Paul D. Blankenship

Interrelationship of kernel water activity, soil temperature, maturity, and phytoalexin production in preharvest aflatoxin contamination of drought-stressed peanuts

Samples of Florunner peanuts were collected throughout a period of late-season drought stress with mean geocarposphere temperatures of 29 and 25 °C, and determinations of maturity, kernel water activity (aw), percent moisture, capacity for phytoalexin production, and aflatoxin contamination were made. Results showed an association between the loss of the capacity of kernels to produce phytoalexins and the appearance of aflatoxin contamination. Kernel aw appeared to be the most important factor controlling the capacity of kernels to produce phytoalexins. Mature peanuts possessed additional resistance to contamination that could not be attributed solely to phytoalexin production. Kernel moisture loss was accelerated in the 29 °C treatment compared to the 25 °C treatment, and data indicated that the higher soil temperature also favored growth and aflatoxin production by Aspergillus flavus in peanuts susceptible to contamination.

Use of a biocompetitive agent to control preharvest aflatoxin in drought stressed peanuts

A three-year study was conducted to evaluate the use of a nonaflatoxin-producing strain of Aspergillus parasiticus (NRRL 13539) as a biocompetitive agent for the control of preharvest aflatoxin contamination of peanuts. The agent was added to the soil of the environmental control plot facility at the National Peanut Research Laboratory and tested by subjecting peanuts to optimal conditions for the development of aflatoxin contamination. Edible peanuts from the treated soil contained aflatoxin concentrations of 11, 1, and 40 ppb for crop years 1987, 1988, and 1989, respectively, compared to untreated peanuts with 531, 96, and 241 ppb, respectively. In addition, treatment in 1989 with low and high inoculum levels of a UV-induced mutant from the NRRL 13539 strain resulted in aflatoxin concentrations of 29 and 17 ppb, respectively, in edible peanuts. Soil populations of the biocompetitive agents were not higher than populations of wild strains of A. flavus/parasiticus in untreated soil subjected to late-season drought stress. This is an important ecological consideration relative to the utilization of this biocontrol system.

Mean geocarposphere temperatures that induce preharvest aflatoxin contamination of peanuts under drought stress.

Apparently undamaged peanuts grown under environmental stress in the form of drought and heat become contaminated with Aspergillus flavus and aflatoxin in the soil prior to harvest. The upper mean temperature limit for aflatoxin contamination in undamaged peanut kernels grown under drought stress the latter 4–6 weeks of the growing season was between 29.6–31.3°C. The lower limit was between 25.7–26.3°C. That is, peanuts grown under drought stress with a mean geocarposphere temperature of 29.6°C were highly contaminated while those at 31.3°C were not contaminated. Likewise, those grown under drought stress with a mean geocarposphere temperature of 25.7°C were not contaminated while those subjected to a mean geocarposphere temperature of 26.0°C resulted in some categories becoming contaminated. Increasing the mean temperature up to 29.6°C caused increasing amounts of contamination.

Effect of geocarposphere temperature on pre-harvest colonization of drought-stressed peanuts by Aspergillus flavus and subsequent aflatoxin contamination

Florunner peanuts grown in research plots were subjected to 5 soil temperature and moisture treatment regimes resulting in A. flavus infestation and subsequent aflatoxin contamination in drought-stressed peanuts. Treatments imposed beginning 85 days after planting were drought, drought with heated soil and 3 drought treatments with cooled soil. The incidence of A. flavus in drought-stressed, unshelled, sound mature kernels (SMK) decreased with decreases in the mean 5 cm deep soil temperature. The incidence of A. flavus was greater in inedible categories and in damaged kernels than in SMK. The mean, threshold, geocarposphere temperature required for aflatoxin development during the latter part of the peanut growth cycle was found to be between 25.7° C and 27° C.

Effect ofAspergillus parasiticus soil inoculum on invasion of peanut seeds

Environmental control plots adjusted to late season drought and elevated soil temperatures where inoculated at peanut planting with low and high levels of conidia, sclerotia, and mycelium from a brown conidial mutant ofAspergillus parasiticus. Percentage infection of peanut seeds from undamaged pods was greatest for the subplot containing the high sclerotial inoculum (15/cm2 soil surface). Sclerotia did not germinate sporogenically and may have invaded seeds through mycelium. In contrast, the mycelial inoculum (colonized peanut seed particles) released large numbers of conidia into soil. Soil conidial populations of brownA. parasiticus from treatments with conidia and mycelium were positively correlated with the incidence of seed infection in undamaged pods. The ratio ofA. flavus to wild-typeA. parasiticus in soil shifted from 7:3 to 1:1 in the uninoculated subplot after instigation of drought, whereas in all subplots treated with brownA. parasiticus, the ratio of the two species became approximately 8:2. Despite high levels of brownA. parasiticus populations in soil, nativeA. flavus often dominated peanut seeds, suggesting that it is a more aggressive species. Sclerotia of wild-typeA. parasiticus formed infrequently on preharvest peanut seeds from insect-damaged pods.

Effect of Drought and Temperature Stress on Peanut (Arachis hypogaea L.) Seed Composition

Summary Peanut ( Arachis hypogaea L.) plants were subjected to drought and temperature stress for various periods, and the seeds from these plants were separated into Jumbo, Medium and No. 1 market size categories and analyzed for soluble and total carbohydrates, α-amino nitrogen, total protein, and oil. The results showed that soluble and total carbohydrate content of the seed increased due to drought and temperature stress, with Jumbo and Medium market categories showing the highest increase. The α-amino nitrogen content of the Jumbo market category decreased while that of the Number 1 market category increased following a 30-day stress exposure. Protein and oil content of all the market categories were not affected after exposure to drought and temperature stress. The protein and polypeptide profiles showed that a polypeptide with a molecular weight of 70,000 and a pi between 6.2 and 7.0 increased with increasing periods of drought and temperature stress. It is suggested that drought and temperature stresses cause increased accumulation and/or synthesis of carbohydrates and certain polypeptides thus may enhance Aspergillus invasion and aflatoxin production.

A low-cost microcomputer system to monitor and control an environmental control plot facility☆

Since 1980, six 67.1-m2 environmental control plots with apparatus for soil temperature manipulation and motorized shelters for rainfall exclusion have been used for the study of Aspergillus flavus invasion and aflatoxin contamination of peanuts during drought stress. Manual controls and the absence of an alarm system for the plots have mandated daily monitoring and adjustments of thermostats for soil temperature control and visual inspection for proper shelter operation, especially during severe thunderstorms. Prior to 1987 experiments, a microcomputer-based control system was assembled to control the operation of the facility and provide telephone alarms for soil temperature discrepancies, improper shelter operation, or electrical power interruptions. Commercially available components were used to construct the system which cost approximately US

Effect of curing temperature and seed maturity status on peanut seed and paste composition

2000. At the end of a 50-day evaluation period, the system had provided an average plot soil temperature of 27.4°C with a setting of 26.7°C. The 95% confidence interval for the treatment period average during the tests was between 25.9°C and 27.9°C. Alarm annunciation and telephone notification were also provided as expected in response to plot temperature extremes and a power outage.

Effect of Inoculum Rate of Biological Control Agents on Preharvest Aflatoxin Contamination of Peanuts

Abstract The effect of curing temperature on composition of peanut seed and paste was determined by analyzing for the reaction precursors of roasted flavor such as α-amino nitrogen, protein, and sugars. The data showed that seed cured at 8.4 and 16.8°C above ambient air temperature contained higher amounts of α-amino nitrogen than peanut cured at ambient temperature. The temperature effect was more pronounced on less mature seed than on mature seed. Curing temperature had no effect on total protein, and soluble- and insoluble-carbohydrate content of the seed. However, higher curing temperatures caused a reduction in the arachin polymer content and also loss of three polypeptides with molecular weights of 35,000 and 25,000 Da, and isoelectric points between 5.1 and 5.3, and 5.8 and 6.4, respectively.