No abstract

Plants must be established quickly on replenished beaches in order to stabilize the sand and begin the dune-building process. The objective of this research was to determine whether inoculation of sea oats (Uniola paniculata L.) with bacteria (indigenous rhizosphere bacteria and N2 fixers) alone or in combination with vesicular-arbuscular mycorrhizal fungi would enhance plant growth in beach sand. At two fertilizer-N levels, Klebsiellapneumoniae and two Azospirillum spp. did not provide the plants with fixed atmospheric N; however, K. pneumoniae increased root and shoot growth. When a sparingly soluble P source (CaHPO4) was added to two sands, K. pneumoniae increased plant growth in sand with a high P content. The phosphorus content of shoots was not affected by bacterial inoculation, indicating that a mechanism other than bacterially enhanced P availability to plants was responsible for the growth increases. When sea oats were inoculated with either K. pneumoniae or Acaligenes denitrificans and a mixed Glomus inoculum, there was no consistent evidence of a synergistic effect on plant growth. Nonetheless, bacterial inoculation increased root colonization by vesiculararbuscular mycorrhizal fungi when the fungal inoculum consisted of colonized roots but had no effect on colonization when the inoculum consisted of spores alone. K. pneumoniae was found to increase spore germination and hyphal growth of Glomus deserticola compared with the control. The use of bacterial inoculants to enhance establishment of pioneer dune plants warrants further study.

Preharvest aflatoxin contamination (PAC) of peanut occurs under prolonged periods of drought and heat stress. Evaluation of peanut germplasm may identify valuable sources of resistance to PAC, but will require a large scale screening system. The objective of this research was to develop a large-scale field system for screening peanut germplasm for resistance to PAC at Yuma, Arizona. Yuma is located in a desert and has great potential as a site for evaluating germplasm for resistance to PAC. Field studies were conducted in 1990 to determine if aflatoxin contamination would occur in drought stressed peanuts grown at Yuma. Aflatoxin levels up to 2,260 ppb were observed, however, 52% of the plots escaped contamination and the coefficient of variation (C.V.) were unacceptably large. During testing at Yuma in 1990 it was noted that drought stressed plants died quickly due to the rapid exhaustion of soil moisture. A subsurface irrigation system was installed in 1991 to alleviate this problem and allow for an extended period of drought stress in the pod zone. Results for 1991 showed a greatly increased mean aflatoxin contamination, a 50% reduction in the C.V., and a virtual elimination in the occurrence of escapes. A study was conducted in 1992 to compare plots with and without subsurface irrigation to determine if the differences observed between 1990 and 1991 were due to the use of subsurface irrigation. The use of subsurface irrigation in 1992 increased the mean contamination by over 100%, reduced the C.V. by over 50%, and reduced the percentage of escapes by over 90%.

Beach replenishment is a widely used method of controlling coastal erosion. To reduce erosional losses from wind, beach grasses are often planted on the replenishment sands. However, there is little information on the microbial populations in this material that may affect plant establishment and growth. The objectives of this research were to document changes in the populations of vesicular-arbuscular mycorrhizal (YAM) fungi and other soil microorganisms in replenishment materials and to determine whether roots of transplanted beach grasses become colonized by beneficial microbes. The study was conducted over a 2-year period on a replenishment project in northeastern Florida. Three sampling locations were established at 1-km intervals along the beach. Each location consisted of three plots: an established dune, replenishment sand planted with Uniola paniculata and Panicum sp., and replenishment sand left unplanted. Fungal and bacterial populations increased rapidly in the rhizosphere of beach grasses in the planted plots. However, no bacteria were recovered that could fix significant amounts of N2. The VAM fungi established slowly on the transplanted grasses. Even after two growing seasons, levels of root colonization and sporulation were significantly below those found in the established dune. There was a shift in the dominant VAM fungi found in the planted zone with respect to those in the established dunes. The most abundant species recovered from the established dunes were Glomus deserticola, followed by Acaulospora scrobiculata and Scutellospora weresubiae. The VAM fungi that colonized the planted zone most rapidly were Glomus globiferum, followed by G. deserticola and Glomus aggregatum.

Efforts are being made to screen peanut (Arachis hypogaea L.) genotypes for resistance to invasion of seed by the Aspergillus flavus (Link ex Fries) group fungi and subsequent contamination by aflatoxins. Field inoculation studies are needed to develop reliable screening techniques that insure infection of susceptible peanut seed by the A. flavus group fungi and minimize the number of escapes. The objectives of this study were to evaluate the effects of A. parasiticus (Speare) inoculum formulation and timing of application on shell and seed colonization and aflatoxin contamination. Soil population densities of A. flavus and plant survival were also measured. Plots were inoculated using various rates either at-plant or at mid-bloom of an organic-matrix infested with A. parasiticus NRRL 2999, and/or using an aqueous conidial suspension of the fungus. Mid-bloom inoculation produced an increase of A. flavus group population densities in the soil. There were no statistical differences between treatments in aflatoxin content, or colonization of shells or seed. At-plant inoculation was associated with reduced plant emergence or survival. Inoculation at mid-bloom with the organic-matrix resulted in peanuts that ranked high in aflatoxin content. There were no correlations between aflatoxin content, colonization of seed or shells, and population densities of A. flavus in soil. Inoculation at mid-bloom with the organic-matrix, resulting in greater soil population densities and relatively higher aflatoxin levels, was an effective method to use in screening for resistance to invasion by A. flavus group fungi and contamination by aflatoxins.