods for accelerated solvent extraction (ASE) of fluometuron from soils were developed. In one experiment, the addition of glyphosate reduced C-mineralization in soils treated with fluometuron, aldicarb, or mefenoxam + PCNB formulations. However, in a second experiment, C-mineralization increased when glyphosate was applied with fluometuron relative to fluometuron applied alone. Accelerated solvent extraction was used in experiments which demonstrated that application of glyphosate with fluometuron increased the rate of fluometuron degradation in soil relative to fluometuron alone. When glyphosate was added to minimal medium, degradation of fluometuron by R. solani was reduced and less fungal biomass was produced. The total amount of 14C-glyphosate mineralized was reduced when glyphosate was applied 5 times relative to 1, 2, 3, or 4 times. Incorporation of 14Cglyphosate residues into soil microbial biomass was greater following five glyphosate applications than one application 3 and 7 days after application (DAA). Soil fatty acid methyl ester (FAME) profiles were altered by five glyphosate applications relative to one application. Additionally, FAMEs common to gram-negative bacteria were present in higher concentrations following five applications relative to 1, 2, 3, or 4 applications both 7 and 14 DAA. These studies indicated that: 1) glyphosate altered the soil microbial response to other pesticides; 2) fluometuron-degrading microorganisms in soil responded differently to glyphosate; 3) changes in the dissipation or distribution of glyphosate following repeated glyphosate applications were associated with changes in the structural diversity of the soil microbial community.