S. A. Omar

The impact of pesticides on arbuscular mycorrhizal and nitrogen-fixing symbioses in legumes

Effects of the pesticides Afugan, Brominal, Gramoxone, Selecron and Sumi Oil on growth, nodulation and root colonisation by arbuscular mycrrhizal (AM) fungi of the legumes cowpea (Vigna sinensis L.), common bean (Phaseolus vulgarisL.) and Lupin (Lupinus albus L.) were determined. The growth of all plants was inhibited by pesticide application, but this effect varied with the pesticide and plant species. Nodule formation was significantly inhibited in cowpea after 20 days of planting by all pesticides tested. Following the initial decrease, there was recovery from the inhibitory effects at 40 and 60 days after planting. Although the number of nodules on common bean and lupin did not differ from control at 20 days after planting, differences were evident during the later stages of plant growth. The pesticides significantly inhibited AM root colonisation and the number of spores in all legumes, but on the other hand, spore formation was stimulated in pesticide-treated cowpea 60 days after planting. The accumulation of N, P and K in pesticide-treated plants was lower than in control plants. Growth and nutrient status of the legumes varied with nodulation and AM colonisation. The results suggest that pesticides affect plant growth, Rhizobium/Bradyrhizobium and AM fungi at different stages of plant growth and effects varied with pesticide and plant species.

Microbial Populations and Enzyme Activities in Soil Treated with Pesticides

The effect of soil treatment with brominal (a herbicide) and theinsecticide selecron (the equivalent field rates and five-fold) on population counts of bacteria, actinomycetes and celluloyticfungi in soil was tested throughout 10 weeks incubation at28 °C. Also, tested their effect on four soil enzymes:cellulase, acid phosphatase, alkaline phosphatase andarylsulphatase. Bacterial and actinomycetes populations in soiltreated with the two pesticides were promoted at fieldapplication rates and inhibited at higher levels. The twopesticides significantly decreased the total number ofcellulolytic fungi and most fungal species after most incubationperiods either by one or the two used levels but the effect ofselecron was more pronounced.Cellulase activity in soil treated with brominal and selecronwas inhibited after most incubation periods. The effect of soiltreatment with the two pesticides on acid phosphatase waspromotive at field application rates after some incubationperiods but the enzyme activity was delayed at the higherapplication doses. Alkaline phosphatase activity in treated soilwas accelerated with both pesticides even at the higherapplication rates, suggesting a direct role of alkaline soil pHin increasing resistance of alkaline phosphatase to pesticides.The effect of soil treatment with pesticides on arylsulphataseactivity fluctuated between promotion and inhibition, butinhibition was predominant.

MICROBIAL POPULATIONS AND ENZYME ACTIVITY IN SOIL TREATED WITH HEAVY METALS

Effects of two concentrations (200 and 2000μg g−1 soil) of two heavy metals (copper and zinc as sulphates) applied to clay or sandy soil for 12 weeks on the total counts of fungi, bateria and actinomycetes were studied. Activities of three soil enzymes (urease, nitrate reductase and amidase) were also investigated. Application of heavy metals to the clay soil reduced the microbial populations. However, although neither heavy metal showed any significant increasing effect on microorganisms populations in clay soil samples, some stimulatory effects were noted in sandy soil. Activities of urease and nitrate reductase were inhibited by heavy metal application in both soils. Amidase activity was inhibited only with the higher application rate after some experimental periods.

Growth and Enzyme Activities of Fungi and Bacteria in Soil Salinized with Sodium Chloride

In soil salinized with different concentrations of sodium chloride, the total counts of soil fungi were significantly decreased during 11 weeks of incubation. Similarly, the total count of bacteria and actinomycetes were severely depressed by increasing salinity level above 5%. Invertase and urease activities were severely decreased as NaCl concentration increase during the incubation. Also, the effect on nitrate reductase was inhibitory with most treatments.

SURVIVAL OF RHIZOBIA/BRADYRHIZOBIA AND A ROCK-PHOSPHATE-SOLUBILIZING FUNGUS ASPERGILLUS NIGER ON VARIOUS CARRIERS FROM SOME AGRO-INDUSTRIAL WASTES AND THEIR EFFECTS ON NODULATION AND GROWTH OF FABA BEAN AND SOYBEAN

Coculturing of eight rhizobial strains (Bradyrhizobium japonicum USDA 110, USDA 3447, RCR 3407, RCR 3442, Rhizobium meliloti TAL 1372, TAL 1373, R. leguminosarum biovar viceae RCR 1001, and RCR 1044) and five rock-phosphate-solubilizing fungi (Aspergillus egyptiacus, A. flavus, A. niger, A. ochraceus, and Penicillium citrinum) was tested on yeast extract-mannitol agar. B. japonicum strains USDA 110, USDA 3447, RCR 3442, R. leguminosarum biovar viceae RCR 1001, and RCR 1044 grew successfully in cocultures with the tested fungal species in yeast extract-mannitol agar without antagonism. Among these fungi, A. niger showed the highest potentialities to solubilize rock- phosphate at all incubation temperatures examined (15, 28, 38, and 42°C), and phosphate dissolution maximized at 28°C. Survival of Rhizobium/Bradyrhizobium strains and the best rock-phosphate-solubilizing fungus (A. niger) was evaluated monthly in four carrier materials (Peat, wheat bran, sugarcane baggase, and wheat straw) at 28 and 38°C. The number of colony-forming units recovered on agar plates for all cocultured microorganisms was the highest with peat as a carrier, followed by bran and sugarcane baggase. On the other hand, survival of cocultured microorganisms strongly reduced in wheat straw. Survival of all microorganisms was the best at 28°C. Cocultures of B. japonicum USDA 3447/A. niger and R. leguminosarum RCR 1044/A. niger were the more tolerant for storage conditions whereas the number of colony-forming units in cocultures of B. japonicum USDA 110/A. niger and R. leguminosarum RCR 1001/A. niger was strongly reduced. Peat or bran inoculants of R. leguminosarum RCR 1044/A. niger and B. japonicum USDA 3447/A. niger significantly increased dry matter yield, nitrogen (N) and phosphorus (P) contents of faba bean and soybean, respectively, when grown in alkaline soil amended with rock-phosphate.

Physiological aspects of fungi isolated from root nodules of faba bean (Vicia faba L.)

The present study was made to isolate and assess some physiological characteristics of root nodule-colonizing fungi. During this study, 17 fungal species were isolated from root nodule samples taken from faba bean plants (Vicia faba L.) collected from different sites at Assiut area (Egypt). The growth of faba bean plants in pots was significantly promoted by soil inoculation with most fungi. Growth was checked in pots with inocula of Cladosporium cladosporioides, Fusarium moniliforme, F: oxysporium, F solani, Macrophominia phaseolina and Rhizoctonia solani which were added separately. All growth-promoting fungi were capable of producing cellulase, pectin lyase, polygalacturonase, protease, urease, amidase, acid phosphatase, alkaline phosphatase and arylsulfatase in growth medium supplemented with the corresponding substrates. Four fungal species, Aspergillus awamori, A. flavus, Penicillium chrysogenum and Trichoderma koningii showed the highest rates of enzyme formation. The effect of the addition of six trace elements to the growth media at 30 micromol/ml on enzyme production revealed some dependency on species, enzyme and metal ion. Cd2+, Hg2+ and Zn2+ generally inhibited enzyme activity. Cu(1+), Fe3+ and Al3+ showed a stimulatory effect. Fungicides (afugan and tilt) and herbicides (brominal and fusilade) at 50 ppm generally promoted enzyme activity, but insecticides (kelthane and fenvalerate) caused some inhibition to enzyme activities. Salinization of the growth media with NaCl strongly inhibited the enzymatic activity of all fungi at concentrations between 0.5 and 1.5%.

Biocontrol of fungal root rot diseases of crop plants by the use of rhizobia and bradyrhizobia

Twenty-oneRhizobium andBradyrhizobium strains were testedin vitro against the mycelial growth of three pathogenic fungi on solid and liquid media. All tested rhizobia and bradyrhizobia significantly suppressed the growth of the three soil-borne root-infecting fungi (Fusarium solani, Macrophominia phasolina andRhizoctonia solani) either in the absence or presence of iron. This indicates that the siderophore played a minor role in the biocontrol potential ofRhizobium andBradyrhizobium against pathogenic fungi. Pot experiments revealed that the numbers of propagules causing disease after 4 weeks of planting varied with species and host plant. The three most activeRhizobium andBradyrhizobium strains (R. leguminosarum bv.phaseoli TAL 182,B. japonicum TAL 377 andBradyrhizobium sp. (lupin) WPBS 3211 D) tested under greenhouse conditions for their ability to protect one leguminous (soybean) and two non-leguminous (sunflower and okra) seedlings from root rot caused byFusarium solani, Macrophominia phaseolina andRhizoctonia solani provided significant suppression of disease severity compared with nonbacterized control in both leguminous and non-leguminous seedlings.Bradyrhizobium sp. (lupin) WPBS 3211 D provided the lowest degree of resistance against all the tested pathogens with all host plants. *** DIRECT SUPPORT *** A00EN058 00013

Microbial populations, ammonification and nitrification in soil treated with urea and inorganic salts

Soil fertilization with urea at rates of 0.2 and 0.5 mg N/g soil was toxic for total counts of bacteria and fungi except with cellulolytic fungi where growth was promoted by addition of urea after 90-d incubation. Also, the population numbers of both bacteria and fungi were significantly decreased when soil was amended with CaCl2 and K2SO4 applied at two levels (50 and 100 μmol/g soil). Some alleviation of the toxic effect of either urea or the inorganic salts was observed when they were applied in combination. Fungal species composition was found to be affected in the treated soil. The most tolerant fungi wereAlternaria alternata, Aspergillus flavus, A. fumigatus, A. niger, A. terreus, Eurotium amstelodami, E. chevalieri, Nectria hœmatococca, Pœcilomyces variotii andStachybotrys chartarum. Other species of fungi were either sensitive to all treatments or sensitive to some and tolerant to others. Ammonium nitrogen was found to be more accumulated in soil treated with urea mixed with inorganic salts compared with soil treated only with urea whereas NO3−-N (and NO2−-N) was decreased. The overall effect of the addition of inorganic salts on total mineralized nitrogen, however was promotive. Soil pH was increased in ureatreated soil but was depressed by application of CaCl2 or K2SO4 to values lower than that of untreated soil. The results may be of agronomical interest in overcoming problems encountered with urea application as N fertilizer.

Effect of soil treatment with the organophosphorus insecticide Profenfos on the fungal flora and some microbial activities

Tests were conducted to determine the effects of Profenfos [(0-(4-bromo-2-chlorophenyl) 0-ethyl S-n-propyl-phosphorothioat] on fungal populations and some activities in soil. Profenfos (at 5.4 micrograms active ingredient/g dry soil), has a significant adverse effect on the count of total fungi after 2, 4 and 6 weeks after treatment. This effect was completely alleviated after longer incubation. Incorporation of this insecticide into the agar medium inhibited the total count of soil fungi at 6.4 and 38.4 micrograms ml-1. Initial activation followed by a decrease in CO2 output occurred in soil treated with 5.4 micrograms a.i./g. The two doses of Profenfos accelerated urease activity for 6 weeks after soil treatment, but inhibited the enzyme activity after longer periods. An inhibitory effect on nitrate reductase activity was observed with some insecticide treatments in the early stages of incubation followed by an activation in certain cases.

Wheat straw and cellulolytic fungi application increases nodulation, nodule efficiency and growth of fenugreek (Trigonella foenum-graceum L.) grown in saline soil

Abstract This study was performed to evaluate the ability of cellulolytic fungi and wheat straw incorporation to improve the nodulation, growth and nitrogen status of fenugreek grown in saline soils. NaCl addition to the growth medium at rates of 0.5 and 1% strongly decreased the enzymatic activity of the ten tested moulds. Three of these fungi, Aspergillus niger, Chaetomium globosum and Trichoderma harzianum, showed the highest enzymatic activity. The three moulds have the ability to degrade straw in the presence of NaCl and T. harzianum was the best straw degrader. Inoculating the plants with Rhizobium meliloti strain TAL1373 and cellulolytic fungi slightly promoted nodulation, growth and nitrogen accumulation when plants were grown with the addition of 0.5% NaCl when compared to plants inoculated with R. meliloti alone. However, application of wheat straw with cellulolytic fungi significantly enhanced growth, nodulation and nodule efficiency at 0.5 and 1.0% salinity. The greatest values of nodulation and growth parameters were obtained with a straw-Trichoderma harzianum combination. Cellulolytic fungi and wheat straw increased the concentration of Ca, Mg and K in the shoots and roots of plants. The increase in dry matter production and N content was mainly due to improved N2 fixation reflected by enhanced formation and growth of nodules as well as nitrogenase activity.