Pythium Root Rot Research Articles

Effect of Marker Aided Pyramiding of Anthracnose and Pythium Root Rot Resistance Genes on Plant Agronomic Characters among Advanced Common Bean Genotypes

One of the factors that accounts for the low yields in common bean is the simultaneous occurrence of diseases on the common bean crop. Bean root rots and anthracnose are the most important common bean diseases that simultaneously occur on the bean crop in Uganda. Moreover, Colletotrichum lindemuthianum, the pathogen that causes bean anthracnose, possesses a high genetic variability which makes it easily break down single gene resistance. Pyramiding resistance genes for both diseases in commercial varieties would ensure reduction of yield losses resulting from the two diseases. However, the effect of marker assisted gene pyramiding on plant agronomic characters is not well understood. Three-way crosses were made to pyramid three anthracnose and one Pythium root rot resistance genes in four susceptible market class varieties. Sequence characterized amplified regions (SCAR) markers were used to facilitate the pyramiding scheme. Correlation analysis and Path coefficient analysis were used to assess the association between number of pyramided genes and different plant agronomic characters. Number of pyramided genes was negatively correlated with number of pods per plant (-0.32), number of seeds per plant (-0.25), number of seeds per pod (-0.18), pod length (-0.17), days to 50% flowering (-0.09) and 100-seed weight (-0.02). Path coefficient analysis showed that number of pyramided genes, plant height, days to 50% flowering, number of seeds per pod and number of pods per plant had negative direct effects on seed weight per plant. Number of seeds per plant had the highest positive direct effects (0.98) followed by 100-seed weight (0.28) while days to maturity had the least positive direct effect (0.03) on seed weight per plant. Only number of seeds per plant had its correlation coefficient (0.94) almost equal to the direct path coefficient (0.97). Number of pyramided genes had significant (P < 0.05) negative indirect effect on seed weight per plant only through number of seeds per plant (-0.25). Therefore, pyramiding higher numbers of resistance genes may cause a grain yield reduction via number of seeds per plant. Therefore, it is important for breeders to simultaneously select for number of pyramided genes with number of seeds per plant and other highly associated traits.

Analysis of AFLP Markers for Screening Resistance to Common Bean Roots Rot (Pythium spp.)

Aims: To investigate AFLPs for suitability as potential markers for identification of sources of Pythium resistance in bean genotypes preferred by poor small holder farmers. Place and Duration of Study: Department of biological sciences Masinde Muliro University of Science and Technology, Kenya and Bangor University, North Wales, United Kingdom, between September 2010 and December 2012. Methodology: 45 common bean accessions comprising 35 seed samples collected from farmers, market centers, as well as seed stockists and ten common bean lines generated from a conventional breeding program with differential resistance to the pathogen Pythium were subjected to AFLP analysis. Genetic characterization using cluster and principal component analysis were conducted to determine segregating patterns of bean accessions in relation to tolerant and Original Research Article Maryrose et al.; AJEA, 5(4): 374-391, 2015; Article no.AJEA.2015.039 375 susceptible bean lines. Average genetic distances were calculated and similarity coefficients subjected to unweighted pair group method of arithmetic averages to generate dendograms. Results: Two informative AFLP primer combinations yielded 194 polymorphic loci. Genetic distance of bean samples from KARI Kakamega had 56 to 414 base pairs with a variability index of 0.63 to 0.90. Combined analysis of bean accessions from KARI breeding program and market class common bean samples revealed a variability index range of 0.62 to 0.90 with 56 to 420 base pairs. PCA contributed about 51.58% on the genetic variation. Cluster analysis of the 10 KARI-Kakamega bean lines revealed that resistant bean varieties were genetically different from the susceptible bean varieties. The dendogram generated revealed four sub-groups and with the exception of Alulu, a mildly resistant cultivar, that segregated alongside resistant cultivars, resistant varieties clustered differently from susceptible cultivars. However, screening with farmers’ germplasm produced a dendogram that revealed a mixture of distinct and relatively non-distinct categorization with regard to resistance. Only resistant cultivars AN1062, R2075, R719 and R1946 and susceptible cultivars GLP2 and GLP585 clustered together as expected. The others segregated randomly alongside the farmers’ germplasm. Resistant varieties AND1062, R2075, R719, R1946 and SCAM80 were more genetically related to marketable class of beans. R1946 R719, R2075 and AND1062 are closely related genetically compared to Mw001, KK15, Alulu and GLP2. Conclusion: AFLP is a relatively informative technique that has a great potential of delineating susceptible and resistant Pythium root rot dry bean varieties, and can be used as a preliminary guide to carry out further analysis. It is notable that the AFLP markers used were not able to clearly distinguish all the cultivars comprehensively and should not be used alone in determining resistance levels. The information generated in this study will contribute to the propagation of acceptable market class bean lines with resistance to Pythium root rot for improved livelihood and increased food security.

Potential of Organic Farming in Production of Environment Friendly Sugarcane

Sugarcane draws heavily from soil and requires corresponding replenishment of nutrients. With the increasingly crop residue and farm waste, finding alternative uses for supplementing the soil has become more acute. Long-term use of inorganic fertilizers and pesticides to enhance the productivity of sugarcane results in deterioration of soil quality and the decline in productivity. It also contributes to the accumulation of heavy metals in soil and raising concern about crop production and potential impact on human health. Stubble decline, characterized by progressive reductions of yield in successive crops, is a major constraint on productivity and profitability of the sugarcane industry due to poor soil aeration, drainage, weed competition and Pythium root rot. To facilitate sugarcane cultivation on a sustainable basis, and at the same time obtain product more acceptable in international market, it is advisable to apply biofertilizers and promote organic farming. Application of different biofertilizers such as Azotobacter, Pseudomonas, Azospirillum, Trichoderma, Gliocladium sp. AM fungi (Glomus fasciculatum, Glomus intraradices and Gigaspora albida) and Seaweeds (Caulerpa racemosa, Ulva lactuca, Sargassum wightii, Gelidiella acerosa and Gracilaria edulis), has shown significant improvement in sugarcane yield. Biofertilizers improve soil physical properties, such as porosity, aeration and water infiltration by forming and stabilizing soil aggregates. Composted organic materials have been utilized with varying success for suppression of root rot in sugarcane that would result in increased production and productivity. Studies have also indicated that it is feasible to make a comparable yield of sugarcane by meeting the nutrient demand entirely through organics. The farm produce will not contain traces of hazardous substances and will be accepted across the globe as a premium organic product. Keywords: AM fungi, biofertilizers, cane yield, seaweeds, soil fertility, organic product.

First Report of Pythium Root and Stalk Rot of Forage Corn Caused by Pythium arrhenomanes in Japan.

Corn (Zea mays L.) is the most important forage crop in Japan. It was cultivated on 92,000 ha in 2011 and was mainly used as whole crop silage for cattle feed. In September 2009, a root and stalk rot disease was detected on corn plants cultivated in Tochigi, located in the central region of Japan. The symptoms of the disease included wilting of whole plants after the R5 (dent) stage (2) with drooping ears. Roots turned black and their number decreased. Further, the stalks became hollow and soft and harbored white hyphae. This tissue deterioration made machine harvest difficult. We obtained seven isolates of a Pythium-like organism by single hypha isolation from surface-sterilized pieces of diseased roots and stems on water agar and deposited one of the isolates at the NIAS genebank, Japan, under the accession no. MAFF511547. The isolate was grown in the dark on V8 juice agar medium for 10 days to produce oogonia. The oogonia were globose, light brown to yellow, smooth, 23.9 to 30.5 μm in size, and had 1 to 8 antheridia. Oospores were mostly plerotic, and oogonia walls were 1.3 to 2.7 μm thick. The morphology of the isolates was similar to that of Pythium arrhenomanes Drechsler and consistent with the species description (3). We analyzed the rDNA-ITS region sequences of the isolate as described by Kageyama et al. (1). The sequence (GenBank Accession No. AB903904) showed 99.1% (783/790 bp) similarity with that of P. arrhenomanes (AY598628). On the basis of morphological and rDNA sequence similarities, we identified the isolates obtained from corn as P. arrhenomanes. The pathogenicity of the isolate was confirmed by planting corn seedlings of the commercial Pioneer Brand hybrid 36B08 immediately after germination in five replicate pots containing soil mixed with 5% boiled barley grain by weight, incubated with or without the isolate for 7 days. After 10 days of incubation in a greenhouse at 20 to 25°C, only the inoculated plants exhibited symptoms of root and stalk rot. Since the inoculated organism was readily re-isolated from the diseased stems and roots, the pathogenicity of the isolate was confirmed. For field observation, the same hybrid of forage corn was sown in the fields in Nasushiobara, Tochigi, on 16 May 2011. The hybrid was sown in a row of 2 m, with 20 seeds planted at a distance of 10 cm with two replicates. For inoculum, the isolate was cultured on 5-cm-long wooden toothpicks, previously soaked in potato dextrose broth and placed on a V8 agar plate for 7 days at 25°C in the dark until covered by hyphae. The toothpicks were pierced into wounds made on the stems of corn plants, approximately 10 cm above the ground, using a thin iron needle. The wounds were about 2 mm in diameter and 2 cm deep. Field inoculation was conducted in late July at the R1 (silking) growth stage. Disease symptoms were observed in mid-September at R5, and only those plants that were inoculated with the toothpicks harboring the hyphae exhibited the typical stem rot symptoms. To our knowledge, this is the first report of root and stalk rot caused by P. arrhenomanes in forage corn in Japan.

Genetic Analysis of Resistance to Pythium Root Rot Disease in Common Bean (Phaseolus Vulgaris L.) Genotypes

Bean root rot caused by Pythium spp. is one of the most important diseases affecting common bean (Phaseolus vulgaris L.). A study was carried out to determine the genetics of resistance to Pythium ultimum in two common bean (Phaseolus vulgaris L.) genotypes, identify the allelic relationship of the resistance genes in these genotypes, and evaluate selected genotypes with known tolerance or susceptibility to low soil fertility for reaction to the disease. The resistant genotypes RWR 1946 and RWR 2075 were each crossed to three susceptible genotypes—K 132, Lyamungu, and Rushare to produce F1 seeds. Part of the F1 seeds from each cross was used to produce F2 and backcross seeds. F1 and F2 seeds of the crosses RWR 1946 x RWR 2075, RWR 1946 x RWR 719, and RWR 2075 x RWR 719 were also generated. Genotype RWR 719, a known and previously studied Pythium root rot resistance source was included mainly for the allelism studies. Some of the low soil fertility-tolerant genotypes included those from the Bean Improvement for Low Soil Fertility in Africa (BILFA) nursery, aluminum toxicity, and low soil phosphorous-tolerant and susceptible genotypes from CIAT, Colombia. All parental genotypes, F1s, F2s, backcrosses, and the low soil fertility-tolerant and susceptible genotypes were evaluated for Pythium root rot severity. Resistance to Pythium ultimum was controlled by a single dominant gene present at the same locus in all the genotypes. We conclude that the BILFA nursery is a potential source of Pythium root rot resistance.

COMPOST APPLICATION TO REPLACE SPHAGNUM PEAT AND TO SUPPRESS PYTHIUM ROOT ROT IN TURF

In the Municipality of Doha in Qatar, the application of sphagnum peat and animal manure compost at rates of 200 m3/ha/yr and 150 t/ha/yr are specified for turf production in sand. Application of the selective fungicide Ridomil is also specified to control Pythium root rot. In this trial, a cured, agronomically defined compost was applied as the sole soil conditioner, at a rate calculated to replace all fertilizer phosphorus required during the establishment period for growing Bermuda grass. The rate of application of fertilizer potassium and nitrogen was also adjusted, to improve soil health. At half the volume, the water and nutrient-holding properties conferred to the soil by the cured compost were equivalent to the full 200 m3/ha application of light peat. In combination with the adjusted fertilizer regime, the cured compost also reduced the severity of Pythium root rot by 50%. The results of this trial prove that the soil conditioning properties of compost can be objectively quantified, enabling the calculation of agronomically appropriate application rates. However, if the fertilizer contribution of the compost is not accounted for in the nutrient management schedule, the desired outcome of reducing disease severity may not be realised.

Characterization of Pythium spp. from soil samples in Illinois

Pythium root rot is widely distributed in major soybean (Glycine max) production areas throughout the world. There are many species of Pythium described on soybean and other crops, although not all species are pathogenic on these crops. The objectives of this study were to isolate and identify Pythium isolates obtained from field soils across Illinois and evaluate their pathogenicity on soybean seedlings. Soil samples were collected from 12 corn-soybean rotation fields in six Illinois counties. All isolates of Pythium were recovered through a baiting technique, identified to the species taxon using morphological and molecular techniques, and evaluated using an in vitro pathogenicity assay on soybean seedlings. Twenty-seven species of Pythium were identified, and P. cryptoirregulare, P. irregulare, P. sylvaticum, P. ultimum var. sporangiiferum and P. ultimum var. ultimum were highly pathogenic on soybean seedlings.

Suppression of Rhizoctonia and Pythium root rot of wheat by new strains of Pseudomonas

This study is part of an effort to identify new biological control agents with broad spectrum activity against pathogenic soilborne fungi and pests. New strains of Pseudomonas spp. were isolated from agricultural soils, river silt, and rhizosphere soils from herbarium specimens. Provisional species assignments based on 16S ribosomal DNA typing were: Pseudomonas borealis, Pseudomonas chlororaphis, Pseudomonas fluorescens, Pseudomonas mandelii, Pseudomonas marginalis, Pseudomonas poae, Pseudomonas putida, Pseudomonas syringae and Pseudomonas vranovensis. We evaluated the strains for activity against root rot pathogens of wheat, with the view to discovery and deployment of new suppressive activities. Our objectives were to compare the strains relative to: (i) production of antifungal metabolites, (ii) suppression of Rhizoctonia and Pythium damage in greenhouse assays, (iii) plant growth promotion, and (iv) ability to colonize the wheat rhizosphere. Colonization assays showed that 11 strains rapidly established and maintained rhizosphere populations of ⩾log5CFUg−1 root. Strains 14B2R, 15G2R, 29G9R, 39A2R, 48G9R and Wood3R reduced disease symptoms of both Rhizoctonia solani AG-8 and Pythium ultimum, and the latter four also suppressed Rhizoctonia oryzae and Pythium irregulare. Four strains increased seedling shoot length and root weight but these growth promotion effects were correlated to disease suppression only for two strains. Finally, the strains differed in demonstrated and potential antifungal metabolite activities, indicating that no single factor could be correlated to disease suppression. We have identified several strains for genome sequencing and for long-term development of integrated management of soilborne diseases of wheat.

Pythium Root Rot of Flue-Cured Tobacco Seedlings Produced in Greenhouses: Factors Associated with its Occurrence and Chemical Control

Pythium root rot is an economically important disease of tobacco seedlings produced in greenhouses. In a survey conducted in 1998 and 1999, fivePythium species were found to cause Pythium root rot symptoms in tobacco seedlings. In pathogenicity studies the most aggressive species were Pythium volutum and P. myriotylum, although these were not the most frequently isolated species. We conducted studies with potting media from different commercial sources. Media were not found to be a source of primary inoculum for the disease in those studies. Infested trays had the potential to carry inoculum from one season to the next. The fungicides etridiazole, mefenoxam, and azoxystrobin along with acibenzolar-S-methyl and Joy detergent were evaluated for controlling Pythium root rot. Etridiazole and azoxystrobin applied to the float water provided the best control. Accepted for publication 20 July 2012. Published 25 September 2012.

Partial saturation under ebb and flow irrigation suppresses Pythium root rot of ornamentals

The timing of ebb and flow cycles for flooded floor irrigation systems was manipulated to alter the water content of the root medium. Ornamental species were grown under the different irrigation regimes to determine if the spread and severity of Pythium root rot was affected by altered sub-irrigation delivery practices. One floor delivered and drained water quickly to achieve a partial saturation of the root medium (approximately 25% less soil water) so that minimal leachates drained from the pot. The other floor operated more slowly as a standard commercial delivery system allowing the pots to imbibe more water. During 2007–2009, poinsettia, geranium or chrysanthemum crops were grown on each floor in five separate experiments. For each experiment, 15 plants were artificially inoculated with Pythium aphanidermatum or Pythium ultimum and set on the floor in a line immediately adjacent to a row of drainage ports. Disease was monitored by evaluating foliar symptoms and by rating the root systems for percent root rot. In three of the five experiments, disease was significantly reduced under partial saturation as compared to the standard saturation. In some instances, root infection as evidenced by re-isolation of the pathogen on agar, was less on plants receiving partial saturation than on standard saturation. Plants were smaller, more compact and of higher horticultural value under partial saturation than with standard saturation. These findings should encourage growers to adopt partial saturation management practices when using ebb and flow irrigation in order to reduce damage from disease after Pythium inoculum is present.

Biocontrol traits of plant growth suppressive arbuscular mycorrhizal fungi against root rot in tomato caused by Pythium aphanidermatum

Arbuscular mycorrhizal (AM) fungi known to cause plant growth depressions in tomato were examined for their biocontrol effects against root rot caused by Pythium aphanidermatum. The main hypothesis was that plant growth suppressive AM fungi would elicit a defence response in the host plant reducing Pythium root rot development. To test this hypothesis a fully factorial experiment was performed with AM fungi (Glomus intraradices, G. mosseae, G. claroideum or nonmycorrhizal), Pythium (± P. aphanidermatum) and harvest (7 and 14 days after pathogen inoculation (dapi)) as the main factors. Two weeks after AM fungi inoculation, roots were challenged with P. aphanidermatum. Variables evaluated at each harvest were root colonization levels of the interacting fungi, plant growth responses, and expression of a plant pathogenesis related protein gene (PR-1). All of the tested AM fungi caused marked growth suppressions, but did not affect PR-1 gene expression or the phosphorous concentration in the host plant. Plants singly inoculated with P. aphanidermatum had an increased PR-1 expression and phosphorous concentration. Among the AM fungi included in the study only G. intraradices reduced the pathogen root infection level, measured both in terms of Pythium ELISA and by recovery on selective media and only at the first harvest. Likewise, P. aphanidermatum root infection reduced colonization levels of G. intraradices, but not that of the two other AM fungi. In conclusion, plant growth suppressive AM fungi may offer plant beneficial traits in terms of biocontrol of root cortical pathogens.

Quantitative relationships of Pseudomonas chlororaphis 63-28 to Pythium root rot and growth in hydroponic peppers

The ability of Pseudomonas chlororaphis 63-28 to suppress Pythium root rot (Pythium aphanidermatum) and promote plant growth was investigated in hydroponic peppers that were predisposed or not predisposed to the disease. The biocontrol agent was introduced into the nutrient solution 10 days before the roots were inoculated with the pathogen. The root zone was maintained at 23oC except when roots were exposed to 33oC for three days before inoculation to induce predisposition to root rot. At constant 23oC (no predisposition) application of P. chlororaphis at 107 CFU mL-1 nutrient solution delayed root browning more effectively than did higher or lower densities. In predisposed plants, densities of 106, 107 and 108 CFU mL-1 were equally superior. When applied at 107 CFU mL-1, the density of P. chlororaphis on roots of the two temperature regimes ranged from log10 5.88 to 6.45 CFU g-1 fresh roots at seven to 19 days after application. The agent delayed root browning, re-mediated predisposition to root rot, and increased growth of inoculated and non-inoculated plants. Leaf expansion was a sensitive marker of root rot and remediation by P. chlororaphis. We conclude that P. chlororaphis 63-28 has substantial potential for managing the disease regardless of predisposition.

English

A series of 231 samples of bean plants affected by bean root rot were collected from different areas of Rwanda in order to characterize the causal agents. The collected samples were used to isolate 96 typical Pythium colonies which were classified into 16 Pythium species according to their respective molecular sequences of the ribosomal ITS fragments. Inoculation assays carried out on a set of 10 bean varieties revealed that all identified species were pathogenic on common bean. However, the bean varieties used in this investigation showed differences in their reaction to inoculation with the 16 Pythium species. In fact, the varieties CAL 96, RWR 617-97A, URUGEZI and RWR 1668 were susceptible to all the Pythium species while the varieties G 2331, AND 1062, MLB 40-89A, VUNINKINGI, AND 1064 and RWR 719 showed a high level of resistance to the all Pythium species used in our study. This high level of resistance to Pythium root rot disease found in diverse varieties of common bean grown in Rwanda constitutes a real advantage to be exploited as source of resistance in breeding programs aiming to increase resistance to the disease in the most popular bean varieties grown in Rwanda.

A simple detached leaf assay provides rapid and inexpensive determination of pathogenicity of Pythium isolates to ‘all year round’ (AYR) chrysanthemum roots

A detached leaf assay was developed to determine the pathogenicity of Pythium isolates to cut‐flower chrysanthemum roots. Leaves from young plants were excised and inoculated by insertion of a plug of mycelium into a slit cut in the excised petiole. After incubation leaves were assessed for presence and extent of necrosis. Necrosis indicated pathogenicity and was consistently confirmed by comparisons with whole plant inoculations. The rate of necrosis spread also gave some indication of virulence. Isolates of Pythium sylvaticum, P. ultimum and HS group were the most virulent, with a mean rate of spread of 14·6 mm per day, significantly (P &lt; 0·05) faster than the mean rate of spread, 1·6 mm per day, of less virulent isolates. Less virulent isolates included P. irregulare, P. oligandrum and P. aphanidermatum. The latter was unexpected, as P. aphanidermatum is an important species in pythium root rot epidemics in chrysanthemums elsewhere. The value of the detached leaf assay for screening large numbers of isolates was demonstrated in a survey of isolates from clinic samples from chrysanthemum nurseries and in a series of dilution‐plating experiments looking at numbers of Pythium propagules in commercial chrysanthemum beds showing root rot. In the survey, the predominant pathogenic species was identified as P. sylvaticum and the most likely source of infection was contaminated soil as opposed to blocking media or irrigation water, whilst in soil colonization studies the use of detached leaf assays demonstrated a relationship between pathogenic inoculum concentration in soil and the expression of root rot symptoms.

Introgression of Pythium root rot resistance gene into Rwandan susceptible common bean cultivars

A breeding scheme was carried out to introgress resistance genes to bean Pythiumroot rot in various commercial varieties grown in Rwanda. The achieved crosses were performed between three selected susceptible varieties (R617-97A, RWR 1668 and Urugezi) which are adapted to the various ecological production zones of Rwanda and two known sources of resistance to Pythium root rot(RWR719 and AND 1062). Following each inter varietal hybridization generation, a series of 4 successive back-crosses was achieved by using the susceptible parents as the initial parent lines to be improved for their respective behavior to Pythium root rot disease. At each back-cross generation, the PYAA 19800 SCAR marker linked toPythium root rot resistance in the two sources of resistance (varieties RWR 719 and AND 1062) was used to identify and to proceed to early selection of progenies possessing the gene of interest. The target materials serving for the molecular analyses were prepared from young trifoliate leaves of 2-weeks bean plantlets. It was observed that at each back-cross generation, there were variable proportions of plants exhibiting presence of the resistance gene according to the SCAR marker profiles. In addition to that observation mainly based to molecular profile, it was also revealed that the proportion of bean seeds having the same color as the susceptible parent line was increasing progressively. Finally, to assess if the individual plants exhibiting the SCAR marker are effectively resistant to the Pythium root rot disease, inoculation tests were carried out with a Pythium ultimum strain on each of them. This ultimate biological evaluation revealed that all the plants showing the SCAR marker were resistant to development of Pythium root rot symptoms after inoculation, confirming thus real introgression of the resistance characteristics through the breeding scheme adopted in our work. Key words: Phaseolus, vulgaris, Pythium, molecular, root rot, bean, genotype, resistance.

Pythium Root Rot

This fungal root disease affecting all warm season turfgrasses is associated with wet soil conditions and causes nonspecific decline in turf quality. This revised 2-page fact sheet was written by M. L. Elliott and P. F. Harmon, and published by the UF Department of Plant Pathology, February 2011. SS-PLP-11/LH050: Pythium Root Rot (ufl.edu)

First Report of Pythium myriotylum Causing Root and Stem Rot on Tobacco in Zimbabwe.

Pythium species have a wide host range and are important pathogens of many agricultural crops. In Zimbabwe, 15 isolates of Pythium have been obtained from symptomatic tobacco (Nicotiana tabacum) in the new float seedling production system. This production system now accounts for 25 to 30% of the tobacco industry's annual requirement of 975 million seedlings. Disease symptoms are observed usually 5 to 6 weeks after sowing as wilting and yellowing of leaves followed by rotting of the roots, and in severe cases, seedling death. Up to 70% seedling loss has been reported in commercial seedbeds. In a study to fulfill Koch's postulates and to determine the susceptibility of 16 commercially grown tobacco cultivars, seedlings were produced in float trays and inoculated individually with 1 ml of Pythium (Isolate Py 19) spores (1 × 104 CFU/ml) and mycelium pipetted around the base of the stem of each seedling 9 weeks after sowing. First symptoms appeared 7 to 10 days after inoculation as yellowing and wilting of leaves. When seedlings were pulled, the lower portion of the stem and roots were brown and rotted. Seedling mortality averaged 29% and disease incidence was 96 to 100% among cultivars. All 16 tobacco cultivars were susceptible to Pythium root and stem rot and there were no significant (P > 0.05) differences in their susceptibility to the disease. The pathogen was reisolated from the inoculated seedlings. The representative isolate (Py 19) sent to Centraalbureau voor Schimmelcultures, the Netherlands was identified as Pythium myriotylum Drechsler (CBS Accession No. 125021) (2). P. aphanidermatum, causing black stem rot, and P. debaryanum and P. ultimum, responsible for damping-off (3), have been reported in the predominant conventional soil-based tobacco seedling production system, but do not cause economic losses. However, stem and root rot caused by P. myriotylum threaten the float seedling production system in Zimbabwe, although a chemical curative control of the disease has been recommended and is now widely practiced. P. myriotylum has previously been reported in the tobacco float seedling production system in South Carolina (1). To our knowledge, this constitutes the first published report of P. myriotylum on tobacco in Zimbabwe.

First report of pythium root rot on celery (Apium graveolens var. dulce) caused by Pythium dissotocum Drechsler in Japan

First report of pythium root rot on celery (Apium graveolens var. dulce) caused by Pythium dissotocum Drechsler in Japan

First report of Pythium root rot of chrysanthemum in Vietnam and control with metalaxyl drench

This is the first report of Pythium root rot of chrysanthemum, and the pathogen responsible, Pythium aphanidermatum, from Quang Nam and TT Hue Provinces in Vietnam. The typical symptoms included stunting, wilting, and necrosis of the main, lateral and feeder rootlets. A black necrotic lesion girdling the lower stem was present in some diseased plants. The root rot commonly caused death of diseased plants and affected plants at all growth stages. The pathogen was also isolated from transplants affected by root rot, and sampled directly from boxes transported from Dalat, Lam Dong Province, the source of most transplants used in Quang Nam. Metalaxyl was shown to provide effective control of Pythium root rot in a field trial when used to dip transplants before planting, and as a regular drench. Metalaxyl also provided effective control of the disease in commercial crops on the farm where the field was located.

Pythium Root Rot and Growth Responses of Organically Grown Geranium Plants to Beneficial Microorganisms

Pythium root rot, caused by Pythium ultimum, is responsible for important losses in geranium plant production, mainly as a result of the decrease in the plant overall quality. An organic production system for geranium plants based on fertilization using a filtered suspension of dehydrated hen manure was compared with a conventional fertilization system to evaluate their tolerance to root disease. Under typical greenhouse conditions, geranium plants were inoculated with a suspension of Pseudomonas putida, Trichoderma atroviride, a mixture of both or with Trichoderma harzianum, and a commercially available product, Rootshield®, 1 and 4 weeks after planting. Four weeks after the first inoculation, Pseudomonas putida and Trichoderma atroviride stimulated plant growth (shoot and root dry weight) compared with the control regardless of the fertilization. The results also showed that the colonization of geranium roots by Pythium spp. was significantly lower for organically grown plants for all treatments compared with the inoculated control under conventional fertilization. Inoculation with T. atroviride under conventional fertilization was the only treatment that did not significantly reduce root colonization by Pythium spp. compared with the conventional control. For both organically and conventionally grown plants, the coinoculation with both P. putida and T. atroviride resulted in the weakest colonization of roots by the pathogen. The inoculation of P. putida, T. atroviride, and the mixture of the bacterium and the fungus also significantly increased the fresh and dry weight of roots regardless of the fertilization used. All microorganism treatments in conventionally grown plants significantly increased the fresh and dry weight of the shoot compared with the control.