Somatic Hybrids Between Potato and S. berthaultii Show Partial Resistance to Soil‐Borne Fungi and Potato Virus Y

Three tetraploid somatic hybrid lines produced by protoplast fusion between a dihaploid potato, Solanum tuberosum, cultivar BF15 and the wild potato species Solanum berthaultii were evaluated here for their response to different soil‐borne pathogens, that is Fusarium solani, Pythium aphanidermatum and Rhizoctonia solani as well as to infection by potato virus Y (PVY). Both hybrid and BF15 plants grown in vitro were inoculated with the tested pathogen strains, that is R. solani, P. aphanidermatum, or F. solani. The growth level and disease severity index of these plants were compared to the susceptible commercial cultivar Spunta. A better growth of inoculated hybrid plants and restricted disease symptoms were observed in comparison with the commercial plants. Under glasshouse conditions and after inoculation with R. solani and P. aphanidermatum, improved resistance of the hybrid plants to these pathogens was confirmed. Indeed, these plants showed no significant damage following inoculation and a better development in R. solani‐infected plants. The susceptibility of the hybrid tubers to R. solani, P. aphanidermatum, and to F. solani infection was also determined. A significant reduction of tissue colonisation was observed in all the hybrid lines compared to the cultivated cultivars. The STBc and STBd hybrids also showed improved resistance to the PVY ordinary strain (PVY^o) under glasshouse conditions.     

Pathogenic and Genetic Variation among the Isolates of Rhizoctonia solani (AG 1‐IA), the Rice Sheath Blight Pathogen

Sheath blight disease of rice caused by Rhizoctonia solani is one of the most dreaded plant diseases faced by the rice farmers all over the world. None of the commercially cultivated rice varieties have sufficient level of field resistance, and the disease is presently being managed by chemical pesticides. In this study, 40 isolates of rice sheath blight pathogen, collected from diverse rice ecosystems from 12 different states of India, were characterized for their morphological, pathological and genetic variation. The isolates showed wide morphological variation in terms of size of sclerotia and abundance of sclerotia production. The virulence of each pathogen isolate was studied on four rice varieties, that is TN1, IR 64, Tetep and Swarnadhan in glasshouse, and observations were taken by measuring the relative lesion height. The relative lesion heights produced by these isolates on four different rice varieties varied widely. Genetic variation of the isolates was analysed using ISSR markers. The primers based on AG, GA, AC and CA repeats were informative and revealed polymorphism among the isolates. The polymorphism information content (PIC) of the primers ranged from 0.80 to 0.96, while the resolving power (Rp) ranged from 3.7 to 15.35. Largely, grouping of the isolates happened based on their geographical origin. One isolate from Titabar, Assam, and another from Adialabad, Telangana, were quite distinct from rest of the isolates.     

Genetic Structure and Aggressiveness of Rhizoctonia solani AG1‐IA,the Cause of Sheath Blight of Rice in Southern China

One hundred and eighty isolates of Rhizoctonia solani AG1‐IA, the causal agent of rice sheath blight, were obtained from six locations in southern China. The genetic structure of R. solani isolates was investigated using random amplified polymorphic DNA (RAPD) markers, and a considerable genetic variation among R. solani isolates was observed. Most of the genetic diversity was distributed within populations, rather than among them. The distribution pattern of the genetic variation of R. solani appears to be the result of high gene flow (Nm) and low‐genetic differentiation among populations. The aggressiveness of R. solani was visually assessed by rice seedlings of five different cultivars in the glasshouse. All isolates tested were found to induce significantly different levels of disease severity, reflecting considerable variation in aggressiveness. The isolates were divided into highly virulent, moderately virulent and weakly virulent groups, and the moderately virulent isolates were dominant in R. solani population. No significant correlation was observed among the genetic similarity, pathogenic aggressiveness and geographical origins of the isolates. Information obtained from this study may be useful for breeding for improved resistance to sheath blight.     

Enhanced Defence Responses of Chickpea Plants Against Rhizoctonia solani by Pre‐Inoculation with Rhizobia

The aim of this work was to study the antagonist effect of two Rhizobium strains Pch Azm and Pch S.Nsir2 to Rhizoctonia solani and for an evaluation of the relative impact of rhizobia on the expression of the plant's defence response against Rhizoctonia. First, these strains reduced fungal growth observed in vitro using the same or separately Petri dishes. Moreover, these isolates led to reduced chickpea infection by R. solani, resulting from the direct effect of rhizobia on pathogens and possible induced resistance in chickpea. Concomitantly, reduction in infection was accompanied by enhanced level of defence‐related enzymes, phenylalanine ammonia lyase (PAL) and peroxidase (POX). An increased level of phenol content was recorded in the roots of bacterized plants grown in the presence of pathogen. The results promise the use of rhizobia for protection of chickpea against R. solani.     

Greek indigenous streptomycetes as biocontrol agents against the soil‐borne fungal plant pathogen Rhizoctonia solani

Aims

To examine the biocontrol potential of multiactive Greek indigenous Streptomyces isolates carrying antifungal activity against Rhizoctonia solani that causes damping‐off symptoms on beans.

Methods and Results

A total of 605 Streptomyces isolates originated from 12 diverse Greek habitats were screened for antifungal activity against R. solani DSM843. Almost one‐third of the isolates proved to be antagonistic against the fungus. From the above isolates, six were selected due to their higher antifungal activity, identified by analysing their 16S rRNA gene sequence and studied further. The obtained data showed the following: firstly, the isolates ACTA1383 and ACTA1557 exhibited the highest antagonistic activity, and therefore, they were selected for in vivo experiments using bean seeds as target; secondly, in solid and liquid culture experiments under optimum antagonistic conditions, the medium extracts from the isolates OL80, ACTA1523, ACTA1551 and ACTA1522 suppressed the growth of the fungal mycelium, while extracts from ACTA 1383 and ACTA1557 did not show any activity.

Conclusions

These results corresponded important indications for the utility of two Greek indigenous Streptomyces isolates (ACTA1557 and ACTA1383) for the protection of the bean crops from R. solani damping‐off symptoms, while four of them (isolates OL80, ACTA1523, ACTA1551 and ACTA1522) seem to be promising producers of antifungal metabolites.

Significance and Impact of the Study

This is the first study on the biocontrol of R. solani using multiactive Streptomyces isolates originated from ecophysiologically special Greek habitats. Our study provides basic information to further explore managing strategies to control this critical disease.     

Leaf Spot on Sweet Potato (Ipomoea batatas) Caused by Stemphylium solani,a New Disease in China

During 2010–2011, a severe leaf spot disease of sweet potato (Ipomoea batatas) was found in Haikou City, Hainan province of China. The disease is characterized with large, irregular, brown, necrotic lesions on the margin or in the centre of leaves. A species of Stemphylium was consistently recovered from pieces of symptomatic tissues on PDA. Based on morphological characteristics and molecular identification by rDNA‐ITS gene analysis, the fungal species was identified as Stemphylium solani Weber, and its pathogenicity was confirmed by Koch's postulates. This is the first report of leaf spot on sweet potato caused by S. solani in China.     

Epidemics of Rhizoctonia Root Rot in Association with Biological and Physicochemical Properties of Field Soil in Bean Crops

During two growing seasons (2008 and 2009), the associations of Rhizoctonia root rot (RRR) with a number of soil properties were determined at different growth stages in 122 commercial bean fields in Zanjan, Iran. Mean RRR incidence at a level of 4–25% sand content was lower than that at 45–65% level. Damage by fly puparia had no significant effect on RRR incidence and occurrence. A greater RRR incidence was detected in field soils treated with fungicides compared with non‐treated soils. A lower RRR incidence was associated with the highest level of soil organic matter (1.2–1.8) compared with the lowest level, 0.4–0.8. The highest RRR incidence corresponded with no rhizobial nodulation compared with highly nodulated bean roots. RRR incidence was negatively correlated with soil silt and organic matter content at R6–7 and R9 growth stages. RRR‐affected fields were recognized with a greater soil pH (V3) and sand content (R9), and a lower silt (R9) and organic matter content (R6–7 and R9) in comparison with RRR‐free fields. Loadings and linear regressions between RRR incidence and principal component scores indicated that the most effective soil characteristic linked to the disease was silt at V3, sand at R6–7 and organic matter at R9 stage. This new epidemiological information extends our knowledge of the bean–RRR–soil interaction on a regional basis.     

Assessment of genetic diversity among soybean genotypes differing in response to aerial blight (Rhizoctonia solani Kuhn) using SSR markers

Forty‐seven genotypes and one wild relative of soybean, Glycine soja, were screened for resistance against aerial blight under epiphytotic conditions in the field during the Kharif season of two consecutive years viz., 2016 and 2017. Out of the 48 genotypes screened, only 18 genotypes exhibited a moderately resistant response to aerial blight during both the years of study. In order to perform molecular screening of the genotypes for aerial blight resistance, the genomic DNA obtained from the seedlings of the forty‐eight soybean genotypes was subjected to PCR amplification with 12 SSR markers. The SSR markers Satt 119, Sat_076, Satt 433, Satt 281, Satt 277, Satt 245 and Satt 520 were able to clearly amplify different banding pattern for resistant and susceptible genotypes, out of which Satt 433 and Satt 520 were found to exhibit a pattern, highly similar to the results of field screening of the genotypes with respect to resistant and susceptible reaction to the disease. The eighteen soybean genotypes that exhibited moderately resistant reaction to RAB under field conditions during both the years showed a banding pattern similar to resistant check PS‐1583 in the amplification profile produced by the SSR markers. The polymorphism information content (PIC) from the analysis of amplification profile of the SSR markers used in the study, ranged from 0.58 to 0.95. The dendrogram constructed using UPGMA cluster analysis clearly differentiated the resistant and susceptible genotypes of soybean into two separate groups.     

Occurrence of the New York Strain of Soil‐Borne Wheat Mosaic Virus in Northern Germany

Sequence analysis has shown that diseased wheat plants in Northern Germany were infected with the New York strain of soil‐borne wheat mosaic virus (SBWMV). This is in contrast to the only other confirmed site of SBWMV occurring in Germany, where a variant closely related to the Nebraska‐type strain of SBWMV was found. The results indicate that there have been at least two separate introductions of SBWMV strains to Germany. A survey is required to study the actual distribution of SBWMV in Germany.     

Effect of Soil‐applied Zinc and Manganese on the Development of Rhizoctonia Aerial Blight of Soybean

This study aimed to investigate the effect of soil‐applied zinc (Zn) and manganese (Mn) rates on the development of aerial blight, caused by Rhizoctonia solani Kühn, in soybean. Plants (cv. ‘Conquista’) were grown in a typical Acrustox red‐yellow latosol amended with Zn rates (applied as ZnSO_4·7H₂O; 24% Zn) of 0, 1, 2, 4, 8 and 16 mg/dm³ of soil and Mn rates (applied as MnSO_4·H₂O; 36% Mn) of 0, 1.5, 3 and 6 mg/dm³ of soil and inoculated with R. solani. The relationship between Zn and Mn concentrations on leaf tissues and the rates of these micronutrients was linear. The incubation period was not affected by Zn and Mn rates. The relationship between application rates and the area under aerial blight progress curve was best described with a positive linear regression model for Zn and with a positive quadratic regression model for Mn. Results from this study showed that high foliar concentrations of Zn and Mn do not increase soybean resistance to aerial blight.     

Linkages of Farmers' Operations with Rhizoctonia Root Rot Spread in Bean Crops on a Regional Basis

The associations of Rhizoctonia root rot (RRR) with a number of agronomic and yield variables were characterized at different growth stages in 122 commercial bean fields in Zanjan, Iran. A lower RRR incidence was detected in red beans compared with white beans. RRR incidence was greater in drought‐exposed fields compared with drought‐free fields. RRR incidence was higher following frequent irrigations at 2‐ to 3‐day intervals than at 4‐ to 9‐day intervals. The highest RRR incidence was associated with the densest category of plant populations and with the deepest plantings at 10–22 cm. Beans grown following alfalfa, bean and maize had a lower disease than rotations with potato. RRR incidence was greater in fields that received 50–500 kg/ha of urea compared with nonfertilized fields. RRR‐affected fields were recognized with a closer irrigation, earlier and deeper planting, denser weed population and lower yields than RRR‐free fields. According to loadings for second principal component, planting density and depth, urea usage and weed density corresponded with RRR incidence. Findings extend our understanding of RRR epidemics in diverse bean cropping systems.     

Effect of Magnesium on the Development of Sheath blight in Rice

This study investigated the effect of magnesium (Mg) on sheath blight, caused by Rhizoctonia solani, development on rice plants from cultivars BR‐IRGA 409 and Labelle grown in nutrient solution containing 0.062, 0.125, 0.25 and 0.50 mm of Mg. Sheath blight progress on inoculated sheaths was evaluated by measuring lesions expansion (mm) at 24, 48, 72 and 96 h after inoculation. Data were used to calculate the area under lesion expansion progress curve (AULEPC). The relationship between the foliar Mg concentration and the Mg rates was quadratic. The Mg concentration on leaf sheaths tissue was highest at the Mg rates of 0.389 and 0.400 mm , respectively, for cultivars BR‐IRGA 409 and Labelle. A linear model best described the relationship between the AULEPC and the Mg rates. The AULEPC decreased by 48.7 and 26.2% for plants of cultivars BR‐IRGA 409 and Labelle, respectively, as the Mg rates in the nutrient solution increased. The results permitted to conclude that high foliar Mg concentration played a pivotal role to decrease sheath blight lesions expansion.     

Identification and characterization of rhizosphere fungal strain MF‐91 antagonistic to rice blast and sheath blight pathogens

Aim

To examine the inhibition effects of rhizosphere fungal strain MF‐91 on the rice blast pathogen Magnaporthe grisea and sheath blight pathogen Rhizoctonia solani.

Methods and Results

Rhizosphere fungal strain MF‐91 and its metabolites suppressed the in vitro mycelial growth of R. solani. The inhibitory effect of the metabolites was affected by incubation temperature, lighting time, initial pH and incubation time of rhizosphere fungal strain MF‐91. The in vitro mycelial growth of M. grisea was insignificantly inhibited by rhizosphere fungal strain MF‐91 and its metabolites. The metabolites of rhizosphere fungal strain MF‐91 significantly inhibited the conidial germination and appressorium formation of M. grisea. Moreover, the metabolites reduced the disease index of rice sheath blight by 35·02% in a greenhouse and 57·81% in a field as well as reduced the disease index of rice blast by 66·07% in a field. Rhizosphere fungal strain MF‐91 was identified as Chaetomium aureum based on the morphological observation, the analysis of 18S ribosomal DNA internal transcribed spacer sequence and its physiological characteristics, such as the optimal medium, temperature and initial pH for mycelial growth and sporulation production.

Conclusions

Rhizosphere fungus C. aureum is effective in the biocontrolling of rice blast pathogen M. grisea and sheath blight pathogen R. solani both in in vitro and in vivo conditions.

Significance and Impact of the Study

This study is the first to show that rhizosphere fungus C. aureum is a potential fungicide against rice blast and sheath blight pathogens.     

Molecular analysis of the site for 2‐arachidonylglycerol (2‐AG) on the β2 subunit of GABAA receptors

2‐arachidonyl glycerol (2‐AG) allosterically potentiates GABA_A receptors via a binding site located in transmembrane segment M4 of the β₂ subunit. Two amino acid residues have been described that are essential for this effect. With the aim to further describe this potential drug target, we performed a cysteine scanning of the entire M4 and part of M3. All four residues in M4 affecting the potentiation here and the two already identified residues locate to the same side of the α‐helix. This side is exposed to M3, where further residues were identified. From the fact that the important residues span > 18 Å, we conclude that the hydrophobic tail of the bound 2‐AG molecule must be near linear and that the site mainly locates to the inner leaflet but stretches far into the membrane. The influence of the structure of the head group of the ligand molecule on the activity of the molecule was also investigated. We present a model of 2‐AG docked to the GABA_A receptor.     

Induced Resistance in Cucumbers against Fusarium oxysporum f. sp. cucumerinum and Rhizoctonia solani AG2–2 by Infection of the Cotyledons

Localized infection in cucumber cotyledons with Colletotrichum lagenarium induced resistance against infection after challenge inoculation with Rhizoctonia solani AG2–2 and Fusarium oxysporum f. sp. cucumerinum in the roots. The plants were unprotected in soil that was infested heavily with R. solani or in contact with the mycelium, and induced resistance was not observed. Wounding of the root also negated the effect of induced resistance to F. oxysporum .     

Development of specific PCR primers for the detection of Rhizoctonia solani AG 2-2 LP from the leaf sheaths exhibiting large-patch symptom on zoysia grass

Detection of Rhizoctonia solani AG 2-2 LP isolates causing large-patch disease on zoysia grass was done using polymerase chain reaction (PCR). Specific primers were designed based on an amplified region using random amplified polymorphic DNA (RAPD)-PCR. Fifteen primers and three cultural types of R. solani AG 2-2 (types IIIB, IV and LP) were used for RAPD-PCR. The banding patterns by RAPD-PCR showed that the three cultural types were clearly distinguishable. A dendrogram constructed from the results of RAPD-PCR showed that the three cultural types of AG 2-2 clustered separately. The sequence of one PCR-amplified region which appeared only in LP isolates using primer A09 was selected for designing specific primers. Primer pair A091-F/R gave a single product from pure fungal DNA of LP isolates but not from those of the other two types (IIIB and IV), R. solani AG 1, 2-1, 2-3, 2-tulip, 3-10 and BI isolates and other turfgrass fungal pathogens. Primer pair A091-F/R also gave a single product from diseased leaf sheaths and this product was in accordance with those of pure fungal DNA of LP isolates. Primer pair A091-F/R did not yield PCR product from healthy leaf sheaths. The frequencies of detection of LP isolates from leaf sheaths of zoysia grass using PCR with primer pair A091-F/R were higher than those of the conventional isolation technique. These results showed that the PCR-based technique using specific primers A091-F/R is useful for the rapid detection of LP isolates from leaf sheaths of zoysia grass exhibiting large-patch symptoms.