Comparative colonisation by virulent versus avirulent <Emphasis Type="Italic">Pyricularia oryzae</Emphasis> on wild <Emphasis Type="Italic">Oryza australiensis</Emphasis>

Pyricularia oryzae (rice blast) conidial development at pre-penetration stage determines success or otherwise of infection inside the rice host plants. Studies on conidial germination and growth on the leaf surface in commercial rice (Oryza sativa) report differently, dependent upon host type and level of blast resistance. Although wild rice (O. australiensis) is known to be an alternative host of blast, the interaction between P. oryzae conidia and wild O. australiensis on its leaf surface has not been previously studied. We found significant (P?<?0.001) differences in conidial development between two blast isolates with different virulence in terms of conidial germination, germ tube growth and appressoria formation on both wild and cultivated rice. Conidial germination at 6 h post-inoculation (hpi) for the virulent isolate was significantly (P?<?0.001) delayed. Germ tubes of the avirulent isolate conidia grew significantly (P?<?0.001) faster and with significantly (P?<?0.001) longer germ tubes than from virulent conidia. Appressoria development for the virulent isolate was significantly (P?<?0.001) faster at its later growth stages of 12 and 18 hpi when approximately 100% of germ tubes formed appressoria. In contrast, formation rate of appressoria for the avirulent isolate was significantly (P?<?0.001) slower and only reached 76% of germ tubes forming appressoria. Appressoria formation on O. australiensis was significantly (P?<?0.001) greater than the formation on O. sativa for both virulent and avirulent P. oryzae at 12 hpi, a clear indication that host type influences the extent of appressoria formation.     

Races of <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis> in Australia and genes with resistance to these races revealed through host resistance screening in monogenic lines of <Emphasis Type="Italic">Oryza sativa</Emphasis>

Rice blast is the most serious disease threat to rice production worldwide. It is difficult to control due to the complex diversity and wide geographic distribution of the causal pathogen Magnaporthe oryzae. In Australia, rice blast occurs in northern Australia but remains exotic to the main south-eastern rice growing area; however, there is the potential for rice blast to threaten this area; in addition, rice production is currently expanding from south-eastern Australia into northern Australia, which makes rice blast a major concern and challenge to rice industry in Australia. Prior to this study, there was lack of information on the race status of M. oryzae present in Australia and on how to manage the disease through host resistance. The races of rice blast isolates collected in northern Australia was characterised based on the disease reactions of eight standard rice differentials used in an international race differential system. The following studies revealed genes conferring resistance to these races through investigating the responses of 25 monogenic rice lines with targeted resistance gene against different races. The rice blast isolates were characterised into five races: IA-1, IA-3, IA-63, IB-3 and IB-59. Genes Pi40, Piz-t, Pi9, Pi5(t) and Pi12(t) exhibited resistance to all the isolates belonging to five races. In addition, two genes showed complete resistance to multiple races, viz. Pi9 that showed complete resistance to races IA-1, IA-3, IA-63 and IB-3 and Pita2 that had complete resistance to races IA-3, IB-3 and IB-59. This study provides information about the races of M. oryzae in Australia. Genes identified conferring resistance to multiple races will not only streamline the identification via molecular markers of imported rice varieties with resistance to rice blast in Australia, but will also allow the Australian rice breeding program to develop new varieties with broad-spectrum resistance to rice blast and pyramid multi-gene resistance into Australian rice varieties.     

Antifungal activity of a novel compound purified from the culture filtrate of <Emphasis Type="Italic">Biscogniauxia</Emphasis> sp. O821 against the rice blast fungus <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis>

Rice blast is a devastating fungal disease resulting in major losses to rice crops. Owing to continuous acquisition of resistance by the causal fungus, several fungicide chemicals are no longer effective. Therefore, there is a need to identify natural components and develop new agents to control fungal pathogens. We previously demonstrated that the culture filtrate of Biscogniauxia sp. O821 inhibited infection behavior of Magnaporthe oryzae and subsequent blast lesion formation. In the present study, we isolated a new compound, (3aS,4aR,8aS,9aR)-3a-hydroxy-8a-methyl-3,5-dimethylenedecahydronaphto[2,3-b]furan-2(3H)-one (HDFO), from the culture filtrate of Biscogniauxia sp. O821 and determined its molecular weight as 248. The HDFO structure was determined by electrospray ionization-mass spectrometry and nuclear magnetic resonance spectroscopy after purification with column chromatography and high-performance liquid chromatography. The structure of this antifungal compound was similar to that of alantolactone and isoalantolactone. The growth inhibition zone against M. oryzae in presence of HDFO was observed at Rf 0.5–0.6 on a thin layer chromatography plate. HDFO inhibited conidial germination of M. oryzae in a dose-dependent manner (1–200 ppm). Furthermore, blast lesion formation was significantly suppressed by HDFO at over 5 ppm. These results suggest that HDFO from the culture filtrate of Biscogniauxia sp. O821 can protect rice from rice blast disease caused by M. oryzae. This is the first report that HDFO produced by Biscogniauxia sp. can serve as an antifungal compound against M. oryzae.     

Effect of fumonisin B<Subscript>1</Subscript> on the emergence,growth and ceramide synthase gene expression of cowpea (<Emphasis Type="Italic">Vigna unguiculata</Emphasis> (L.) Walp)

Rice production is currently expanding from the south-eastern regions of Australia into northern Australia where indigenous species of wild rice occur widely. A survey of fungal diseases on wild (Oryza australiensis, Oryza spp.) and cultivated rice (Oryza sativa) in North Queensland, Australia, in May 2014 revealed a diverse range of fungal genera species, including important pathogens of cultivated rice. Whilst a single isolate of Magnaporthe oryzae (causal agent of rice blast) was obtained from wild rice, Bipolaris oryzae (causal agent of brown spot) was the predominant pathogen detected under North Queensland conditions. For the first time for Australia, we report Rhizoctonia oryzae-sativae (causal agent of aggregate sheath spot) occurring on wild rice. Other pathogens detected on wild rice included Curvularia lunata, Cochliobolus intermedius, Cochliobolus geniculatus, and Fusarium equiseti present in the majority of wild rice samples. Nearby cultivated rice fields harboured additional pathogens not found in wild rice including Fusarium graminearum, Leptosphaeria spegazzinii and Cochliobolus lunatus, causing scab disease, glume blight and leaf blight, respectively. We also confirmed that Bipolaris oryzae from wild rice can infect cultivated rice. This study highlights the importance of wild rice species as alternative hosts harbouring pathogens of cultivated rice and the likely disease threats to expansion of cultivated rice into the same region(s) where wild rice is endemic.     

Sensitivity of <Emphasis Type="Italic">Cochliobolus heterostrophus</Emphasis> to three demethylation inhibitor fungicides,propiconazole, diniconazole and prochloraz,and their efficacy against southern corn leaf blight in Fujian Province,China

Southern corn leaf blight (SCLB) caused by Cochliobolus heterostrophus is a fungal disease that impacts production of corn in China. Fungicides have been the main strategy to manage SCLB. In this study, 276 isolates of C. heterostrophus from seven locations in Fujian Province of China were tested for sensitivity to three demethylation inhibitor (DMI) fungicides. The results indicated that most of the isolates of C. heterostrophus tested were exceptionally sensitive to the three DMI fungicides. Correlation analysis revealed positive association between propiconazole and diniconazole (r?=?0.8145, P?<?0.0001), propiconazole and prochloraz (r?=?0.6190, P?<?0.0001), and diniconazole and prochloraz (r?=?0.5784, P?<?0.0001). However, there was no cross-resistance between these three DMI fungicides and the other six fungicides tested, which included carbendazol, chlorothalonil, mancozeb, iprodione, fluazinam, and pyraclostrobin. In a preventive pot experiment, one spray of 25% propiconazole emulsifiable concentrate (EC) with 250 μg active ingredient (a.i.) mL^?1 applied 12 and 24 h before inoculation at the seedling (V6) stage reduced severity of SCLB by 85.60–89.21%. Nevertheless, the curative activity of propiconazole was much weaker (P?<?0.05) than its preventive efficacy. In greenhouse pot assays, one dose of propiconazole at 250 μg a.i. mL^?1 was the most efficacious for controlling SCLB at the seedling and tasseling (VT) stages of corn, decreasing severity by 80.31%–84.85%, which was higher (P?<?0.05) compared to diniconazole, prochloraz, and other reference fungicides. Therefore, propiconazole appears to be very effective in reducing SCLB and should be applied as a preventive rather than therapeutic fungicide. Our findings provide essential information on the evolution of DMI resistance in C. heterostrophus in Fujian Province of China and may serve as a guide for early resistance monitoring in the future.     

Identification of blast resistance genes in 358 rice germplasms (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) using functional molecular markers

Rice blast, caused by the fungus Magnaporthe oryzae, is one of the most devastating diseases of rice (Oryza sativa L.), and neck blast is the most destructive phase of this disease. Although neck blast causes tremendous yield loss, little is known about the molecular mechanisms underlying the neck blast resistance. To address this issue, we collected 358 rice varieties from different ecotypes in China and assessed them for the neck blast resistance under natural conditions favoring disease development in Jining, Shandong Province. Our results showed that 124 (34.6%) and 234 (65.4%) varieties were resistant and susceptible to M. oryzae under natural field conditions, respectively. Among the 358 rice varieties that were screened for the presence of 13 major blast resistance (R) genes against M. oryzae by using functional markers, 259 varieties contained one to seven R genes. In addition, the relationship between the presence of R genes and the disease reactions was also investigated by integrative analysis of phenotyping and genotyping based on functional markers. Our results showed that the rice blast resistance gene Pi2 was significantly correlated with neck blast resistance. Furthermore, any of the 13 major blast R genes was absent from 32 rice varieties exhibited obvious neck blast resistance, which would be the potential materials for identifying novel neck blast R genes. Taken together, our findings provide insight into the distribution of the 13 major blast R genes in the tested Chinese rice germplasm resources, which will serve as a basis for developing rice blast resistant. Furthermore, 32 rice varieties exhibited neck blast resistance, but they did not harbor any of the 13 major blast R genes. In the future, these varieties may be used to identify novel neck blast R genes.     

Fungicide sensitivity,growth rate,aggressiveness and frost hardiness of <Emphasis Type="Italic">Monilinia fructicola</Emphasis> and <Emphasis Type="Italic">Monilinia laxa</Emphasis> isolates

Monilinia fructicola, the most destructive pathogen of the genus Monilinia, has recently been introduced into Serbia and many other European countries. Since then, many studies have been conducted to evaluate the characteristics of Monilinia species that have a role in the establishment and survival of the pathogen in new areas. The present study assessed the capacity of M. fructicola to repress and replace Monilinia laxa in Serbia based on: fungicide sensitivity, growth rate and aggressiveness at different temperatures, as well as frost hardiness of the isolates of both species. The results showed that the isolates of M. fructicola, compared to M. laxa, were significantly less sensitive to the following fungicides: iprodione, tebucanozole, chlorothalonil, azoxystrobin, fluopyram, and boscalid. In addition, M. laxa isolates exhibited little variation in sensitivity to all of the tested fungicides, whereas M. fructicola isolates displayed a wide range of sensitivity. The temperature of 5°C favored M. laxa growth and aggressiveness, while at 30°C M. fructicola grew faster and had higher lesion expansion rate. These results support an assumption that M. fructicola will continue to spread in Serbian orchards in coming years, particularly on stone fruits harvested during hot summer weather.     

Leaf blast disease reduction by rice-phyllosphere actinomycetes producing bioactive compounds

Fungal leaf blast, caused by Pyricularia oryzae, is a devastating disease of rice plants that annually causes severe production losses worldwide and is one of the top 10 fungal diseases that threaten global food security. Thus, a reliable control strategy against this disease is essential. In this study, the antagonistic activity of indigenous phyllosphere actinomycetes was elucidated against P. oryzae in vitro and in planta to develop an efficient, effective and environmental friendly approach to protect rice plants against P. oryzae. Of 75 isolates of actinomycetes isolated from the rice phyllosphere, 18 isolates inhibited P. oryzae by >45%. According to analysis of their 16 S rRNA gene sequences, the majority of the 18 isolates belonged to Streptomyces genera; others were identified as belonging to Saccharothrix, Gordonia, or Lentzea. Isolates that potentially produced a bioactive compound(s) were identified among the 18 isolates: 17 isolates (94.44%) had a domain marker for nonribosomal peptide synthetase (NRPS) gene and 12 (66.67%) had type-I polyketide synthase (PKS) gene in their corresponding genome. Interestingly, isolates JSN1.9, SKB2.14, and SKB2.3 suppressed disease suppression by approximately 88%. To our knowledge, this is the first report on the application of rice-phyllosphere actinomycetes producing bioactive compounds to control leaf blast disease in Indonesia. Thus, these findings have escalated the potential application of phyllosphere actinomycetes as a supreme biocontrol agent against fungal leaf blast disease.     

Assessing the Belgian potato <Emphasis Type="Italic">Alternaria</Emphasis> population for sensitivity to fungicides with diverse modes of action

Early blight and brown spot, caused by respectively Alternaria solani and Alternaria alternata, can lead to severe yield losses in potato-growing areas. To date, fungicide application is the most effective measure to control the disease. However, in recent years, a reduced sensitivity towards several active ingredients has been reported. To shed light on this issue, Alternaria isolates were collected from different potato fields in Belgium during two growing seasons. Subsequently, the sensitivity of these isolates was assessed using four widely used fungicides with different modes of action. Demethylation inhibitors, quinone outside inhibitors, a dithiocarbamate and a carboxylic acid amide were included in this study. Although all fungicides reduced spore germination and vegetative growth of Alternaria species to some extent, the interspecies sensitivity was very variable. In general, A. solani was more suppressed by the fungicides compared to A. alternata. The effectiveness of the dithiocarbamate mancozeb was high, whereas the quinone outside inhibitor azoxystrobin showed a limited activity, especially towards A. alternata. Therefore, a subset of the A. alternata and A. solani isolates was tested for the presence of, respectively, the G143A substitution and the F129L substitution in the cytochrome b. The frequency of A. alternata isolates bearing the resistant G143A allele (approximately 65%) was comparable in both sampling years, although sensitivity of isolates decreased during the growing season. This finding points to a shift of the population towards resistant isolates. Both the European genotype I and American genotype II were present in the A. solani population, with genotype I being the most prevalent. None of the genotype I isolates carried the F129L substitution, whereas in 83% of the genotype II isolates this substitution was present. Our results demonstrate for the first time that the Belgian Alternaria population on potato comprises a considerable broad spectrum of isolates with different sensitivity to fungicides.     

Overwintering of <Emphasis Type="Italic">Pyricularia oryzae</Emphasis> in wild infected foxtails

The rice blast fungus Pyricularia oryzae mainly overwinters in infested rice organs stored indoors, whereas it is difficult or impossible for the pathogen to overwinter outdoors. By contrast, blast pathogens infecting weed grasses must overwinter outdoors every winter to continue their life cycle. In this study, we investigated the overwintering location of P. oryzae infecting wild, green, and giant foxtails to identify the mechanism that enables them to overwinter. Recovery of P. oryzae was tested in seeds of wild foxtail collected from the soil surface from December to April over three winters. No P. oryzae was recovered from the seed samples of any wild foxtail collected at the ends of the three experimental periods in April. Recovery was also tested from blast lesions on leaves and seeds sampled from withered green foxtail in the experimental field of Saga University from November to April during two winters. In contrast to seeds on the soil surface, P. oryzae survived in lesions and seeds at the ends of the two experimental periods during April, suggesting that withered host plants could be the overwintering site of the pathogen. Rice plants are reaped and removed from paddy fields after harvesting. Thus, withered, standing plants may be available solely to blast pathogens infecting wild grasses, possibly explaining the higher winter survival frequency of weed pathogens than that of rice blast pathogens outdoors.     

A climatic risk analysis of the threat posed by the South American leaf blight (SALB) pathogen <Emphasis Type="Italic">Microcyclus ulei</Emphasis> to major rubber producing countries

In 2010, symptoms of cobweb disease were observed on cultivated Pleurotus eryngii crops in Spain. Based on morphological and genetic analyses, the causal agent of cobweb was identified as Cladobotryum mycophilum. Pathogenicity tests on fruit bodies were performed using conidial suspensions of three C. mycophilum isolates. The causal agent was re-isolated in 80–85 % of the fruit bodies inoculated internally and 15–40 % of those fruit bodies inoculated on the cap surface. The results pointed to a certain resistance of the P. eryngii cap surface to the mycelium of C. mycophilum. Two cropping trials inoculated with C. mycophilum were set up to evaluate the pathogenicity of the causal agent of cobweb in two casings. At the end of the growth cycle, 50–60 % of the inoculated blocks cased with mineral soil, and 20–33 % of the inoculated blocks cased with black peat showed cobweb symptoms. This difference in the appearance of the disease and its aggressiveness may be partly explained by different electrical conductivity values of the casing materials used. In vitro sensitivity of the C. mycophilum isolates and P. eryngii strains against four fungicides (chlorothalonil, prochloraz-Mn, thiabendazole and thiophanate-methyl) was assessed in radial growth experiments on fungicide-amended media. The most effective fungicides for inhibiting the in vitro growth of C. mycophilum were prochloraz-Mn and chlorothalonil, while prochloraz-Mn was also the most selective fungicide between P. eryngii and C. mycophilum, and chlorothalonil was the most toxic fungicide against the P. eryngii mycelium.     

Phylogenetic relationship and fungicide sensitivity of members of the <Emphasis Type="Italic">Colletotrichum gloeosporioides</Emphasis> species complex from apple

The members of the Colletotrichum gloeosporioides species complex (CGSC), the dominant pathogens of apple bitter rot in Nagano prefecture, Japan, were reidentified and the relationship between the species and fungicide sensitivity was revealed. Based on phylogenetic analysis of the ApMat locus with the neighbor-joining (NJ) method, isolates from apple contained three species of the CGSC; C. fructicola, C. aenigma, C. siamense, and three clades of the CGSC: Clade V, S and K. Colletotrichum fructicola and Clade S dominated in Nagano Prefecture. Isolates of C. siamense, C. aenigma and Clade V, S and K remained sensitive to benomyl and quinone outside inhibitor (QoI) fungicides, while C. fructicola often developed resistance to benomyl and QoI fungicides. These results suggest that the development of fungicide resistance differs among members of the CGSC.     

Characterization of <Emphasis Type="Italic">Myoviridae</Emphasis> and <Emphasis Type="Italic">Podoviridae</Emphasis> family bacteriophages of <Emphasis Type="Italic">Erwinia amylovora</Emphasis> from Hungary - potential of application in biological control of fire blight

The virulence spectrum of 300 isolates of Xanthomonas oryzae pv. oryzae (Xoo), representing 17 districts of Punjab, Pakistan was elucidated through inoculation on a set of six rice IRRI-differentials. The virulence level was assessed by using principal component and cluster analysis. Among six principal components (PCs), PC-1 exhibited 59.3 % of the total variance. The highly virulent isolates clusters on the positive side of the ordination away from the point of intersection of PC1 and PC2 and classifies the Xoo isolates from slow disease to the highest disease causing entities. The 300 isolates were categorized into 29 pathotypes (Pt1-29) wherein the highly virulent pathotype (Pt-1), comprises of 39 Xoo isolates were widespread in 12 districts. The majority of Xoo isolates were moderately to least virulent (21.7–43 %) and average disease progress curves confirmed the field reactions of these pathotype clusters for an efficient recognition of Xoo isolates. Interaction of the pathogen with differentials harboring different resistant genes was well investigated in the current study for future management approaches for which the surveillance of the new Xoo pathotypes may expedite the disease resistant rice breeding programme in the country.     

Multi-resistance to thiophanate-methyl,diethofencarb, and procymidone among <Emphasis Type="Italic">Alternaria alternata</Emphasis> populations from tobacco plants,and the management of tobacco brown spot with azoxystrobin

In 2014 and 2015, a total of 151 tobacco brown spot (Alternaria alternata) isolates were collected from Guizhou Province in China to evaluate their resistance to the benzimidazole thiophanate-methyl, the carbamate diethofencarb, and the dicarboximide procymidone. Resistance to thiophanate-methyl and diethofencarb was observed in all isolates. Resistance to all the three fungicides, thiophanate-methyl, diethofencarb, and procymidone was detected at a frequency of 6.0%. The F167Y single mutation in the β-tubulin gene was found to be associated with resistance to thiophanate-methyl,but no mutation was found in the coiled-coil region of the histidine kinase-encoding gene OS1, a fungal gene for dicarboximide resistance. Procymidone applied at the rate of 20 mg l^?1 inhibited spot lesion formation on tobacco leaves with an efficacy of 51.7% for the low resistance (LR) isolates and 74.2% for the procymidone-sensitive isolates. Thiophanate-methyl applied at 100 mg l^?1, however, slightly promoted the expansion of disease lesions with an efficacy of ?7.7%. Azoxystrobin applied at 10 and 20 mg l^?1 provided efficacies of 91.1 and 100%, respectively, regardless of whether the isolates were thiophanate-methyl resistant or procymidone-LR. Further studies suggested that azoxystrobin exhibited excellent protective activity and good curative activity against A. alternata in plants. The baseline sensitivity to azoxystrobin was then determined. In the presence SHAM, the mean EC₅₀ values for conidial germination inhibition were 0.49?±?0.22 (Mean?±?SD) mg l^?1. Interestingly, no resistance was recovered through UV irradiation or Agrobacterium tumefaciens-mediated mutagenesis. This research indicated widespread resistance to thiophanate-methyl and diethofencarb, low frequency of (6.0%) resistance to procymidone in A. alternata populations from tobacco, and suggested that azoxystrobin could potentially constitute a good alternative for the management of tobacco brown spot disease.     

Antifungal activity of <Emphasis Type="Italic">Biscogniauxia</Emphasis> sp. culture filtrates against the rice pathogen <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis>

An ethyl acetate extract of a culture filtrate (ECF) from an unidentified fungal isolate O821 was evaluated for antifungal activity against the rice pathogen Magnaporthe oryzae. The O821-ECF significantly inhibited spore germination, appressorium formation, and mycelial growth of M. oryzae, and its antifungal activity was heat-stable. It also significantly suppressed the number and size of blast lesions. In an analysis of the ITS sequence of this isolate, it shared similarities with species of the fungus Biscogniauxia. These results suggest that isolate O821 of the genus Biscogniauxia produces a heat-stable antifungal compound(s) in its culture filtrate.     

Site‐directed mutagenesis of the cytochrome b gene and development of diagnostic methods for identifying QoI resistance of rice blast fungus

BACKGROUND: It is possible that a single nucleotide polymorphism (SNP) (G143A mutation) in the cytochrome b gene could confer resistance to quinone outside inhibiting (QoI) fungicides (strobilurins) in rice blast fungus because this mutation caused a high level of resistance to fungicides such as azoxystrobin in Pyricularia grisea Sacc. and other fungal plant pathogens. The aim of this study was to survey Magnaporthe oryzae B Couch sp. nov. isolates in Japan for resistance to QoIs, and to try to develop molecular detection methods for QoI resistance. RESULTS: A survey on the QoI resistance among M. oryzae isolates from rice was conducted in Japan. A total of 813 single‐spore isolates of M. oryzae were tested for their sensitivity to azoxystrobin using a mycelial growth test on PDA. QoI fungicide resistance was not found among these isolates. The introduction of G143A mutation into a plasmid containing the cytochrome b gene sequence of rice blast fungus was achieved by site‐directed mutagenesis. Molecular diagnostic methods were developed for identifying QoI resistance in rice blast fungus using the plasmid construct. CONCLUSION: As the management of rice blast disease is often dependent on chemicals, the rational design of control programmes requires a proper understanding of the fungicide resistance phenomenon in field populations of the pathogen. Mutation of the cytochrome b gene of rice blast fungus would be specifically detected from diseased leaves and seeds using the molecular methods developed in this study. Copyright © 2009 Society of Chemical Industry     

Rice <Emphasis Type="Italic">terpene synthase 18</Emphasis> (<Emphasis Type="Italic">OsTPS18</Emphasis>) encodes a sesquiterpene synthase that produces an antibacterial (<Emphasis Type="Italic">E</Emphasis>)-nerolidol against a bacterial pathogen of rice

Plant volatile compounds, including terpenes, are known to be involved in the rice defense system. In the present analysis of a terpene synthase, OsTPS18, in rice, we found that OsTPS18 was localized in the cytoplasm and synthesized the sesquiterpenes (E)-nerolidol and (E)-β-farnesene. The amounts of (E)-nerolidol and (E)-β-farnesene increased after jasmonic acid (JA) treatment. (E)-Nerolidol had significant antibacterial activity against Xanthomonas oryzae pv. oryzae (Xoo). These results suggest that (E)-nerolidol plays an important role in JA-induced resistance against Xoo and that it functions as an antibacterial compound in rice.     

Identification of <Emphasis Type="Italic">Magnaporthe oryzae</Emphasis>-elicited rice novel miRNAs and their targets by miRNA and degradome sequencing

MicroRNAs (miRNAs) play an important role in plant growth, development and responses to stresses. Rice blast is one of the most devastating diseases in rice production. However, little is known regarding the effects of miRNAs response to rice blast. Herein, by deep sequencing small RNA from the susceptible line ZhongerRuanzhan and its space-induced blast resistant mutant line H4 under normal conditions and upon Magnaporthe oryzae (M. orzyzae) infection, several known miRNAs were detected and their expression profiles were found to be negatively correlated with their targets. And, a total of 50 novel miRNAs induced by M. oryzae infection were also identified from four small RNA libraries. Moreover, 176 putative targets of 23 novel miRNAs, which are involved in the various functions, were validated by two degradome analysis. Six novel miRNAs were selected for further validation with qRT-PCR analysis and the results showed that their expression levels were associated with blast response. The knowledge obtained in this study will help us understanding the functions of miRNAs and their targets in regulating blast resistance.     

Management of laurel wilt of avocado,caused by <Emphasis Type="Italic">Raffaelea lauricola</Emphasis>

Laurel wilt is caused by Raffaelea lauricola, a nutritional symbiont of an Asian ambrosia beetle, Xyleborus glabratus. American members of the Lauraceae plant family are most susceptible and 300 million trees have been killed by the disease in the southeastern USA since 2003. Recently, commercial production of an important crop in the laurel family, avocado (Persea americana), has been affected in southern Florida. We summarize studies in which diverse measures were tested for managing the disease. In all studies, trees were treated with potential laurel wilt control measures and subsequently inoculated with R. lauricola. On potted plants in greenhouse experiments, commercial nutritional products (Greenstim and Keyplex 350) and SAR products (Agri-Fos and Nutri-Phite), when applied as soil drenches or foliar sprays, had either no impact on, or increased laurel wilt symptom development compared to non-treated control treatments. Bark applications of Tilt (a propiconazole fungicide for which emergency registration had been obtained in 2010) in a surfactant (Pentrabark) enabled significant laurel wilt protection in greenhouse studies on small potted plants, but Pentrabark and other surfactants moved little propiconazole into the xylem of fruit-bearing trees in field studies. In efficacy studies in the field, Propiconazole Pro (an injectable formulation of propiconazole), Tilt, and two experimental formulations of another triazole fungicide, tebuconazole, decreased the development of laurel wilt compared to nontreated control trees when applied as undiluted injections into branches and scaffold limbs (microinjection), or injection of dilute fungicide into tree flare roots (macroinfusion). However, symptoms developed in all treated trees by 10–11 months after inoculation with R. lauricola, indicating that trees would need to be re-treated at least on an annual basis. Regardless of how fungicides were applied, insignificant levels of different active ingredients entered fruit. Although fungicide treatment of fruit-bearing avocado trees is not a concern for food safety and several triazole and DMI fungicides can protect avocado trees from laurel wilt, cost-effective measures with which the xylem could be loaded with and protected by these products remains a challenge. Management of laurel wilt in commercial avocado production areas is discussed.     

Virulence profiling of <Emphasis Type="Italic">Xanthomonas oryzae</Emphasis> pv. <Emphasis Type="Italic">oryzae</Emphasis> isolates,causing bacterial blight of rice in India

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo), remains a major production constraint in rice cultivation especially in irrigated and rainfed lowland ecosystems in India. The pathogen is highly dynamic in nature and knowledge on pathotype composition among the Xoo population is imperative for designing a scientific resistance breeding program. In this study, four hundred isolates of Xoo collected from diverse rice growing regions of India were analyzed for their virulence and genetic composition. Virulence profiling was carried out on a set of differentials consisting of 22 near isogenic lines (NILs) of IR24 possessing different BB resistance genes and their combinations along with the checks. It was observed that different NILs possessing single BB resistance gene were susceptible to about 59–94% of the Xoo isolates except IRBB 13 (containing BB resistance gene xa13), which showed susceptibility to about 35% of the isolates. Based on the reaction of the Xoo isolates on the differentials, they were categorized into 22 pathotypes. Among the 22 pathotypes, IXoPt-1 and IXoPt-2 were least virulent and IXoPt # 18–22 were highly virulent. Pathotype IXoPt-19 which was virulent on all single BB resistance genes except xa13 constituted the major pathotype (22.5% isolates) and was widely distributed throughout India (16 states). This was followed by pathotype IXoPt-22 (17.25%) which was virulent on all the NILs possessing single BB resistance genes. Molecular analysis was carried out using two outwardly directed primers complementary to sequence of IS1112, a repetitive element of Xoo. A high level of genetic polymorphism was detected among these isolates and the isolates were grouped into 12 major clusters. The data indicated complex nature of evolution of the Xoo pathotypes and there was no strong correlation between pathotypes and genetic clusters as each genetic cluster was composed of Xoo isolates belonging to different pathotypes. The study indicated that none of the single BB resistance genes can provide broad spectrum resistance in India. However, two-gene combinations like xa5 + xa13 and different 3 or 4 genes combination like Xa4 + xa5 + xa13, Xa4 + xa13 + Xa21, xa5 + xa13 + Xa21 and Xa4 + xa5 + xa13 + Xa21 are broadly effective throughout India.