Thermal sensitivity of Phytophthora cinnamomi and long-term effectiveness of soil solarisation to control avocado root rot

Root rot caused by the fungus Phytophthora cinnamomi is a major disease of avocados worldwide. Heat sensitivity of a collection of P. cinnamomi isolates was determined by exposing agar discs containing mycelium or mycelium plus chlamydospores at various temperatures for different periods. Long‐term effectiveness of soil solarisation to control Phytophthora root rot was evaluated in two field trials. In the first, soil disinfestation by solarisation was applied in 1990 to a naturally infested plot before planting avocado (Persea americana) and viñatigo (Persea indica) seedlings. In the second trial, established avocado trees were solarised for four consecutive summers (1996–1999). Results for heat sensitivity showed that fungal mycelium was inactivated after 1–2 h at 38°C. However, 1–2 h at 40°C was needed to kill all propagules when chlamydospores were present. Fungal growth inhibition after thermal treatments was related to levels of time and temperature, and detrimental effects occurred as consequence of sublethal thermal doses. Soil solarisation presented long‐term positive effects when applied as a preplanting treatment. Five years after solarisation, disease severity (0–5 scale where 0 = healthy and 5 = dead plant) of avocado and viñatigo planted in solarised soil was 2.03 and 0.71, respectively, compared with 4.65 and 4.84 in controls. Eleven years after solarisation, the percentage of dead plants in solarised soil was 73% for avocado and 43% for viñatigo but 100% in controls. In contrast, an insufficient level of control was observed in established orchards, probably because of the lower temperature reached during solarisation under the shade of tree canopy. In this situation, maximum temperatures at 5‐cm depth were 10–13.7°C lower than under solar‐heated mulch, only exceeding 40°C in 1997.     

Soil solarisation, amendments and bio-control agents for the control of Macrophomina phaseolina and Fusarium oxysporum f.sp. cumini in aridisols

The effects of soil solarisation, residue incorporation, summer irrigation and biocontrol agents singly or in combination on survival of Macrophomina phaseolina and Fusarium oxysporum f.sp. cumini were ascertained in the 2000 and 2001 summer seasons. In amended plots, temperature increased by 2.5°C over non‐amended plots (42–51°C) at various soil depths. Combining amendments and soil solarisation elevated the soil temperatures by 0.5–5°C and 2.5–13.0°C compared to non‐amended solarised and non‐solarised plots, respectively. These treatment combinations significantly reduced M. phaseolina and Fusarium propagules compared to control. Of these, combining mustard pod residues with soil solarisation almost eliminated viable propagules of both the pathogens at 0–30 cm soil depth. However, a combination of mustard pod residue and oil‐cake (2.5 + 0.5 ton ha^?1) with only one summer irrigation also caused pronounced reduction in pathogenic propagules, which was equal to that recorded in non‐amended solarised plots. The effect of surviving propagules of M. phaseolina and Fusarium on incidence of dry root rot on clusterbean and wilt on cumin was studied in subsequent rainy and winter seasons, respectively. Significant reductions in both diseases were recorded in residue and biocontrol amended plots with or without polyethylene mulching compared to non‐amended control. The lowest plant mortality in both the crops was recorded in mustard residue amended solarised plots in a two year field experiment. However, the disease indices in the plots having a combination of mustard residues and oil‐cake amendment with one summer irrigation was equal to that achieved in the treatment having polyethylene mulching. These results suggest that in hot arid regions use of Brassica residues can be a practical and feasible substitute for polyethylene mulching in managing soil‐borne diseases.     

Effects of preplant inoculation of apple (Malus domestica Borkh.) with arbuscular mycorrhizal fungi on population growth of the root-lesion nematode,Pratylenchus penetrans

Six species of arbuscular mycorrhizal (AM) fungi (Glomus aggregatum, G. clarum, G. etunicatum, G. intraradices, G. mosseae and G. versiforme) were evaluated, in three greenhouse experiments, for their effects on reproduction of the root-lesion nematode, Pratylenchus penetrans, and growth of Ottawa 3 apple rootstock. Glomus mosseae increased total dry weights of nematode-inoculated and non-inoculated rootstock in all three greenhouse experiments, and G. intraradices increased dry weights in two of three greenhouse experiments. Plants inoculated with G. mosseae generally supported fewer P. penetrans per gram of root than plants inoculated with other AM fungi, but did not differ significantly from the controls in any greenhouse experiment. Colonization of roots by AM fungi was reduced by P. penetrans at initial inoculum densities greater than 250 nematodes/L soil. In field trials, preplant inoculation with either G. intraradices or G. mosseae increased rootstock growth and leaf concentrations of P, Mg, Zn and Cu in fumigated plots but not in non-fumigated plots, indicating that colonization by native AM fungi in non-fumigated plots may have been sufficient for adequate nutrient acquisition. The abundance of vesicles and arbuscules was greater in roots of plants inoculated with AM fungi before planting than in roots of non-inoculated plants, in both fumigated and non-fumigated plots. P. penetrans per gram of root and per 50 ml soil were significantly lower for G. mosseae- inoculated plants than for non-inoculated plants in fumigated soil but not in non-fumigated soil.     

Chemical alteration of the rhizosphere of the mycorrhizal-colonized wheat root

Plexiglass pot growth chamber experiments were conducted to evaluate the chemical alterations in the rhizosphere of mycorrhizal wheat roots after inoculation with Glomus intraradices [arbuscular mycorrhizal fungus (AMF)]. Exchange resins were used as sinks for nutrients to determine whether the inoculated plant can increase the solubility and the uptake of P and micronutrients. Treatments included: (1) soil (bulk soil); (2) AMF inoculation no P addition (I–P); (3) no inoculation with no P addition (NI–P); (4) AMF inoculation with addition of 50 mg P (kg soil)^–1 (I+P), and (5) no inoculation with addition of 50 mg P (kg soil)^–1 (NI+P). The AMF inoculum was added at a rate of four spores of G. intraradices (g soil)^–1. The exchange resin membranes were inserted vertically 5 cm apart in the middle of Plexiglass pots. Spring wheat (Triticum aestivum cv. Len) was planted in each Plexiglass pot and grown for 2 weeks in a growth chamber where water was maintained at field capacity. Rhizosphere pH and redox potential (Eh), nutrient bioavailability indices and mycorrhizal colonization were determined. Mycorrhizal inoculation increased the colonization more when P was not added, but did not increase the shoot dry weight at either P level. The rhizosphere pH was lower in the inoculated plants compared to the noninoculated plants in the absence of added P, while the Eh did not change. The decrease in pH in the rhizosphere of inoculated plants could be responsible for the increased P and Zn uptake observed with inoculation. In contrast, Mn uptake was decreased by inoculation. The resin-adsorbed P was increased by inoculation, which, along with the bioavailability index data, may indicate that mycorrhizal roots were able to increase the solubility of soil P.     

Effect of<Emphasis Type="Italic">Glomus intraradices</Emphasis> isolated from Pb-contaminated soil on Pb uptake by<Emphasis Type="Italic">Agrostis capillaris</Emphasis> is changed by its cultivation in a metal-free substrate

Development and heavy metal tolerance of two cultivation lineages of the indigenous isolate of arbuscular mycorrhizal fungus (AMF)Glomus intraradices PH5 were compared in a pot experiment in soil from lead (Pb) smelter waste deposits. One lineage was sub-cultured in original Pb-contaminated soil; the second one was maintained for 13 months in an inert substrate (river sand) without Pb stress. The contribution of these cultivation lineages to the Pb uptake and accumulation by the host plantAgrostis capillaris was investigated. The experiment was conducted in a compartmented system where the lateral compartments withAgrostis seedlings were separated from the central pot containing 4-week olderAgrostis plants by a nylon mesh for allowing out-growing of extraradical mycelium (ERM) from the pot. No differences in mycorrhizal colonization, ERM length and viability were observed between the two lineages ofG. intraradices PH5 in the soil of the isolate origin. However, the ability to support plant growth and Pb uptake differed between the lineages and also between the plants in the central pots and the lateral compartments. The growth of the plants in the central pots was positively affected by AMF inoculation. The plants inoculated with the lineage maintained in original soil showed larger shoot biomass and higher shoot P content as compared to the other inoculation treatments. The shoot Pb concentration of these plants was lower when compared to the plants inoculated with the lineage sub-cultured in the inert substrate. However the concentration did not differ from non-mycorrhizal control or from the reference isolateG. intraradices BEG75 from non-contaminated soil. Also shoot Pb contents were similar for all inoculation treatments. The development ofG. intraradices BEG75 in the contaminated soil was very poor; this isolate was not able to initiate colonization of seedlings in lateral compartments. In lateral compartments, growth of seedlings in contaminated soil was inhibited by theG. intraradices PH5 lineage maintained in the inert substrate. Pb translocation from the seedling roots to shoots was increased for plants inoculated with either lineage as compared to the non-mycorrhizal control; however, the increase for the lineage cultivated in the inert substrate was significantly higher in comparison with that maintained in the original soil. After 13 months of cultivation in a metal free substrate, theG. intraradices isolate from Pb contaminated soil did not lose its tolerance to Pb as regards colonization of plant roots and growth of ERM in the soil of its origin. However, its ability to support plant growth and to prevent Pb translocation from the roots to the shoots was decreased.     

Field microplot performance of the peach-almond hybrid GF-677 after inoculation with arbuscular mycorrhizal fungi in a replant soil infested with root-knot nematodes

The effects of arbuscular mycorrhizae (AM) on the development and nutrition of the peach almond hybrid GF-677 rootstock in a replant soil heavily infested with Meloidogyne javanica were evaluated in field microplot conditions for two growing seasons. There was a significant beneficial effect of mycorrhizal inoculation on plant growth and nutrition in previously pasteurized replant soil. In natural replant soil, early inoculation with a mixed AM inoculum of Glomus intraradices, Glomus mosseae and Glomus etunicatum did not affect growth parameters. Whilst inoculation with these AM fungi led to suppression of root-knot nematode reproduction, natural mycorrhizal colonization of the replant soil with native AM fungi did not. Accepted: 6 December 2000     

Mycorrhizal symbiosis and phosphorus fertilization effects on Zea mays growth and heavy metals uptake

Arbuscular mycorrhizal fungi (AMF) can promote plant growth and reduce plant uptake of heavy metals. Phosphorus (P) fertilization can affect this relationship. We investigated maize (Zea mays L.) uptake of heavy metals after soil AMF inoculation and P fertilization. Maize biomass, glomaline and chlorophyll contents and uptake of Fe, Mn, Zn, Cu, Cd and Pb have been determined in a soil inoculated with AMF (Glomus aggregatum, or Glomus intraradices) and treated with 30 or 60 µg P-K₂HPO₄ g^?1 soil. Consistent variations were found between the two mycorrhizal species with respect to the colonization and glomalin content. Shoot dry weight and chlorophyll content were higher with G. intraradices than with G. aggregatum inoculation. The biomass was highest with 30 µg P g^?1 soil. Shoot concentrations of Cd, Pb and Zn decreased with G. aggregatum inoculation, but that of Cd and Pb increased with G. intraradices inoculation. Addition of P fertilizers decreased Cd and Zn concentrations in the shoot. AMF with P fertilization greatly reduced maize content of heavy metals. The results provide that native AMF with a moderate application rate of P fertilizers can be exploited in polluted soils to minimize the heavy metals uptake and to increase maize growth.     

Cold-storage of mixed inoculum of Glomus intraradices enhances root colonization,phosphorus status and growth of hot pepper

Growth response of hot pepper (Capsicum annuum L.) inoculated with the arbuscular mycorrhizal (AM) fungus, Glomus intraradices Schenck and Smith was evaluated in a greenhouse study. Three treatments in a soil-based medium amended with rock phosphate were: (1) control (CON), (2) inoculation of G. intraradices as a freshly prepared soil mixture of spores, hyphae and colonized roots of Sorghum vulgare (FM), and (3) inoculation of the fungus as cold-stored mixed inoculum (CM). Colonization at 14 weeks after inoculation with CM was 42.5%, but was significantly lower with FM (14.5%). Inoculation with G. intraradices as FM and CM increased growth of pepper, and total phosphorus and nitrogen uptake in shoots and roots compared with the CON treatment. Inoculation with CM resulted in significant increases in plant dry weight and chlorophyll concentration compared to the FM and CON treatments. Acid phosphatase activity in the rhizosphere was generally increased by AM fungal treatments. Highest acid phosphatase activity occurred at 14 weeks after inoculation with CM. Alkaline phosphatase activity in the CM treatment was significantly higher compared to that in CON and FM treatments throughout the growth period. Thus, cold storage of mixed inoculum enhanced colonization and growth-promoting activity of G. intraradices compared to freshly prepared inoculum.     

Effects of Preconditioning Through Mycorrhizal Inoculation on the Control of Melon Root Rot and Vine Decline Caused by Monosporascus cannonballus

The use of inoculum of arbuscular mycorrhizal fungi (AMF) in nursery represents a promising field in horticulture because of its known benefits in terms of plant growth and bioprotection. The present work was undertaken to determine the effect of mycorrhizal inoculation with Rhizophagus irregularis in a nursery medium on the containment of melon root rot and vine decline (MRRVD) caused by the soil‐borne pathogen Monosporascus cannonballus. The percentage of mycorrhization, biomass and yield following mycorrhizal inoculation were also evaluated. Biocontrol activity was assessed in greenhouse pot experiments upon artificial inoculation of M. cannonballus and in a two‐season field experiment under production conditions in an unheated greenhouse with a history of MRRVD. On the basis of the mycorrhization parameters, the interaction appeared to be established within 30 days after inoculation. The total shoot growth in the mycorrhized plants was significantly higher when compared to the control, while the root growth was unaffected. Upon artificial inoculation of M. cannonballus, mycorrhization provided complete protection against the pathogen. Greenhouse experiments under production conditions during spring cropping season showed that pretransplanting inoculation with R. irregularis significantly decreased the severity of the disease. Also, the average fruit weight of mycorrhized plants was significantly higher than the untreated control. Nevertheless, in summer crop, the bioprotection activity of AMF failed. Present results indicate that the use of AMF in a nursery setting can contribute to the prevention of the onset of this problematic soil‐borne disease within a sustainable and integrated soil‐borne disease management.     

Establishment of Two Ectomycorrhizal Shrub Species in a Semiarid Site after in Situ Amendment with Sugar Beet, Rock Phosphate, and Aspergillus niger

A field experiment was carried out to assess the effectiveness of the addition of sugar beet, rock phosphate, and Aspergillus niger directly into the planting hole, and the mycorrhizal inoculation of seedlings with Scleroderma verrucosum, for promotion of plant growth of Cistus albidus L. and Quercus coccifera L. and enhancement of soil physicochemical, biochemical, and biological properties, in a degraded semiarid Mediterranean area. One year after planting, the available phosphorus content in the amended soils of both species was about fourfold higher than in the nonamended soil. Amendment addition increased the aggregate stability of the rhizosphere of C. albidus (by 56% with respect to control soil) while the mycorrhizal inoculation increased only the aggregate stability of the rhizosphere of Q. coccifera (by 13% with respect to control soil). Biomass C content and enzyme activities (dehydrogenase, urease, protease-BAA, acid phosphatase, and -glucosidase) of the rhizosphere of C. albidus were increased by amendment addition but not by mycorrhizal inoculation. Both treatments increased enzyme activities of the rhizosphere of Q. coccifera. The mycorrhizal inoculation of the seedlings with S. verrucosum was the most effective treatment for stimulating the growth of C. albidus (by 469% with respect to control plants) and Q. coccifera (by 74% with respect to control plants). The combined treatment, involving mycorrhizal inoculation of seedlings and addition of the amendment directly into soil, had no additive effect on the growth of either shrub species.     

Effectiveness of arbuscular mycorrhizal fungi in phytoremediation of lead- contaminated soil by vetiver grass

A greenhouse experiment was conducted to evaluate the effectiveness of arbuscular mycorrhizal (AM) fungi in phytoremediation of lead (Pb)-contaminated soil by vetiver grass. Experiment was a factorial arranged in a completely randomized design. Factors included four Pb levels (50, 200, 400, and 800 mg kg^?1) as Pb (NO₃)₂, AM fungi at three levels (non mycorrhizal (NM) control, Rhizophagus intraradices, Glomus versiforme). Shoot and root dry weights (SDW and RDW) decreased as Pb levels increased. Mycorrhizal inoculation increased SDW and RDW compared to NM control. With mycorrhizal inoculation and increasing Pb levels, Pb uptake of shoot and root increased compared to those of NM control. Root colonization increased with mycorrhizal inoculation but decreased as Pb levels increased. Phosphorus concentration and uptake in shoot of plants inoculated with AM fungi was significantly higher than NM control at 200 and 800 mg Pb kg^?1. The Fe concentration, Fe and Mn uptake of shoot in plants inoculated with Rhizophagus intraradices in all levels of Pb were significantly higher than NM control. Mycorrhizal inoculation increased Pb extraction, uptake and translocation efficiencies. Lead translocation factor decreased as Pb levels increased; however inoculation with AM fungi increased Pb translocation.     

Biological Control of Fusarium Wilt in Tomato Caused by Fusarium oxysporum f. sp. lycopersici by AMF Glomus intraradices and some Rhizobacteria

In the present study, the effects of the arbuscular mycorrhizal fungus (AMF) Glomus intraradices Schenck & Smith and four rhizobacteria (RB; 58/1 and D/2: Pseudomonas fluorescens biovar II; 17: P. putida; 21: Enterobacter cloacae), which are the important members of the rhizosphere microflora and biological control agents against plant diseases, were examined in the pathosystem of Fusarium oxysporum f. sp. lycopersici [(Sacc) Syd. et Hans] (FOL) and tomato with respect to morphological parameters (fresh and dry root weight) and phosphorous (P) concentration in the roots. Treatments with single and dual inoculation with G. intraradices and RB strains reduced disease severity by 8.6–58.6%. Individual bacteria inoculations were more effective than both the single AMF and dual (G. intraradices + RB) inoculations. In addition, the RB and G. intraradices enhanced dry root weight effectively. Significant increases in root weights were recorded particularly in the triple inoculations compared with single or dual inoculations. Compared with the non‐treated controls all biological control agents increased P‐content of treated roots of plants. Colonization with RB increased especially in triple (FOL + G. intraradices + RB) inoculations whereas colonization of G. intraradices was significantly decreased in treatment of FOL + G. intraradices compared with triple inoculations. The results suggest that suitable combinations of these biocontrol agents may ameliorate plant growth and health.     

Does the sequence of plant dominants affect mycorrhiza development in simulated succession on spoil banks?

The aim of this field study was to examine how the development of arbuscular mycorrhizal fungi (AMF) on coal mine spoil banks is affected by the presence of plants with different mycorrhizal status. A 3-year trial was conducted on the freshly created spoil bank Vršany, North-Bohemian coal basin, the Czech Republic. Three plant species – non-mycotrophic annual Atriplex sagittata, highly mycotrophic annual Tripleurospermum inodorum (both dominants of early stages of succession) and facultatively mycotrophic Arrhenatherum elatius (a perennial grass species of the later stage of succession) – were planted on 1 m² plots over 3 years in different sequences that simulated the progress of succession on spoil banks. The development of AMF populations was monitored by evaluation of mycorrhizal colonization of plant roots and by measurement of the mycorrhizal inoculation potential (MIP) of soil. These two parameters were compared between plots inoculated with the mixture of three AMF isolates – Glomus mosseae BEG95, G. claroideum BEG96 and G. intraradices BEG140 – (“inoculated plots”) and plots exposed only to natural dispersal of AMF propagules (“uninoculated plots”). Highly colonized roots of plants together with a high MIP of soil in uninoculated plots were already found at the end of the first season, indicating rapid natural dispersal of AMF propagules. Root colonization of facultatively mycotrophic and non-mycotrophic plants in later years was affected by the mycorrhizal status of the previous plant species. The MIP of soil continuously increased throughout the experiment; in uninoculated plots, the MIP was temporarily decreased if plant species of higher mycotrophy were replaced by species of lower mycotrophy. The results lead to the conclusion that AMF colonize freshly formed sites very quickly and reproduce or accumulate in the soil, which leads to increasing MIP values. However, this infective potential can be decreased if non-mycotrophic plants predominate on the site.     

Field application of vesicular-arbuscular mycorrhizal fungi improved garlic yield in disinfected soil

 The production of certified garlic propagation material requires measures to be taken against pathogenic nematodes. Methyl bromide (MB) may be used for this purpose, but is known to cause stunting in Allium spp. Vesicular-arbuscular mycorrhizal (VAM) fungal inoculum was applied to the planting furrow after MB treatment. VAM-inoculated plants were larger, had more green leaves, an increased photosynthesis rate, especially at low light intensities, and higher fresh and dry weights than plants in uninoculated plots. The mean bulb weights from uninoculated and VAM-treated plots were 27 g and 51 g respectively. The native or an improved VAM population should be reintroduced after soil disinfection to ensure satisfactory garlic yields. Accepted: 15 January 1997     

The plant growth‐promoting fungus Fusarium equiseti and the arbuscular mycorrhizal fungus Glomus mosseae stimulate plant growth and reduce severity of anthracnose and damping‐off diseases in cucumber (Cucumis sativus) seedlings

To alleviate the environmental contamination due to persistent chemical usage, approaches to integrated pest management were conceived. In this perspective, microbe–microbe interactions such as mycorrhizal relationships with other soil microbiota in the rhizosphere like the plant growth‐promoting fungi (PGPF) are particularly important. Better understanding of the interactions between beneficial microbial groups is imperative in the identification of possible synergistic or antagonistic effects to improve their practical usage as biocontrol agents or biofertilizers. In this study, the consequence of co‐inoculation of the arbuscular mycorrhizal fungus (AMF) Glomus mosseae (Gm) and the PGPF Fusarium equiseti (isolates GF18‐3 and GF19‐1) in terms of plant growth enhancement, root and rhizosphere colonisation, and development of anthracnose (Colletotrichum orbiculare) and damping‐off (Rhizoctonia solani AG‐4) diseases in cucumber plants was investigated under controlled conditions. The amendment of either GF18‐3 or GF19‐1 singly or in combination with Gm indicated a general tendency to significantly enhance the shoot dry weight (SDW) of cucumber plants at 4 weeks after planting (WAP). Similarly, Gm alone significantly enhanced SDW at 4 WAP. Gm showed a tendency to depress root colonisation by F. equiseti but such antagonistic effect was not observed in the rhizosphere soil. Both GF18‐3 and GF19‐1 significantly reduced percent root colonisation of Gm. However, these general tendencies may vary with the inoculum densities of AMF and PGPF. Both F. equiseti and Gm inoculated singly significantly increased percent of protection against anthracnose, but the combined inoculation was more effective in controlling the disease compared to single inoculation. The inoculation of the cucumber seedlings with GF18‐3, GF19‐1 or Gm, 6 or 12 days prior to damping‐off pathogen inoculation, increased percent of protection against damping‐off disease. This study shows that the co‐inoculation of F. equiseti and Gm resulted in additive effect on the suppression of anthracnose disease in cucumber.     

Responses of soil microbial catabolic diversity to arbuscular mycorrhizal inoculation and soil disinfection

Although it is usually admitted that arbuscular mycorrhizal (AM) fungi are key components in soil bio-functioning, little is known on the response of microbial functional diversity to AM inoculation. The aims of the present study were to determine the influence of Glomus intraradices inoculum densities on plant growth and soil microflora functional diversity in autoclaved soil or non-disinfected soil. Microbial diversity of soil treatments was assessed by measuring the patterns of in situ catabolic potential of microbial communities. The soil disinfection increased sorghum growth, but lowered catabolic evenness (4.8) compared to that recorded in the non-disinfected soil (6.5). G. intraradices inoculation induced a higher plant growth in the autoclaved soil than in the non-disinfected soil. This AM effect was positively related to inoculum density. Catabolic evenness and richness were positively correlated with the number of inoculated AM propagules in the autoclaved soil, but negatively correlated in the non-disinfected soil. In addition, after soil disinfection and AM inoculation, these microbial functionality indicators had higher values than in the autoclaved or in the non-disinfected soil without AM inoculation. These results are discussed in relation to the ecological influence of AM inoculation, with selected fungal strains and their associated microflora on native soil microbial activity.     

Growth enhancement of Citrus reshni after inoculation with Glomus intraradices and Trichoderma aureoviride and associated effects on microbial populations and enzyme activity in potting mixes

There have been some scientific reports suggesting that dual inoculations with arbuscular mycorrhizal (AM) and saprophytic soil fungi may cause an additive or synergistic growth enhancement of the inoculated host plant. Some Trichoderma spp. have shown antagonistic potential against pathogenic fungi and a beneficial effect on plant growth. Joint inoculations of the mycorrhizal fungus Glomus intraradices Schenck and Smith, isolated from a citrus nursery (Tarragona, Spain) and a strain of Trichoderma aureoviride Rifai, isolated from an organic compost, were tested on a citrus rootstock, Citrus reshni Hort. ex Tan. The interactions between both microorganisms and their influence on mycorrhizal root colonization and plant growth enhancement, the changes produced in the soil microbial activity, like esterase, trehalase, phosphatase and chitinase activities, and on microbial populations were evaluated in three organic substrates: (1) sphagnum peat and autoclaved sandy soil (1/1, v/v), (2) sphagnum peat, quartz sand and perlite (1/1/1, v/v) and (3) pine bark compost (BVU, Prodeasa Product). Substrate characteristics were more important than the AM inoculation treatment in the determination of enzyme activity. In bark compost, the number of bacterial colonies obtained on soil-dilution plates was significantly higher than in peat and sand mixtures. Inoculation with T. aureoviride alone produced no significant effect on growth enhancement of C. reshni. However, dual inoculation with both, T. aureoviride and G. intraradices significantly increased plant growth in two of the substrates used and was the best treatment in pine bark amended compost. The inoculation with T. aureoviride did not affect the development of mycorrhizal root colonization. These results show a synergistic effect of G. intraradices and T. aureoviride on the growth of C. reshni in organic substrates and indicate the potential benefits of using combined inoculations.     

Field response of wheat to arbuscular mycorrhizal fungi and drought stress

Mycorrhizal plants often have greater tolerance to drought than nonmycorrhizal plants. This study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi inoculation on growth, grain yield and mineral acquisition of two winter wheat (Triticum aestivum L.) cultivars grown in the field under well-watered and water-stressed conditions. Wheat seeds were planted in furrows after treatment with or without the AM fungi Glomus mosseae or G. etunicatum. Roots were sampled at four growth stages (leaf, tillering, heading and grain-filling) to quantify AM fungi. There was negligible AM fungi colonization during winter months following seeding (leaf sampling in February), when soil temperature was low. During the spring, AM fungi colonization increased gradually. Mycorrhizal colonization was higher in well-watered plants colonized with AM fungi isolates than water-stressed plants. Plants inoculated with G. etunicatum generally had higher colonization than plants colonized with G. mosseae under both soil moisture conditions. Biomass and grain yields were higher in mycorrhizal than nonmycorrhizal plots irrespective of soil moisture, and G. etunicatum inoculated plants generally had higher biomass and grain yields than those colonized by G. mosseae under either soil moisture condition. The mycorrhizal plants had higher shoot P and Fe concentrations than nonmycorrhizal plants at all samplings regardless of soil moisture conditions. The improved growth, yield and nutrient uptake in wheat plants reported here demonstrate the potential of mycorrhizal inoculation to reduce the effects of drought stress on wheat grown under field conditions in semiarid areas of the world.     

Penetration and Postinfection Development of Meloidogyne incognita on Cotton as Affected by Glomus intraradices and Phosphorus

The influence of the vesicular-arbuscular mycorrhizal fungus Glomus intraradices (Gi) and superphosphate (P) on penetration, development, and reproduction of Meloidogyne incognita (Mi) was studied on the Mi-susceptible cotton cultivar Stoneville 213 in an environmental chamber at 28 C. Plants were inoculated with Mi eggs at planting or after 28 days and destructively sampled 7, 14, 21, and 28 days after nematode inoculation. Mi penetration after 7 days was similar in all treatments at either inoculation interval. At 28 days, however, nematode numbers were least in mycorrhizal root systems and greatest in root systems grown with supplemental P. The rate of development of second-stage juveniles to ovipositing females was unaffected by Gi or P when Mi was added at planting, but was delayed in mycorrhizal root systems when Mi was added 28 days after planting. Nematode reproduction was lower in mycorrhizal than in nonmycorrhizal root systems at both Mi inoculation intervals. Nematode reproduction was stimulated by P when Mi was added at planting, but was similar to reproduction in the low P nonmycorrhizal treatment when Mi was added 28 days after planting. Eggs per female were increased by P fertility when Mi was added at planting.     

Displacement of an herbaceous plant species community by mycorrhizal and non-mycorrhizal Gmelina arborea, an exotic tree, grown in a microcosm experiment

Gmelina arborea Roxb. (Gmelina, Yemane) is a fast growing tree, native from India and considered as a potentially invasive woody plant in West Africa. Mycorrhizal inoculation of seedlings with Glomus intraradices was performed to study (1) the effect on the growth of G. arborea, (2) the impact on the catabolic diversity of soil microbial communities and (3) the influence on the structure of herbaceous plant species communities in microcosms. Treatments consisted of control plants, pre-planting fertilizer application and arbuscular mycorrhizal (AM) inoculation. After 4 months’ culture in autoclaved soil, G. arborea seedlings were either harvested for growth measurement or transferred into containers filled with the same soil but not sterilized. Other containers were kept without G. arborea seedlings. After 12 months’ further culture, effects of fertilizer amendment and AM inoculation on the growth of G. arborea seedlings were recorded. AM colonization was significantly and positively correlated with plant diversity. The substrate-induced respiration response to carboxylic acids was significantly higher in the absence of G. arborea and in the presence of G. intraradices as compared to the other treatments. The influence of AM symbiosis on plant coexistence and on allelopathic processes of invasive plants are discussed.