Barbara B. Shew

Weed species as hosts of Sclerotinia minor in peanut fields.

Bleached stems and sclerotia were observed on winter annual weed species growing in harvested peanut fields in northeastern North Carolina in March 2001. Each field had a history of Sclerotinia blight caused by Sclerotinia minor. Symptomatic plants were collected and brought back to the laboratory for identification and isolation. S. minor was isolated and Kochs postulates were fulfilled to confirm pathogenicity of S. minor on nine weed species. They included Lamium aplexicaule (henbit), Cardamine parviflora (smallflowered bittercress), Stellaria media (common chickweed), Cerastium vulgatum (mouse-ear chickweed), Coronopus didymus (swinecress), Oenothera laciniata (cutleaf eveningprimrose), Conyza canadensis (horseweed), Brassica kaber (wild mustard), and Arabidopsis thaliana (mouse-ear cress). This is the first report of these species as hosts of S. minor in the natural environment. All isolates of S. minor obtained from the weed species were pathogenic to peanut.

Sclerotinia blight resistance in Virginia-type peanut transformed with a barley oxalate oxidase gene.

Transgenic peanut lines expressing oxalate oxidase, a novel enzyme to peanut, were evaluated for resistance to Sclerotinia blight in naturally infested fields over a 5-year period. Area under the disease progress curve (AUDPC) for transgenic lines in single rows planted with seed from single-plant selections averaged 78, 83, and 90% lower than nontransgenic parents in 2004, 2005, and 2006, respectively. In addition, AUDPC in 14 transgenic lines planted with bulked seed in two-row plots averaged 81% lower compared with nontransgenic parents in 2005 and 86% lower in 16 transgenic lines in 2006. Six transgenic lines yielded 488 to 1,260 kg/ha greater than nontransgenic parents in 2005, and 10 lines yielded 537 to 2,490 kg/ha greater in 2006. Fluazinam (0.58 kg a.i./ha) fungicide sprays in 2008 and 2009 reduced AUDPC in transgenic and nontransgenic lines but AUDPC was lowest in transgenic lines. Without fluazinam, yields of transgenic lines averaged 1,133 to 1,578 kg/ha greater than nontransgenic lines in 2008 and 1,670 to 2,755 kg/ha greater in 2009. These results demonstrated that the insertion of barley oxalate oxidase in peanut conveyed a high level of resistance to Sclerotinia blight, and negated the need for costly fungicide sprays.

Evaluating Isolate Aggressiveness and Host Resistance from Peanut Leaflet Inoculations with Sclerotinia minor

Sclerotinia minor is a major pathogen of peanut in North Carolina, Virginia, Oklahoma, and Texas. Partial resistance to S. minor has been reported based on field screening, but field performance is not always correlated with laboratory or greenhouse evaluations of resistance. More efficient screening methods and better understanding of the mechanisms contributing to Sclerotinia blight resistance are needed, and a detached leaf assay was developed and evaluated. Detached leaflets of 12 greenhouse-grown peanut lines were inoculated on the adaxial surface with a 4-mm-diameter mycelial plug of a single isolate of S. minor. Leaflets were incubated in the dark at 20°C in Nalgene utility boxes containing moistened sand. Lesion length 3 days after inoculation ranged from 11 to 24 mm, with a mean of 19 mm. Lengths differed significantly among the entries, with GP-NC WS 12, an advanced breeding line derived from a cross of NC 6 × (NC 3033 × GP-NC WS 1), being the most resistant. Forty-eight isolates of S. minor obtained from peanut were inoculated on leaflets of the susceptible cultivar NC 7 and aggressiveness was assessed by measuring lesion-length expansion. Three days after inoculation, lesion length differed among the isolates and ranged from 2 to 24 mm, with a mean of 15 mm. Finally, the potential for specific interactions between peanut lines and S. minor isolates was evaluated. A subset of S. minor isolates was selected to represent the observed range of aggressiveness and a subset of peanut entries was selected to represent the range of resistance or susceptibility. Nine-week-old greenhouse- or field-grown plants were compared for five peanut entries. Main effects of isolates and entries were highly significant, but isolate-entry interactions were not significant. The most resistant peanut entry (GP-NC WS 12) performed consistently with all isolates regardless of plant source.

Screening of Virginia-Type Peanut Breeding Lines for Resistance to Cylindrocladium Black Rot and Sclerotinia Blight in the Greenhouse

Abstract Cylindrocladium black rot (CBR) caused by Cylindrocladium parasiticum and Sclerotinia blight caused by Sclerotinia minor are two economically important diseases of peanut (Arachis hypogaea...

A Site-Specific, Weather-Based Disease Regression Model for Sclerotinia Blight of Peanut

In North Carolina, losses due to Sclerotinia blight of peanut, caused by the fungus Sclerotinia minor, are an estimated 1 to 4 million dollars annually. In general, peanut (Arachis hypogaea) is very susceptible to Sclerotinia blight, but some partially resistant virginia-type cultivars are available. Up to three fungicide applications per season are necessary to maintain a healthy crop in years highly favorable for disease development. Improved prediction of epidemic initiation and identification of periods when fungicides are not required would increase fungicide efficiency and reduce production costs on resistant and susceptible cultivars. A Sclerotinia blight disease model was developed using regression strategies in an effort to describe the relationships between modeled environmental variables and disease increase. Changes in incremental disease incidence (% of newly infected plants of the total plant population per plot) for the 2002-2005 growing seasons were statistically transformed and described using 5-day moving averages of modeled site-specific weather variables (localized, mathematical estimations of weather data derived at a remote location) obtained from SkyBit (ZedX, Inc.). Variables in the regression to describe the Sclerotinia blight disease index included: mean relative humidity (linear and quadratic), mean soil temperature (quadratic), maximum air temperature (linear and quadratic), maximum relative humidity (linear and quadratic), minimum air temperature (linear and quadratic), minimum relative humidity (linear and quadratic), and minimum soil temperature (linear and quadratic). The model explained approximately 50% of the variability in Sclerotinia blight index over 4 years of field research in eight environments. The relationships between weather variables and Sclerotinia blight index were independent of host partial resistance. Linear regression models were used to describe progress of Sclerotinia blight on cultivars and breeding lines with varying levels of partial resistance. Resistance affected the rate of disease progress, but not disease onset. The results of this study will be used to develop site- and cultivar-specific spray advisories for Sclerotinia blight.

Evaluation of Microbial, Botanical, and Organic Treatments for Control of Peanut Seedling Diseases

Diseases affecting stand establishment are a major obstacle to organic production of peanut (Arachis hypogaea). Stand losses of 50% or more are possible with untreated seed. Biological, botanical, and organic seed treatments or soil amendments were tested for efficacy against pre- and postemergence damping-off of peanut in greenhouse, microplot, and field plot trials. Seed of the lines Perry, GP-NC 343, and Bailey (tested as N03081T) were used in all trials. Commercial formulations of Bacillus subtilis (Kodiak), B. pumilus (Yield Shield), Trichoderma harzianum (T-22 PB and Plantshield HC), Muscodor albus, and Coniothyrium minitans (Contans); activated charcoal; two separate soil amendments of dried herbage of Monarda didyma cultivars; a commercial fungicide control (Vitavax PC); and an untreated control were tested in natural soil in the greenhouse. Vitavax PC and Kodiak were the only treatments that resulted in higher percent emergence and survival than in untreated seed. A separate greenhouse experiment was conducted in natural soil or natural soil infested with field isolates of Aspergillus niger. Seed were treated with Kodiak, copper hydroxide (Champion), Plantshield HC, Kodiak + Plantshield HC, Streptomyces griseoviridis (Mycostop), hot water, Vitavax PC, or were left untreated. Seedling emergence and survival was much lower in infested versus uninfested soil. Seed treatment with Kodiak increased percent emergence and survival compared to untreated seed, but was not as effective as Vitavax PC. Field microplot studies in 2007 and 2008 at Clayton, NC, evaluated four seed treatments on the peanut lines following small grain cover crops, soil amendment with M. albus, or no cover. Cover crops did not affect emergence or interact with seed treatments. In field studies in 2007 and 2008 at Lewiston, NC, the peanut lines were planted with M. albus infurrow, with Kodiak or T. harzianum seed treatments, or were untreated. In the 2007 trial, none of the treatments improved stands compared to the untreated check. In 2008, the highest stand counts were produced by seed treated with Kodiak. In both years, Bailey produced the greatest stand counts. A. niger was strongly associated with postemergence damping-off in the field. Regardless of peanut line, in many trials, Kodiak seed treatment increased emergence and survival over untreated seed.

Analysis of Factors That Influence the Epidemiology of Sclerotinia minor on Peanut

In North Carolina, sclerotia of Sclerotinia minor germinate myceliogenically to initiate infections on peanut. The effects of soil temperature and soil matric potential (ψM on germination and growth of S. minor have not been well characterized, and little is known about relative physiological resistance in different parts of the peanut plant. Laboratory tests examined the ability of the fungus to germinate, grow, and infect detached peanut leaflets at soil temperatures ranging from 18 to 30°C at ψM of -100, -10, and -7.2 kPa. In addition, detached pegs, leaves, main stems, and lateral branches from three peanut lines varying in field resistance were examined for resistance to infection by S. minor. Sclerotial germination was greatest at 30°C and ψM of -7.2 kPa. Final mycelial diameters decreased with decreasing ψM, whereas soil matric potential did not affect lesion development. Mycelial growth and leaflet lesion expansion were maximal at 18 or 22°C. Soil ψM did not affect leaflet infection and lesion expansion. Lesions were not observed on leaves incubated at temperatures of 29°C or above, but developed when temperatures were reduced to 18 or 22°C 2 days after inoculation. Pegs and leaflets were equally susceptible to infection and were more susceptible than either main stems or lateral branches. Results of this work, particularly the effects of temperature on S. minor, and knowledge of peanut part susceptibility has application in improving Sclerotinia blight prediction models for recommending protective fungicide applications.

Influence of Cover Crops on Weed Management in Strip Tillage Peanut

Abstract Experiments were conducted in North Carolina during 2005, 2006, and 2007 to determine peanut and weed response when peanut was planted in strip tillage after desiccation of cereal rye, Italian ryegrass, oats, triticale, wheat, and native vegetation by glyphosate and paraquat before planting with three in-season herbicide programs. Control of common ragweed and yellow nutsedge did not differ among cover crop treatments when compared within a specific herbicide program. Applying dimethenamid or S-metolachlor plus diclosulam PRE followed by imazapic POST was more effective than a chloroacetamide herbicide PRE followed by acifluorfen, bentazon, and paraquat POST. Incidence of spotted wilt in peanut (caused by a Tospovirus) did not differ when comparing cover crop treatments, regardless of herbicide program. Peanut yield increased in all 3 yr when herbicides were applied POST, compared with clethodim only. Peanut yield was not affected by cover crop treatment. Response to cover crop treatments was comparable, suggesting that growers can select cereal rye, Italian ryegrass, oats, or triticale as an alternative to wheat as a cover crop in peanut systems without experiencing differences associated with in-season weed management. Nomenclature: Acifluorfen; bentazon; glyphosate; imazapic; paraquat; common ragweed, Ambrosia artemisiifolia L.; yellow nutsedge, Cyperus esculentus L.; cereal rye, Secale cereale L.; Italian ryegrass, Lolium multiflorum Lam.; oats, Avena sativa L.; peanut, Arachis hypogaea L.; triticale, Triticale hexaploide Lart.; wheat, Triticum aestivum L.

Influence of Digging Date and Fungicide Program on Canopy Defoliation and Pod Yield of Peanut (Arachis hypogaea L.)

Abstract Although response can be variable and is dependant upon many factors, digging peanut as little as 1 wk prior to or 1 wk following optimum maturity can result in substantial reductions in pod yield. While growers often assume that harvest should be initiated prior to optimum maturity in fields with high levels of canopy defoliation, there is no clear threshold where gains in pod weight and grade are offset by losses due to pod shedding. Moreover, yield loss relationships for foliar diseases have not been characterized for modern virginia market-type cultivars. Determining interactions between digging date and peanut canopy defoliation would assist growers and their advisors in making more informed decisions on when to initiate harvest to avoid yield loss. Research conducted from 2004–2005 in North Carolina established a range of defoliation levels in a total of seven fields. Fungicides were applied on a bi-weekly schedule from early July through mid-September, or were applied two times in July, or...

Comparison of cropping systems including corn, peanut, and tobacco in the North Carolina Coastal Plain.

Research was conducted in North Carolina from 2001 to 2006 to determine disease development, parasitic nematode population in soil, crop yield, and cumulative economic return in rotation systems including corn, peanut, and tobacco. Specific rotations included two consecutive cycles of corn-corn-peanut, corn-tobacco-peanut, or tobacco-corn-peanut; five years of corn followed by peanut, and corn-corn-tobacco-corn-corn-peanut. In the final year of the experiment when only peanut was planted, the Cylindrocladium black rot (caused by Cylindrocladium parasiticum) (CBR)-susceptible cultivar Gregory and the CBRresistant cultivar Perry were included. Increasing the number of years between peanut plantings increased yield of peanut in the final year of the experiment when Gregory was planted but not when Perry was planted. Incidence of CBR was highest when peanut was planted twice during the duration of the experiment compared with only once. No difference in ring nematode was observed regardless of rotation in the final year of the experiment. The highest soil population of stunt nematode was noted when five years of corn was followed by peanut with the lowest soil population of this nematode noted following two cycles of tobacco-corn-peanut. Cropping systems that included tobacco provided higher cumulative economic returns regardless of rotation sequence in most instances.