In 2001, entries from the peanut core collection, a subset of the USDA peanut germplasm collection, were planted in non-replicated plots in a field with a history of Sclerotinia blight caused by Sclerotinia minor . Variability existed among entries for reaction to Sclerotinia blight. Of the 744 entries evaluated, 11% had no disease, nearly 30% had <10% disease incidence, and only 21% had 50% disease incidence or more. Most of the resistant entries had an upright growth habit and were in early and mid-maturity groups. Many of the early maturing entries were susceptible to the foliar disease pepper spot which occurred throughout the study. Entries were selected for further evaluation in replicated plots based on a nil to low (<10%) incidence of Sclerotinia blight, adaptation and/or vigor, and other desirable characteristics such as an intermediate to prostrate growth habit and pepper spot resistance. Selected entries were retested in both 2002 and 2003 (n = 62) and compared to resistant (Tamspan 90), moderately resistant (Tamrun 96), and susceptible (Okrun) reference cultivars. Most entries (55 in 2001 and 46 in 2003) had disease incidence less than Tamrun 96 and similar to Tamspan 90. In 2003 when disease incidence was highest, all 46 entries with resistant reactions similar to that of Tamspan 90 had erect plant growth habits except for entries 208 and 582 which were prostrate, and entries 273, 128, and 804 which were intermediate. Resistance to Sclerotinia blight and yield similar to Tamspan 90, plant habit, and/or reactions to pepper spot and web blotch were used to select the best entries. Entries 208, 128, 804, 582, and 273 combined resistance to Sclerotinia blight, pepper spot, and web blotch with less than erect growth habits. Entry 103 had good Sclerotinia blight resistance and yield, but an upright growth habit. Entry 92 had an upright growth habit and low yield, but good Sclerotinia blight resistance. Entries 92 and 103 had upright growth habits but were among the best entries for resistant to pepper spot and web blotch. Entries 426, 184, and 562 were upright and susceptible to pepper spot, but had resistance to web blotch and the best resistance to Sclerotinia blight. These entries appear to be useful sources of resistance to Sclerotinia blight for breeding programs and for increasing the probability of finding additional sources of resistance in clusters of germplasm identified within the entire USDA collection.

Efficacy of four types of Sclerotinia minor inocula were compared for disease incidence, lesion development, and differentiating a disease response on susceptible and moderately resistant peanut genotypes. Inocula were evaluated on intact and detached peanut main stem tissue. Three inocula lacked an exogenous nutrient source, which included germinating sclerotia produced on peanut stem, 3-day-old dry mycelia, and perlite granules coated with fresh mycelial fragments. Inoculum with an exogenous nutrient source consisted of an agar plug with mycelium. The mycelial agar plug inoculum resulted in a greater disease incidence and a longer mean lesion length/day compared to other inocula on intact stem or detached stem tissue. All inocula produced longer (P < 0.05) lesions 6 d after inoculation on intact stem tissue for susceptible control and shorter lesions on three of the four moderately resistant genotypes. All inocula produced longer (P < 0.05) lesions 6 d after inoculation on detached stem tissue for moderately resistant cultivars compared to susceptible check. Although, all inocula resulted in similar lesion development differentials of susceptible and moderately resistant genotypes, mycelial agar plug inoculum had a consistent disease incidence for both intact stem and detached stem tissue.

Fungal diseases of peanut, such as Sclerotinia blight caused by Sclerotinia minor , are responsible for increased production costs and yield losses of up to 50% for peanut producers in the Southwestern U.S., North Carolina, and Virginia. The literature is replete with information on the pathology, epidemiology, and control of S. minor infection of peanut, however little is known about the physical location of infection on the plant and its effect on plant production. In a 2-yr field trial, the susceptible peanut cultivar Okrun was subjected to high disease intensity without fungicide application for control of S. minor . Location of initial S. minor infection was noted to occur either on the “crown” or “limb” of infected plants, and the date of initial onset was recorded. In general, plants with initial crown infections had reduced yield and seed quality compared to those with initial limb infections. Initial infections occurring in early- and mid-season were more detrimental to plant productivity than those infections occurring late in the season. Mid-season onset of a crown infection had the greatest impact on plant productivity, causing a severe decrease in seed quality and pod yield. This study demonstrates that both infection location and date of disease onset affect plant productivity, and the results may be used to improve Sclerotinia blight management strategies.

Agronomic traits may become adversely affected when field crops are transformed with foreign genes which confer resistance to plant pathogens. Field testing of such transformed plant lines is necessary to determine if desirable yield components have been retained through the transformation and regeneration process. Thirty-four peanut lines, thirty-two of which are transgenic containing anti-fungal genes, were evaluated under field conditions for a 3-year period. Peanut lines were arranged in a complete randomized block design with three replications. Disease incidence was recorded throughout the growing season, the yield components of pod mass, shelling percentage, and seed weight were determined upon harvest, and the data were analyzed for statistical significance. Deterioration of yield components was observed for 5/32 transgenic lines and was not always consistent with increased disease incidence. Yield components measured for the majority of transgenic lines tested were comparable to the parental genotype Okrun, suggesting retention of desirable market traits by these plant lines. Of the plant lines retaining desirable quality traits, 10 demonstrated increased resistance to fungal infection compared to Okrun. These results have identified transgenic peanut lines with potential use in breeding programs for disease resistance coupled with desirable yield components.

Field studies were conducted at three locations in the south Texas peanut growing region to compare regular agricultural calcium with power plant byproduct calcium applied at planting or peanut pegging to runner peanut at rates of 560 to 1680 kg/ha. Both calcium sources decreased Rhizoctonia and Pythium pod disease development up to 62% when compared with the non-treated control and resulted in a yield increase over the non-treated control of up to 25%. When Rhizoctonia or Pythium was not a problem, no increase in peanut yield over the non-treated control was noted with any calcium application. Peanut grade (Total Sound Mature Kernels + Sound Splits) was increased at one location with the use of both calcium sources. Calcium, magnesium, and potassium content of foliage, hull, and kernel tissue was variable and was not related to calcium source.

Fungal diseases of peanut, such as Sclerotinia blight caused by Sclerotinia minor Jagger, are responsible for increased production costs and yield losses of up to 50% for peanut producers in the Southwest, North Carolina, and Virginia. A few cultivars with moderate disease resistance, such as Southwest Runner, have been developed through traditional breeding practices. An urgent need exists for developing peanut cultivars that are resistant to the broad spectrum of fungal pathogens that pose a recurring threat to peanut health. Transgenic peanut plant lines containing anti-fungal genes have been produced from somatic embryos of the susceptible cultivar Okrun and tested under greenhouse conditions for resistance to S. minor by inoculation with a mycelial plug. Disease symptoms of lesion length and vascular collapse were recorded for 30 transgenic peanut lines, non-transgenic Okrun, and Southwest Runner. The reaction of the majority of transgenic peanut lines to S. minor infection was indistinguishable from that of the susceptible cultivar Okrun. However, three transgenic lines had a significant increase in resistance to S. minor as compared to Okrun, and one line demonstrated levels of resistance comparable to the moderately resistant cultivar Southwest Runner.

Fungal diseases of peanut are responsible for increased production costs and yield losses of up to 50% for peanut producers in the U.S. Few cultivars with disease resistance have been developed through traditional breeding practices. There is an urgent need for developing cultivated peanut ( Arachis hypogaea L.) cultivars that are resistant to the broad spectrum of fungal pathogens that pose a recurring threat to peanut health. Hydrolases such as chitinase and β-1-3-glucanase are known to degrade the cell walls of many fungi that attack plants, making them likely candidates for over-expression through genetic engineering to produce disease-resistant crops. Somatic embryos of the peanut cultivar Okrun were transformed with a chitinase gene from rice, and/or a β-1-3-glucanase from alfalfa via microprojectile bombardment. Regenerated Okrun lines were tested for the presence of the transgenes by polymerase chain reaction (PCR) and Southern blot and for transgene expression by colorimetric assays. Transgenic lines exhibited hydrolase activities 0-37% above levels observed in nontransformed Okrun plants.

Leaf surface morphology of untreated peanut leaves and peanut leaves treated with herbicides and adjuvants were examined using scanning electron microscopy. Electron micrographs revealed that the adaxial surface of untreated peanut leaves was covered with crystalline wax platelets above an amorphous layer of wax. Electron micrographs revealed that peanut leaves treated with acifluorfen plus nonionic surfactant, bentazon and lactofen with crop oil concentrate, and 2,4-DB, altered the leaf surface morphology when compared to nontreated peanut leaves. Alterations in the leaf epicuticular wax structures appeared amorphous-like rather than normal plate-like structures. Nonionic surfactant and crop oil concentrate applied alone to peanut leaves altered the epicuticular wax structures similarly to that of herbicides plus adjuvants.

Early leaf spot is a common disease of peanut caused by the fungus Cercospora arachidicola Hori. Experiments were conducted to evaluate the effect of postemergence herbicides on the conidial germination of C. arachidicola and on the incidence of early leaf spot disease in peanut ( Arachis hypogaea L.) in a greenhouse. Conidial germination was enhanced (≥ 100%) at concentrations of 1, 100, and 1000 mg/L of 2,4-DB compared with the untreated control. Lactofen reduced conidial germination by 42% compared with the control at concentrations as low as 100 mg/L and completely inhibited germination at concentrations ≥ 5000 mg/L. A concentration of 10,000 mg/L acifluorfen and 2,4-DB completely inhibited conidial germination. Acifluorfen, acifluorfen plus 2,4-DB, and lactofen decreased the sporulation of early leaf spot lesions. Lactofen reduced leaf spot incidence 12% and decreased sporulation of lesions 22% compared with the control. None of the herbicides increased the incidence of early leaf spot on peanut plants or the number of early leaf spot lesions per leaflet when compared with plants that received no herbicide.

Field screening tests were conducted in 1991, 1992, and 1993 to identify Sclerotinia blight-resistant runner-type peanuts. Selections were made from three populations, “backcross to runner,” “backcross to Tamspan 90,” and “single cross.” Runner parents were chosen for agronomic qualities and resistance to other pathogens. Resistant (Spanish) parents used were cultivar Tamspan 90 and TxAG-5 germplasm. Families within populations were planted as single row 3 × 0.9-m plots. Repeated plant-by-plant inspections (PBPI) were made for assessment of sequence of infection and subsequent calculation of an area under the disease progress curve (AUDPC). Most families in the backcross to Tamspan 90 population consistently had a disease reaction close to or lower than Tamspan 90 but also retained undesirable plant forms (i.e., upright). By 1993, the F 2:5 backcross to runner population had the highest percentage of runner-type families (88%) and a mean disease reaction close to Tamspan 90. Whole plot evaluation of Sclerotinia blight severity was compared with PBPI for effectiveness on a segregating population containing a total of 298 F 4:7 single cross and F 2:5 backcross entries. The whole plot system used a scale of 0 = no visible plot infection to 5 ≥80% infection in the plot. Correlation between area under disease progress curve values of both methods was high (>.85). Remnant seed of selections screened for resistance in the greenhouse were planted in the field and evaluated for resistance. Chi-square test for independence indicated a significant (α = .012) relationship between greenhouse and field performance. Field screening and limited greenhouse screening appears to be an effective way to develop resistant runner-type peanut lines.

The cut ends of 15-cm-long shoot tips from 15 peanut genotypes were immersed individually in 1 × 14 cm test tubes containing Hoagland's solution. Shoots were supported by foam plugs leaving about 12 cm extending above the foam plugs. All leaves were removed leaving about 1 cm of each petiole on the shoot. A 4-mm-mycelial plug of Sclerotinia minor , taken from the periphery of a 2-day old culture grown on potato dextrose agar containing 100 ug/ml streptomycin sulfate (SPDA), was placed between the stem and a petiole in the middle of the shoot. Tubes with shoots were then placed in a polyethylene enclosure on a greenhouse bench where the day and night temperature were 29 ± 2C and 25 ± 2C, respectively. Relative humidity (RH) was maintained at 95 to 100% by lining the bottom of the enclosure with wet burlap. Lesions appeared on shoot tips 3 days after inoculation, and their length was measured at various times. Genotypes with the least percent of symptomatic stems also had the lowest rates of lesion expansion. Two weeks after inoculation, tubes were drained, and shoots remained in the chamber at about 60–70% RH to allow sclerotial production. Sclerotia from each shoot were removed, counted, and their viability determined by germination on SPDA at 25 ± 2C in darkness. This method was effective in differentiating the reaction of peanut genotypes to infection by S. minor.

Sclerotia of Sclerotinia minor were soaked in a conidial suspension (1.3 × 10 7 conidia/mL) of Penicillium citrinum at 25 ± 2 C for 1 h. This resulted in coating each sclerotium with about 3.7 × 10 4 conidia. Treated sclerotia were incubated either in the dark on dry or damp Whatman No. 1 filter paper or in pasteurized and nonpasteurized soil at 25 ± 2 C, for up to eight weeks. Colonization by P. citrinum of sclerotia incubated on damp or dry filter paper was 70 and 25%, respectively. Seventy four percent of sclerotia incubated in pasteurized soil were colonized and destroyed by P. citrinum , whereas 55% colonization and destruction occurred in sclerotia incubated in a nonpasteurized soil. Similarly treated sclerotia of Sclerotinia sclerotiorum variety major and Sclerotium rolfsii were incubated in pasteurized soil and colonized by P. citrinum at 45 and 5%, respectively, over the same period of time. Up to 50% colonization and destruction by P. citrinum has been observed on sclerotia of S. minor recovered from soil in a peanut field in Oklahoma. These findings suggest a potential use of P. citrinum as a biocontrol agent for S. minor.

Western blotting was used to detect infections of peanut cv. Tamnut 74 with peanut mottle virus (PMV) and/or peanut stripe virus (PStV). Leaf samples were ground in electrophoresis sample buffer and heated for 5 min at 95 C prior to electrophoresis in 12% polyacrylamide gels. After electrophoresis, proteins were transferred to nitrocellulose sheets at 100V for 45 min. Western blots were performed by first blocking unbound sites on the nitrocellulose with 5% non-fat dry milk in Tris-buffered saline (TBS), pH 7.4 for 30 min, followed by incubation in a 1/200 dilution of PMV and/or PStV antiserum in TBS (the latter antiserum provided by J. W. Demski, U. of GA) for 45 min. This was followed by incubation in protein-A-peroxidase (2 μg/mL in TBS) for 45 min, followed by 4-chloro-1-napthol plus hydrogen peroxide in TBS. As little as 25 ng of either purified PMV or PStV was detected. This was similar to the limits of detection fo the double sandwich enzyme linked immunosorbent assay (ELISA). Because of the difference in migration of the coat proteins of PMV and PStV, both viruses may be detected in plants infected with PMV and PStV. This assay can be performed in approximately 6 h when mini-gels are used for the initial electrophoretic seperation and does not require the antiserum to be fractionated or bound to an enzyme as is the case with ELISA.

Two-week old seedlings of Tamnut 74 peanut were inoculated with peanut mottle virus (PMV) or left nontreated, and four weeks later were sprayed with a conidial suspension of Cercospora arachidicola (2 × 10 4 conidia/mL) prior to placement in a polyethylene enclosure in a growth chamber (22–30 C, 100% RH). After three weeks, number of lesions/leaflet, leaflet area (mm 2 ), necrotic area (mm 2 )/leaflet, percent necrotic area, conidial density (conidia/mm 2 ) of necrotic area), and conidia/leaflet, was determined on four leaflets for each of 12 PMV-infected and 12 virus-free plants. Conidia/leaflet (one important parameter in evaluating reaction of peanut genotypes to C. arachidicola ), lesions/leaflet, area of lesion, and leaflet area was reduced on PMV-infected plants compared to virus-free plants in each of 4 experiments. Similarly, necrotic area/leaflet were reduced in 3 of 4 experiments. Conidia/mm 2 of necrotic area was reduced in 2 of 4 experiments. PMV was not detected in conidia of C. arachidicola from PMV infected plants by inoculation to a PMV indicator host, serology, electron microscopy or electrophoresis.

Growth of Cercospora arachidicola Hori, the causal agent of early leaf spot disease on peanuts, was completely inhibited on Czapek-Dox broth medium amended with the acaricide cyhexatin (Plictran) at ≤ 73.5 μg/ml; however, at 14.7 μg/ml traces of growth occurred after 6 weeks of incubation at 27±2C and continuous fluorescent light (800 lux). Aqueous preparations of cyhexation at ≤ 147.0 μg/ml completely inhibited the germination of conidia of C. arachidicola. At a concentration of cyhexatin ≤ 73.5 and 14.7 μg/ml, less than two percent of the conidia germinated as compared with more than 95 percent germination in distilled water. Misting of aqueous preparations of cyhexatin at ≤ 147.0 μg/ml on the adaxial or both surfaces of peanut leaflets with mature leafspot lesions was very effective in reducing the sporulating potential of C. arachidicola.

A detached leaf technique was developed for screening peanut genotypes for resistance to leafspot caused by Cercospora arachidicola. Compound leaves of four cultivars of peanut ( Arachis hypogaea ) and four wild peanut genotypes were used in this study. Individual leaf petioles, each supported by a foam plug, were immersed in Hoagland's solution in 1 × 14 cm test tubes. Leaves were inoculated with C. arachidicola by misting both surfaces with a conidial suspension (2 × 10 4 conidia/ml) using a DeVilbiss atomizer (No. 152). Test tubes with innoculated leaves were placed in racks in a clear polyethylene chamber on a greenhouse bench. Temperatures in the chamber averaged 26 ± 2 C and 31 ± 2 C during night and day, respectively. Relative humidity was maintained between 80 and 90% by hanging wicks of cheesecloth with their bases in water on both sides of the chamber. Lesions appeared on leaves of susceptible peanuts 8 to 10 days after inoculation and leaflet defoliation started 18 to 21 days after inoculation. This screening technique is reproducible, and requires a minimum of leaf tissue, space, and fungal inoculum.

Percent leaf necrosis can be used to assess severity of early leafspot caused by Cercospora arachidicola Hori on peanut ( Arachis hypogaea L.). Measurements of leaf necrosis (%), as determined by area, and the corresponding loss (%) in total chlorophyll content were made. Correlations were calculated using linear regression, and logarithmic curve fit analyses. High positive correlation values were obtained between leaf necrosis and loss in total chlorophyll. Therefore, this method is suggested for a reasonably accurate and fast assessment of leaf necrosis of peanut caused by C. arachidicola.