Effects of ammonium-N on development and infectivity of the take-all fungus

Effects of applications of a mixture of ammonium sulphate and mono-ammonium phosphate and of ammonium nitrate on the incidence of take-all disease of wheat (caused by Gaeumannomyces graminis var. tritici) and on subsequent inoculum levels were studied in field and glasshouse experiments. In a field experiment in Western Australia, on a sandy soil at pH 5·4, nitrogen applications had no detectable effect on disease severity at anthesis, but ammonium sulphate treatment increased the number of propagules of the pathogen in the soil. In a pot experiment, in which seed was sown in the field experiment soils, disease was greater in soil from plots treated with ammonium sulphate and least in soil from the nil-nitrogen plots, reflecting the respective inoculum levels in the field plot. However, treatment of the soils of lower inoculum with ammonium sulphate and ammonium nitrate during this pot experiment decreased disease. A second pot experiment confirmed the effectiveness of ammonium sulphate and ammonium nitrate in reducing take-all at lower inoculum levels, and their ineffectiveness at higher inoculum levels.     

Prospects for fungicidal control of take-all of wheat

A range of fungicides and herbicides was tested against Gaeumannomyces graminis (causal agent of take-all) on agar plates, and on wheat seedlings in pots and in liquid culture. Benomyl, the standard in all tests, was consistently most effective: like iprodione, nuarimol and KWG 0599 , it diminished infection from inoculum placed just below the seeds more effectively when applied as a drench than as a soil-mix. Benomyl as a soil-mix was most effective in soils with least organic matter. Some compounds toxic to the pathogen on agar plates and in plants grown in liquid culture were ineffective as soil treatments. The practical limitations of soil treatment with conventional fungicides and application methods are discussed.     

Soil-applied fungicides for controlling take-all in field experiments with winter wheat

Soil treatment fungicides were tested against take-all (Gaeumannomyces graminis var. tritici) in three field experiments with winter wheat. Fungicides were applied as drenches either before sowing in autumn, and incorporated by rotary harrowing, or to the crop in spring. The most effective treatments were autumn applied benomyl (20 kg/ha) and nuarimol (0·55-4·4 kg/ha). However, the highest nuarimol concentration depressed yield. Benomyl sometimes induced a resurgence of take-all in the second wheat crop after treatment. Nuarimol had no adverse effects in subsequent crops, and neither fungicide hindered the onset of take-all decline in a third crop after treatment. The possible value of soil treatment in future control strategies is discussed.     

Effects of wheat volunteers and blackgrass in set-aside following a winter wheat crop on soil infectivity and soil conduciveness to take-all

Two experiments were carried out in France in which disease indices were used to evaluate the effects of wheat volunteers and blackgrass (Alopecurus myosuroides) on soil infectivity and soil conduciveness to take-all caused by Gaeumannomyces graminis var. tritici. Soil infectivity was evaluated by measuring the disease index on susceptible wheat plants grown on soil samples collected from the field. Soil conduciveness to the disease was obtained by measuring disease indices on plants grown on soil samples to which different amounts of take-all fungus inoculum were added. One experiment (Expt. 1) was carried out using soils from farmers' fields (two fields in 1994 and two in 1995); soil infectivity and soil conduciveness were evaluated for three experimental situations: bare soil, soil with wheat volunteers and soil with blackgrass plants. In 1994 the soil infectivity was zero in bare soil, high with the wheat cover, and intermediate with the blackgrass cover. In 1995 the soil infectivity was uniformly low for all three conditions. Soils bearing wheat were less conducive than bare soil, soils bearing blackgrass and bare soils were similarly conducive. A second experiment (Expt. 2) carried out in 1995 compared the soil infectivity and soil conduciveness to take-all of soils planted with wheat or blackgrass in set-aside land after periods of wheat monoculture of 0–6 yr. The soil infectivity was low for all treatments. The soil was more conducive after blackgrass than after wheat. In both cases, the soil conduciveness was less when the monoculture had continued for more than 4 yr. The decline was less after blackgrass than after wheat. Thus, whenever set-aside is set up during the increase phase of the disease in fields with cereal successions, abundant wheat volunteers might hinder the expected positive effect of a break in cereal successions on take-all development. The presence of blackgrass in a set-aside field, with significant soil infectivity and high soil conduciveness, might increase the risks of take-all development in a wheat crop following set-aside.     

Intermittent wetting of soils at high temperature reduces survival of the take-all fungus

Two pot experiments using naturally infested soil and a range of watering regimes were conducted to study the possible effect of level and frequency of wetting of hot soil (to simulate the period between growing seasons in Western Australia) on inoculum of the take-all fungus (Gaeumannomyces graminis var.tritici). In combination with the high soil temperatures, all watering regimes reduced infectivity and propagule number of the take-all fungus, this reduction being absent in dry soils.     

Reduction of Take-all Inoculum by Rotation with Lupins, Oats or Field Peas

The feasibility of use of lupins, oats and field peas as alternative rotation crops to reduce inoculum of the take-all fungus (Gaeumannomyces graminis var. tritici) (under Western Australian field conditions) and disease in following wheat was investigated with a one year field trial, the soil from which was used in two succeeding pot experiments. The possible mechanisms of reduction of inoculum and disease by these crops were examined testing the soil for pathogen and disease suppression. Rotation with lupins or oats for two seasons reduced (P <0.05) inoculum of the take-all fungus and lupins, oats or field peas reduced (P <0.05) disease in following wheat. Lupins alone reduced inoculum and disease, (P <0.1) after one season. No apparent suppression of the pathogen in the absence of host plants was recorded after one season of rotation, but after two seasons, lupins, oats or field peas all suppressed (P <0.02) growth of the pathogen within soil. However only field pea soil suppressed take-all in comparison with the wheat control. Although after two seasons all rotation crops were effective in reducing inoculum and disease the mechanisms of reduction appear to differ between the rotation crops used in this study.     

Control of the take-all fungus byMicrodochium bolleyi,and interactions involvingM. bolleyi,Phialophora graminicola andPericonia macrospinosa on cereal roots

Summary In glasshouse experiments,Microdochium bolleyi (Mb) significantly reduced infection of wheat roots by the take-all fungus,Gaeumannomyces graminis vartritici (Ggt), when inocula were dispersed in soil at ratios of 10∶1 (Mb:Ggt) or more. Spread of take-all lesions up roots from a layer of inoculum also was reduced when Mb was inoculated immediately below the crown. In contrast,Periconia macrospinosa did not control take-all even at an inoculum ratio of 100∶1. M. bolleyi interfered with growth on roots byPhialophora graminicola, a known biocontrol agent of take-all. It is suggested that this phenomenon and control of take-all by these fungi occur by competition for cortical cells that senesce in the normal course of root development.     

Decrease of take-all by a transmissible factor in take-all decline soils

Irrespective of type and cropping history, soils with take-all decline (D soils) contained a take-all-decreasing factor that could be transmitted to cereal soils in which take-all had not declined (U soils). As little as 0.001 % by weight of D soil decreased take-all by 50 % in some seedling assays, but in a longer, outdoor test the decrease proved to be temporary and within 10 wk disease increased to equal that in unamended U soil. In two tests in which D soil was amended with U soil there was an unexpected increase in disease in seedling assays. Particulate fractions (0.2-2.0 μm) from leachates of D soils, but not the coarse fractions (> 1 mm, dry sieved; > 150μm, wet sieved) of these soils, decreased disease in assays. Although moved downwards by water, the transmissible factor in D soil did not spread laterally into adjacent unamended U soil and D soil amendments of U soil were most effective as top dressings and least effective when initially placed below assay seedlings. D soil was slightly less effective when added 14 days after planting wheat in U soil, but incubating soil mixtures for up to 4 wk before planting gave variable results, although there was always some decrease in take-all. The transmissible factor could be eliminated by heating moist soil for 30 min at 70 ^oC (but not 60 ^oC). The evidence suggests that the factor is biological, but is insufficient to suggest that it is the cause of take-all decline.     

Effects of inoculum density and substrate type on saprophytic survival of Phytophthora drechsleri, the causal agent of gummosis (crown and root rot) on pistachio in Rafsanjan, Iran

In this study, sampling was carrid out on soils around pistachio trees in various regions of Rafsanjan, Iran. Following isolation and identification of Phytophthora isolates, the predominant species was found to be P. drechsleri and used for further investigation. For studying saprophytic survival of the fungus, soils collected from different areas were combined and autoclaved. Sterile soil was divided into 10 parts and mixed with fungal inoculum at various concentrations of 0-9% (w/w) separately. Each soil part (100g) was placed in 15cm diameter plastic pot. Some soils in pots were supplemented with sterile wheat straw whereas others were mixed with pistachio leaves surface sterilized with 5% (v/v) sodium hypochlorite. After 3, 6 and 9 weeks of incubation, five leaves or straws samples were taken from each replicate and cultured on CMA-PARPH medium and the fungal colony formation was monitored. The experiment was performed using completely randomized design with factorial experiments including three factors (substrate type, inoculum density and time), 10 treatments (0-9 g inoculum levels) and nine replicates. The results showed that the type of substrate (wheat straw and pistachio leaf) was very important for the fungal saprophytic survival in that this was significantly greater for the pistachio leaves. Time was also considered another critical factor for the fungal survival. With passing incubation time, saprophytic survival of the fungus declined. Further, it was demonstrated that increasing inoculum density would result in longer survivability of P. drechsleri and maximum fungal survival on substrate was obtained when inoculum density was at 9% (w/w).     

Fungicidal control of Rhizoctonia solani in relation to soil texture, organic matter and clay minerals

In pot tests, MEMC, quintozene, captafol, carboxin, thiabendazole, carbendazim, benomyl and thiophanate-methyl used as seed treatments gave much better control of cowpea seedling rot in light-textured sandy and loamy sand soils than in heavy-textured loam and silt loam soils inoculated with Rhizoctonia solani. Disease control by chloroneb was not altered by soil texture. Amendment of sandy soil with montmorillonite reduced disease control with all fungicides, except chloroneb and carboxin; similar amendments with kaolinite decreased efficacy of MEMC and captafol. Green manuring with cluster bean reduced disease control by MEMC, captafol, benomyl and thiophanate-methyl; sunnhemp reduced efficacy of MEMC. Most fungicides gave poor disease control when farm yard manure or biogas sludge was added to soil, the sludge having the more marked effect. All the fungicides tested, except carboxin, were inactivated to different extents by humic acid extracted from farm yard manure.     

Studies on the fungicidal control of stem lesions caused by Botrytis cinerea on glasshouse tomatoes

In glasshouse pot experiments, uptake of benomyl, thiophanate-methyl and carbendazim from equivalent soil applications (in the range 0–003– 0–035 %^a-i- ^atarateof ²&4 ml/plant) gave equal levels of control of Botrytis lesions developing from inoculations of freshly exposed leaf scars on tomato stems. Spray applications of benomyl to exposed leaf scars controlled infection at concentrations down to 0025 % a.i. The effect of lower concentrations of the stem spray could be markedly enhanced either by the addition of a mineral oil (2 % Actipron) or by a prior soil application of benomyl at a low rate which on its own had little effect on lesion development. Protectant spray applications of glycophene and vinclozolin gave levels of control quite comparable to that of benomyl at equivalent concentrations. Evidence was obtained that the lesions formed at the artificially-inoculated leaf scars at the top of the stems of young pot-grown tomato plants were larger than those lower down. In spite of this, the level of disease control with soil applications of fungicides containing or generating carbendazim (MBC) was greater at the top than at the bottom, probably because of the normal migration of the fungicides and their accumulation at the extremities of the plant. In an observation trial in a commercial crop of tomatoes, benomyl applied either as five soil drenches at approximately monthly intervals, or as two drenches followed by five sprays at three-weekly intervals, or as five sprays alone gave marked reductions in plant loss and number of Botrytis stem lesions in both cvs Eurocross BB and Cudlow Cross. Those stem lesions which did develop, however, were generally as large as those on untreated control plants. Five sprays of dichlofiuanid gave similar levels of disease control. All the treatments gave apparently higher yields (statistically untested) in Eurocross BB, but less consistent responses were recorded in Cudlow Cross.     

Identification and manipulation of soil properties to improve the biological control performance of phenazine-producing Pseudomonas fluorescens

Pseudomonas fluorescens 2-79RN(10) protects wheat against take-all disease caused by Gaeumannomyces graminis var. tritici; however, the level of protection in the field varies from site to site. Identification of soil factors that exert the greatest influence on disease suppression is essential to improving biocontrol. In order to assess the relative importance of 28 soil properties on take-all suppression, seeds were treated with strain 2-79RN(10) (which produces phenazine-1-carboxylate [PCA(+)]) or a series of mutants with PCA(+) and PCA(-) phenotypes. Bacterized seeds were planted in 10 soils, representative of the wheat-growing region in the Pacific Northwest. Sixteen soil properties were correlated with disease suppression. Biocontrol activity of PCA(+) strains was positively correlated with ammonium-nitrogen, percent sand, soil pH, sodium (extractable and soluble), sulfate-sulfur, and zinc. In contrast, biocontrol was negatively correlated with cation-exchange capacity (CEC), exchangeable acidity, iron, manganese, percent clay, percent organic matter (OM), percent silt, total carbon, and total nitrogen. Principal component factor analysis of the 16 soil properties identified a three-component solution that accounted for 87 percent of the variance in disease rating (biocontrol). A model was identified with step-wise regression analysis (R(2) = 0.96; Cp statistic = 6.17) that included six key soil properties: ammonium-nitrogen, CEC, iron, percent silt, soil pH, and zinc. As predicted by our regression model, the biocontrol activity of 2-79RN(10) was improved by amending a soil low in Zn with 50 micro g of zinc-EDTA/g of soil. We then investigated the negative correlation of OM with disease suppression and found that addition of OM (as wheat straw) at rates typical of high-OM soils significantly reduced biocontrol activity of 2-79RN(10).     

Seed Treatment with Trichoderma Species for Control of Damping-off of Cowpea Caused by Macrophomina phaseolina

Cowpea seeds treated with three Trichoderma spp. at four inoculum doses, and at four exposure times in three different formulations were planted in soils amended with Macrophomina phaseolin a, and assessed for stand establishment and post-emergence damping off. The highest percentage plant stands at 21 days after planting were 66% for T. koningii and T. harzianum , and 51% for Trichoderma sp., at 6.8 ×10 7 , 2.0 ×10 10 , and 1.0 ×10 7 colony forming units (CFUs) ml -1 , respectively. Across sampling dates and irrespective of time of exposure to the formulations, the T. harzianum and T. koningii formulations resulted in significantly greater percentage plant stands than the seeds treated with a Trichoderma sp. and the controls. Seed treatment formulations with Trichoderma spp. were derived from propagule suspensions at the most effective inoculum dose in Tween 80, in suspension with cooked cassava starch as an adhesive, or in a slurry with uncooked cassava starch. At 21 days, the suspensions with Tween 80 and cooked starch resulted in significantly higher percentage plant stands than the control, while stands from seeds treated in a slurry formulation and starch solutions were not different. Seed exposure to the different formulations for 10, 20, 30, or 40 min, provided mixed results. Seeds treated with benomyl at 0.5 g a.i/50 g resulted in 95 and 100% stands for the two sets of experiments, respectively.     

Field tests of bacteria and soil-applied fungicides as control agents for take-all in winter wheat

Putative biological and chemical treatments for controlling take-all were used in each of three consecutive years at two locations where winter wheat crops were grown in naturally-infested fields. The chemical treatments more often decreased take-all than the biological treatments, but no treatment consistently and significantly decreased take-all, nor did any cause a significant increase in yield. An isolate of Bacillus cereus var. mycoides and one of B. pumilis, applied as soil drenches in autumn or spring, or in the seed furrows, were usually ineffective. Of the few significant effects on disease, half were associated with increases and half with decreases, and most occurred in April and did not persist to late June. Two strains of Pseudomonas pluorescens applied to the seed were ineffective. The fungicide benomyl, applied as a drench in autumn and spring at 20 kg/ha was ineffective, while nuarimol, applied as a drench in autumn at 2 kg/ha was sometimes effective. Nuarimol incorporated into the seed bed at 2 kg/ha was the most effective treatment. In analyses using a functional relationship model for data from treated and untreated plots 12% of 176 data sets for biological treatments, 38% of 96 data sets for chemical treatments and 81% of 16 data sets for combined treatments showed increasing efficiency of the treatment with increasing disease intensity. These findings also demonstrate an additional advantage of the experimental design, namely that treatments are tested at different disease intensity levels within fields.     

The effect of artificially inoculated antagonistic bacteria on the prevalence of take-all disease of wheat in field experiments

Bacteria from wheat field soils were screened in vitro and in glasshouse experiments for antagonism to the take-all fungus Gaeumannomyces graminis var. tritici. Field experiments to test the ability of the selected bacteria to reduce naturally occurring take-all disease gave variable results. In the most successful series the yield of spring wheat was doubled and the amount of disease reduced to half the unprotected control value. Failure to show disease control seemed to be due either to a naturally low incidence of the disease on the trial sites or to especially dry soil conditions.     

The microbiology of soils under successive wheat crops in relation to take-all disease

Abstract Thirty-eight wheat fields in southern England were sampled in an attempt to correlate the amount of take-all disease with 35 microbiological and chemical measurements of soil. There was little correlation between field take-all and pot tests to determine soil infectivity. Myxogastrids were important components of the soil population, being up to half of the amoebal population, and most soils contained dictyostelids, reticulate amoebae and myxobacteria. Amoebae, ciliates, bacteria and saprophytic fungi were recorded for all soils. pH was a major determinant of soil populations, being clearly correlated with fungal abundance and with numbers of ciliates, dictyostelids and bacteria. Principal component analysis separated dictyostelids from the other soil amoebae and again showed the importance of pH in determining soil microbial populations. Take-all was negatively correlated with soil fertility and positively related to nematodes and myxobacteria, but this was probably an effect of take-all, and represented saprophytic growth on dead roots rather than being a cause. Reticulate amoebae and dictyostelids were both correlated with low levels of take-all. This study emphasises the large number of interrelated populations of soil microorganisms which could have an effect on the severity of take-all infections.     

Comparison of fungicides for the control of Rhizoctonia solani causing datnping-off of mung bean (Phaseohis aureus)

Of 41 fungicides tested in the laboratory, copper carbonate, copper sulphate, mercuric chloride, Agrosan GN, quintozene, kasugamycin, carboxin, pyracar-bolid, carbendazim, chloroneb, benomyl, Ohric, RH 893 (2-n-octyl-4-isothiazole-3-one) and Terrazole were most inhibitory to the mycelial growth of Rhizoctonia solani on Czapek's agar plates and had EC₅₀ values of less than 1 μg a.i./ml, while copper oxychloride, Udonkor, zineb, ziram, F 319 (3-hydroxy-5-methyl isoxazole) and anilazine were much less toxic, ziram being least inhibitory with an EC₅₀ of 214 μg a.i./ml. Of 17 fungicides tested in the greenhouse as seed treatments, thiabendazole, carbendazim, benomyl, thiophanate-methyl, dichlozoline and Ohric gave 80–90% control of damping-off of mung bean seedlings. A single soil drench with thiophanate-methyl and two drenches with benomyl gave about 90% disease control, More seedlings with R. solani infection survived when thiophanate-methyl was used as a post-inoculation soil drench than when benomyl or chloroneb were used.     

The effect of carbendazim-generating fungicides with and without a mineral oil additive on tomato stem lesions caused by carbendazim-sensitive and tolerant strains of Botrytis cinerea

Isolates of Botrytis cinerea were obtained from tomatoes in several localities in the West Scotland. Some isolates grew on agar containing 100 mg/1 benomyl (carbendazim-tolerant), while others did not (carbendazim-sensitive). Pot-grown tomato plants treated with benomyl and other carbendazim-generating fungicides, applied either as sprays or soil drenches, were inoculated on the leaf scars with some of these isolates. On treated plants the carbendazim-tolerant isolates formed lesions which were about as large as those on untreated plants. Sensitive isolates formed much smaller lesions on treated plants. There was evidence that the increase in lesion size during the period 7–14 days after inoculation with a carbendazim-sensitive isolate was less on plants sprayed with benomyl or carbendazim with added mineral oil (2% Actipron) than on plants to which the fungicides alone had been applied. No such effect was recorded with thio-phanate-methyl. There was also an indication that the addition of Actipron to a benomyl spray improved the effect of the fungicide against two tolerant isolates, though there was no effect on the relative increase in lesion size during the second week after inoculation. In two tests the addition of 2% and 4% Actipron to benomyl soil drenches did not improve the level of leaf scar lesion control.     

Biological Control of Take-all Disease of Wheat Caused by Gaeumannomyces graminis var. tritici Under Field Conditions Using a Phialophora sp.

A Phialophora sp. (isolate I-52), originally isolated from soil in a wheat field exhibiting suppression of take-all disease caused by Gaeumannomyces graminis var. tritici , was tested under field conditions for its ability to suppress this disease in winter and spring wheat. I-52 was grown on a variety of autoclaved organic substrates, including oat, millet and canola seed. All of these gave significant disease control when added to the seed furrow with inoculum of the take-all fungus. W hole seed of I-52 substrate was as effective as particles < 0.5 mm in diameter. Placing I-52 in powdered form directly on to wheat seed was ineffective in controlling take-all. Rates as low as 2 g of I-52/3.3 m of row added with the seed provided some control of take-all, and nearly complete control in winter wheat was obtained using 15 g/3.3 m. The winter wheat host cultivar did not influence the degree of control of take-all by I-52.     

Identification and Manipulation of Soil Properties To Improve the Biological Control Performance of Phenazine-Producing Pseudomonas fluorescens

Pseudomonas fluorescens 2-79RN₁₀ protects wheat against take-all disease caused by Gaeumannomyces graminis var. tritici; however, the level of protection in the field varies from site to site. Identification of soil factors that exert the greatest influence on disease suppression is essential to improving biocontrol. In order to assess the relative importance of 28 soil properties on take-all suppression, seeds were treated with strain 2-79RN₁₀ (which produces phenazine-1-carboxylate [PCA⁺]) or a series of mutants with PCA⁺ and PCA⁻ phenotypes. Bacterized seeds were planted in 10 soils, representative of the wheat-growing region in the Pacific Northwest. Sixteen soil properties were correlated with disease suppression. Biocontrol activity of PCA⁺ strains was positively correlated with ammonium-nitrogen, percent sand, soil pH, sodium (extractable and soluble), sulfate-sulfur, and zinc. In contrast, biocontrol was negatively correlated with cation-exchange capacity (CEC), exchangeable acidity, iron, manganese, percent clay, percent organic matter (OM), percent silt, total carbon, and total nitrogen. Principal component factor analysis of the 16 soil properties identified a three-component solution that accounted for 87 percent of the variance in disease rating (biocontrol). A model was identified with step-wise regression analysis (R² = 0.96; Cp statistic = 6.17) that included six key soil properties: ammonium-nitrogen, CEC, iron, percent silt, soil pH, and zinc. As predicted by our regression model, the biocontrol activity of 2-79RN₁₀ was improved by amending a soil low in Zn with 50 μg of zinc-EDTA/g of soil. We then investigated the negative correlation of OM with disease suppression and found that addition of OM (as wheat straw) at rates typical of high-OM soils significantly reduced biocontrol activity of 2-79RN₁₀.