Ronald B. Sorensen

Sexual Reproduction in Aspergillus flavus Sclerotia Naturally Produced in Corn

Aspergillus flavus is the major producer of carcinogenic aflatoxins worldwide in crops. Populations of A. flavus are characterized by high genetic variation and the source of this variation is likely sexual reproduction. The fungus is heterothallic and laboratory crosses produce ascospore-bearing ascocarps embedded within sclerotia. However, the capacity for sexual reproduction in sclerotia naturally formed in crops has not been examined. Corn was grown for 3 years under different levels of drought stress at Shellman, GA, and sclerotia were recovered from 146 ears (0.6% of ears). Sclerotia of A. flavus L strain were dominant in 2010 and 2011 and sclerotia of A. flavus S strain were dominant in 2012. The incidence of S strain sclerotia in corn ears increased with decreasing water availability. Ascocarps were not detected in sclerotia at harvest but incubation of sclerotia on the surface of nonsterile soil in the laboratory resulted in the formation of viable ascospores in A. flavus L and S strains and in homothallic A. alliaceus. Ascospores were produced by section Flavi species in 6.1% of the 6,022 sclerotia (18 of 84 ears) in 2010, 0.1% of the 2,846 sclerotia (3 of 36 ears) in 2011, and 0.5% of the 3,106 sclerotia (5 of 26 ears) in 2012. For sexual reproduction to occur under field conditions, sclerotia may require an additional incubation period on soil following dispersal at crop harvest.

INFLUENCE OF PLANT STRUCTURE, ORIFICE SIZE, AND NOZZLE INCLINATION ON SPRAY PENETRATION INTO PEANUT CANOPY

Three spray penetration tests were conducted 48, 68, and 109 days after peanuts were planted on single–row and twin–row beds. Spray was applied with flat fan pattern nozzles 8001VS, 8003VS, and 8005VS at 276 kPa pressure. Leaf area index, foliage density, and plant height and width were measured for each test and correlated with spray deposits at the bottom and middle of peanut canopies. Tests to compare spray penetrations by adjusting spray inclination from vertical to 15³ toward travel direction were also conducted when peanut plants were 68 and 109 days old. Data showed that spray penetration into peanut canopies could be improved by increasing nozzle size from 8001VS to 8003VS but could not be improved by increasing the nozzle size from 8003VS to 8005VS after plants were 68 days old. Spray deposits on the top of canopies from the 8003VS nozzle were 10.5 times higher than at the middle position and 62 times higher than at the bottom positions when plants were 109 days old. The average spray deposits at the middle of canopies from the 8003VS nozzle were 1.251, 0.721 and 0.552 .L/cm2 when plants were 48, 68, and 109 days old, respectively. Spray deposits at the bottom and middle of peanut canopies tended to decrease linearly as the plant structure indicator of growth (square root of the product of plant height, width, leaf area index, and foliage density) increased. Inclining nozzles to discharge sprays from vertical to 15³ toward travel direction did not significantly improve spray penetration.

Determination of Maturity and Degree Day Indices and their Success in Predicting Peanut Maturity1

Abstract The ability to accurately assess and predict peanut maturity is a strong determinant of the economic return to the producer as it governs crop quality, flavor, and yield. However, the currently available methods used to predict peanut maturity are based on hull color determination and are somewhat labor-intensive and subject to the observers ability to finely discriminate color classes. The objectives in this study were: 1) create an index of maturity based on the distribution of peanut pods within the accepted maturity profile board classes that give the best quantifiable correlation with peanut yield, grade, and net value; and 2) test degree day models to determine their efficacy in predicting the optimum maturity index. Peanuts were harvested on 7 and 6 sequential dates in 2003 and 2004, respectively, at two sites in southwest Georgia, USA. Several maturity indices were calculated at each harvest based on the percentage of pods in each color class of the maturity profile board. For both sites...

Sexual Reproduction in Aspergillus flavus Sclerotia: Acquisition of Novel Alleles from Soil Populations and Uniparental Mitochondrial Inheritance.

Aspergillus flavus colonizes agricultural commodities worldwide and contaminates them with carcinogenic aflatoxins. The high genetic diversity of A. flavus populations is largely due to sexual reproduction characterized by the formation of ascospore-bearing ascocarps embedded within sclerotia. A. flavus is heterothallic and laboratory crosses between strains of the opposite mating type produce progeny showing genetic recombination. Sclerotia formed in crops are dispersed onto the soil surface at harvest and are predominantly produced by single strains of one mating type. Less commonly, sclerotia may be fertilized during co-infection of crops with sexually compatible strains. In this study, laboratory and field experiments were performed to examine sexual reproduction in single-strain and fertilized sclerotia following exposure of sclerotia to natural fungal populations in soil. Female and male roles and mitochondrial inheritance in A. flavus were also examined through reciprocal crosses between sclerotia and conidia. Single-strain sclerotia produced ascospores on soil and progeny showed biparental inheritance that included novel alleles originating from fertilization by native soil strains. Sclerotia fertilized in the laboratory and applied to soil before ascocarp formation also produced ascospores with evidence of recombination in progeny, but only known parental alleles were detected. In reciprocal crosses, sclerotia and conidia from both strains functioned as female and male, respectively, indicating A. flavus is hermaphroditic, although the degree of fertility depended upon the parental sources of sclerotia and conidia. All progeny showed maternal inheritance of mitochondria from the sclerotia. Compared to A. flavus populations in crops, soil populations would provide a higher likelihood of exposure of sclerotia to sexually compatible strains and a more diverse source of genetic material for outcrossing.

Economic Returns of Irrigated and Non-Irrigated Peanut Based Cropping Systems

Proper crop rotation is essential to maintaining high peanut yield and quality. However, the economic considerations of sustainable cropping systems must incorporate commodity prices, production costs, and yield responses of the crops within the cropping system. Research was conducted at the USDA/ARS National Peanut Research Laboratorys Multi-crop Irrigation Research Farm in Shellman, Georgia to determine the average net returns of irrigated and non-irrigated cropping systems consisting of peanut (Arachis hypogea L.), cotton (Gossypium hirsutum L.), and corn (Zea mays L.). Five replicated cropping systems provided data on yield responses from irrigated and non-irrigated rotation sequences defined as: continuous peanuts (PPP), cotton/peanuts/cotton (CPC), corn/peanuts/corn (MPM), cotton/cotton/peanuts (CCP), and cotton/corn/peanuts (CMP). The peanut yield in the PPP rotation was 3300 kg/ha in the non-irrigated treatment. Non-irrigated yields in CPC and MPM rotation sequences were 3940 and 3890 kg/ha, respectively and yields in CCP and CMP rotation sequences were 4770 and 4710 kg/ha, respectively. The peanut yield in the PPP rotation was 4080 kg/ha in the irrigated treatment. Irrigated yields in CPC and MPM rotation sequences were 5280 and 5230 kg/ha, respectively and yields in CCP and CMP rotation sequences were 5940 and 6010 kg/ha, respectively. The economic returns of the cropping systems were analyzed for 3 different price level combinations. Production costs (variable and fixed) were obtained from partial budgets. Returns were defined as the 3 year average net returns of each cropping system and were calculated for each price level combination which resulted in 57 comparable average net returns for the irrigated and non-irrigated treatments. Net returns were influenced by rotation sequence, price, and irrigation.

Five Years of Subsurface Drip Irrigation on Peanut: What Have We Learned?1

Abstract Long term peanut yield with various crop rotations and irrigated with subsurface drip irrigation (SDI) is not known. A subsurface drip irrigation system was installed in 1998 on a Tifton loamy sand (Fine-loamy, kaolinitic, thermic Plinthic Kandiudults) with five crop rotations, two drip tube lateral spacings, and three irrigation levels. Crop rotations ranged from continuous peanut (Arachis hypogaea L.) to four years between peanut rotated with either cotton (Gossypium hirusutum L.) and/or corn (Zea mays L.). Laterals were installed underneath each crop row (narrow) and alternate row middles (wide). Crops were irrigated daily at 100, 75 and 50% of estimated crop water use. Continuous peanut yields averaged 3107 kg ha−1 while peanut in rotation averaged 4031 kg ha−1. Yield of peanut in any rotation and with narrow spaced drip tube laterals averaged 4883 kg ha−1 and wide spaced laterals averaged 4592 kg ha−1. Peanut in any rotation and irrigated at 75% had the same pod yield as the 100% irrigated i...

Leaf ontogeny strongly influences photosynthetic tolerance to drought and high temperature in Gossypium hirsutum

Temperature and drought are major abiotic limitations to crop productivity worldwide. While abiotic stress physiology research has focused primarily on fully expanded leaves, no studies have investigated photosynthetic tolerance to concurrent drought and high temperature during leaf ontogeny. To address this, Gossypium hirsutum plants were exposed to five irrigation treatments, and two different leaf stages were sampled on three dates during an abnormally dry summer. Early in the growing season, ontogenic PSII heat tolerance differences were observed. Photosystem II was more thermotolerant in young leaves than mature leaves. Later in the growing season, no decline in young leaf net photosynthesis (PN) was observed as leaf temperature increased from 31 to 37°C, as average midday leaf water potential (ΨMD) declined from -1.25 to -2.03MPa. In contrast, mature leaf PN declined 66% under the same conditions. Stomatal conductance (gs) accounted for 84-98% of variability in leaf temperature, and gs was strongly associated with ΨMD in mature leaves but not in young leaves. We conclude that young leaves are more photosynthetically tolerant to heat and drought than mature leaves. Elucidating the mechanisms causing these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future.

Pod Yield and Mineral Concentration of Four Peanut Cultivars Following Gypsum Application With Subsurface Drip Irrigation

Abstract A 2-year study (2004 and 2005) was conducted where gypsum was applied to four peanut (Arachis hypogaea L.) cultivars and irrigated with subsurface drip to determine pod yield and mineral concentration of peanut plants and kernels. Gypsum rates were none, 560 and 1120 kg/ha. Peanut cultivars were C99R, Georgia Green (GG), NCV-11 (NCV), and GA-O2C (O2C). Irrigation was applied daily with subsurface drip irrigation except when precipitation exceeded the estimated daily water requirement. Average soil Ca and S concentrations increased as gypsum was applied, 5% and 20%, respectively, compared with the non-treated control. The average soil calcium to potassium (Ca∶K) ratio increased to 9.8∶1 compared with 7.6∶1 prior to applying calcium. When averaged across calcium rates, peanut leaves had 3 and 14 times higher calcium and 1.4 times higher S concentrations compared with pegs and pods, respectively. The cultivars GG and NCV had the same pod yield. Cultivars C99R and O2C had the same yield as NCV but we...

Predawn respiration rates during flowering are highly predictive of yield response in Gossypium hirsutum when yield variability is water-induced

Respiratory carbon evolution by leaves under abiotic stress is implicated as a major limitation to crop productivity; however, respiration rates of fully expanded leaves are positively associated with plant growth rates. Given the substantial sensitivity of plant growth to drought, it was hypothesized that predawn respiration rates (RPD) would be (1) more sensitive to drought than photosynthetic processes and (2) highly predictive of water-induced yield variability in Gossypium hirsutum. Two studies (at Tifton and Camilla Georgia) addressed these hypotheses. At Tifton, drought was imposed beginning at the onset of flowering (first flower) and continuing for three weeks (peak bloom) followed by a recovery period, and predawn water potential (ΨPD), RPD, net photosynthesis (AN) and maximum quantum yield of photosystem II (Fv/Fm) were measured throughout the study period. At Camilla, plants were exposed to five different irrigation regimes throughout the growing season, and average ΨPD and RPD were determined between first flower and peak bloom for all treatments. For both sites, fiber yield was assessed at crop maturity. The relationships between ΨPD, RPD and yield were assessed via non-linear regression. It was concluded for field-grown G. hirsutum that (1) RPD is exceptionally sensitive to progressive drought (more so than AN or Fv/Fm) and (2) average RPD from first flower to peak bloom is highly predictive of water-induced yield variability.

Impact of Sprinkler Irrigation Amount on Peanut Quality Parameters

Abstract Peanut quality parameters were analyzed across four irrigation levels during the 2002 through 2007 crop years. The peanut quality parameters consisted of total sound mature kernels and sound splits (farmer stock grade), shelling outturn by commercial edible size, accept and reject kernels by commercial edible size, seed germination, and aflatoxin. The four irrigation levels consisted of a full level (100%), two reduced levels (66% and 33%), and a non-irrigated control. The research was conducted at the USDA/ARS National Peanut Research Laboratorys Multi-crop Irrigation Research Farm in Shellman, Georgia. By year, significant differences in the irrigation treatments depended upon precipitation distribution for the specific quality parameters. For the average over the six years in the study, farmer stock grade was not significantly different in the 100, 66, and 33% treatments while all were significantly higher than the non-irrigated control. Total shelling outturn and total edible outturn were hi...