Causal Agent Of Root Rot Research Articles

Biological control of Helminthosporium sativum the causal agent of root rot in wheat by some antagonistic fungi

The present study has been undertaken to determine the efficacy of some antagonistic fungi isolated from the rhizosphere of wheat plants grown in Summer region, Diwaniya Governorate, Mid Iraq for the biological control of Helminthosporium sativum, the causal organism of root rot disease of wheat in vitro. Three different species from the genus Trichoderma: T. harzianum, T. pseudokoningii and T. lignorum, in addition to Stachybotrys atra and Penicillium sp. were isolated from the rhizosphere of wheat plants. Laboratory experiments indicated that T. harzianum and S. atra were highly antagonistic to the pathogen when grown together on potato dextrose agar in Petri plates. Microscopic examination of the mycelia showed that hyphae of T. harzianum were parasitized H. sativum, coiled around its hyphae and caused its lysis, but did not penetrate inside the hyphae. However, S. atra was invaded colonies of H. sativum and caused severe hyphal damage. In the experiments of culture filtrates of the antagonistic fungi, T. harzianum and S. atra, were able to suppress growth of H. sativum, if incorporated in the medium and proved to be effective in controlling the pathogen. Results of effect of filtrate on spore germination showed that about 80 and 95% of them are unable to germinate with high concentrations (15 or 20 %) of culture filtrates of T. harzianum and S. atra respectively. However, the other antagonists: T. pseudokoningii, T. lignorum and Penicillium sp. were less effective in inhibiting spore germination of the pathogen. Results of antagonistic effect of the culture filtrates on wheat seed infection, except T. lignorum, showed that seed colonization by the root rot fungus was decreased significantly at the concentrations of 15 and 20 % compared with control. Seed coating with antagonistic fungi was the best biological seed treatment for reducing seed rot and diseased seedling caused by H. sativum. Antagonistic fungi have no side effects on seed germination except Penicillium sp. which reduced the seed activity and germination as compared with the other antagonists and with the control. It can be concluded that T. harzianum is a strong mycoparasite and S. atra is a good antagonistic agent to control H. sativum, but in fields may be their activity are conditioned by soil environment specially the microflora.

Root Rot of Balloon Flower (Platycodon grandiflorum) Caused by Fusarium solani and Fusarium oxysporum.

Balloon flower (Platycodon grandiflorum) is a kind of mountain herbs whose roots have restorative properties and the cultivating acreage of balloon flower has been steadily increasing in Korea. More frequent rain and high amount of rainfalls as a result of climate changes predisposed balloon flower to the outbreaks of root rot at high-density cultivation area in recent years. Root crowns were usually discolored into brown to blackish brown at first and the infected plants showed slight wilting symptom at early infection stage. Severely infected roots were entirely rotted and whole plants eventually died at late infection stage. The overall disease severities of root rot of balloon flower were quite variable according to the surveyed fields in Jeonnam, Gyeongnam and Jeju Provinces, which ranged from 0.1% to 40%. The root rot occurred more severely at the paddy or clay soils than the sandy soils and their severities were much higher at lowland than upland in the same localty. The disease increased with aging of the balloon flower. The causal fungi were identified as Fusarium solani and F. oxysporum on the basis of their mycological characteristics. The optimum temperature ranges of their mycelial growths was found to be 24°C. The pathogenic characters of F. solani and F. oxysporum treated by artificial wounding inoculation on healthy roots of balloon flower revealed that F. solani was more virulent than F. oxysporum. This study identified the causal agents of root rot of balloon flower as Fusarium solani and F. oxysporum, probably for the first time.

Identify and relative virulence of seven Phytophthora spp. causing crown and root rot of citrus trees in Kerman, Iran

In the identification and investigation virulence of Phytophthora species, the causal agent of root rot and gummosis citrus was investigated in Kerman from 2004 to 2007. Twenty-seven of 70 isolates were Phytophthora citrophthora, and 33 isolates were identified as Phytophthora parasitica. These species were isolated from soil, crown and root trees. Also, 10 isolates were identified as Phytophthora cryptogea. This species was isolated from soils of around declining citrus trees in Bam region using citrus leaf discs. The subset of these isolates was tested for pathogenicity on Citrus aurantium. Results of canker length and weight of fresh root indicated a variation in virulence among Phytophthora species. P. parasitica sh-1 from Shahdad and P. citrophthora o-25 from Orzuyeh were the most virulent on fresh weight of root and Canker length on crown. The P. parasitica isolates from three regions significantly produced the most virulent on roots relative to other isolates and P. citrophthora isolates significantly produced the most virulent on crown, while P. cryptogea b-5 from Bam was the least virulent. P. parasitica sh-1 created the most reduction in weight of fresh roots. Furthermore, P. citrophthora o-25 significantly caused the most virulent relative to P. citrophthora b-57 and b-5 isolates. All P. parasitica and P. citrophthora isolates produced root rot and crown canker but P. cryptogea no pathogenic on crown but produced weak root rot on citrus seedlings. The low virulent of P. cryptogea b-5 suggested that this pathogen is not a serious threat to C. aurantium.

Suppression of root rot of mung bean (Vigna radiata L.) by Streptomyces sp. is associated with induction of peroxidase and polyphenol oxidase

Five strains of Streptomyces sp. were evaluated in vitro for their ability of inhibiting the mycelial growth of Macrophomina phaseolina, the causal agent of root rot of mung bean (Vigna radiata L.). Among the Streptomyces sp. strains tested, PDK showed the maximum in vitro inhibition of mycelial growth of M. phaseolina and recorded an inhibition zone of 21 mm. The strains CBE, MDU, SA and ANR recorded inhibition zones of 18, 16, 13 and 11 mm, respectively. These Streptomyces sp. strains were tested for their growth-promoting efficiency on mung bean seedlings. Among them, CBE and PDK recorded the maximum increase in shoot length, root length and seedling vigour compared with control, followed by MDU. Three Streptomyces sp. strains (CBE, MDU and PDK) that showed higher levels of inhibition of growth of M. phaseolina in dual culture assay and plant growth-promoting activity were tested for their biocontrol activity against root rot under greenhouse and field conditions. Seed treatment or soil application with powder formulation of Streptomyces sp. strains CBE, MDU and PDK was effective in controlling root rot disease; but, combined application through seed and soil increased the efficacy in both the greenhouse and field trials. Among the treatments, seed treatment plus soil application with powder formulation of Streptomyces sp. strain CBE proved to be most effective, which reduced the root rot incidence from 26.8% (with non-bacterised seeds) to 4.0% in Trial I and from 32.0 to 4.9% in Trial II. The above treatment recorded the highest yield in both the field trials, and the yield increase was 78 and 74% over control in Trial I and Trial II, respectively. Isozyme analysis of the Streptomyces sp.-treated plants indicates that seed treatment plus soil application strongly induce the activities of peroxidase (PO-1 and PO-2) and polyphenol oxidase (PPO-2 and PPO-3) in mung bean. Among the three strains tested, Streptomyces sp. strain MDU- treated plants showed higher levels of activities of PO and PPO. Based on the above findings, it can be concluded that both the direct inhibition of pathogen and induced resistance might be involved in the control of root rot of mung bean by Streptomyces sp.

Do jasmonates play a role in arbuscular mycorrhiza-induced local bioprotection of Medicago truncatula against root rot disease caused by Aphanomyces euteiches?

Bioprotective effects of mycorrhization with two different arbuscular mycorrhizal (AM) fungi, Funneliformis mosseae and Rhizophagus irregularis, against Aphanomyces euteiches, the causal agent of root rot in legumes, were studied in Medicago truncatula using phenotypic and molecular markers. Previous inoculation with an AM-fungus reduced disease symptoms as well as the amount of pathogen within roots, as determined by the levels of A. euteiches rRNA or transcripts of the gene sterol C24 reductase. Inoculation with R. irregularis was as efficient as that with F. mosseae. To study whether jasmonates play a regulatory role in bioprotection of M. truncatula by the AM fungi, composite plants harboring transgenic roots were used to modulate the expression level of the gene encoding M. truncatula allene oxide cyclase 1, a key enzyme in jasmonic acid biosynthesis. Neither an increase nor a reduction in allene oxide cyclase levels resulted in altered bioprotection by the AM fungi against root infection by A. euteiches. These data suggest that jasmonates do not play a major role in the local bioprotective effect of AM fungi against the pathogen A. euteiches in M. truncatula roots.

Linking fungal communities in roots, rhizosphere, and soil to the health status of Pisum sativum.

Changes in fungal communities associated with healthy and diseased pea roots were investigated using deep amplicon pyrosequencing in three spatial compartments: roots, rhizosphere, and surrounding soil. Thirty root systems were collected from three fields, half of which showing clear symptoms of root rot. In total, 500,461 internal transcribed spacer-1 sequences were obtained that were clustered into 123 (roots), 271 (rhizosphere), and 440 (bulk soil) nonsingleton operational taxonomic units (OTUs). Species richness was highest in bulk soils and lowest in roots; however, no notable differences in richness were observed between samples associated with diseased and healthy roots. Health status and field both had significant effects on fungal community structures in roots, whereas only field had significant effects on communities in rhizosphere and bulk soils. Indicator species analysis across the three fields identified a number of OTUs that were more abundant in healthy roots. Pathogens such as Fusarium oxysporum were abundant in diseased roots in some fields. Patterns of disease and causal agents of root rot were different among the three fields, which were also reflected in fungal communities. In conclusion, health status of roots was only vaguely reflected in rhizosphere and bulk soil fungal communities, whereas health status was more important for shaping root communities.

Biopotential of some Trichoderma spp. against cotton root rot pathogens and profiles of some of their metabolites

The antagonistic activity of five selected isolates of Trichoderma spp. (Trichoderma harzianum CCM340, Trichoderma koningii CCM341, Trichoderma longibranchiatumNRRL11236, Trichoderma viride DSM63065 and Trichoderma viride) against Fusarium oxysporum f.sp. vasinfectum and Rhizoctonia solani, the causal agents of root rot of cotton plants grown in saline and non-saline soil, was studied in vitro and in vivo. Results indicated that F. oxysporum and R. solani were mostly inhibited by T. viride and least inhibited by T. longibranchiatum NRRL11236. Greenhouse test showed that for both pathogens, T. koningii CCM341 gave the highest disease control percentage (100%) in saline and non-saline soil and that T. harzianum CCM340, T. longibrachiatum NRRL11236 and T. viride DSM63065 gave the same disease control percentage (100%) in non-saline soil. In addition, the effect of Trichoderma isolates on several growth parameters of cotton plants was also studied. On the other hand, study on metabolites such as amino acids (AA), fatty acids (FA) and plant growth hormones (PGH) produced by these Trichoderma isolates showed that T. viride was the highest in AA production and the only isolate that produced lauric acid (FA). The highest amount of gibberellic acid, indol-3-acetic acid and abscisic acid was produced by T. viride DSM63065, T. harzianum CCM340 and T. koningii CCM341, respectively. Also, the pH and temperature effects on Trichoderma mycelial growth were also evaluated. However, the application of Trichoderma spp. to soil infested with F. oxysporum and R. solani provided a good way for protection of cotton plants. Also,Trichoderma metabolites such as amino acids, fatty acids and plant growth hormones may be developed into several commercial biological control products to prevent development of several pathogenic fungi and / or to induce plant growth response. Evaluation of pH and temperature effects on Trichoderma isolates mycelial growth showed that Trichoderma strains were found to be able to display activities under a wider range of pH values and that they are mesophili. Key words: Fusarium oxysporum, Rhizoctonia solani, cotton growth parameters, biocontrol agent, salinity.

An Environment Friendly Approach for Controlling Pathogenic Fusarium solani (Mart.) Sacc., The causal Agent of Root Rot of Medicinal Coleus by Methyl Jasmonate

Effects of ‘methyl jasmonate’ were studied in vitro on colony growth, sporulation, spore germination and germ tube elongation of phytopathogenic Fusarium solani (Mart.) Sacc., the causal organism of root rot of medicinal Coleus. Three different concentrations of methyl jasmonate- 0.05%, 0.10%, and 0.20% were used as amendment with ‘potato dextrose agar’ medium. At 0.10% and 0.20% concentration, methyl jasmonate was able to significantly increase percent growth inhibition of the fungal colonies, as compared to the control, in a concentration dependent manner, after 48 and 96 hours incubation. Inhibition was more severe after 48 hours than 96 hours. Highest percent growth inhibition (76.00) was with 0.20% methyl jasmonate after 48 hours. Methyl jasmonate treatments also had significant reducing effects on spore formation, spore germination frequency and germ tube growth of the pathogen in a concentration dependent manner. 0.20% methyl jasmonate had most severe effects which caused lowest spore count (8 x 10 4 /ml culture filtrate), germination percent (4.80) and germ tube length (54.52μ). In the present study, methyl jasmonate showed fungistatic and fungicidal activity against Fusarium solani , under in vitro conditions. This is the first report of methyl jasmonate showing inhibitory effects on this particular fungus which in turn supports the possibility of future use of methyl jasmonate as a bio-control agent against root rot of medicinal Coleus caused by Fusarium solani , under field conditions.

Causal Agents of Root Rot and the Effect of Vesicular-Arbuscular Mycorrhizal Fungi in Seedlings of Rhodiola rosea in Alberta, Canada

Causal Agents of Root Rot and the Effect of Vesicular-Arbuscular Mycorrhizal Fungi in Seedlings of Rhodiola rosea in Alberta, Canada

Fusarium oxysporum f. sp. lycopersici races and their genetic discrimination by molecular markers in West Mediterranean region of Turkey

Fusarium oxysporum f. sp. lycopersici (FOL) races and F. oxysporum f. sp. radicis lycopersici (FORL), the causal agents of root rot and crown rot diseases, respectively, cause serious economic losses in tomato greenhouses where production is intensive in the West Mediterranean region of Turkey. The isolates were collected from West Mediterranean region of Turkey and were characterized by specific primers based on three races (r1, r2, r3), besides pathogenicity tests in in vivo conditions Additionally, a scheme was developed using newly tested ISSR and SRAP markers to a genotyping database and to determine the possible origin of these pathogens. The present study provided new information on these pathogens based on their races and their dominant existence in this region that has not been reported before. Genetic diversity detected in the same races of the pathogen may be associated with difficulties in controlling the pathogen and a possible resistance formation effort exerted by the pathogen to chemicals used in plant protection in tomato greenhouses. Molecular analyses indicated genetic diversity in pathogen isolates identified as r3, r2 and FORL, which may be associated with abiotic stress to which the pathogens were exposed.

Improved extraction of Rhizoctonia and Pythium DNA from wheat roots and soil samples using pressure cycling technology

Soilborne pathogens are important biotic factors in yield reduction in the dryland cereal production region of the Pacific Northwest. Rhizoctonia solani AG-8, Rhizoctonia oryzae, and Pythium spp. are causal agents of root rot, bare patch, and damping-off of wheat (Triticum aestivum) and barley (Hordeum vulgare). Although these pathogens can be rapidly and specifically quantified using quantitative real-time PCR, the extraction of Rhizoctonia DNA from agricultural samples is often inconsistent, especially at low pathogen population densities. Using a novel extraction system that uses pressure cycling technology (PCT), we improved the extraction of R. solani AG-8 DNA up to 16-fold and of P. abappressorium DNA up to 2-fold from three types of agricultural soils compared with a bead beating extraction method. PCT also yielded quantifiable amounts of R. solani AG-8 and R. oryzae DNA from lyophilized wheat roots that were otherwise recalcitrant to homogenization. Furthermore, the extractions were so consistent that pathogen quantification generally could be derived from two rather than three or four replicated extracts. Because PCT is performed in a closed system and minimizes sample shearing and heating, it confers a substantial advantage over conventional extraction systems. Here, we report for the first time the application of PCT in a laboratory setting for the improved extraction and quantification of three types of soilborne pathogens in soil samples. The effectiveness of PCT for three soils suggests that it will be beneficial for other hard-to-extract pathogen samples.

Determination of Culture Condition for Polygalacturonase Production by Rhizoctonia solani AG2-2, Causal Agent of Root Rot in Sugar Beet

Determination of Culture Condition for Polygalacturonase Production by Rhizoctonia solani AG2-2, Causal Agent of Root Rot in Sugar Beet

Biological Control of Phytophthora drechsleri Tucker, the Causal Agent of Pistachio Gummosis, under Greenhouse Conditions by Use of Actinomycetes

Actinomycetes enhance soil fertility and have antagonistic activity against wide range of plant root-pathogens. These microorganisms were isolated from agricultural soils of Kerman as pure cultures. Phytophthora drechsleri Tucker, causes gummosis and root rot of pistachio trees worldwide. From 130 Actinomycetes isolates, 12 inhibited growth of the pathogen of pistachio gummosis in culture plates and four of the most active isolates exhibited biological control of the pathogen under greenhouse conditions. When plants were grown in sterile soil mix and treated both with Actinomycetes and the pathogen, the number of healthy plants increased dramatically and the symptoms on diseased plants were less severe in comparison with seedlings treated with the pathogen alone. From the collected data it was well conclusive that in greenhouse tests, soil applications of Actinomycetes controlled causal agent of root rot of pistachio seedlings. Antifungal activity was of fungicidal type on the pathogen mycelia. From the stand point of biotechnological goals, the results indicate that the active isolates can be investigated for use as biofertilizers, biofungicides and use in future development of recombinant DNA in pistachio trees bearing elevated resistance to gummosis. Field trials of the active isolates are under investigation

The Role of Antagonism in the Rhizospheric Region for Chaetomium globosum and Trichoderma harzianum against Fusarium spp. Attacking Tomato Plant.

Chaetomium globosum and Trichoderma harzianum were isolated from healthy tomato plants at the rhizospheric area. These isolates were evaluated as potential bio-agents in control of the soil-borne fungi, Fusarium solani the causal agent of root rot, Fusarium oxysporum f. sp. lycopersici, Fusarium oxysporum f. sp. radicis-lycopersici and Fusarium semitectum wereisolated from diseased tomato plants in Alexandria and Giza Governorates. The Pathogenicity test on tomato plant was conducted under greenhouse conditions. It indicated that, all tested fungi have a pathogenic effect on tomato plant. After one week the death percentages caused by F. solani, F. oxysporum f. sp. lycopersici, F. oxysporum f. sp. radicis-lycopersici and F. semitectum were 43.8, 31.3, 68.8 and 56.3%, respectively. However, after 45 days, the obtained data percentages for the same fungi were 62.5, 73, 87.5 and 83.8, respectively. The tested bio-agents affected all tested pathogens ,however, C. globosum at zero time of the pathogen inoculation reduced the radial growth of F. oxysporum f. sp. lycopersici, F. oxysporum f. sp. radicis-lycopersici, F. semitectum and F. solani by 53, 24.4, 48.9 and 22.2%, respectively, while C. globosum (3 days before the pathogen inoculation) reduced the radial growth of F. oxysporum f. sp. lycopersici, F. oxysporum f. sp. radicis-lycopersici, F. semitectum and F. solani by 68.9, 48.9, 72.2 and 38.8%, respectively, on the other hand, T. harzianum reduced the radial growth of the same tested pathogens (F. oxysporum f. sp. lycopersici, F. oxysporum f. sp. radicis-lycopersici, F. semitectum and F. solani ) by 66.7, 67.8, 100 and 61.1, respectively. T. harzianum was the most effective bio-agent against Fusarium spp. Comparing with C. globosum. It had recorded the complete growth reduction When it had been tested on Fusarium semitectum with 100% percentage.

The plant antifungal isoflavone genistein is metabolized by Armillaria mellea Vahl to give non-fungitoxic products

Armillaria mellea, the causal agent of root rot, is a fungal pathogen which proved able to convert the leguminous plant antifungal compound 4′,5,7-trihydroxyisoflavone (genistein) into intermediate metabolites. After suitable periods of incubation, the metabolites were extracted and concentrated from liquid culture media, containing both the isoflavone and the fungus. After purification by column chromatography, the molecular structure of the metabolites was determined by means of mass spectrometry and nuclear magnetic resonance analyses. Five different compounds were identified: 1,3,5-trihydroxybenzene, 4-hydroxyphenylacetic acid, 2,5-dihydroxyphenylacetic acid (homogentisic acid), its lactone 5-hydroxy-2(3H)-benzofuranone, and 1,4-benzoquinone. In vitro experiments showed that while the starting compound, i.e. genistein, has some activity in inhibiting the growth of the fungal pathogen, the degradation products are devoid of any appreciable fungitoxic activity. Moreover, results show that the isoflavone metabolites can be, at least partially, utilized by A. mellea as a carbon source.

First Report of Root Rot of Cocoyam Caused by Pythium myriotylum in Sri Lanka

Root rot of cocoyam (Xanthosoma sagittifolium L. Schott) caused by Pythium myriotylum Drechsler is a major disease of this crop in Africa (1,2) but is unreported from other regions of the world. During September 1999, commercially grown cocoyam (cv. Ratu-kiri-ala) in Gampaha (7°05'N, 80°00'E), Sri Lanka suffered from severe root rot. Initial symptoms were water-soaked lesions at the root tips that gradually enlarged to rot the entire root system and tuber. Wilting and yellowing of leaves were observed in advanced stages of disease. A Pythium sp. was regularly isolated from the affected roots and an isolate, SC5, was identified as P. myriotylum on the basis of morphology and the internal transcribed spacer (ITS) rDNA sequence. Characteristics of isolate SC5, grown on a grass-leaf water culture (3) were main hyphae up to 8.5 μm wide, oogonia terminal or intercalary (22.5 to 33.8 μm in diameter), antheridia diclinous occasionally monoclinous, one to eight per oogonium, stalks branched, often more or less loosely enveloping the oogonium, antheridium clavate or crook-necked, making apical contact with the oogonium, breadth of antheridium 2.5 to 7.0 μm, oospores aplerotic (17.0 to 22.5 μm in diameter), oospore wall 0.8 to 2.0 μm in thickness, sporangia terminal or intercalary, filamentous, inflated lobulate, and digitate, of variable length, breadth of sporangia 7.0 to 17.5 μm, formed in water; zoospores formed at 25°C, and diameter of encysted zoospores 10.0 to 12.5 μm. Cardinal temperatures on potato carrot agar 8°C minimum, 34°C optimum, and 37°C maximum with daily radial growth rate for 34°C at 32.8 mm. The ITS rDNA sequence of the isolate matched the sequences of P. myriotylum in GenBank (Accession Nos. AB095051 and AF452156) and isolate CBS254.70 used for the species description by van der Plaats-Niterink (3). The sequence of SC5 has been deposited in GenBank, Accession No. DQ102701. Pathogenicity tests used potted cocoyam plants (20 cm high), planted in an autoclaved potting mix. Four agar disks (8 mm in diameter) of isolate SC5 grown at 25°C for 48 h on potato dextrose agar was mashed and injected at a depth of 2 to 3 cm in the soil around the roots. Inoculated plants were placed in transparent plastic bags and kept for 7 days in a growth chamber maintained at 24 to 26°C with continuous light (52 to 98 μmol m-2·s-1). The experiment was carried out twice with three replications for each test. Dark brown rotting on roots and wilting of leaves were observed in 7 days after the inoculation. P. myriotylum was reisolated from diseased tissues and found to be morphologically identical to the original isolate SC5. Noninoculated control plants remained healthy. On the basis of the symptoms, morphological and molecular characteristics and confirmation of pathogenicity, P. myriotylum is the causal agent of root rot of cocoyam. To our knowledge, this is the first report of P. myriotylum causing root rot of cocoyam in Sri Lanka.

In vitro toxin production by Fusarium solani f. sp. piperis

Fusarium solani f. sp. piperis (teleomorph: Nectria haematococca f. sp. piperis), causal agent of root rot and stem blight on black pepper (Piper nigrum), produces secondary metabolites with toxigenic properties, capable of inducing vein discoloration in detached leaves and wilting in transpiring microcuttings. Production of F. solani f. sp. piperis (Fsp) toxic metabolites reached a peak after 25 days of static incubation on potato sucrose broth at 25 ºC under illumination. Changes in the pH of the culture filtrate did not alter the effect of toxic metabolites. However, when the pH was changed before the medium had been autoclaved, a more intense biological response was observed, with an optimum at pH 6.0. Isolates that produced red pigments in liquid cultures were more efficient in producing biologically active culture filtrates than those which produced pink coloured or clear filtrates suggesting that these pigments could be related to toxigenic activity. Detached leaves of seven black pepper cultivars and Piper betle showed symptoms of vein discoloration after immersion in autoclaved and non-autoclaved Fsp culture filtrates indicating the thermostable nature of these toxic metabolites.

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.

A novel tetratricopeptide repeat-containing J-protein localized in a plasma membrane-bound protein complex of the phytopathogenic oomycete Phytophthora megasperma

Phytoalexins originating from plant tissues may cause within cells of fungi or oomycetes a change in the localization of actin, tubulin and chaperones. To test the hypothesis in a filamentously growing oomycete, we compared the distribution of cellular markers in the presence and absence of hydroxystilbene phytoalexins. Using cDNA from the phytopathogenic organism Phytophthora megasperma, the causal agent of root rot on soybean and many other plants, and including probes for Hsp70 and Hsp40, we cloned a DnaJ-protein (Jcp) with the capacity of interacting with both a particular Hsp70 isoform via its J-domain and with other proteins via its tetratricopeptide repeat (TPR) domain. Antisera raised against the bacterially expressed protein Jcp allowed the analysis of its intracellular localization during hyphal growth. Following the subfractionation of cell homogenates, we detected virtually all immunoreactive Jcp in the plasma membrane-enriched fraction and as constituent of a membrane-associated protein complex. In agreement with the biochemical findings, immunocytochemical stains of hyphae showed Jcp as part of cortical patches positioned along the plasma membrane similar to the distribution of actin patches. Confocal microscopy, however, revealed that the Jcp-containing patches did not generally co-localize with the patches visualized by the actin stain. The 59-kDa Jcp, characterized by a large 8-fold TPR domain at the N-terminal region and a J-domain close to the C-terminus, is a good candidate for bridging the gap between Hsp70 and Hsp90 by protein-protein interactions. By administration of plant-derived phytoalexins it was shown that the presence of resveratrol or piceatannol significantly reduces the amount of the Jcp-containing patches, but does not lead to a relocalization of intracellular Jcp.

Fusarium oxysporum and F. sambucinum Associated with Root Rot of Amaranthus hybridus in South Africa.

Amaranthus hybridus has been identified as an important alternative vegetable crop with potential for increased commercial production in South Africa (1). In summer 1999, severe losses occurred in a large plot of 2-month-old A. hybridus plants on an experimental farm near Bloemfontein, South Africa. More than 90% of the plants were severely stunted, with chlorotic foliage that was wilted in most cases. Root rot was present in all symptomatic plants and was clearly visible as an amber to brown discoloration of tap and secondary roots; in severe cases, white mycelium was clearly visible on diseased root tissue. Isolations from symptomatic roots were made on potato dextrose agar (PDA) amended with streptomycin sulfate. Isolates (N = 121) were recovered from diseased roots (n = 89). The two most frequently isolated fungi were transferred to carnation leaf agar and identified as Fusarium oxysporum (n = 90, 74%) and F. sambucinum (n = 29, 24%). Pathogenicity tests with one isolate of each species were performed in the greenhouse on 1-month-old potted A. hybridus seedlings (10 plants per treatment). A single hyphal tip of each isolate was transferred to PDA and incubated at 25°C for 7 days in the dark. Five 4-mm-diameter mycelial plugs were taken and placed directly on the taproot of each plant, halfway along the length and ≈30 mm below the soil surface. Control plants were treated with uncolonized PDA plugs. Seedlings inoculated with either fungus exhibited wilting within 7 days; stunting, chlorosis (pale green to yellow), and root necrosis after 2 weeks; and mortality after 4 weeks. Inoculated plants were removed from pots after 3 weeks, roots were washed free of potting soil, and necrotic lesion length was measured. Necrosis and discoloration of root tissue were similar to those observed in field plants. The mean length of tissue necrosis induced by the fungi was 22.5 and 34.8 mm for F. oxysporum and F. sambucinum, respectively. F. sambucinum, thus, was more pathogenic than F. oxysporum despite being recovered significantly less often from field plants. Control plants inoculated with noninfested PDA plugs remained healthy. The presence of both pathogens was confirmed by reisolation from artificially inoculated taproots of all plants. No Fusarium spp. were recovered from the 10 control treatments. F. oxysporum has been reported on diseased red root pigweed (A. retroflexus) in the United States (2), but this is the first report of both F. oxysporum and F. sambucinum as causal agents of root rot in A. hybridus. These pathogens, therefore, must be considered a potential threat to commercial production of A. hybridus in South Africa and elsewhere.