Rhizoctonia spp. Causing Root and Hypocotyl Rot in Phaseolus vulgaris in Cuba

Sixty isolates of Rhizoctonia spp. were obtained from Cuban bean fields during the period 2004–2007. Isolates were characterized with different techniques, including nuclei staining, pectic zymogram, PCR–RFLP analysis of the rDNA–ITS region and sequencing of the rDNA–ITS region. The majority of the isolates were identified as multinucleate Rhizoctonia solani isolates, representing two different anastomosis groups (AGs), AG 2‐2 WB and AG 4 HGI; the remaining isolates were binucleate Rhizoctonia isolates and belonged to AG F and AG A. AG 4 HGI isolates were equally distributed in all soil types; AG 2‐2 isolates were more frequently isolated from cambisols, whereas AG F isolates were related to calcisols. Pathogenicity experiments in vitro and in the greenhouse, revealed that binucleate isolates only caused root rot, whereas R. solani isolates were able to cause root rot and hypocotyl rot. Furthermore, differences in virulence level were observed between R. solani and binucleate isolates and among different AGs. Isolates of R. solani AG 4 HGI and R. solani AG 2‐2 WB were the most aggressive, binucleate isolates of AG F were intermediate aggressive, whereas a binucleate isolate of AG A was weakly aggressive. In contrast with other reports about R. solani in bean, web blight symptoms were never observed during this study.     

Molecular characterization of Rhizoctonia solani isolates the incitant of soybean root rot

Rhizoctonia solani isolates used in this investigation were identified as anastomosis-4 (AG-40), collected from different localities from Assiut governorate in Egypt. Pathogenicity test of seven isolates of R. solani was evaluated on soybean Giza 111 cultivar under greenhouse conditions. All tested isolates were able to infect soybean plants causing root rot with different degrees of severities, isolate No. 1, 2 and 3 showed significantly highest root rot severity, while isolate No. 5 gave the lowest percentage of root rot rating. The sodium dodecyl sulphate polyacrylamide gel electrophoresis patterns were used to compare three isolates of R. solani. There are no variations among R. solani isolates except a few exceptions according to their protein patterns. DNA markers obtained from all isolates showed genetic similarity among different isolates obtained from different geographical regions barring few exceptions. Correlation between DNA patterns of R. solani isolates and their virulence was detected, but no correlation with anastomosis groups (AG).     

Genetic Diversity of Thanatephorus cucumeris Infecting Tomato in Iran

The necrotrophic fungus Thanatephorus cucumeris (anamorph Rhizoctonia solani) is among the most important soil‐borne pathogens which causes tomato foot and root rot worldwide. We investigated virulence and genetic relationships among and within different taxonomic groups of R. solani from the tomato‐growing regions in the north‐east of Iran. Characterization of R. solani taxonomic groups revealed that, of 56 isolates, four were AG‐2‐1, 16 were AG‐3 PT, 21 were AG‐4 HG‐I and 15 were AG‐4 HG‐II. Because interprimer binding site (iPBS), which is based on amplification of retrotransposons, is known as novel and powerful DNA fingerprinting technology, we selected four iPBS primers, which can detect polymorphisms of tomato foot root and root rot pathogen, for investigating genotypic variability of the isolates. The iPBS analyses separated various taxonomic groups of R. solani and showed great diversity among the isolates, demonstrating that the R. solani isolates obtained from tomato were not a clonal population. Crop rotation strategies and geographic location seem to be important factors affecting genetic structure of the isolates. Pathogenicity tests on tomato cultivar ‘Mobil’ showed significant differences in the virulence of various isolates. The overall results indicated that isolates of AG‐3 and AG‐4 were more virulent than AG‐2‐1. There was no significant correlation between genetic diversity and virulence of the isolates. This is the first report of R. solani AG‐4 HG‐II, causing tomato foot and root rot. Also, our research is the first in assessment of genetic diversity in fungal populations using iPBS molecular markers.     

Variability in the Sensitivity of Rhizoctonia solani Anastomosis Groups to Fungicides

Tolclofos-methyl, iprodione and cyproconazole, among the eleven fungicides tested in vitro, gave consistently strong inhibition against all ten anastomosis groups (AGs) of Rhizoctonia solani. Carboxin, furmecyclox, thiabendazole, fenpropimorph and vinclozolin also inhibited all AGs but with wide variations in toxicity levels (EC90 values). Pencycuron showed strong activity against four AGs but was ineffective against the other six AGs. Generally, R. solani AGs were insensitive to fenarimol and imazalil. Tolclofos-methyl strongly inhibited 23 AG2-1 and 20 AG4 rapeseed/canola R. solani isolates from different locations in Saskatchewan, Alberta and Manitoba. The same isolates were also sensitive to iprodione, cyproconazole and carboxin. All AG4 canola isolates were insensitive to pencycuron (EC90 > 500 mg/l) while AG2-1 isolates showed highly variable levels of sensitivity with EC90s ranging from 0.5 to 220 mg/l. Tolclofos-methyl, applied to Brassica napus (canola) cv. Westar seed at 1 g a.i./kg, provided 75—100 % control of seedling damping-off in pots infested with AG2-1 or AG4 isolates. In parallel experiments, pencycuron (1 g a.i./kg seed) failed to control damping-off by AG4 canola isolates and gave variable disease control against AG2-1 isolates.     

Sulphur and phosphorus requirements of three fungi causing diseases in storage

Summary The effect of different sulphur and phosphorus compounds on the growth and reproduction of three fungi causing storage rot, viz.,Fusarium solani, Botryodiplodia ananassae andMacrophomina phaseoli has been studied. Sixteen different sources of sulphur were used and out of them magnesium sulphate was found to be most favourable for the growth and reproduction of all the three fungi. Sodium sulphite and sodium bisulphite were toxic. Potassium metabisulphite prevented growth ofF. solani and M. phaseoli while it supported moderate growth ofB. ananassae. Only magnesium sulphate could induce the sporulation ofB. ananassae while sporulation and sclerotial development ofF. solani andM. phaseoli respectively varied with the type of sulphur sources used. Optimum concentration of magnesium sulphate was also determined and it was found that the growth and sporulation ofF. solani andB. ananassae were best at 0.375 g/l and 0.75 g/l.M. phaseoli tolerated higher doses of this substance as the best growth and excellent sclerotial development were recorded at 3.0 g/l (the maximum concentration used). Phosphorus was found to be essential for the present fungi as none of them could grow in complete absence of this substance. Onthophosphates and nucleic acid, were found to be favourable sources for growth and reproduction of the 3 organisms.     

First Report of Rhizoctonia solani AG‐7 on Cotton in Egypt

Eighty‐two isolates of Rhizoctonia solani were recorded from roots of naturally‐infected seedlings of the Egyptian cotton (Gossypium barbadense L.). Anastomosis groups (AGs) of the isolates were determined by using 13 different AGs testers. Three (3.7%) of the isolates were identified as R. solani AG7, while the remaining isolates were belonging to the AG 2‐1, AG4 and AG5. The identification of the three isolates was based on the frequency of the C2 reaction with the AG7 tester isolate. No fusion was observed between AG7 and isolates representing the other 13 AGs. Colonies of AG7 isolates grown on potato dextrose agar (PDA), malt yeast agar (MYA) and melt peptone agar (MPA) were brown to dark brown with aerial mycelium and sclerotia. The isolates had pitted sclerotial clusters and brownish exudates after 21 days of culturing on PDA, but without clear zonation. Pathogenicity test under greenhouse conditions revealed that AG7 caused the common symptoms of damping–off, which included seed rot, lesions on the hypocotyls and root rot.     

Effect of mycorrhiza and biofertilisers on reducing the incidence of Fusarium root and pod rot diseases of peanut

Pathogenicity tests of twenty-six fungal isolates were tested on peanut plants (Giza 5 cv.) and the results revealed that, Fusarium oxysporum isolate (No. I) followed by F. solani (No. II) then F. moniliforme (No III) significantly caused highest incidence of root rot disease. Also, F. moniliforme (No III) followed by F. solani (No II) then F. oxysporum (No I) gave the highest incidence of pod rot disease. The effectiveness of vescular arbuscular-mycorrhiza (VAM) at different application rates on the incidence of root rot, pod rot diseases and plant growth parameters of peanut was studied. All soil treatments with each rate of VAM significantly reduced root and pod rot diseases compared with control (rate 0%). The best reduction in the severity of both diseases with VAM was found at the rate of 3%. Application of rhizobacterin, microbin and cerialin biofertilisers at the different concentrations decreased the severity of both root rot and pod rot severity diseases compared with non-treated seeds. The greatest reduction in both diseases was achieved at a concentration of 8/100?g seeds. The highest number of pods and fresh weight (g) was achieved in seed supplemented with each biofertiliser at concentration of 8/100?g seed.     

First Report of Rhizoctonia solani AG 8 on Wheat in Turkey

A survey was conducted in Ankara and Eskisehir provinces of Turkey for determining anastomosis groups and pathogenicity of Rhizoctonia species associated with root and crown rot of wheat. Pathogenicity tests revealed that Rhizoctonia solani AG 8 caused the common symptoms of damping‐off and stunting.     

Growth promotion and disease suppression ability of Pseudomonas fluorescens in acid lime

Biochemical analysis was carried out for 10 isolates of Pseudomonas fluorescens. All isolates were found positive for siderophore and indole acetic acid production (except Pf IX) and phosphate solubilisation (except Pf VII). Biochemically efficient strains (Pf I, Pf IV, Pf VII and Pf IX) were selected for management of root rot, collar rot and damping off caused by Phytophthora parasitica, Pythium sp. and Fusarium solani. The activity of defence-related enzymes like esterase, peroxidase and polyphenol oxidase was also detected by polyacrylamide gel electrophoresis (PAGE). Consistent appearance of esterase isozyme bands was visualised, in the range of 0.193–0.349. The highest Rm value 0.349 was observed on Pf IV. Whereas, for peroxidase, Rm value ranged between 0.302 and 0.373 and for polyphenol oxidase, it was in the range of 0.211–0.800. P. fluorescens Pf IV was found significantly effective to arrest the per cent mycelial growth of P. parasitica (55.20%), Pythium sp. (65.33%) and F. solani (64.67%). Among bioagents, seed treatment and soil drenching with Pf IV at 15 and 30 days after sowing were found effective to reduce per cent disease incidence (30.55%) at 120 days after emergence. Seed treatment with copper oxychloride at 3g/kg seed and metalaxyl at 2 g/kg seed were also found effective.     

Response of different forms of propagules of Rhizoctonia solani AG2-1 (ZG5) exposed to the volatiles produced in soil amended with green manures

The sensitivity of different forms of propagules of Rhizoctonia solani anastomosis group (AG)2‐1/zymogram group (ZG)5 to volatile compounds produced in soil amended with green manure will influence the efficacy of green manures used to manage the disease. In laboratory experiments, we determined the impact of volatiles arising from residues of five species of Brassicaceae, and Avena sativa (oat), a non‐Brassicaceae species, and volatiles of pure allyl isothiocyanate (AITC) or 2‐phenylethyl isothiocyanate (2‐PEITC) in either their soluble or vapour phase on the hyphal growth of R. solani arising from different propagules. The brassicaceous species were Brassica napus var. Karoo, B. napus B1, B. napus B2, B. nigra and Diplotaxis tenuifolia (a brassicaceous weed). Colony growth and hyphal density on water agar were measured up to 7 days. The amendment of a sandy soil with green manures at a high (100 g kg^?1, 10%) concentration generally suppressed the growth of the pathogen, but at a low (10 g kg^?1, 1%) concentration, the amendment had little effect on the radial fungal growth of the pathogen but increased the density of hyphae through more branching. The inhibition by volatiles from the residues of Brassicaceae species at 10% concentration was stronger (82–86%) than that by volatiles from oat (64%) amendment at the same rate. Hyphae arising from sclerotia and precolonised ryegrass seed were less sensitive than hyphae growing out of potato dextrose agar plugs. This indicates that thick‐pigmented cell walls may have protected the fungus from these unfavourable conditions. Pure AITC and 2‐PEITC in the range of 0.5–2.0 mM inhibited the growth of R. solani from all forms of propagules, but hyphae originating from agar plugs were the most vulnerable compared with the two others. Thus, hyphae arising from the medulla of the sclerotia may be relatively tolerant to volatile compounds emanating from decomposing Brassica green manure amendments in the field and in vitro inhibition of the vegetative growth of the pathogen may not reflect its response to the amendments in the field.     

Interactions Between Rhizoctonia Root Rot and the Foliar Common Bean Diseases Anthracnose and Rust

The effects of co‐inoculation of Rhizoctonia solani and Colletotrichum lindemuthianum or Uromyces appendiculatus at different inoculum levels were studied on the disease dynamics and on the growth of bean plants under greenhouse conditions. Bean seeds were sown in R. solani‐infested soil. Additional experiments in which seedlings were transplanted to infested soil were also carried out. Conidial suspensions of C. lindemuthianum or uredospores of U. appendiculatus were inoculated onto leaves at plant developmental stages V2 and V3, respectively. Interactions between root rot and the aerial diseases were observed depending on the inoculum levels and on the timing of R. solani inoculation. Anthracnose severity tended to be higher on R. solani‐infected plants. Conversely, R. solani infection significantly reduced diameter of pustules and rust severity. When seedlings were transplanted to soil infested with low levels of R. solani, root rot severity and density of R. solani in the soil were magnified at high levels of C. lindemuthianum or U. appendiculatus. In these experiments, a synergistic interaction between root rot and anthracnose was observed to affect the plant dry weight. Antagonistic effects on the plant dry weight were found for the combination root rot/rust only when seeds were sown in infested soil.     

Pseudomonas induces salinity tolerance in cotton (Gossypium hirsutum) and resistance to Fusarium root rot through the modulation of indole-3-acetic acid

Abiotic stresses cause changes in the balance of phytohormones in plants and result in inhibited root growth and an increase in the susceptibility of plants to root rot disease. The aim of this work was to ascertain whether microbial indole-3-acetic acid (IAA) plays a role in the regulation of root growth and microbially mediated control of root rot of cotton caused by Fusarium solani. Seed germination and seedling growth were improved by both NaCl and Mg₂SO₄ (100 mM) solutions when treated with root-associated bacterial strains Pseudomonas putida R4 and Pseudomonas chlororaphis R5, which are able to produce IAA. These bacterial strains were also able to reduce the infection rate of cotton root rot (from 70 to 39%) caused by F. solani under gnotobiotic conditions. The application of a low concentration of IAA (0.01 and 0.001 μg/ml) stimulated plant growth and reduced disease incidence caused by F. solani (from 70 to 41–56%, respectively). Shoot and root growth and dry matter increased significantly and disease incidence was reduced by bacterial inoculants in natural saline soil. These results suggest that bacterial IAA plays a major role in salt stress tolerance and may be involved in induced resistance against root rot disease of cotton.     

Impact of Continuous Cropping of Lettuce on the Disease Dynamics of Bottom Rot and Genotypic Diversity of Rhizoctonia solani AG 1‐IB

The impact of continuous cropping of lettuce on the disease dynamics of bottom rot and genotypic diversity of the causal pathogen Rhizoctonia solani AG 1‐IB was studied over 3 years with two crops per year within a field naturally infested with R. solani the pathogen. This field had not had lettuce cultivated in it for 7 years. The disease incidence (DI) and disease severity (DS) were assessed at each harvest and mapped. Surprisingly, a high DI was already observed in the first crop of year one of this field study. In addition, the pathogen was also found to be evenly distributed. Severely infected plants occurred mainly in patches, and the position varied between crops. A significant increase in DS was already observed in the second year, and both temperature conditions and continuous cropping influenced the DS on average over time. Rhizoctonia isolates were randomly collected from the first crop in 1999 and the sixth crop in 2001. The genotypic diversity within the subgroup of R. solani AG 1‐IB was analysed by BOX‐PCR genomic fingerprinting and the aggressiveness of isolates by bioassay. The fingerprints revealed a high level of genotypic diversity within the AG 1‐IB field population. However, continuous cropping was found not to have an impact on genotypic diversity and aggressiveness.     

Identification and Pathogenicity of Rhizoctonia spp. Causing Wirestem of Betula nigra in China

During July 2004, wirestem was frequently observed on the seedlings of Betula nigra at Dehong district in Yunnan Province, China. Isolates of Rhizoctonia spp. consistently obtained from their diseased leaves, roots and stems were identified as belonging to binucleate Rhizoctonia anastomosis groups (AG) AG‐P and AG‐R, and R. solani AG‐I IB and AG‐4 HG‐I, based on cultural characteristics, nuclear staining, anastomisis reaction and analysis of their ITS rDNA region. The percentage of recovery of AG‐P, AG‐1, AG‐R and AG‐4 was 48%, 39%, 8% and 3%, respectively. This is the first report of wirestem of red birch cause by binucleate Rhizoctonia AG‐P and AG‐R, and R. solani AG‐1 IB and AG‐4 HG‐I in China.     

Cowpea Root Rot Severity and Metabolic Changes in Relation to Manganese Application

Severity of root rot (Rhizoctonia solani and Rhizoctonia bataticola) of cowpea (Vigna unguiculata L.) was reduced by 42.7 and 42.0%, respectively over control following the application of 10 μg/g Mn as manganese sulphate. Reduction in disease incidence was associated with increased levels of polyphenol oxidase (PPO), peroxidase (PO) and total phenols. PO activity was several times more as compared with PPO‐specific activity and increased markedly after infection either with R. solani or R. bataticola. Contrary to PPO and PO, the specific activity of catalase declined sharply. Infection also caused an increase in the content of reducing sugars, Cu, Zn and Mn but a decrease in o‐dihydric phenols, flavanols, total soluble sugars, non‐reducing sugars and Fe contents. It is suggested that Mn at the rate of 10 μg/g soil can be used to manage the root rot of cowpea.     

Potential of Allyl Isothiocyanate to Control Rhizoctonia solani Seedling Damping Off and Seedling Blight in Transplant Production

The synthetic mustered flavouring essential oil, allyl isothiocyanate (AITC), was evaluated for its effect on suppression of Rhizoctonia solani growth in vitro, and in field soils for reducing inoculum density, saprophytic substrate colonization and seedling damping off and blight using snap bean and cabbage as indicator plants. In vitro growth was completely inhibited at the concentration of 50 μl/l. Inoculum density and saprophytic substrate colonization by the fungus in soil were not affected by AITC concentrations of 50 or 75 μl/kg soil. The inoculum density estimation by the use of soil‐drop technique created an artefact leading to an erroneous conclusion that the fungus was eradicated from soil within 1–3 days after AITC treatment at 150 or 200 μl/kg soil. The saprophytic substrate colonization showed that although the activity of R. solani was greatly reduced, the fungus still colonized 45% of the substrate units at these concentrations, and up to 100% at lower concentrations within 1 day after treatment. At higher concentrations the recovery rate from the substrates gradually declined over time to <6%. Drenching R. solani infested sandy‐loam or silty‐clay‐loam soil with water containing the emulsified AITC to provide 150 or 200 μl/l soil, a few days prior to planting, gave over 90% disease control in snap bean and cabbage, with no apparent phytotoxic effect. The effect of AITC was not influenced by the physical soil texture. AITC appears to have a good potential to replace methyl bromide fumigation of the substrate used for transplant production.     

Dryopteris crassirhizoma Dryocrassin ABBA for Postharvest Control of the Potato Dry Rot Pathogen Fusarium solani var. coeruleum

Dryopteris crassirhizoma dryocrassin ABBA treatment was tested for its effectiveness in controlling Fusarium solani var. coeruleum growth in vitro and for prevention of postharvest dry rot of potato tubers and slices. Dryocrassin ABBA strongly inhibited mycelial growth, resulting in reductions in both mycelium dry weight and per cent spore germination of F. solani var. coeruleum at concentrations of 2.0, 0.5 and 0.1 mg/ml. Scanning electron microscopy (SEM) observations showed that treatment induced abnormal, tightly twisted morphological changes in hyphae. Moreover, in vivo experiments demonstrated that dryocrassin ABBA treatment at 2 mg/ml effectively controlled dry rot of potato tubers inoculated with mycelial discs of F. solani var. coeruleum. After dryocrassin ABBA treatment, the content of soluble proteins, peroxidase (POD) and superoxide dismutase (SOD) activity increased, and malondialdehyde (MDA) content remained the stable situation. In addition, the expression level of plant lipid‐transfer proteins (LTPs) genes – StLTPa1, StLTPa7, StLTPb1 and StLTPb3 – was upregulated. These results collectively demonstrate that dryocrassin ABBA can inhibit growth of Fusarium pathogens to induce disease resistance. On the other side, dryocrassin ABBA maybe induce potato defence responses.     

Evaluation of Clove Essential Oil as a Mycobiocide Against Rhizopus stolonifer and Fusarium solani,Tuber Rot Causing Fungi in Yam (Dioscorea rotundata Poir.)

To exploit natural products for plant disease control, the essential oil of Syzygium aromaticum (L.) Merr. & Perr. (clove) was investigated for its antifungal activity against Rhizopus stolonifer and Fusarium solani, the postharvested yam tuber rot pathogens. The essential oil was obtained by hydrodistillation using a Clevenger‐type apparatus. The chemical composition of the oil was determined by gas chromatography and gas chromatography coupled with mass spectrometry. Antifungal activities of the oil were tested in vitro against mycelia growth and spores germination. In situ tests were conducted on healthy yam tubers, and necrosis symptoms were assessed. Results showed that eugenol (79.4%), eugenylacetate (9.2%) and isocaryophyllene (7.0%) were the major components. The oil exerted antifungal activities with total inhibition (TI) of the mycelia growth of R. stolonifer and F. solani was recorded at 200 and 300 ppm, respectively, while TI of spores germination was recorded at 31.25 and 250 ppm, respectively. For the standard fungicide (Ridomil^®), TI value of mycelia growth was 1600 ppm for the both pathogens, while TI of spores germination were 200 ppm and 1600 ppm, respectively, for Rhizopus and Fusarium. In situ tests showed complete inhibition of yam tuber rot when the essential oil was applied at 2000 ppm for preventive tests. This oil also reduced significantly (P ≤ 0.05) necrosis development on yam tuber for curative test at the same concentration. Total inhibition of the necrosis by Ridomil (3000 ppm) was observed only for Rhizopus on preventive test. There were positive correlations between the oil concentration and the reduction of necrosis cause by R. stolonifer and F. solani. These findings showed that clove essential oil may serve as environmental friendly bio‐fungicide for the management of postharvest yam tuber rot.     

Efficiency of different application methods of biocontrol agents and biocides in control of Fusarium root rot on some citrus rootstocks

Abstract

The efficiency of two biocontrol agents (Trichoderma harzianum NB and Bacillus subtilis NB) and two commercial biocides (Plant Guard and Rhizo-N) in controlling Fusarium root rot disease on some citrus rootstocks was evaluated under artificially infested soil in green house.

Fusrium root rot on citrus rootstocks seedlings i.e. sour orange (SO), volkamer lime (VL), rangpur lime (RP) and cleopatra mandarin (CL) was successfully controlled by dipping the root system of such seedlings in water suspensions of each biological treatment i.e. Trichoderma harzianum (spore suspension 5×10⁶ spore/ml), Bacillus subtilis (cell suspension 8×10⁷ cell/ml), Plant Guard (3 g/l) and Rhizo-N (4 g/l), then transplanted into artificially infested soil with Fusarium solani and drenched with enough water suspension of such biological treatments. Plant Guard (3 g/l) and Rhizo-N (4 g/l) were highly effective treatments in decreasing infection and severity of the disease, Fusarium density in rhizosphere soil and colonization of Fusarium solani in the roots of all tested seedlings.

Meanwhile, root dipping or soil drenching with the same treatments individually gave the least effect in reducing root rot incidenceon all tested rootstocks compared with application of the two methods together.

It should be noted that using biocontrol agents and commercial biocides could be successfully used in controlling root rot pathogens on citrus in commercial greenhouses or under field conditions before transplanting in new reclaimed lands in the desert.     

Characterization and Pathogenicity of Rhizoctonia spp. from Onion in Amasya, Turkey

Forty‐two isolates of Rhizoctonia spp. were obtained from onion in Amasya, Turkey. Of these, 29% were Rhizoctonia solani (AG‐4), 69% were Waitea circinata var. zeae (Rhizoctonia zeae) and 2% were binucleate Rhizoctonia (AG‐B). Most of the isolates were recovered from rhizosphere soil. In pathogenicity tests on onion, R. solani AG‐4 caused the greatest disease severity, those of W. circinata var. zeae were moderately virulent but binucleate Rhizoctonia isolates were of low virulence. This is the first report of binucleate Rhizoctonia AG‐B and W. circinata var. zeae occurring on onion in Turkey.