Major Soil-borne Pathogen Research Articles

Piriformospora indica modifies cucumber’s tolerance to Meloidogyne incognita by regulating various agro-physiological traits, antioxidant enzymes, and abscisic acid pathway genes

Root knot nematode (RKN), Meloidogyne incognita, is considered a major soil-borne pathogen that can cause severe yield losses for vegetables and diverse crops. Usually, reducing of M. incognita damage is mainly relies on the application of nematicides and good agricultural practices. However, the use of synthetic nematicides is restricted due to concerns about their impact on the environment and human health. As a result, the use of alternative strategies is becoming necessary to combat RKN resistance. This study evaluates the antagonistic impact of the root mutualistic fungus Pirforomospora indica on M. incognita. It also assesses its influence on the nutritional status, photo-synthesis, antioxidant enzyme activity, endogenous abscisic acid (ABA) levels, and selected ABA related-responsive genes in cucumber plants. Roots of cucumber seedlings were inoculated with P. indica and the second-stage juveniles (1000 J2 per plant). The results demonstrated that P. indica significantly reduced M. incognita invasion in roots, resulting in a 24% reduction in root galling and 42.6% decline in final population. Inoculating plants with both P. indica and RKN increased performance of root fresh and dry weight, as well as improved photochemical efficiency of PSII (Fv/Fm), photosystem II efficiency (PSII), catalase (CAT), peroxidase (POD), and superoxide dis-mutase (SOD). Furthermore, P. indica colonization, either alone or in combination with M. incognita, significantly improved number of fruits per plant, average fruit weight, the plant's marketable yield, and leaf nutrient content (N, P, K and Mg), Moreover, there was an increase in IAA content combined with a decrease in ABA content in roots of dual inoculation plants, if compared to M. incognita infested plants. The highest ABA content was recorded in the root of RKN-cucumber plants. The decline in ABA content due to P. indica treatment was consistent with the modulation of ABA pathway genes, specifically PP2C, PYL1, RK2,1, and RK2,2. The mixed of P. indica and M. incognita led to a decrease in the expression of PP2C, PLY1, RK2,1, and RK2,2 in comparison to the control group. These results indicate that P. indica application could help reduce the negative effects of RKN on important crops.

Abiotic stress tolerance and antifungal activities of rhizobacteria for the management of soil-borne pathogens

Cotton production is negatively affected by both biotic (diseases and insects) and abiotic (high temperature, salinity, water deficit, and extreme pH) factors. Soil-borne diseases, especially wilts and rots, significantly reduce cotton yield. Thus, we aimed to isolate and identify multi-stress tolerant bacterial antagonistic agents (AGAs) against two major soil-borne pathogens, Macrophomina phaseolina and Fusarium oxysporum. A total of 132 isolates with distinct morphologies were recovered from 25 different rhizospheric soil samples of cotton. A dual culture plate and broth assay confirmed the antagonistic activity of the isolates against these phytopathogens. Four selected AGAs thrived in salt stress induced by different NaCl concentrations, up to 1.71 M, except for isolate 62, which survived up to 0.85 M. Under osmotic stress, all the AGAs were tolerant of up to −1.03 MPa. Similarly, all the AGAs were able to survive over a temperature range of 20–50 °C except for isolate 62, which survived up to 45 °C and was regarded as thermotolerant. All four AGAs were able to grow at pH values ranging from 5 to 9. AGA 18 and S46-7 survived under highly acidic conditions (pH 4). These multi-stress tolerant AGAs also exhibited different plant growth-promoting activities, such as mineral solubilization, ACC-deaminase production, and IAA production. Molecular identification revealed the following AGAs: Bacillus siamensis SSVP1 (18), Bacillus halotolerans SSVP2 (34), Pseudomonas aeruginosa SSVP3 (62), and Bacillus tequilensis SSVP4 (S46-7). AGAs with multiple stress tolerance traits can serve as potential biocontrol agents in the field to reduce pesticide consumption in cotton-growing areas.

Effect of Sequential Crop Termination and Bed Fumigation on Verticillium dahliae Soil and Plant Density in Strawberry

ABSTRACT Verticillium dahliae, the causal agent of Verticillium wilt of strawberry, is a major soilborne pathogen in California strawberry production. Recent shifts in disease management have necessitated alternative methods for disease control. Two field experiments were conducted to examine the efficacy of sequential fumigant applications for crop termination and bed fumigation to reduce Verticillium dahliae inoculum density in a naturally infested field. Fumigant efficacy was evaluated for crop termination and for reducing pathogen survival in host tissue. Crop termination was conducted using metam potassium and metam sodium. Bed fumigation was conducted using both metam products and chloropicrin. For plots that were subject to both crop termination and bed fumigation, inoculum levels were somewhat reduced (3–6 CFU/g of soil) but not to a level below the disease threshold for strawberry (2 CFU/g of soil). Plots subjected to crop termination with metam potassium and bed fumigation with chloropicrin resulted in the lowest V. dahliae inoculum density. Injury due to crop termination ranged from 53% to 77% at 7 days after fumigation (DAF) and 73–89% at 14 DAF. The odds ratio of V. dahliae survival in the terminated crop were lower than in crops not terminated and V. dahliae survival was less likely at 14, compared to 7 DAF. In the crop terminated with metam potassium, the odds ratio of pathogen survival in the petiole versus the crown was 0.9–1.4. This research shows that crop termination could constitute management strategies for Verticillium wilt of strawberry.

Survey of Soilborne Pathogens Infecting Strawberry in Santa Maria, California

ABSTRACTTo determine the prevalence of the four major soilborne pathogens of strawberry in the Santa Maria growing district in California, symptomatic plant samples (n = 100) were collected from 68 fields between May and September 2022. Samples comprised 28.3% of 2021 fall-planted strawberry fields and 14.0% of 2022 summer-planted strawberry fields in the district. Recombinase polymerase amplification was used to detect Macrophomina phaseolina, Fusarium oxysporum f. sp. fragariae, Verticillium dahliae, and Phytophthora spp. in plant crown material. Macrophomina phaseolina was most prevalent, detected in 52 samples, followed by Verticillium dahliae in 17 samples, F. oxysporum f. sp. fragariae in 16 samples, and Phytophthora spp. in 14 samples. Macrophomina phaseolina was more likely in eastern field locations (P = .0159) and in instances where strawberries were planted consecutively (P = .0100). Disease incidence was more likely to be higher in samples from organic fields than conventional fields (P = .0304) and in samples from flat-fumigated fields than fields fumigated in-line (P = .0124).

Evaluation of Wheat Cultivars and Germplasm Lines for Resistance to Pratylenchus neglectus Populations Collected in North Dakota.

Root-lesion nematode (RLN; Pratylenchus neglectus) is a migratory endoparasite and a major soilborne pathogen that affects wheat (Triticum spp.) production worldwide. Genetic resistance is one of the most economical and effective ways to manage P. neglectus in wheat. This study evaluated 37 local cultivars and germplasm lines in seven greenhouse experiments, including 26 hexaploid wheat, six durum wheat, two synthetic hexaploid wheat, one emmer wheat, and two triticale for P. neglectus resistance from 2016 to 2020. North Dakota field soils infested with two RLN populations (350 to 1,125 nematodes per kilogram of soil) were used for resistance screening under controlled greenhouse conditions. The final nematode population density for each cultivar and line was counted under the microscope to categorize the resistance ranking of these entries as resistant, moderately resistant, moderately susceptible, and susceptible. Out of the 37 cultivars and lines, one was classified as resistant (Brennan); 18 were moderately resistant (Divide, Carpio, Prosper, Advance, Alkabo, SY Soren, Barlow, Bolles, Select, Faller, Briggs, WB Mayville, SY Ingmar, W7984, PI 626573, Ben, Grandin, and Villax St. Jose); 11 were moderately susceptible; and seven were susceptible to P. neglectus. The resistant to moderately resistant lines identified in this study could be used in breeding programs after the resistance genes or loci are further elucidated. This research provides valuable information about P. neglectus resistance among wheat and triticale cultivars used in the Upper Midwest region of the United States.

Major Soilborne Pathogens of Field Processing Tomatoes and Management Strategies.

Globally, tomato is the second most cultivated vegetable crop next to potato, preferentially grown in temperate climates. Processing tomatoes are generally produced in field conditions, in which soilborne pathogens have serious impacts on tomato yield and quality by causing diseases of the tomato root system. Major processing tomato-producing countries have documented soilborne diseases caused by a variety of pathogens including bacteria, fungi, nematodes, and oomycetes, which are of economic importance and may threaten food security. Recent field surveys in the Australian processing tomato industry showed that plant growth and yield were significantly affected by soilborne pathogens, especially Fusarium oxysporum and Pythium species. Globally, different management methods have been used to control diseases such as the use of resistant tomato cultivars, the application of fungicides, and biological control. Among these methods, biocontrol has received increasing attention due to its high efficiency, target-specificity, sustainability and public acceptance. The application of biocontrol is a mix of different strategies, such as applying antagonistic microorganisms to the field, and using the beneficial metabolites synthesized by these microorganisms. This review provides a broad review of the major soilborne fungal/oomycete pathogens of the field processing tomato industry affecting major global producers, the traditional and biological management practices for the control of the pathogens, and the various strategies of the biological control for tomato soilborne diseases. The advantages and disadvantages of the management strategies are discussed, and highlighted is the importance of biological control in managing the diseases in field processing tomatoes under the pressure of global climate change.

Survey of Late-Season Soilborne Pathogens Infecting Strawberry in Watsonville-Salinas, California

There are four major soilborne pathogens of strawberries in California, but their distribution and prevalence in the Watsonville-Salinas production district are unknown. To fill this knowledge gap, 74 symptomatic strawberry plant samples were collected from 69 fields in the Watsonville-Salinas growing district between 11 August and 15 October 2021. Each sample consisted of eight plants exhibiting moderate to severe plant collapse. Crown tissue from each plant was excised and pooled for recombinase polymerase amplification (RPA) to detect Macrophomina phaseolina, Fusarium oxysporum f. sp. fragariae, Verticillium dahliae, and Phytophthora spp. Root, petiole, and crown tissue from plant samples in which no pathogens were detected by RPA was plated on semiselective media to verify the absence of the four pathogens and screen for other pathogenic fungi. At least one of the four pathogens was detected in 55 of the 74 samples (74.3%). All four of the major soilborne pathogens are prevalent in this growing district, as F. oxysporum f. sp. fragariae, M. phaseolina, Phytophthora spp., and V. dahliae were detected in 23 samples (31.1%), 22 samples (29.7%), 18 samples (24.3%), and 16 samples (22.0%), respectively. No strong associations were found between the pathogens and growing practices.

Characterization of Actinobacterial Strains as Potential Biocontrol Agents against Macrophomina phaseolina and Rhizoctonia solani, the Main Soil-Borne Pathogens of Phaseolus vulgaris in Cuba.

Macrophomina phaseolina and Rhizoctonia solani are considered two major soil-borne pathogens of Phaseolus vulgaris in Cuba. Their management is difficult, not only due to their intrinsic biology as soil-borne pathogens, but also because the lack of active ingredients available against these pathogens. Actinobacteria, a heterogeneous bacterial group traditionally known as actinomycetes have been reported as promising biological control agents (BCAs) in crop protection. Thus, the main objective of this study was to evaluate the effectiveness of 60 actinobacterial strains as BCAs against M. phaseolina and R. solani in vitro by dual culture assays. The most effective strains were characterized according to their cellulolytic, chitinolytic and proteolytic extracellular enzymatic activity, as well as by their morphological and biochemical characters in vitro. Forty and 25 out of the 60 actinobacteria strains inhibited the mycelial growth of M. phaseolina and R. solani, respectively, and 18 of them showed a common effect against both pathogens. Significant differences were observed on their enzymatic and biochemical activity. The morphological and biochemical characters allow us to identify all our strains as species belonging to the genus Streptomyces. Streptomyces strains CBQ-EA-2 and CBQ-B-8 showed the highest effectiveness in vitro. Finally, the effect of seed treatments by both strains was also evaluated against M. phaseolina and R. solani infections in P. vulgaris cv. Quivicán seedlings. Treatments combining the two Streptomyces strains (CBQ-EA-2 + CBQ-B-8) were able to reduce significantly the disease severity for both pathogen infections in comparison with the non-treated and inoculated control. Moreover, they showed similar effect than that observed for Trichoderma harzianum A-34 and with Celest® Top 312 FS (Syngenta®; Basilea, Switzerland) treatments, which were included for comparative purposes.

Effect of Setophoma terrestris, Sclerotium cepivorum, and Trichoderma spp. on in vitro onion (Allium cepa) root tissues and the final yield at the field

Trichoderma is a rhizosphere fungus widely used in agriculture due to the variety of mechanisms of biological control. It can establish a direct relationship with the plant root cells, modifying the morphology and physiological processes, conferring a better defensive capacity against the attack of pathogens in the soil. This research aimed to study the interaction of T. asperellum, T. harzianum, T. virens, Setophoma terrestris and Sclerotium cepivorum on onion roots (Allium cepa), both in vitro and in field trials, to evaluate the histological modifications and the effect on in vitro growth promotion, and to test the effect of Trichoderma in the field, over the incidence of these pathogens and the crops harvest. In vitro plant promotion assay was made using T. asperellum, T. harzianum, and T. virens to test their effect on the development of onion seedlings from disinfected seeds. Roots of these plants were subjected to histological analysis using Transmission Electronic Microscopy (TEM) to examine changes in cell structure. This analysis also included the pathogens S. cepivorum and S. terrestris, the major soilborne pathogens of onion worldwide. To verify the effect of the Trichoderma species used in the study, a field experiments were performed where the fresh and dry weight of onion bulbs and the incidence of pathogens were measured. Histological modifications were observed in the root cells in the different treatments and were related to the effects caused for Trichoderma. It was shown that although T. asperellum did not stimulate in vitro root growth it can have an important effect in the field by reducing the incidence of S. cepivorum and S. terrestris while improving the onion’s harvest. On the contrary, species that have a root promoting effect do not necessarily improved yield. Besides, rather than this study, there are no other histological studies published in the onion- S. terrestris pathosystem.

Etiology, epidemiology, and management of white rot on onion and garlic: current knowledge and future directions for Brazil

Stromatinia cepivora (= Sclerotium cepivorum ), causal agent of the white rot, is a major soilborne pathogen that attacks garlic ( Allium sativum L.), onion ( Allium cepa L.), and other plants of the Alliacea family. The pathogen is difficult to control because it survives as sclerotia for decades in soil. White rot can cause total crop losses when sclerotia levels are high in soil and environmental conditions favorable for disease development. Aspects of the biology and epidemiology of S . cepivora have been investigated extensively worldwide. These studies have pro­vided essential information to develop different control strategies. Currently, white rot management is based pri­marily on the application of fungicides and biocontrol agents to protect the crop against infection, and the use of natural and synthetic germination stimulants of sclerotia, soil fumigation and solarization to reduce sclerotia den­sity in soils. In Brazil, few studies have been conducted to understand white rot epidemiology and effectiveness of control measures currently available, despite the disease being an economically important in garlic and onion production regions for many years. This review provides updated information on the biology of S. cepivora , and epidemiology and control of white rot to identify important knowledge gaps and future research directions for white rot in Brazil.

Challenges and Prospects for Building Resilient Disease Management Strategies and Tactics for the New York Table Beet Industry

The New York table beet industry is expanding and has unique challenges to minimize crop loss in both conventional and organic production. Diseases may reduce plant population density and increase heterogeneity in a stand, reduce the duration of time foliage is healthy, and decrease the yield of marketable roots. Rhizoctonia solani Kuhn and Pythiumultimum Trow are dominant in the pathogen complex affecting crop stand and root health. Cercospora leaf spot (CLS) caused by the fungus, Cercospora beticola Sacc., is a highly destructive disease affecting foliar health. In conventional table beet production, fungicides are applied in-furrow and at emergence for early season and root disease control, and applied to foliage periodically thereafter for foliar disease control. Resistance within C. beticola populations to single-site mode-of-action fungicides poses the most significant threat to the resilience of conventional disease management. An integrated approach to reduce pesticide application when not economically warranted (i.e., a false positive) is urgently required. For foliar disease, improved scheduling of fungicides may reduce usage without loss of disease control. For soilborne diseases, pre-plant quantification of soilborne inoculum may support the selection of fields with lower inoculum densities to minimize risk of early season and root disease. For organic production, some approved products have moderate efficacy for foliar disease control, but strategies to reduce inoculum and select fields at lowest risk of disease will be paramount. Crop rotation has shown promise for disease management, but broad host range of several of the major soilborne pathogens limits the utility of this method in the production region. Enhanced knowledge of cultivar susceptibility to local populations of fungal pathogens responsible for foliar and root diseases is paramount, and adoption of commercially acceptable cultivars with improved resistance to CLS and Rhizoctonia crown and root rot has potential to transform disease management strategies for the New York table beet industry.

Rhizosphere actinobacteria for combating Phytophthora capsici and Sclerotium rolfsii, the major soil borne pathogens of black pepper (Piper nigrum L.)

The experiment was conducted with an objective to survey potential black pepper growing tracts of Kerala and Karnataka for the isolation and characterization of rhizosphere actinobacterial strains for exploiting its antagonistic potential against major pathogens of black pepper as well as for growth promotion. Accordingly fifty actinobacterial strains were isolated and were morphologically characterized and studied for its antagonism against major soil borne pathogens of black pepper viz., Phytophthora capsici and Sclerotium rolfsii. Three isolates (IISRBPAct1, IISRBPAct25 and IISRBPAct42) showed more than 90% inhibition against the targeted pathogens. The isolate IISRBPAct1 showed 91–94% inhibition to both of the pathogens followed by IISRBPAct42 (68–94%) and IISRBPAct25 (86–90%). The potential isolates were characterized morphologically using light microscopy and Scanning Electron Microscopy (SEM). Molecular characterization was done by 16S-rDNA sequencing using two sets of actinomycetes specific primers viz., 1) S-C-Act-235-S-20 and S-C-Act-878-A-19 and 2) 27f and1525r were identified as belonging to Streptomyces sp. The isolates exhibited production of different hydrolytic enzymes such as amylases, proteases, lipases, and cellulases. Further the isolates were evaluated for their Plant Growth Promoting (PGP) traits and biocontrol traits such as production of Indole Acetic Acid (IAA) and siderophores. IISRBPAct1 showed production of both IAA and siderophore while IISRBPAct25 and IISRBPAct42 produced only siderophore. In planta experiment was conducted to evaluate the growth promotion activity as well as pathogen suppression. Out of the three potential Streptomyces spp IISRBPAct1 showed maximum growth promotion in terms of shoot biomass, shoot height and number of laterals where as maximum root biomass was observed with IISRBPAct42. Highest reduction of disease incidence was observed on treatment with IISRBPAct1 (98.10%) against Sclerotium rolfsii while IISRBPAct25 showed highest reduction of foot rot incidence (80.73%). The results of the study clearly revealed the biocontrol and PGPR properties of three Streptomyces sp. which can be developed as potential candidates for the biological control of major black pepper pathogens.

Association of Neonectria macrodidyma with Dry Root Rot of Citrus in California

During yearly surveys that started in 2010 to identify pathogens associated with dry root rot disease of citrus in California, samples with root rot symptoms were collected in Tulare County. Small pieces of tissue from root samples were plated onto potato dextrose agar amended with 0.01% tetracycline and incubated at 25°C. Pure cultures of fungal isolates were identified by morphology and sequence analysis of the Internal Transcribed Spacer and Beta Tubulin regions. Neonectria macrodidyma (Cylindrocarpon macrodidymum) was first recovered in 2011 and has subsequently been recovered multiple times from citrus samples. The pathogen appeared to be widely distributed in association with citrus dry root rot and possibly interacts with Fusarium solani, Phytophthora nicotianae and P. citrophthora, the major soil borne pathogens that were frequently identified from plant samples with root and/or crown rot of citrus in California. The fungal genus Cylindrocarpon (Teleomorph: Neonectria wolenw=Dactylonect ria=Ilyonectria) contains ubiquitous soil borne pathogens that cause black foot disease on a wide range of hosts, including grapevine, strawberry, apple, and conifers. Hosts typically become infected through natural wounds on roots and other below ground parts. In this report, we present strain UCR3312, which is the most recently isolated pathogenic strain in 2015. Considering the potential damage that this organism may cause to the citrus industry, detailed studies are recommended to better understand its distribution, epidemiology, and the general pathogen biology to improve the disease management practices.

EFFECTS OF FARMING SYSTEM ON ROOT-ZONE FUNGAL POPULATIONS IN WHEAT

Effects of farming system on the composition of the root-zone microbiota and on the occurrence of root and stem-base diseases in winter wheat cv. Rywalka were analysed. The wheat was grown in organic, integrated and conventional systems of production and in monoculture over three years. Using classical methods, saprotrophic and pathogenic microorganisms, mainly fungi, were isolated from the root zone and identified on the basis of morphology and, where possible, designated as potential pathogens or antagonists. In roots, rhizoplane, and rhizosphere and non-rhizosphere soil the total frequencies of dominants (with frequency >5%) was 80.7- 98.7% and subdominants (with frequency 1–5%) was 0.5-14.5% and were similar in all production systems. Potential major soil-borne pathogens included Fusarium + Haematonectria. Potential pathogen antagonists included Chaetomium, Clonostachys, Gliomastix, Sarocladium and Trichoderma species. The low ratio of fungal pathogens : antagonists (Fusarium + Haematonectria : Clonostachys + Trichoderma spp.) in soil associated with lowest incidence of root and stem-base diseases, lower occurrence of Fusarium and Rhizoctonia in root and stem-base region and highest yield of grain in a conventional system for production of wheat cv. Rywalka confirms the cultivar’s suitability for high-input conventional farming. Benefits of microbiological approaches in the study of suitability of cultivar to a particular farming system is demonstrated.

Efficacy and safety of generic azoxystrobin at controlling Rhizoctonia solani in sugar beet

Rhizoctonia solani Kühn is a major soil-borne pathogen limiting sugar beet (Beta vulgaris L.) production in the United States and other countries worldwide. There is no commercial cultivar which is immune from infection by R. solani and highly resistant cultivars typically have a yield penalty. Growers use cultivars with some levels of resistance and rely on the timely application of fungicides to control R. solani. Azoxystrobin (Quadris®) has been the most widely used fungicide for control of R. solani on sugar beet since its registration in the United States in 1997. In 2015, growers wanted to know whether generic azoxystrobin, which became available on the market and at a lower price than Quadris®, would provide effective control of R. solani. The objectives of this greenhouse study were to evaluate the efficacy and safety of generic azoxystrobins at controlling R. solani on sugar beet compared to the industry's standard Quadris. Three generic azoxystrobins – Satori®, Aframe™ and Equation™ - and two application methods - in-furrow at planting and band applications to 4-leaf stage sugar beet - were evaluated. Inoculation with R. solani AG 2-2IIIB was done after fungicide applications. To check for phytotoxicity, fungicides were applied but with mock inoculation. Plant survivors, plant height, root weight and disease severity were evaluated. Results indicated that the generic azoxystrobins were safe to use on sugar beet seeds and plants and provided similar disease control of R. solani as the industry's standard Quadris treatment. This study demonstrated under greenhouse conditions that Satori®, Aframe™ and Equation™ have the potential to be used by growers for disease management in sugar beet fields with a history of R. solani.

Studies on comparative efficacy of commercially available talc formulations of Trichoderma spp. and fungicide against root rot of chilli (Capsicum annuum L.)

Rhizoctonia solani causing root and stem rot in young transplanted plants is a major soil borne pathogen of chilli ( Capsicum annuum L.). Poor growth of affected plants, yellowing and drying of foliage accompanied with partially or fully damaged root system are major symptoms of the disease. Studies were carried out to evaluate efficacy of soil application of ten commercially available formulations of Trichoderma harzianum and T. viride , under laboratory and field conditions for efficacy in suppressing Rhizoctonia root rot and promoting plant growth in chilli. Soil drenching by carbendazim 75 per cent WP (0.2%) was also taken as standard chemical check. Except BF 10 and BF 5 all the formulations which were tested in the field experiment were effective in reducing Rhizoctonia rot incidence in chilli as compared to control. However, disease incidence was least (12%) for the BF4. Reduction in disease incidence in this treatment was comparable to soil drenching by carbendazim (12%). Among other treatments BF3 was second most effective bioagent against Rhizoctonia root rot All the bioagents promoted plant growth in terms of plant height, root length, shoot dry weight and root dry weight. Maximum shoot dry weight was recorded for BF4 (60.5 g) followed by BF3 (60.00 g), BF6 (56.5 g) and BF2 (55.6 g). Similar trend of root dry weight was recorded. Highest rhizosphere soil population was recorded in case of bioagent formulation BF4 (4.1x106 cfu/g soil) followed by BF3 (3.8x10 6 cfu/g soil) and BF6 (3.4x10 6 cfu/g soil).

Influence of brassicaceous soil amendments on potentially beneficial and pathogenic soil microorganisms and seedling growth in Douglas-fir nurseries

Fusarium, Cylindrocarpon and Pythium spp. are the major soil-borne pathogens of conifer seedlings. Soil fumigation with methyl bromide and chloropicrin has been the most effective method for reducing the population density and disease pressure of these organisms. Due to safety and environmental concerns, use of methyl bromide as a pre-plant soil fumigant has been abolished in the majority of cropping systems. However, the conifer seedling industry continues to use methyl bromide under a quarantine pre-shipment exemption due to a lack of effective alternatives. Toward identifying alternatives to methyl bromide for management of soil microbial populations, a three-year field study was conducted in northwest USA. The objective of this study was to examine the effects of brassica seed meals and green manures on potentially pathogenic and beneficial microorganisms, soil health, and seedling growth in conifer nursery fields. The study treatments were Brassica juncea seed meal, B. carinata seed meal, Sinapis alba seed meal, B. juncea green manure, methyl bromide/chloropicrin fumigation, and a non-treated control, with four replications in a randomized complete block design. The treatments were incorporated into soil in autumn or early spring, and Douglas-fir (Pseudotsuga menziesii) seedlings were transplanted into plots in late spring. Population densities of Fusarium, Cylindrocarpon, Pythium, actinomycetes, and Trichoderma; mineralizable nitrogen; and dehydrogenase enzyme activity in soil were assessed at pre-transplant, post-transplant, and seedling harvest. The pre-treatment soil pathogen count was similar among study plots. At transplant time, Fusarium spp. densities in soil were generally similar among most brassica treatments but fumigated plots generally had less Fusarium. Treatment with S. alba, however, increased soil densities of Fusarium spp. In 2012, Fusarium spp. density was significantly lower after B. juncea green manure incorporation [1.8 log CFU (colony forming units)] than after chemical fumigation (2.4 log CFU) or in the untreated control (2.6 log CFU); whereas the soil density of potentially antagonist Trichoderma spp. was significantly greater in fumigated plots (3.7 log CFU) followed by B. juncea green manure (3.4 log CFU) and lowest in control (3.2 log CFU). Fumigation produced the largest seedlings but B. juncea green manure also produced significantly larger seedlings than control. Dehydrogenase activity, an indicator of soil microbial activity, was greatest with B. juncea green manure and lowest in fumigated soil. Mineralizable nitrogen in soil followed the same trend. These results suggest that B. juncea green manure may have a suppressive effect on soil-borne pathogens, and maintain or improve soil and seedling health.

In Vitro Evaluation of Commercial Fungicides against some of the Major Soil Borne Pathogens of Soybean

Four Strobilurin, two premix of Strobilurin and Triazole, and one pyrazole-carboxamide foliar fungicides were tested in a modified in vitro culture plug technique against Colletotrichum truncatum (CT), Fusarium virguliforme (FV), Macrophomina phaseolina (MP), Pythium irregulare (PI), Rhizoctonia solani (RS), and Sclerotinia sclerotiorum (SS) and three Strobilurin and two premixes against Septoria glycines (SG). Under aseptic conditions, a single 6-mm culture plug of actively growing individual fungus was placed inverted on one end inside periphery of 9-cm PDA plates and on the opposite end 6-mm sterilized blotter disc with 50-μl fungicide solution was placed. Tests against SG were by spreading 50-μl spore suspension (1×108 spores/ml) on to PDA and placing blotter disc with 50-μl fungicide in the center. During 12-day incubation in 12-h photoperiod, assessed in vitro (i) effects of fungicides on growth of pathogens, (ii) sensitivity of pathogens to fungicides and (iii) persistence of fungicide tolerance in pathogens. All the fungicides except Sercadis, significantly (P<0.05) reduced radial growth of CT, while Headline EC, Priaxor and Stratego YLD significantly reduced growth of FV, MP, RS and SS. Similarly, Sercadis was effective against RS, and Aproach and Quadris against FV. SG and CT showed significant (P<0.05) sensitivity to most of the fungicides, FV, RS and SS showed significant sensitivity by forming inhibition zone between their growth ends and Headline EC, Priaxor and Stratego YLD discs. CT, MP and RS showed significant (P<0.05) persistence to all the fungicides that is considered fungistatic effect.

The Concentration of 2-Propenyl Glucosinolate in Biofumigant Crops Influences Their Anti-Fungal Activity (In-Vitro) against Soil-Borne Pathogens

This study investigated the biofumigation potential of nine Brassica species/cultivars by determining the levels of 2-propenyl glucosinolate in their roots and shoots, and their in-vitro suppression of four major soil-borne pathogens of vegetable crops. Hydrolysis of 2-propenyl GSL produces volatile isothiocyanate (ITC) compounds which are known to have anti-fungal activity. HPLC results showed that 2-propenyl GSL only occurred in root and shoot residue of flowering plants of four Brassica cultivars developed for green manuring (Caliente 199?, Mustclean?, Nemfix? and BQ Mulch?) and in the standard (mustard seed meal) treatment Fumafert?. Levels of 2-propenyl GSL varied several fold within the four Brassica cultivars, with 77-88% of the total concentrations recorded in the shoot tissues. In in vitro assays, the level of fungal suppression by volatiles emitted by hydrated shoot and root residues related to their content of 2-propenyl GSL, and the dose of residue applied to five soilborne test pathogens (S. minor, Rhizoctonia solani, Fusarium oxysporum, Pythium dissotocum and Rhizoctonia solani). The variation in 2-propenyl GLS levels found in the Brassica green manure crops tested provides scope for selecting cultivars with greater potential for biofumigation, and to control multiple soil-borne disease problems in vegetable farms.

Colonization of lettuce rhizosphere and roots by tagged Streptomyces.

Beneficial microorganisms are increasingly used in agriculture, but their efficacy often fails due to limited knowledge of their interactions with plants and other microorganisms present in rhizosphere. We studied spatio-temporal colonization dynamics of lettuce roots and rhizosphere by genetically modified Streptomyces spp. Five Streptomyces strains, strongly inhibiting in vitro the major soil-borne pathogen of horticultural crops, Sclerotinia sclerotiorum, were transformed with pIJ8641 plasmid harboring an enhanced green fluorescent protein marker and resistance to apramycin. The fitness of transformants was compared to the wild-type strains and all of them grew and sporulated at similar rates and retained the production of enzymes and selected secondary metabolites as well as in vitro inhibition of S. sclerotiorum. The tagged ZEA17I strain was selected to study the dynamics of lettuce roots and rhizosphere colonization in non-sterile growth substrate. The transformed strain was able to colonize soil, developing roots, and rhizosphere. When the strain was inoculated directly on the growth substrate, significantly more t-ZEA17I was re-isolated both from the rhizosphere and the roots when compared to the amount obtained after seed coating. The re-isolation from the rhizosphere and the inner tissues of surface-sterilized lettuce roots demonstrated that t-ZEA17I is both rhizospheric and endophytic.