Control Of Soil-borne Pathogens Research Articles

Solarização para controle de Phytophthora parasitica em mudas de citros

O uso de solarização tem-se mostrado eficiente para o controle de fitopatógenos habitantes do solo. No caso de Phytophthora parasitica, agente causal de podridão de raízes em viveiros de citros (Citrus spp.), utiliza-se, normalmente, desinfestação com brometo de metila, produto altamente tóxico ao homem e à comunidade microbiana do solo. Neste trabalho, verificou-se a eficiência da solarização em substrato pré-colonizado com P. parasitica por meio de dois métodos: sacos plásticos e coletor solar. Os experimentos foram realizados no inverno e no verão. No inverno, o delineamento foi em blocos ao acaso com oito tratamentos (coletor solar por 24 h e 48 h, saco plástico por 24 h e 48 h, coletor solar por 48 h + Trichoderma spp., Trichoderma spp., testemunha à sombra inoculada e não inoculada) e quatro repetições com 15 plantas. A avaliação foi feita pelo teste de isca para recuperação do patógeno e pelo desenvolvimento das plântulas de citros após três meses do transplantio para os tubetes. No verão, os tratamentos foram um, dois, sete e 14 dias de solarização em sacos plásticos, e a avaliação foi feita apenas pelo teste de isca para recuperação do patógeno. A solarização do substrato para produção de mudas em coletor solar (tubos com 10 cm de diâmetro) por 24 h (inverno e verão) e em sacos plásticos (20 x 25 x 4 cm³) por 48 h no verão eliminou P. parasitica propiciando melhor desenvolvimento das mudas.

Biological control in greenhouse systems.

The controlled environment of greenhouses, the high value of the crops, and the limited number of registered fungicides offer a unique niche for the biological control of plant diseases. During the past ten years, over 80 biocontrol products have been marketed worldwide. A large percentage of these have been developed for greenhouse crops. Products to control soilborne pathogens such as Sclerotinia, Pythium, Rhizoctonia and Fusarium include Coniothyrium minitans, species of Gliocladium, Trichoderma, Streptomyces, and Bacillus, and nonpathogenic Fusarium. Products containing Trichoderma, Ampelomyces quisqualis, Bacillus, and Ulocladium are being developed to control the primary foliar diseases, Botrytis and powdery mildew. The development of Pseudomonas for the control of Pythium diseases in hydroponics and Pseudozyma flocculosa for the control of powdery mildew by two Canadian research programs is presented. In the future, biological control of diseases in greenhouses could predominate over chemical pesticides, in the same way that biological control of greenhouse insects predominates in the United Kingdom. The limitations in formulation, registration, and commercialization are discussed, along with suggested future research priorities.

Plant mechanisms that give defence against soilborne diseases

Little is known about factors that determine the outcome of interactions between soilborne pathogens and their hosts, despite the economic importance of many root diseases. Preformed defences, disease suppression and induced resistance mechanisms are all likely to contribute to plant protection. This overview highlights some key aspects of resistance mechanisms and experimental approaches that are relevant to the control of soilborne pathogens.

A NOTE Direct microscopy of Bacillus endospore germination in soil microcosms

Antagonistic endospore-forming Bacillus spp. offer a large potential as seed inoculants for control of soil-borne pathogens. In the soil, however, inoculated Bacillus endospores may remain dormant without germination, and plant protection can therefore be inefficient and unpredictable. A method based on direct fluorescence microscopy in soil microcosms was used to determine whether low-cost organic additives incorporated into seed coating material could stimulate endospore germination. Complex organic additives supported a high level of endospore germination of the fungal antagonist Paenibacillus polymyxa CM5-5. Skim milk is a low-cost additive that may be incorporated into seed coating material for efficient induction of Bacillus endospore germination in soil.

Combined Soil Treatments and Sequence of Application in Improving the Control of Soilborne Pathogens

ABSTRACT The effects of reduced doses of methyl bromide (MB) or metham sodium, heating, short solarization, and soil microbial activity, alone or in combination, on survival of soilborne fungal pathogens were tested in a controlled-environment system and field plots. Sublethal doses of heating or MB delayed germination of Sclerotium rolfsii sclerotia. Combining MB and heating treatments was more effective than either treatment alone in controlling S. rolfsii and Fusarium oxysporum f. sp. basilici. The application heating followed by fumigation with MB, was significantly more effective in delaying and reducing germination of S. rolfsii sclerotia and in controlling F. oxysporum f. sp. basilici than the opposite sequence. Further, incubation in soil and exposure to microbial activity of previously heated or MB-treated sclerotia increased the mortality rate, indicating a weakening effect. Similarly, incubation of chlamydospores of F. oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici in soil in the field after fumigation further reduced their survival, confirming the laboratory results. In field tests, combining MB or metham sodium at reduced doses with short solarization was more effective in controlling fungal pathogens than either treatment alone. Treatment sequence significantly affected pathogen control in the field, similar to its effect under controlled conditions. This study demonstrates a frequent synergistic effect of combining soil treatments and its potential for improving pathogen control and reducing pesticide dose, especially when an appropriate sequence was followed.

Inhibition of Pythium ultimum and Rhizoctonia solani by Shredded Leaves of Brassica Species

The U.S. Clean Air Act bans the use of methyl bromide after 2005. Consequently, the development of alternative methods for control of soilborne pathogens is imperative. One alternative is to exploit the pesticidal properties of Brassica L. species. Macerated leaves (10 g) from `Premium Crop' broccoli [B. oleracea L. (Botrytis Group)], `Charmant' cabbage [B. oleracea L. (Capitata Group)], `Michihili Jade Pagoda' Chinese cabbage [B. rapa L. (Pekinensis Group)], `Blue Scotch Curled' kale [B. oleracea L. (Acephala Group)], Indian mustard [B. juncea (L.) Czerniak, unknown cultivar] or `Florida Broadleaf' mustard [B. juncea (L.) Czerniak] were placed in 500-mL glass jars. Petri dishes with either Pythium ultimum Trow or Rhizoctonia solani Kühn plugs on potato-dextrose agar were placed over the jar mouths. Radial growth of both fungi was suppressed most by Indian mustard. Volatiles were collected by solid-phase microextraction (SPME) and analyzed by gas chromatography-mass spectrometry. Allyl isothiocyanate (AITC) comprised >90% of the volatiles measured from `Florida Broadleaf' mustard and Indian mustard whereas (Z)-3-hexenyl acetate was the predominant compound emitted by the other species. Isothiocyanates were not detected by SPME from `Premium Crop' broccoli and `Blue Scotch Curled' kale although glucosinolates were found in freeze-dried leaves of all species. When exposed to AITC standard, P. ultimum growth was partially suppressed by 1.1 μmol·L-1 (μmol AITC/headspace volume) and completely suppressed by 2.2 μmol·L-1 R. solani was partially suppressed by 1.1, 2.2, and 3.3 μmol·L-1 AITC. Use of Brassica species for control of fungal pathogens is promising; the presence of AITC in both lines of B. juncea suppressed P. ultimum and R. solani but some Brassicas were inhibitory even when isothiocyanates were not detected.

Reduced dosages of methyl bromide applied under gas-impermeable plastic films for controlling soilborne pathogens of vegetable crops

Four experimental trials were carried out at Albenga (Northern Italy) to evaluate the effectiveness of reduced dosages of methyl bromide (MB) applied under gas-impermeable films in controlling soilborne pathogens of vegetable crops. Forty and thirty g/m 2 of MB applied under different types of gas-impermeable films controlled Rhizoctonia solani on bean, lettuce and basil, Fusarium oxysporum f. sp. basilici on basil and Sclerotinia sclerotiorum on lettuce. Such treatments greatly decreased survival of artificially introduced pathogen inoculum in the soil, with Fusarium oxysporum f. sp. basilici being the most difficult pathogen to control. Gas impermeable films permitted increased retention of MB, thus resulting in higher C×T (concentration × time) product values and reduced emissions into the atmosphere, as indirectly shown by measurements carried out by using a portable Fumiscope gas detector.

622 Toxicity of Indian Mustard (Brassica juncea) and Allyl Isothio-cyanate to Masked Chaffer Beetle Larvae (Cyclocephala sp.)

The development of alternative methods for control of soilborne pathogens is imperative since the U.S. Clean Air Act bans the use of methyl bromide after 2005. One possibility is to exploit the pesticidal properties of compounds released by macerated Brassica tissues. In this study, masked chaffer beetle larvae were placed in sealed 473-mL jars with 335 g of soil amended with 1%, 2%, 4%, or 8% (g·g–1) Brassica tissue. The most prevalent volatile toxic compound of Brassica juncea (PI 458934) is allyl isothiocyanate (AITC). AITC production was measured in the jars at 0.25, 4, 8, 24, and 48 h using a solid-phase microextraction device (SPME) and gas chromatography. After 7 days, larvae mortality was determined. Control treatments included untreated soil, soil amended with 8% tomato plant tissue, soil amended with pure AITC, and untreated soil with an atmosphere of ≈20% O2 and 0% CO2 changing over 48 h to 2% O2 and 20% CO2. AITC levels were positively correlated to larvae mortality. The estimated lethal concentration for 50% kill (LC50) was 3.6 μg AITC/L soil atmosphere. AITC levels may be influenced by Brassica mass added, soil bulk density, and environmental factors including temperature and moisture. B. juncea has a high tissue AITC concentration. However, the mass of Brassica tissue required for insecticidal application against Cyclocephala sp. is also high, between 4% and 8% of soil mass. Development and selection of Brassica species that produce higher concentrations of isothiocyanate would increase the effectiveness of Brassica biofumigation as an alternative to methyl bromide for controlling soilborne insects.

Effect of Population Density of Pseudomonas fluorescens on Production of 2,4-Diacetylphloroglucinol in the Rhizosphere of Wheat

ABSTRACT The role of antibiotics in biological control of soilborne pathogens, and more generally in microbial antagonism in natural disease-suppressive soils, often has been questioned because of the indirect nature of the supporting evidence. In this study, a protocol for high pressure liquid chromatography/mass spectrometry is described that allowed specific identification and quantitation of the antibiotic 2,4-diacetylphloroglucinol (Phl) produced by naturally occurring fluorescent Pseudomonas spp. on roots of wheat grown in a soil suppressive to take-all of wheat. These results provide, for the first time, biochemical support for the conclusion of previous work that Phl-producing fluorescent Pseudomonas spp. are key components of the natural biological control that operates in take-all-suppressive soils in Washington State. This study also demonstrates that the total amount of Phl produced on roots of wheat by P. fluorescens strain Q2-87, at densities ranging from approximately 10(5) to 10(7) CFU/g of root, is proportional to its rhizosphere population density and that Phl production per population unit is a constant (0.62 ng/10(5) CFU). Thus, Phl production in the rhizosphere of wheat is strongly related to the ability of the introduced strain to colonize the roots.

Inhibition of Fungal Growth by Macerated Plant Tissues in a Closed Environment

The U.S. Clean Air Act bans the use of methyl bromide after 2001. Consequently, the development of alternative methods for control of soilborne pathogens is imperative. One alternative is to exploit the pesticidal properties of macerated tissues of Brassica spp. This study tested the potential of several Brassica spp. for control of fungal pathogens. Pythium ultimum Trow or Rhizoctonia solani Kühn plugs on potato-dextrose agar on petri dishes were sealed in 500-ml glass jars (at 22 °C) containing macerated leaves (10 g) from one of six Brassica spp. Radial growth was measured 24, 48, and 72 h after inoculation. Indian mustard (B. juncea) was the most suppressive, followed by `Florida Broadleaf' mustard (B. juncea). Volatile compounds in the jars were sampled with a solid-phase microextraction device (SPME) and identified by gas chromatography-mass spectrometry (GC-MS). Allyl isothiocyanate (AITC) comprised over 90% of the total volatiles measured from Indian mustard and `Florida Broadleaf' mustard. Isothiocyanates were detected in jars with all plants except broccoli. (Z)-3-hexenyl acetate was emitted by all plants and was the predominant volatile of `Premium Crop' broccoli (B. oleracea L. var. italica), `Michihili Jade Pagoda' Chinese cabbage (B. pekinensis), `Charmant' cabbage (B. oleracea L. var. capitata), and `Blue Scotch Curled' kale (B. oleracea L. var. viridis). To assess the influence of AITC on radial growth of P. ultimum and R. solani, AITC was added to jars to give headspace concentrations of 0.10, 0.20, and 0.30 mg·L–1 (mass of AITC per volume of headspace). Growth of both fungi was inhibited by 0.10 mg·L–1 AITC. 0.20 mg·L–1 AITC was fungicidal to P. ultimum although the highest AITC level tested (0.30 mg·L–1) did not terminate R. solani growth. These results indicate that residues from some Brassica spp. may be a viable part of a soilborne pest control strategy.

Control of soilborne pathogens of watermelon by solar heating

In field tests during September and October 1991–1992 solarization of pre-irrigated soil by covering with transparent plastic tarps was examined for effectiveness in controlling soilborne pathogens in watermelon. Progagules of Fusarium solani, Phytophthora drechsleri and Pythium aphanidermatum were greatly reduced or completely eliminated between 0 and 25 cm depth in soil covered for 30–60 days. For the two seasons combined, disease incidence was reduced to 12, 17 and 10% after solarizing periods of 30, 45 and 60 days, respectively compared to 62% for the plants in non-solarized plots.

Evaluation of Bacterial Antagonists for Reduction of Summer Patch Symptoms in Kentucky Bluegrass

Ten bacterial strains previously identified for their ability to control soilborne pathogens on agronomic crops were evaluated for their ability to suppress summer patch disease caused by Magnaporthe poae in Kentucky bluegrass (Poa pratensis L). Bacterial strains varied in the ability to inhibit the growth of M. poae in agar plate bioassays, although most strains inhibited the fungus to some degree. Three strains originally isolated from wheat, Pseudomonas fluorescens 2-79, P. fluorescens 13-79, and Bacillus subtilis D-39Sr, and two strains originally isolated from cotton, Enterobacter cloacae EcH-1 and EcCT-501, significantly reduced summer patch symptoms by between 29 and 46% compared to untreated control plants after a 5-week period in the growth chamber. The five strains did not reduce summer patch disease in field trials in 1990. However, in 1991, B. subtilis D-39Sr and E. cloacae EcH-I reduced summer patch severity by 53 and 49%, respectively, over the entire season compared to untreated control plots. In 1994, both B. subtilis D-39Sr and E. cloacae EcH-1 also reduced summer patch severity over the entire season by up to 39 and 34%, respectively, compared to the untreated control in field plots. All five bacteria that reduced summer patch in growth chamber studies were present in the rhizosphere of greenhouse/growth chamber grown turfgrass at 10 4 to 10 6 CFU/g of fresh weight sample 2 weeks after application to plants. A single sampling of field plots during 1991 and four separate samplings of field plots during 1994 indicated that introduced bacteria were present within the turf at populations above 10 3 CFU/g of sample.

John Rishbeth, 10 July 1918 - 1 June 1991

Plant pathology is about diseases of plants caused by infectious agents such as viroids, mycoplasmas, viruses, bacteria and fungi and, to a far less extent, by non-infectious agents such as aerial pollutants. Almost always, the diseases studied have been and are of economically important higher plants, and in contrast to medical and veterinary pathology, plant pathology covers all aspects of disease including control. Its scope, therefore, is unusually large, even for a biological discipline, ranging from the molecular, biochemical and physiological analyses of interactions between plants and the disease-causing agents, at sub-cellular and cellular levels through intermediate stages to the study of interactions at whole organism levels in populations of crop or wild plants. These latter were John Rishbeth's main interests whereas mine were at the other end of the scale. However, I did have an interest, albeit secondary, in biological control of plant diseases, a field in which John became an international authority. So I followed closely his work on biological control of soil-borne pathogens and also his studies of the biology and ecology of these pathogens upon which his methods of control were so firmly based. Thus I accepted with great pleasure the invitation to write about John Rishbeth despite the fact that I am far removed from forest pathology and thus, perhaps, less well placed than others to pay full tribute to his outstanding research in this field.

Changes in populations of soil microorganisms, nematodes, and enzyme activity associated with application of powdered pine bark

Evaluation of enzyme activities in combination with taxonomic analyses may help define the mechanisms involved in microbial decomposition of orgaic amendments and biological control of soilborne pathogens. In this study, powdered pine bark was added to nematode-infested soil at rates of 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 g kg−1. Total fungal populations did not differ among treatments immediately after application of pine bark. After 7 days, fungal populations were positively correlated with increasing levels of pine bark. This increase was sustained through 14 and 21 days.Penicillium chrysogenum andPaecilomves variotii were the predominant fungal species isolated from soil amended with pine bark. Total bacterial populations did not change with addition of pine bark at 0, 7, and 14 days after treatment. At 21 and 63 days, total bacterial populations declined in soil receiving the highest rates of pine bark. Addition of pine bark powder to soil caused a shift in predominant bacterial genera fromBacillus spp. in nonamended soil, toPseudomonas spp. in amended soil. Soil enzyme activities were positively correlated with pine bark rate at all sampling times. Trehalase activity was positively correlated with total fungal populations and with predominant fungal species, but was not related to bacterial populations. The number of non-parasitic (non-stylet bearing) nematodes andMeloidogyne arenaria in soil and roots were not correlated with pine bark rate. However,Heterodera glycines juveniles in roots, and the number of cysts g−1 root, declined with increasing levels of pine bark.

Replacing pesticides with nonchemical tools for the control of soilborne pathogens — A realistic goal?

Replacing pesticides with nonchemical tools for the control of soilborne pathogens — A realistic goal?

Delayed and enhanced biodegradation of soil‐applied diphenamid, carbendazim, and aldicarb

Recent studies have demonstrated that interaction between various agrochemicals and soil microorganisms may either slow down or enhance processes of degradation. Soil disinfestation is employed for the control of soil-borne pathogens and weeds. Soil application of such broad-spectrum biocides, as well as some more selective chemicals, has a strong effect on microbial activity, which may result in drastic reduction in the rate of degradation of pesticides applied to such treated soils. Application of pesticides to previously disinfested soils may extend their biological activity, which in the case of herbicides could cause phytotoxic damage to the next crop. In contrast, repeated application of the same or structurally related pesticides may result in a selective buildup of microbial populations capable of degrading the pesticide at much faster rates. Cases of accelerated degradation were reported for pesticides belonging to various chemical groups. Studies were conducted to evaluate the mechanisms of accelerated degradation. For several pesticides it has been shown that soil fungi are involved in their normal degradation, but not in their accelerated degradation. The shift in the rate of degradation of pesticides such as diphenamid, benomyl, and S-ethyl dipropylthiocarbamate, in soils that have acquired accelerated degradation, seems to be associated with the buildup of populations of bacterial degraders. Moreover, it has been shown that for the herbicide diphenamide, accelerated degradation is apparently linked to the induction of an oxidative demethylation process in soil bacteria, which might be analogous to the development of resistance in pests. Contrary to studies demonstrating accelerated degradation of the systemic insecticide aldicarb and accumulation of aldicarb sulfoxide in non-history soils, our work has shown that repeated application of this carbamate at several locations in Israel over a 10-year period did not induce accelerated degradation. It should be pointed out that in the Israeli soils there was only minimal formation of aldicarb sulfoxide. These studies were performed in soils with a pH ranging from 7.8 to 8.3, which is higher than the pH reported for soils where accelerated degradation was detected.

INFLUENCE OF TRICHODERMA ON THE GROWTH OF BEDDING PLANTS

Trichoderm a spp. are currently being investigated for biological control of soil-borne pathogens and their potential to enhance plant growth and development. The influence of T. harzianum and T. hamatum on growth of 7 bedding plant species was Investigated. Trichoderm a formulated in peat moss and wheat bran, was mixed into germination and growing media at 1 × 106 cfu per gram of medium. Seeds were germinated in plugs and later grown in cellpacks containing a treated and non-treated medium until market stage. Plants were evaluated by measuring height, fresh and dry weight, and number and timing of flowering. Growth enhancement was found in marigold (14.8% dw), petunia (15.5% dw) and tomato (38.2% dw), however, no significant differences were seen in celosia, impatiens, salvi a and vinca. Results suggest that growth enhancement by Trichoderm a is species dependent and that Trichoderm a applied in the plug mix remains-effective through marketing stage.

Biological Control of Soilborne Plant Pathogens in the Rhizosphere with Bacteria

Biological control of soilborne pathogens by introduced microorganisms has been studied for over 65 years (9, 49), but during most of that time it has not been considered commercially feasible. Since about 1 965, however, interest and research in this area have increased steadily (9), as reflected by the number of books (10, 47,49, 152) and reviews about it (11,26,30, 106, 143, 153, 173, 174, 183) that have appeared . Concurrently, there has been a shift to the opinion that biological control can have an important role in agriculture in the future, and it is encouraging that several companies now have programs to develop biocontrol agents as commercial products. This renewed interest in biocontrol is in part a response to public concern about hazards associated with chemical pesticides. Microorganisms that can grow in the rhizosphere are ideal for use as biocontrol agents, since the rhizosphere provides the front-line defense for roots against attack by pathogens. Pathogens encounter antagonism from rhizosphere microorganisms before and during primary infection and also during secondary spread on the root. In some soils described as microbiologi­ cally suppressive to pathogens (172), microbial antagonism of the pathogen is especially great, leading to substantial disease control. Although pathogen­ suppressive soils are rare, those identified are excellent examples of the full potential of biological control of soilborne pathogens.

Biocontrol of soil-borne plant pathogens with introduced inocula

Despite more than 60 years of research on biocontrol of plant pathogens, introduced inocula of only two control agents are used widely and successfully against soil-borne or root-infecting pathogens in current commercial practice. Several others are undergoing exploratory commercial development or are used on a limited or local commercial scale. This review adopts a critical approach to the strategies for control of soil-borne pathogens with applied antagonists, and identifies some areas in which rapid developments could occur. In most instances it will be necessary to combine the use of microbial inocula with management practices designed to minimize disease losses. Also, in most instances, biocontrol strategies should be targeted against small pathogen populations, to prevent or delay the build up of disease, rather than to control existing high levels of pathogens. Natural, rapid senescence of cereal root cortices, which is influenced by both genetic and environmental factors, offers prospects for developing the use of weak parasites for control of take-all disease, caused by Gaeumannomyces graminis var. tritici . Recent studies on biocontrol of take-all by the fungus Microdochium bolleyi are presented, and special emphasis is given to the strategies most likely to be of value for take-all control.

Solar Heating (Solarization) of Soil for Control of Soilborne Pests

Soilborne pathogens cause heavy losses to most agricultural crops. Re­ peated plantings of a crop in the same plot of land, which is usual with valuable and successful crops, sooner or later results in a high inoculum buildup, forcing the farmer to change either the crop or the land. Thus, effective control of soilborne pathogens increases not only yield and quality, but available lands too, by prolonging their use for crop production. Patho­ gen control is often hampered by the fact that both the inocula of the pathogens and the lethal agents applied to the soil are affected by the soil's physical, chemical, and biological environment. However, the persistence of these organisms in the soil may serve as a clue for controlling them by interrupting their life cycle. In Israel, extension workers and growers suggested that the intensive heating that occurs in mulched soil might be used for disease control. By mulching the soil with transparent polyethylene sheets in the hot season prior to planting, a team of Israeli workers developed a solar heating approach for soil disinfestation (40). The solar heating of soil is a soil disinfestation method for disease control. As such, it aims to eradicate or reduce, prior to planting, using drastic means, the inoculum existing in the soil. The final goal is to achieve an economic reduction in disease incidence for at least one season. Many studies using this approach to disease and weed control have been carried out in Israel (15, 18, 26, 27, 32-34, 39-42, 48-50, 69, 70, 81) and