Baruch Sneh

Rhizoctonia species: taxonomy, molecular biology, ecology, pathology and disease control

Preface B. Sneh, et al. Introduction: The Genus Rhizoctonia A. Ogoshi. I: Taxonomy and Evolution of Rhizoctonia spp. I.A. Classical Methods. I.B. Biochemical and Molecular Methods. II: Genetics, and Pathogenicity of Rhizoctonia spp. III: Plant-Pathogen Interactions of Rhizoctonia spp. IV: Ecology of Rhizoctonia spp., Population and Disease Dynamics. V: Characterization of Rhizoctonia spp. Isolates, Disease Occurrence and Management in Various Crops. VI: Control of Disease Caused by Rhizoctonia Species. VI.A. Cultural Control. VI.B. Biological Control. VI.C. Plant Germ Plasm for Resistance Against Rhizoctonia. VI.D. Chemical Disease Control of Rhizoctonia Species. VI.E. Integrated Control of Rhizoctonia Species. Index.

Digestion of δ-endotoxin by gut proteases may explain reduced sensitivity of advanced instar larvae of Spodoptera littoralis to CryIC

The present study describes the correlation between gut protease activity of lepidopteran larvae of different instars, the inactivation of Bacillus thuringiensis delta-endotoxins in crystalline and noncrystalline forms, and the reduced susceptibility of advanced larval instars of Spodoptera littoralis to the toxin. The original assembly of delta-endotoxins in a crystal structure is essential for causing efficient larval mortality. Denaturation and renaturation (D/R) of delta-endotoxin crystals increased the vulnerability of the toxin molecules to proteolysis, reduced their capability to kill neonate larvae of S. littoralis, but sustained most of their larval growth-inhibition activity. E. coli-produced CryIC delta-endotoxin applied as a fraction of inclusion bodies exerted a growth inhibition effect, similar to the molecules released from the crystals by denaturation and subsequent renaturation. Incubation of CryIC with gut juice of 1st or 2nd instar larvae, left part of the CryIC toxin intact, while the toxin was completely degraded when incubated with gut juice of 5th instar larvae. The degradation rate was consistent with the increase of protease specific activity of the gut juice during larval development. This increase in toxin degradation may account for the loss of sensitivity of 5th instar larvae to CryIC. Specific protease inhibitors such as PMSF and Leupeptin were shown to inhibit gut proteases activity in all instar larvae, while, 1,10 phenanthroline, TLCK and TPCK were effective only in young instar larvae. The differential effect of protease inhibitors on proteases obtained from different larval instars indicated that gut juice protease profiles change with larval age. The observed quantitative and qualitative differences in degradation of delta-endotoxin by larval gut proteases that occur during larval maturation may account for the difference in susceptibility to the delta-endotoxin. This finding should be taken into consideration when designing strategies for the development of transgenic crops expressing delta-endotoxins as potent insecticidal proteins.

Purification and characterization of pectolytic enzymes produced by virulent and hypovirulent isolates of Rhizoctonia solani Kuhn

Four pectolytic enzymes were purified from an isolate of Rhizoctonia solani (No. 82, AG-4) virulent on a wide range of hosts (Ichielvich-Auster et al. 1985, Phyloparasilica vol. 13, 103–112). The enzymes designated as endopolygalacturonase I and II (endoPG-I and endoPG-II), pectinesterase (PE) and endopectinlyase (endoPL) have been purified to homogeneity by a single chromatographic step on a cross linked polypectate column. These enzymes were identified also in two virulent isolates of R. zeae and two virulent binucleate Rhizoctonia spp. The endoPG-I, endoPG-II and PE but not endoPL were identified in three hypovirulent isolates of R. solani and two of R. zeae. These enzymes were purified to homogeneity from R. solani (No. 521, AG-4). The molecular weight (mol.wt), pH optimum, isoelectric point (pI) and optimal temperature (T) for each enzyme were endoPG-I, mol.wt 34 000, pH 4·8, pI 6·8, T 50°C); endoPG-II, mol.wt 37 000, pH 5·4, pI 7·4, T 42°C; PE, mol.wt 26 000, pH 7·7, pI 6·2, T 48°C; and endoPL (mol.wt. 45 500, pH 8·4, pI 8·1, T 53°C.

Isolation of a Virus from Virulent Strains of Rhizoctonia solani

Summary Double-stranded RNA viruses were detected in virulent strains of Rhizoctonia solani. The virus particles were 33 nm in diameter, contained two or three major segments of dsRNA (mol. wt. 1.6 × 106, 1.45 × 106 and 1.25 × 106), had a density of 1.34 g/ml and had an estimated sedimentation coefficient of 161S. The major coat protein had a mol. wt. of 46000. An RNA-dependent RNA polymerase activity was associated with the viral capsids. Native hypovirulent strains of R. solani were devoid of these viruses and attenuation of virulent strains to hypovirulence appeared to be correlated with the loss of some or all of the segments of dsRNA. Transmission of dsRNA from a virulent to a hypovirulent strain by heterokaryon formation was associated with the transmission of virulence.

Biological control of black scurf on potato under organic management

Abstract Field experiments showed that Trichoderma harzianum , nonpathogenic Rhizoctonia (np-R) and cattle manure compost amendment (CMC-H) applied in furrow could reduce black scurf incidence in organically grown potatoes. Incorporation of T. harzianum applied to the soil surface had a relatively small effect compared to the in-furrow treatment. Application of two isolates of nonpathogenic-binucleate Rhizoctonia (RS 521 and RU 56-8-AG-P) also significantly reduced the incidence of infected tubers in field experiments. Although treatments significantly reduced disease incidence and severity, total yield was unaffected. For the first time the efficiency of T. harzianum and np-R in reducing the incidence of black scurf on daughter tubers was demonstrated using naturally infested soil and contaminated seed tubers.

Lethality and developmental delay in Drosophila melanogaster larvae after ingestion of selected Pseudomonas fluorescens strains.

Background The fruit fly, Drosophila melanogaster, is a well-established model organism for probing the molecular and cellular basis of physiological and immune system responses of adults or late stage larvae to bacterial challenge. However, very little is known about the consequences of bacterial infections that occur in earlier stages of development. We have infected mid-second instar larvae with strains of Pseudomonas fluorescens to determine how infection alters the ability of larvae to survive and complete development. Methodology/Principal Findings We mimicked natural routes of infection using a non-invasive feeding procedure to study the toxicity of the three sequenced P. fluorescens strains (Pf0-1, SBW25, and Pf-5) to Drosophila melanogaster. Larvae fed with the three strains of P. fluorescens showed distinct differences in developmental trajectory and survival. Treatment with SBW25 caused a subset of insects to die concomitant with a systemic melanization reaction at larval, pupal or adult stages. Larvae fed with Pf-5 died in a dose-dependent manner with adult survivors showing eye and wing morphological defects. In addition, larvae in the Pf-5 treatment groups showed a dose-dependent delay in the onset of metamorphosis relative to control-, Pf0-1-, and SBW25-treated larvae. A functional gacA gene is required for the toxic properties of wild-type Pf-5 bacteria. Conclusions/Significance These experiments are the first to demonstrate that ingestion of P. fluorescens bacteria by D. melanogaster larvae causes both lethal and non-lethal phenotypes, including delay in the onset of metamorphosis and morphological defects in surviving adult flies, which can be decoupled.

INDUCED RESISTANCE OF CUCUMBER SEEDLINGS CAUSED BY SOME NON-PATHOGENIC RHIZOCTONIA (NP-R) ISOLATES

Among 153 isolates ofRhizoctonia spp. obtained from 95 soil samples collected from different fields in the USA, 42 (27.5%) isolates were hypovirulent or non-pathogenic on cabbage (tested on tap water agar plus 250 μg/ml chloramphenicol plates). Of these, 14 (33.3% of the np-R) isolates protected >60% of the cabbage seedlings againstR. solani, and the best eight isolates protected 73–95% of the cucumber seedlings. The np-R isolates RU56-8 (AG-P) and RU89-1 [AG-B(o)] induced the highest resistance against hypocotyl challenge inoculation with virulentR. solani (38.3–85.7%), whereas most of the challenged control seedlings (85–100%) collapsed. Similarly, isolates RU56-8 and RU89-1 induced the highest resistance (22.2–87.5%) against hypocotyl challenge inoculation withPythium aphanidermatum, whereas most of the challenged control seedlings collapsed (90–100%). Isolates RU56-8 and RU89-1 significantly reduced the lesion numbers and area/leaf (to 8.9–42.0% of the control) caused by challenge inoculation of the first true leaves withPseudomonas syringae pv.lachrymans. No np-R isolate could be recovered from the upper hypocotyls or from the leaves, indicating that there was no contact between the inducer and the pathogen. Root colonization with some np-R increased seedling tolerance to low soil moisture levels.

Pathogenicity, host specificity and anastomosis groups ofRhizoctonia spp. isolated from soils in Israel

One hundred and twenty-nine isolates ofRhizoctonia spp. were obtained from soil samples in Israel and from culture collections in the U.S.A. and Japan. The isolates varied in host range and disease severity when tested on six to eleven different host plants. Approximately 30% of the isolates were nonpathogenic to all the host plants tested. Mycelial growth rate of the nonpathogenic strains did not differ significantly from that of the virulent isolates. The 107 Israeli isolates represented anastomosis groups (AG) 1, 2, 3, 4, 5 and 6 ofR, solani, two groups ofR. zeae, and three groups of binucleateRhizoctonia AG: A, F and K.

The allelopathic potential ofCoridothymus capitatus L. (Labiatae). Preliminary studies on the roles of the shrub in the inhibition of annuals germination and/or to promote allelopathically active actinomycetes

SummarySuppression of annuals at various intensities was observed around some shrubs ofCoridothymus capitatus growing on kurkar formation in the coastal hills of Israel. The phenomenon was clearly observed as annuals-free belts of 15–20 cm around ‘aggressive’ shrubs. Quantitatively, density of annuals decreased by 16 fold in the annual-free belts as compared to a distance of 60–80 cm from the canopies of the shrubs. Their dry matter was decreased by 5.4 fold around the shrubs. Suppression rate of emergence of planted seeds of annuals (Plantago psyllium andErucaria hispanica) early in the season was 45% higher around ‘aggressive’C. capitatus than that around ‘non-aggressive’ ones.In the laboratory, seed germination of the annuals was strongly suppressed by diffusates and volatiles from shoots, as well as from their water extracts and their essential oils.Incubation of fresh shoots ofC. capitatus in soil collected from around ‘non-aggressive’ shrubs, for 7 days, increased population levels of actinomycetes by 9.6 fold and by 36.7 fold when soil was collected from around ‘aggressive’ shrubs. Isolates of some soil-borne actinomycetes inhibited germination of the test plantsLactuca sativa andAnastatica hierochuntica on agar plates (4–98%). The preliminary results indicate a possible synergistic inhibitory effect induced by essential oils of the aromatic shrub and the phytototic activity of actinomycetes.

Quantitative evaluation of the microbial nutrient sink in soil in relation to a model system for soil fungistasis

Abstract A quantitative approach was devised to evaluate the influence of soil microbial activity as a sink for nutrients exuded by fungal spores as a factor in soil fungistasis. The approach was based on measuring the CO 2 evolved from microbial respiration of 14 C-labelled exudates from conidia of Cochliobolus victoriae incubated on soil. The amount of exudate lost by spores on soil was greater than the amount lost by spores incubated on a bed of sand undergoing leaching at a flow rate of 110 ml h −1 . where restriction of germination was similar to that on soil. Increasing flow rates in the leaching system increased spore exudation and reduced germination. Germination of C . victoriae conidia on membrane filters floated on distilled water decreased as the volume of water increased. The results indicate that the microbial nutrient sink of soil is sufficient to impose soil fungistasis.