Assessing ectomycorrhizal fungal spore banks of truffle producing soils with pecan seedling trap-plants

Background and Aims

Recently, the truffle species Tuber lyonii Butters was found to be dominant in ectomycorrhizal (EcM) fungal communities of cultivated pecan (Carya illinoinensis (Wangenh.) K. Koch). Many truffle fungi exhibit the trait of effectively colonizing plant roots via spores. We hypothesized that T. lyonii would be well represented in the spore bank of pecan orchard soils where it is found.

Methods

We used axenically-grown pecan seedlings as trap-plants to bait for EcM associates in soils collected beneath truffle-producing pecan trees. EcM fungi on seedlings were characterized through rDNA sequencing and were compared to EcM communities of adult trees in these orchards.

Results

Tuber lyonii mycorrhizas were well formed on seedlings inoculated with truffle spores, but were limited to just a few of the trap-plants grown in field soils. We compared EcM communities of adult pecan orchard trees to those on trap-plants and found distinct communities on each, with a high degree of similarity at the ordinal but not species level.

Conclusions

Although species of Pezizales are abundant in pecan EcM communities and as propagules in their soil spore banks, only a low level of T. lyonii was detected in soil spore banks beneath orchard trees naturally colonized by T. lyonii. Other factors including land-use history or orchard management may better explain this truffle species presence and abundance in pecan EcM communities.

     

Arbuscular fungi and mycorrhizae (Glomales) of the Hel Peninsula,Poland

In the years 1985–1989, the occurrence of arbuscular fungi and mycorrhizae on the Hel Peninsula (Poland) was investigated with the help of 45 soil and root samples collected under 20 plant species of eight families. Except for Zea mays, the other plant species were from uncultivated sites. All soil samples contained spores of arbuscular fungi, of which about 45% were of the genus Glomus. Acaulospora spp. preferred members of the Cupressaceae. Spores of Gigaspora occurred rarely and only in two plant families. Glomus spp. were most frequently associated with plants of the Rosaceae, and species of Scutellospora were found at markedly higher frequencies among roots of plants of the Gramineae and Cupressaceae. A total of 29 spore-forming species and Glomus tenue (a fungus recognizable by its distinctive infections) were found. The most frequently recovered fungus, Glomus tenue, was present in roots of 56.8% of examined plants. Of the spore-forming fungi, the most frequently isolated spores were those of Scutellospora dipurpurascens, then Glomus constrictum, Acaulospora 61, and Glomus microcarpum. The overall spore density in examined samples averaged 99.8 in 100 g dry soil in the range 1 to 547, and was highest in a sample taken from around roots of Festuca arundinacea. The dominant fungi forming spores in sampled soils were Glomus constrictum, Glomus microcarpum, and Scutellospora dipurpurascens. The average species density was 3.9 in 100 g dry soil in the range 1 to 10, and was highest in Corynephorus canescens, Rosa canina, and Thuja occidentalis. Levels of colonization by arbuscular fungi ranged from 0.0 to94.0% (mean 23.3%) of the root length and were highest in Festuca arundinaceae and Zea mays.     

Effectiveness of VAM fungi in nonsterile soils before and after optimization of P in soil solution

Five tropical soils were either not inoculated or inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus aggregatum. The degree to which VAM effectiveness was expressed in the soils was evaluated prior and after solution P status was adjusted for optimal VAM activity. VAM effectiveness determined by monitoring P concentrations of pinnules of Leucaena leucocephala leaves as a function of time and as dry matter yield determined at the time of harvest, indicated that in three of the soils VAM effectiveness was either very restricted or altogether unexpressed irrespective of vesicular-arbuscular mycorrhizal fungal (VAMF) inoculation if soil solution P was not optimized for VAM effectiveness. After P optimization, effectiveness was significantly increased by VAMF inoculation although in four of the soils, densities of indigenous VAMF propagules greatly exceeded that attained by the inoculum after it was mixed with soil. Mycorrhizal fungal inoculation effects varied from soil to soil, depending on the extent to which the effectiveness of indigenous and introduced endophytes was enhanced by P optimization and the similarity of inherent soil solution P concentrations to the range known to be optimum for VAM effectiveness. Of the indicator variables monitored, VAMF colonization was least sensitive to treatment effects followed by shoot P concentration measured at the time of harvest.Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal series No. 3781.Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal series No. 3781.     

Diversity and efficiency of arbuscular mycorrhizal fungi in soils from organic chili (Capsicum frutescens) farms

No previous studies have been conducted on the diversity and population of arbuscular mycorrhizal fungi (AMF) in relation to organically grown chili (Capsicum frutescens L.) in Thailand. This study was carried out to investigate the diversity and status of AMF populations at four organically managed farms in Ubon Ratchathani and Sisaket provinces. The effects of each AMF species on the growth and nutrient uptake of chili grown in sterile, organically managed soil were determined. Fourteen AM fungal taxa belonging to the genera Acaulospora (4 spp.), Entrophospora (1 sp.), Glomus (7 spp.) and Scutellospora (2 spp.) were found. Among these, Glomus was the dominant genus found at all sites, followed by Acaulospora. The spore density and root colonization of AMF on chili did not vary significantly among the sites. The effects of ten selected AMF species on the growth of chili showed that Gl. clarum RA0305 increased the growth, flowering, and fruit production of chili, and also increased the P uptake significantly, compared to non-mycorrhizal plants. This fungus showed the highest potential as a promoter of growth, flowering and yield in organically managed chili production.     

Long-term orchard groundcover management systems affect soil microbial communities and apple replant disease severity

Apple replant disease (ARD) is a soil-disease syndrome of complex etiology that affects apple tree roots in replanted orchards, resulting in stunted tree growth and reduced yields. To investigate whether different groundcover management systems (GMSs) influence subsequent ARD severity, we grew apple seedlings in an outdoor nursery in pots containing orchard soil from field plots where four GMSs had been maintained for 14 years in an orchard near Ithaca, NY, USA. The GMS treatments were: (1) pre-emergence herbicide (Pre-H), bare soil strips maintained by applying tank-mixed glyphosate, norflurazon and diuron herbicides annually; (2) post-emergence herbicide (Post-H), sparse weed cover maintained by applying glyphosate in May and July each year; (3) mowed sod grass (Mowed Sod); and (4) bark mulch (Mulch). Soils were also sampled from the grass drive lane maintained between the trees in the orchard (Grass Lane). Sampled soils (Orchard soil) were either pasteurized or left untreated, placed into 4-L pots, and planted with one apple seedling per pot. After 3 months of growth, soil (Bioassay soil) and apple tree roots (Bioassay roots) were sampled from each pot and microbial populations colonizing samples were characterized. Seedling growth was reduced in soils sampled from all four GMS treatments compared to the Grass Lane soils. Among the GMS treatments, seedling biomass was greater in Pre-H than in the Post-H soil. Soil microbial communities and nutrient availability differed among all four GMS treatments and the Grass Lane. Root-lesion (Pratylenchus sp.) nematode populations were higher in the Mowed Sod than in the other GMS treatments. Soil bacterial and fungal community composition was assessed in Orchard and Bioassay soils and Bioassay roots with a DNA fingerprinting method (T-RFLP). Redundancy analysis indicated that soils sampled from the different GMS treatments differentially influenced seedling biomass. A clone library of 267 soil bacteria was developed from sampled Orchard soils and Bioassay roots. These communities were dominated by Acidobacteria (25% of sequences), Actinobacteria (19%), δ-Proteobacteria (12%), β-Proteobacteria (10%), and these ratios differed among the GMS soils. Members of the family Comamonadaceae were detected only in tree-row soil, not in the Grass Lanes. The dominant sequences among 145 cloned fungi associated with apple seedling roots were Fusarium oxysporum (16% of sequences), an uncultured soil fungus submitted under DQ420986 (12%), and Rhodotorula mucilaginosa (9%). In a redundancy analysis, factors including fungal and oomycete community compositions, soil respiration rates, population sizes of culturable bacteria and fungi, soil organic matter content, and nutrient availability, were not significant predictors of apple seedling biomass in these soils. Different GMS treatments used by apple growers may influence subsequent ARD severity in replanted trees, but edaphic factors commonly associated with soil fertility may not reliably predict tree-root health and successful establishment of replanted orchards.     

Effect of vesicular-arbuscular mycorrhizal fungi on verticillium wilt of cotton

The development of vesicular-arbuscular mycorrhizal fungi (VAMF): Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe, Glomus versiforme (Karsten) Berch, Sclerocystis sinuosa Gerdemann and Bakhi and Verticillium dahliae and the effects of the VAMF on the verticillium wilt of cotton (Gossypium hirsutum L. and Gossypium barbadense L.) were studied with paper pots, black plastic tubes and clay pots under natural growth conditions. All of the tested VAMF were able to infect all the cotton varieties used in the present experiment and typical vesicles and arbuscules were formed in the cortical cells of the cotton roots after inoculation. The cap cells, meristem, differentiating and elongating zones of the root tip were found to be colonized by the VAMF. In the case of most V. dahliae infection, the colonization occurred mostly from the root tip up to 2 cm. VAMF and V. dahliae mutually reduced their percentage of infection when inoculated simultaneously. VAMF inoculation reduced the numbers of germinable microsclerotia in the soil of the mycorrhizosphere, while the quantity of VAM fungal spores in the soil was not influenced by infection of with V. dahliae. The % of arbuscule colonization in roots was negatively correlated with the disease grades, while the numbers of vesicles in roots were not. These results suggest that certain vital competition and antagonistic reactions exist between VAMF and V. dahliae. VAMF reduced the incidence and disease indices of verticillium wilt of cotton during the whole growth phase. It is evident that cotton seedling growth was promoted, flowering was advanced, the numbers of flowers and bolls were increased, and this resulted in an increase in the yield of seed cotton. Among the VAMF species, Glomus versiforme was the most effective, and Sclerocystis sinuosa was inferior. So far as the author is aware, such an effect of VAMF on the increase of cotton wilt tolerance/resistance is reported here far the first time.     

Biological Control of Anastrepha spp. (Diptera: Tephritidae) in Mango Orchards through Augmentative Releases of Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae)

Diachasmimorpha longicaudata (Ashmead) parasitoids were released by air on a weekly basis over 1600 ha of commercial mango orchards, backyard orchards, and patches of native vegetation, at a density of ca. 940 parasitoids/ha. Releases were made during 2 consecutive years, beginning at flower onset and lasting until the end of the production cycle. Two areas, 7 km apart, were compared. In one area parasitoids were released, whereas the other area was used as a control. During the 2nd year treatments were reversed. Fruit was sampled in commercial mango orchards and in backyard orchards to assess levels of parasitism in fruit fly larvae. Highly significant differences in percentage parasitism were found in release and control zones in backyard orchards. Furthermore, trapping results indicated that D. longicaudata releases were associated with ca. 2.7-fold suppression of Anastrepha spp. populations in backyard orchards. Results suggest that suppression might be affected by environmental conditions and by the parasitoid:fly ratio achieved. Anastrepha obliqua McQuart populations were suppressed more effectively by use of parasitoids than those of Anastrepha ludens Loew, perhaps due to the type of host fruits used by each species. Augmentative parasitoid releases in marginal areas surrounding commercial orchards (backyard orchards, wild vegetation) can substantially suppress fly populations. Through this approach, the number of flies that later move into commercial orchards can be significantly reduced. Such a strategy, when combined with sound orchard management schemes, can allow growers to produce clean fruit without the need to resort to the widespread use of insecticides.     

Allelochemicals and their transformations in the Ageratum conyzoides intercropped citrus orchard soils

Intercropping Ageratum conyzoides in citrus orchards may effectively suppress weeds and control other pests. Investigations showed that the inhibition of major weeds and soil pathogenic fungi in citrus orchards was significantly correlated with the allelochemicals released into the soil by intercropped A. conyzoides. Three flavones, ageratochromene, and its two dimers were isolated and identified from the A. conyzoides intercropped citrus orchard soil. These allelochemicals had different biological actions on major weeds and soil pathogenic fungi in the citrus orchard. Three flavones and ageratochromene could significantly inhibit the growth of weeds Bidens pilosa, Digitaria sanguinalis and Cyperus difformis, and spores germination of soil pathogenic fungi Phytophthora citrophthora, Pythium aphanidermatum and Fusarium solani. However, two dimers of ageratochromene had no inhibitory actions on them. The presence of these allelochemicals in soils suggests that they may be able to make a major contribution to control some weeds and diseases in citrus orchards. Further studies revealed that dynamic transformation between ageratochromene and its two dimers in the A. conyzoides intercropped citrus orchard soil was reversible, that is, ageratochromene released from ground A. conyzoides plants was transformed into its dimers, and the dimers can be remonomerized in the soils. However, this dynamic transformation did not occur in the soil with low organic matter and fertility. The dimerization was not correlated with microorganisms in the soil, but the biodegradation of both ageratochromene and its two dimers may have occurred, particularly in the soil with low organic matter and fertility. Our results strongly suggest that the reversible transformation between ageratochromene and its dimers in the A. conyzoides intercropped citrus orchard soil can be an important mechanism maintaining bioactive allelochemicals at an effective concentration, thus, sustaining the inhibition of weeds and pathogenic fungi in soil.     

VA MYCORRHIZAE IN STRAND VEGETATION OF HAWAII: EVIDENCE FOR LONG-DISTANCE CODISPERSAL OF PLANTS AND FUNGI

Vesicular-arbuscular mycorrhizal fungi (VAMF) were a nearly constant component of the coastal strand of the Hawaiian Islands, occurring in beach sand, driftline debris, in roots of 23 of 31 species of vascular plants examined, and in association with rhizomes of two native species, Sporobolus virginicus and Jacquemontia sandwicensis. Mycorrhizae were most frequent and intensity of VAM development was greatest in endemic plants, less in indigenous species, and least in alien species. Spores of VAMF were produced in abundance between the rhizome and the leaf sheaths of Sporobolus. Roots of two strand species, including roots of Sporobolus that were immersed in seawater for 7 days, functioned as inocula of VAMF in pot-culture studies. The close association between propagules of VAMF and vegetative fragments of indigenous plants found in the present study suggests a mechanism of codispersal that appears to ensure the maintenance of the symbiosis in nutrient-deficient sites where it is most beneficial. The codispersal of fungus and plant may explain the high frequency of mycotrophy in strand species in the tropical Pacific.     

Indigenous Populations of Three Closely Related <Emphasis Type="Italic">Lysobacter</Emphasis> spp. in Agricultural Soils Using Real-Time PCR

Previous research had shown that three closely related species of Lysobacter, i.e., Lysobacter antibioticus, Lysobacter capsici, and Lysobacter gummosus, were present in different Rhizoctonia-suppressive soils. However, the population dynamics of these three Lysobacter spp. in different habitats remains unknown. Therefore, a specific primer–probe combination was designed for the combined quantification of these three Lysobacter spp. using TaqMan. Strains of the three target species were efficiently detected with TaqMan, whereas related non-target strains of Lysobacter enzymogenes and Xanthomonas campestris were not or only weakly amplified. Indigenous Lysobacter populations were analyzed in soils of 10 organic farms in the Netherlands during three subsequent years with TaqMan. These soils differed in soil characteristics and crop rotation. Additionally, Lysobacter populations in rhizosphere and bulk soil of different crops on one of these farms were studied. In acid sandy soils low Lysobacter populations were present, whereas pH neutral clay soils contained high populations (respectively, <4.0–5.87 and 6.22–6.95 log gene copy numbers g⁻¹ soil). Clay content, pH and C/N ratio, but not organic matter content in soil, correlated with higher Lysobacter populations. Unexpectedly, different crops did not significantly influence population size of the three Lysobacter spp. and their populations were barely higher in rhizosphere than in bulk soil.     

Resident bacteria, nitric oxide emission and particle size modulate the effect of Brassica napus seed meal on disease incited by Rhizoctonia solani and Pythium spp

Amendment of orchard soil with low-glucosinolate Brassica napus (rape) seed meal (RSM) suppresses infection of apple roots by Rhizoctonia solani but increases incidence of Pythium spp. infection. Following incorporation of Brassica sp. seed meals, soils were monitored for changes in populations of selected saprophytic and plant pathogenic microorganisms. When conducted in pasteurized soil, which possessed high numbers of Bacillus spp. and lower than detectable numbers of Streptomyces spp., RSM amendment did not provide control of R. solani. Populations of streptomycetes in RSM-amended soil increased to stable levels >20-fold higher than in non-amended soil. Disease suppressiveness was restored to pasteurized RSM-amended soil by adding any of several Streptomyces strains. Maximal rates of nitrification in orchard soil, determined by nitric oxide emission, were observed within two weeks following RSM amendment and inhibition of nitrification via application of nitrapyrin abolished the capacity of RSM to suppress R. solani infection of apple roots when seedlings were planted one day after soil amendment. Apple seedling mortality and Pythium spp. root infection were highest for seedlings planted immediately following incorporation of B. napus cv. Athena RSM, particularly when meal was added in a flake rather than powder form. Lower infection frequencies were observed for seedlings planted four weeks after RSM incorporation, even for soil in which densities of culturable Pythium spp. had not declined. Our results demonstrate that suppression of Rhizoctonia root rot in response to RSM amendment requires the activity of the resident soil microbiota and that initial disease control is associated with the generation of nitric oxide through the process of nitrification.     

Phytophthora and Pythium Associated with Feeder Root Rot of Citrus in the Transvaal Province of South Africa

Mild to extensive feeder root rot was present in all 23 orchards, with trees showing symptoms of citrus decline from nine areas in the Transvaal Province of South Africa. Phytophthora nicotianae and Pythium spp. were isolated from diseased roots and rhizosphere soils in all areas sampled. Isolations from diseased feeder roots showed P. nicotianae present in 26% of orchards during Spring and 61% of orchards during Autumn, while Pythium spp. were present in 56% of orchards during Spring and 65% of orchards during Autumn. In isolations from baited rhizosphere soils, P. nicotianae was present in 56% of orchards during Spring and 52% of orchards during Autumn, while Pythium spp. were present in 69% of orchards during Spring and 82% of orchards during Autumn. In rhizosphere soils, the mean population density of Pythium spp. was higher than that of P. nicotianae throughout the season. Only P. nicotianae was consistently isolated during thesurvey. Different Pythium spp. were isolated of which two were tentatively identified as P. paroecandrum and Pythium‘Gp.G’.     

Nachweis signifikant verschiedener Pilzpopulationen bei gleicher Bodennutzung

Summary The soil fungus flora in two crop-rotation experiments was studied using the soil-washing technique. Numerous analyses were carried out at regular intervals during three years.Data of abundance and frequence are tabulated for the 91 most common species. Although these figures showed a significant correlation (r=0.790 for 91 species) between the two fungus floras, significant differences were found in the abundance of 37 species (atP=0.01). It is concluded, that there are still considerable differences in similar soils under the same plant cover. Even stronger differences were found, when four Dutch polder soils were included in the comparison.It is suggested, that these differences reflect the different previous cultivations rather than the influence of physical or chemical soil factors.     

Distribution,occurrence and characterization of entomopathogenic fungi in agricultural soil in the Palestinian area

The occurrence of entomopathogenic fungi was investigated in irrigated vegetable fields and citrus orchards soils, over a nine-month period (April-December 1999),using the Galleria bait method (GBM). Entomopathogenic fungi were found to occur in 33.6% of the soil samples studied, with positive samples yielding 70 fungal isolates, belonging to 20 species from 13 genera. Conidiobolus coronatus was the most frequent and abundant entomopathogenic species recovered, comprising 31.4% of the total number of isolates. Soil pH, soil moisture content and the geographical location had minor or no effect on the isolation of entomopathogenic fungi in the fields studied. On the other hand, organic matter content of soil, and vegetation type were found to significantly affect the occurrence of entomopathogenic fungi in soil habitats, with orchard fields yielding larger numbers of isolates than vegetable fields. Using Koch's postulates the pathogenicity of fungal isolates to Galleria larvae was found to range from 16–100% (mortality rate). Isolates of C. coronatus proved to be the most virulent isolates recovered. The effect of media and temperature on mycelial growth rate, conidial production and conidial germination of six entomopathogenic fungal species (C. coronatus, Entomophaga grylli, Erynia castrans, Hirsutella jonesii, Paecilomyces farinosus and Sporodiniella umbellata) was also studied. Mycelial growth rate, spore production and spore germination were significantly affected by media, temperature and isolates. In view of the present results, C. coronatus appears to be a good candidate for pest control in agricultural soils, as it has a wide tolerance to agricultural practices, has frequently been isolated from both vegetable and orchard fields, and is characterized by high mycelial growth rate, conidial production and conidial germination.This revised version was published online in October 2005 with corrections to the Cover Date.     

The effect of groundcover and herbicide treatment on twospotted spider mite density and dispersal in southern Oregon pear orchards

A total of 49 groundcover plant species representing 47 genera in 22 families were identified from a survey of 5 pear orchards. Density of twospotted spider mite (Tetranychus urticae Koch) inhabiting these plants was estimated visually several times during the field season. Plants were ranked in 1 of 3 categories depending on mite densities found on these plants during the summer. T. urticae was found to be highly abundant (category 3) on 26 species, at lower densities on 10 species (category 2), and was rarely or never found on the remaining 12 species (category 1). Dispersal of mites from groundcover plants into trees was found to be highly variable within and between orchards. Within orchard dispersal appeared to be related to the distribution and abundance of category 3 host plants in the orchard. Variability between orchards may also be affected by groundcover management techniques and levels of acaricide resistance in T. urticae. The use of herbicides to control groundcover plants significantly increased the dispersal of T. urticae into the orchard trees.     

Growth and yield of lentil and wheat inoculated with three Glomus isolates from Saskatchewan soils

The vesicular-arbuscular mycorrhizal fungi (VAMF) Glomus clarum (Nicol. and Schenck) isolate NT4, G. mosseae (Nicol. and Gerd.) Gerd. and Trappe isolate NT6 and G. versiforme (Karst.) Berch isolate NT7 coexist in wheat field soils in Saskatchewan. This study assessed the response of lentil (Lens esculenta L.) and wheat (Triticum aestivum L.) to monospecific and mixed cultures of these VAMF isolates. Seedlings were inoculated with 100 spores of a VAMF isolate, or an equal mixture of spores of two isolates, and grown in a sterile soil mix in a growth chamber. Both crops responded differently to these different VAMF isolates. In the case of lentil, G. clarum NT4 was more effective than G. mosseae NT6 and G. versiforme NT7, and significantly increased (P<0.05) the shoot dry weight (43%) and grain yield (57%) compared with the uninoculated control. There was a significant positive correlation between the percentage of VAMF colonized roots and shoot dry weight (r=0.672^***) and shoot phosphorus concentration (r=0.608^***) of lentil. In the case of wheat, G. clarum NT4 had no effect on shoot dry weight, but produced significant (P<0.08) increases in grain yield (12%) and the phosphorus concentration of the shoot and grain. Although G. clarum NT4 and G. mosseae NT6 both produced similar levels of VAM colonization in wheat, the only response of wheat to isolate NT6 was an increase in plant height at harvest. The efficacy of G. clarum NT4 on both crops appeared to be related to its ability to produce more arbuscular colonization than G. mosseae NT6. Dual inoculation of seedlings with G. clarum NT4 and G. mosseae NT6 resulted in competition between these two isolates. This was evident from a comparison of plant shoot dry weight and grain yield, and VAMF spore production on the two crops inoculated either with isolate NT4 alone or in combination with NT6. G. mosseae NT6 reduced the efficacy of G. clarum NT4 by 16% when dual inoculated on lentil, but had no effect when the host was wheat. Based on spore production, it was found that G. clarum NT4 was more competitive than G. mosseae NT6 when dual inoculated on lentil or wheat. Isolate NT4 produced ca. 2000 and 500 spores/ 100 g substrate, respectively, in the lentil and wheat pots, which was approximately 2–3 times more spores than those produced by isolate NT6 with either crop. When the plants were dual inoculated, there was a 15–19% reduction in spore production by G. clarum NT4 and a 50–70% decrease in spore production by G. mosseae NT6. Our results show that G. clarum NT4 was more competitive and effective in its ability to colonize and increase the growth and yield of lentil and wheat than G. mosseae NT6 or G. versiforme NT7. The relative performance of isolate NT4 with different host plants suggests that this VAMF isolate exhibits a host preference for lentil.     

Diversity patterns of arbuscular mycorrhizal fungi associated with cacao in Venezuela

The arbuscular mycorrhizal fungal (AMF) communities associated with cacao in Venezuela were studied. The species of AMF spores present in sixteen cacao plantations and in one nursery were isolated and identified when possible. The spore densities, species richness, diversity, Shannon-Wiener diversity index and dominance concentration index for the AMF communities were calculated. Acaulospora scrobiculata was associated with cacao plants in all study sites. No Scutellospora spp. were found in the analyzed soils. The spore number found in cacao plantations was relatively lower as compared with other tropical crops (38 spores 100 g^–1 soil up to 1674). Soils that were cultivated with cacao for more than 40 years showed the lowest spore numbers. Species richness and diversity of AMF communities associated with cacao, were negatively correlated with available P in soils. The Shannon-Wiener diversity index was positively correlated with soil organic matter. These results indicate that the traditional cacao cultivation practices used in Venezuela, maintain mycorrhizal infection on cacao plants. The diversity of the AMF community is similar to that found in natural undisturbed ecosystems from Venezuela.     

Mechanisms of salt tolerance in transgenic Arabidopsis thaliana constitutively overexpressing the tomato 14-3-3 protein TFT7

A hydroponics experiment was conducted to investigate the effects of iron plaque on root surfaces with respect to selenite uptake and translocation within the seedlings of two cultivars of rice (Oryza sativa L. cv Xiushui48 and Bing9652). Different amounts of iron plaque were formed by adding 0, 10, 30, 50, 70 mg Fe l⁻¹ in the nutrient solution. After 24 h of growth, the amount of iron plaque was positively correlated with the Fe²⁺ addition to the nutrient solution. These concentrations of Fe, inducing plaque, had no significant effect on the shoot and root growth of rice plants in 50 μg Se l⁻¹ nutrient solution. The amount of Se accumulated in iron plaque was positively correlated to the amount of iron plaque. Increasing iron plaque decreased the selenium concentration in shoots and in roots. At the same time, the translocation of Se from roots to shoots was reduced with increasing amounts of iron plaque. At both the shorter and longer exposure times, the ratio of root- to-shoot selenium was higher than in the controls. More Se stayed in the roots at the longer exposure time than at the shorter time. The concentration of selenium in the xylem sap was sharply decreased with increasing amount of iron plaque on the rice roots. The DCB (dithionite-citrate-bicarbonate)-extracted Se was up to 89.9–91.1% of the total Se when roots with iron plaque (Fe 70) were incubated in 50 μg Se l⁻¹ solution for 30 min. This DCB-extracted Se, however, accounted for only 21.9–28.7% of total Se when roots with iron plaque were incubated in the same solution for 3 days. Se adsorbed in iron plaque can be desorbed by low-molecular-weight organic acids, similar to the desorption of Se from ferrihydrite. These results suggest that iron plaque might act as a ‘buffer’ for Se in the rhizosphere.     

Soil calcium and plant disease in serpentine ecosystems: a test of the pathogen refuge hypothesis

Ecologists have long sought mechanistic explanations for the patterns of plant distribution and endemism associated with serpentine soils. We conducted the first empirical test of the serpentine pathogen refuge hypothesis, which posits that the low levels of calcium found in serpentine soils provide associated plants with a refuge from attack by pathogens. We measured the range of soil calcium concentrations experienced by 16 wild population of California dwarf flax (Hesperolinon californicum) and experimentally recreated part of this range in the greenhouse by soaking serpentine soils in calcium chloride solutions of varying molarity. When flax plants grown in these soils were inoculated with spores of the rust fungus Melampsora lini we found a significant negative relationship between infection rates and soil calcium concentrations. This result refutes the pathogen refuge hypothesis and suggests that serpentine plants, by virtue of their association with low calcium soils, may be highly vulnerable to attack by pathogens. This interaction between plant nutrition and disease may in part explain demographic patterns associated with serpentine plant populations and suggests scenarios for the evolution of life history traits and the distribution of genetic resistance to infection in serpentine plant communities.     

Soil application of Beauveria bassiana GHA against apple sawfly,Hoplocampa testudinea (Hymenoptera: Tenthredinidae): Field mortality and fungal persistence

Low impact alternatives to synthetic insecticides for the control of apple sawfly (Hoplocampa testudinea Klug) are scarce encumbering pest management in organic apple orchards. We investigated the soil persistence and field efficacy of the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin (BotaniGard) against apple sawfly under common organic orchard practices. We also assessed the efficacy of B. bassiana GHA and Metarhizium brunneum Petch (indigenous strain) against sawfly in the laboratory. Larvae treated with either fungus in the laboratory died faster than control larvae and displayed 49.4%–68.4% mycosis. In the field, B. bassiana density remained high in the week after application, during larval descent to the soil. Fungal density decreased to 25% at 49 d after application and to 0.4% after 55 weeks. Molecular markers revealed that the majority of fungal isolates recovered comprised the applied B. bassiana strain GHA. Larvae pupating in soil cages in the orchard for 49 d displayed 17% mycosis. The high efficacy under laboratory conditions was not seen in the field. B. bassiana application resulted in densities above the upper natural background level during the growing season, but reversion to background levels occurred within a year. It remains to be investigated whether this has a detrimental effect on nontarget organisms. Additional work is needed to bridge the knowledge gap between laboratory and field efficacy in orchards.