Browsing by Author "David Jordan, Committee Co-Chair"

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    Distribution, Biology, and Management of Glyphosate-resistant Palmer amaranth in North Carolina
    (2009-04-23) Whitaker, Jared Ross; Alan York, Committee Chair; David Jordan, Committee Co-Chair; Randy Wells, Committee Member; Jim Burton, Committee Member
    The introduction of glyphosate-resistant (GR) crops allowed for the topical applications of the herbicide glyphosate. This herbicide revolutionized weed control and crop management. Widespread adoption of this technology and extensive use of glyphosate led to intense selection pressure for evolution of GR weeds. In 2005, GR Palmer amaranth was suspected in North Carolina. A survey detected GR populations in 49 of 290 fields sampled. ALS-inhibitor resistance was also detected in 52 fields. Five fields had populations exhibiting multiple resistance to both glyphosate and ALS-inhibitors. Experiments were conducted to determine the resistance mechanism of GR Palmer amaranth. A GR biotype exhibited a 20-fold level of resistance compared to a glyphosate-susceptible (GS) biotype. Shikimate accumulated in GS but not GR plants after glyphosate application. Maximum absorption was observed by 12 hours after treatment (HAT), and was similar among biotypes except at 6 HAT, where GS plants absorbed 67% more than GR plants. Distribution of 14C was similar among biotypes in (42%), above (30%), and below (22%) the treated leaf and in roots (6%). This work did not lead to a suggestion a resistance mechanism. Field experiments were conducted to develop management strategies for GR Palmer amaranth in cotton. One evaluated residual control of Palmer amaranth by various herbicides. Of herbicides typically applied PRE or pre-plant, fomesafen, flumioxazin, and pyrithiobac were most effective. Pyrithiobac and S-metolachlor were the most effective postemergence (POST) herbicides. Flumioxazin and prometryn plus trifloxysulfuron were the most effective options for postemergence-directed applications. Integration of these herbicides into glyphosate-based systems could increase Palmer amaranth control. An experiment was conducted to evaluate PRE herbicides in a season-long system. All PRE herbicides increased late-season control. Among individual herbicides, fomesafen and pyrithiobac were most effective. Combinations of fomesafen plus pyrithiobac or diuron and diuron plus pyrithiobac were the most effective PRE applications. Another experiment investigated herbicide systems with residual herbicides applied pre-plant, PRE and POST. Pre-plant applications of flumioxazin and PRE applications of fomesafen increased late-season control, but applications of both were more effective than either herbicide alone. Applications of glyphosate plus pyrithiobac POST were more effective than glyphosate alone. Glyphosate plus S-metolachlor was more effective than glyphosate alone at one of two locations. These data suggest early-season control of GR Palmer amaranth is critical for successful management in cotton. Glufosinate is another herbicide effective on Palmer amaranth. However, growers were reluctant to plant glufosinate-tolerant cotton cultivars. Widestrike cotton is GR and also contained a glufosinate tolerance gene used as a selectable marker, however glufosinate tolerance in production situations had not been investigated. Experiments were conducted to evaluate Widestrike cotton tolerance to glufosinate and yield was reduced by glufosinate in only one of 11 trials by 4%, suggesting acceptable tolerance. Another experiment evaluated weed control with glufosinate and glyphosate in Widestrike cotton. Control of GR Palmer amaranth by glufosinate-based systems was higher than glyphosate-based systems, which demonstrated that glufosinate-based systems could be used to control GR Palmer amaranth in Widestrike cotton. In soybean, several glyphosate alternative herbicides could be used to control Palmer amaranth. An experiment was conducted to evaluate control of GS and GR Palmer amaranth from a glyphosate-only system compared to several alternative systems. Glyphosate alone applied once POST was very effective on GS Palmer amaranth and alternative systems with two PREs followed by fomesafen POST provided similar control from glyphosate. In fields with GR Palmer amaranth, greater than 80% late-season control was obtained only with systems of two PREs followed by fomesafen POST.
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    Potential for Management of Sclerotinia Blight of Peanut (Arachis hypogaea L.) caused by Sclerotinia minor with the Biological Control Agent Coniothyrium minitans
    (2006-04-05) Partridge, Darcy Erin; Marc Cubeta, Committee Member; Barbara Shew, Committee Member; Turner Sutton, Committee Co-Chair; David Jordan, Committee Co-Chair
    Sclerotinia blight of peanut (Arachis hypogaea L.), caused by Sclerotinia minor (Jagger) Kohn, is an important disease in North Carolina and Virginia. Sclerotia are the main overwintering propagules of S. minor and serve as the primary source of inoculum for Sclerotinia blight. The effectiveness of the fungal mycoparasite Coniothyrium minitans, which is capable of colonizing sclerotia of Sclerotinia spp., was evaluated in a 5-year field study and in eight short-term field studies in northeastern North Carolina. Control of Sclerotinia blight was highest when C. minitans was applied for 3 consecutive years. However, application of C. minitans for 1 or 2 years also reduced disease in the long-term study. A single application of C. minitans was less effective when applied 4 to 6 months prior to planting and sclerotia numbers were only reduced in two of the eight short-term field studies. Sclerotia used as baits placed in the long-term field study as well as the sclerotia isolated from soil were infected by C. minitans, and the number of sclerotia was reduced where C. minitans was applied. Moderate resistance in the cultivar Perry and application of the fungicide fluazinam provided adequate control of Sclerotinia blight in all plots. The integration of C. minitans with current peanut management practices is needed for successful biological control of Sclerotinia blight. Laboratory experiments evaluated the effects of nine pesticides commonly used in peanut production on mycelial growth, conidia germination, and mycoparasitic activity of C. minitans on sclerotia of S. minor. The commercial formulations of the fungicides azoxystrobin, chlorothalonil, fluazinam, pyraclostrobin, and tebuconazole, and the herbicide flumioxazin reduced mycelial growth and conidia germination of C. minitans. Eight of nine pesticides, azoxystrobin, chlorothalonil, fluazinam, pyraclostrobin, tebuconazole, diclosulam, flumioxazin, and pendimethalin applied to soil plates reduced but did not inhibit the mycoparasitic activity of C. minitans on sclerotia of S. minor. Temperature and moisture effects on mycoparasitism were also evaluated to determine optimum conditions for infection of sclerotia of S. minor by C. minitans. Optimum temperatures for infection of sclerotia of S. minor by C. minitans ranged from 14 to 22°C and soil moisture –0.33 to –1 kPa x 10². These results indicate that C. minitans should remain active throughout most of the year in North Carolina, except during the hot summer months of June, July and August. Soil fauna such as collembola may aid in the reduction of sclerotia through direct predation and the movement of inocula of mycoparasites from infected to noninfected sclerotia. Collembola diversity and abundance were compared in four peanut fields. The most prevalent collembola families were Isotomidae, Smithurididae, Poduridae, and Hypogastruridae, with Isotomidae isolated most frequently from all sites. Abundance and diversity of collembola increased from August to October with sampling date and location having the greatest effect on the composition of the population. Determining the abundance and diversity of collembola in the field can help increase our understanding of the soil community structure. These studies show that C. minitans is able to persist and infect sclerotia of S. minor in peanut fields of North Carolina when applied in the fall or early winter across crop residue and incorporated into the top layers of the soil. C. minitans will not eradicate infestations by S. minor, but over time has the potential to reduce inoculum levels and ultimately decrease the incidence of the disease.