Leandro José Dallagnol

Defective active silicon uptake affects some components of rice resistance to brown spot.

Rice is known to accumulate high amounts of silicon (Si) in plant tissue, which helps to decrease the intensity of many economically important rice diseases. Among these diseases, brown spot, caused by the fungus Bipolaris oryzae, is one of the most devastating because it negatively affects yield and grain quality. This study aimed to evaluate the importance of active root Si uptake in rice for controlling brown spot development. Some components of host resistance were evaluated in a rice mutant, low silicon 1 (lsi1), defective in active Si uptake, and its wild-type counterpart (cv. Oochikara). Plants were inoculated with B. oryzae after growing for 35 days in a hydroponic culture amended with 0 or 2 mMol Si. The components of host resistance evaluated were incubation period (IP), relative infection efficiency (RIE), area under brown spot progress curve (AUBSPC), final lesion size (FLS), rate of lesion expansion (r), and area under lesion expansion progress curve (AULEPC). Si content from both Oochikara and lsi1 in the +Si treatment increased in leaf tissue by 219 and 178%, respectively, over the nonamended controls. Plants from Oochikara had 112% more Si in leaf tissue than plants from lsi1. The IP of brown spot from Oochikara increased approximately 6 h in the presence of Si and the RIE, AUBSPC, FLS, r, and AULEPC were significantly reduced by 65, 75, 33, 36, and 35%, respectively. In the presence of Si, the IP increased 3 h for lsi1 but the RIE, AUBSPC, FLS, r, and AULEPC were reduced by only 40, 50, 12, 21, and 12%, respectively. The correlation between Si leaf content and IP was significantly positive but Si content was negatively correlated with RIE, AUBSPC, FLS, r, and AULEPC. Single degree-of-freedom contrasts showed that Oochikara and lsi1 supplied with Si were significantly different from those not supplied with Si for all components of resistance evaluated. This result showed that a reduced Si content in tissues of plants from lsi1 dramatically affected its basal level of resistance to brown spot, suggesting that a minimum Si concentration is needed. Consequently, the results of this study emphasized the importance of an active root Si uptake system for an increase in rice resistance to brown spot.

Silicon reduces brown spot severity and grain discoloration on several rice genotypes

This study aimed to investigate the effect of silicon (Si) on the reduction of brown spot severity (BSS), caused by Bipolaris oryzae, and grain discoloration in several rice genotypes. An experiment was conducted in a greenhouse where eight genotypes were evaluated for their response to Si rates and decrease on BSS. The relationship between Si rates and BSS was linear negative. Additionally, a field experiment was conducted including forty-eight genotypes which were grown in upland conditions and evaluated for their response to Si rates and reduction on grain discoloration. The relationship between Si rates and grain discoloration was quadratic negative. Weight of filled grains per panicle increased as the Si rates in the soil increased. Genotypic differences for Si concentration in husk were evident for both non-amended and Si-amended plots. While the Si concentration in husk increased to all genotypes, there was no relationship between grain discoloration and Si concentration in husk for plants from non-amended and Si-amended plots. The genotype CAN-7024 with the highest resistance to leaf brown spot also showed the lowest grain discoloration in comparison to the genotypes Casado and Caqui.

Silicon Potentiates Host Defense Mechanisms Against Infection by Plant Pathogens

Several agronomic and horticultural crops, such as barley, cucumbers, oats, rice, sugarcane, and wheat, benefit from applications of silicon. Growth enhancements results, in part, from reductions in the intensities of plant diseases. For the rice-Pyricularia oryzae model pathosystem, the mechanical barrier formed from silicon polymerization below the cuticle and in the cell walls was the first proposed hypothesis to explain how this element reduced the number of blast lesions and the lesion sizes. However, new insights have revealed that silicons effect on plant resistance to a number of diseases may also occur through mediated host plant resistance mechanisms against pathogen infection. Plants supplied with silicon exhibit potentiated activation of the phenylpropanoid pathway resulting in increases in total soluble phenolics and lignin. The activities of defense enzymes, such as chitinases and β-1,3-glucanases, are maintained at higher levels during infection and the transcription of defense related genes occur faster and with greater output. When plants are supplied with silicon and then challenged with a pathogen, there is an enhanced activation in antioxidant metabolism, which in turn, suppresses the damaging cytotoxic effect of the reactive oxygen species that causes lipid peroxidation in the cell membrane. At the physiological level, leaf gas exchange parameters of silicon-treated plants are higher upon pathogen infection for crops, such as common beans, rice, sorghum and wheat, indicating the ameliorating effect of this element on photosynthesis. Although our understanding of how silicon affects plants in response to infection has advanced, the exact mechanism(s) by which silicon modulates plant physiology through the potentiation of host defense mechanisms still requires further investigation at the genomics, proteomics, and metabolomics levels.

Mycotoxin and fungicide residues in wheat grains from fungicide-treated plants measured by a validated LC-MS method☆

Wheat (Triticum aestivum) is an annual crop, cultivated in the winter and spring and susceptible to several pathogens, especially fungi, which are managed with fungicides. It is also one of the most consumed cereals, and can be contaminated by mycotoxins and fungicides. The objective of this study was to validate an analytical method by LC-MS for simultaneous determination of mycotoxins and fungicide residues in wheat grains susceptible to fusarium head blight treated with fungicides, and to evaluate the relationship between fungicide application and mycotoxin production. All parameters of the validated analytical method were within AOAC and ANVISA limits. Deoxynivalenol was the prevalent mycotoxin in wheat grain and epoxiconazole was the fungicide residue found in the highest concentration. All fungicidal treatments induced an increase in AFB2 production when compared to the control (without application). AFB1 and deoxynivalenol, on the contrary, were reduced in all fungicide treatments compared to the control.

Rice grain resistance to brown spot and yield are increased by silicon

Brown spot, caused by Bipolaris oryzae, is one of the most important diseases of rice and can cause a reduction in yield and grain quality. The effect of silicon (Si) on the resistance of rice grains to brown spot was investigated. Plants from cv. Oochikara and its mutant, defective in the Lsi1 transporter (lsi1 mutant), were grown in hydroponic culture either with Si (+Si; 2 mM) or without Si (-Si). Panicle inoculation with B. oryzae was carried out at the beginning of the milk-grain stage. Panicles were harvested at physiological grain maturity. The supply of Si significantly increased Si concentration in husks compared to -Si plants. Si concentration in husks from cv. Oochikara was up to three times greater than the lsi1 mutant. In the presence of Si, brown spot severity was reduced by 88% in grains from cv. Oochikara and by 53% in grains from lsi1 mutant. Brown spot severity was 77% lower for grains of cv. Oochikara than for the lsi1 mutant, both plant types were grown in the presence of Si. Panicle inoculation reduced significantly the following yield components: number of grains per panicle, the weight of 1000 grains and the percentage of filled grains. Si significantly increased these yield components, especially for inoculated panicles. Considering kernel quality, the panicle inoculation with B. oryzae significantly reduced the yield of husked kernel, yield of whole kernel and kernel diameter, especially for grains from -Si plants. For panicles from +Si plants, the kernel quality was improved under inoculation, compared to -Si plants. Results from this study show that Si improved rice yield and kernel quality in panicles inoculated with B. oryzae. Furthermore the functional Lsi1 gene contributed significantly for increasing the yield of whole kernel and kernel diameter, possibly due to the increasing Si concentration in husks.

Silicon Control of Foliar Diseases in Monocots and Dicots

One of the most notable effects of silicon on plants is a reduction in the severity of multiple plant diseases caused by pathogenic organisms. For instance, the severity of several rice diseases, such as bacterial blight, brown spot, grain discoloration, leaf scald, leaf and panicle blast, stem rot and sheath blight were suppressed by applying silicon. The reduction in symptom expression is believed to be due to silicon’s effects on multiple defense mechanisms in rice that govern the latent period, lesion size, lesion number, and inoculum production. Although foliar-applied silicon is effective in reducing many foliar diseases, applying silicon to the roots is more effective because it mediates the plant’s defense responses to both foliar and root infections. Applications of silicon can perform as well as fungicides for suppressing plant diseases as well as enhance the resistance of susceptible cultivars to the same level as those that have complete genetic resistance. The application of silicon has value for inclusion in an integrated disease management strategy since this often overlooked, quasi-essential element clearly has the potential to reduced plant disease epidemics.

Root-Knot Nematodes (Meloidogyne spp.) Parasitizing Resistant Tobacco Cultivars in Southern Brazil

Our study surveyed Meloidogyne spp. from tobacco cultivars in Brazil carrying the Rk gene, which confers resistance to Meloidogyne incognita races 1 and 3. Identification of Meloidogyne populations from 39 tobacco fields was based on the analysis of esterase phenotypes, perineal patterns, and 18S-internal transcribed spacer (ITS)1-5.8S ribosomal RNA sequences. Biometric characterization and differential host tests of isolates were determined as well. We detected M. incognita, M. javanica, M. enterolobii, M. arenaria, and M. inornata in 18 (46.2%), 16 (41%), 10 (25.6%), 2 (5.1%), and 1 (2.6%) samples, respectively. Mixtures of species were found in 25.6% of the samples. This is the first report of M. inornata parasitizing tobacco in Brazils southern region. Two morphological and biochemically unusual populations had host ranges and ITS1 sequences nearly identical to M. enterolobii and M. incognita, respectively. Pathogenic and biometric intraspecific variations were observed. Based on our results and considering the limited efficiency of the Rk gene, it is indispensable not only to search for resistance sources to M. incognita virulent strains but also to consider resistance to M. enterolobii, M. inornata, M. arenaria, and M. javanica in tobacco breeding programs. Changes in integrated management procedures should be considered to avoid increased crop damage in the future.

First report of powdery mildew caused by Erysiphe quercicola on Cinnamomum camphora in Brazil

The camphor tree (Cinnamomum camphora (L.) J.) (Lauraceae) is used for industrial and pharmaceutical purposes, and also an evergreen shade tree (Singh and Jawaid, 2012). In 2016, intense defoliation and reduced seedling development was observed in camphor plants showing signs of powdery mildew in the municipality of Capao do Leao, Rio Grande do Sul, Brazil. White to gray rounded irregular patches, consisting of conidia and mycelia of the fungus were observed primarily on the adaxial surface of the leaves of camphor seedlings. When the disease was severe, the entire leaf surface was colonized by the pathogen, resulting in leaf necrosis and defoliation. For Kochs postulates, five healthy seedlings were inoculated on their adaxial surface using an eyelash brush to place conidia on the leaf surface (10 to 15 conidia cm-2). Five non-inoculated seedlings served as controls. Inoculated and non-inoculated seedlings were kept in a greenhouse, but in a separate compartment, with relative humidity around 80% and te...

First report of powdery mildew caused by Podosphaera xanthii on Lagenaria siceraria in Brazil

The bottle gourd (Lagenaria siceraria Mol.) is a plant in the Cucurbitaceae that is grown for its fruit which can either be harvested immature for consuming as a vegetable, or mature, dried and used for handcrafts as well as pipes and bottles. Powdery mildew on bottle gourd was observed on plants under field condition on three farms in the city of Capao do Leao (Rio Grande do Sul State – Brazil) during the spring/summer of 2016/2017. In the three areas, all bottle gourd plants were affected by powdery mildew, and foliar disease severity reached up to 50%. Powdery mildew symptoms characterized by circular white colonies on both leaf surfaces and on petioles were easily observed by the abundant production of mycelia, conidiophores, and conidia. On the oldest leaves, colonies coalesced, often covering the entire leaf surface causing chlorosis and eventual death. Mycelia was superficial with nipple-shaped to almost absent apressoria and producing erect conidiophores with three to six immature conidia produced...

First Report of Powdery Mildew Caused by Erysiphe platani on Platanus × acerifolia in Rio Grande do Sul, Brazil

Platanus × acerifolia (Aiton) Willd. (London planetree) is a tree commonly used as an ornamental and in the furniture industry. In the summer of 2013, powdery mildew was observed on shoots of P. × acerifolia plants in the cities of Pelotas and Canela (State of Rio Grande do Sul, Brazil). Voucher specimens (n = 2) were deposited in the Phytopathological Museum Manoel Alves Oliveira at Federal University of Pelotas. Dense white powdery masses of conidia and mycelium were observed on leaves (abaxial and adaxial surfaces), petioles, and young stems. Leaves with high disease severities (≥70%) were deformed with curved edges to the adaxial side, and they often died. Mycelia were superficial with lobed appressoria. Conidiophores were straight, sometimes curved at the base, unbranched, cylindrical, 98 to 236 μm long (137.3 ± 41.2 μm) and composed of a cylindrical foot cell 49 to 102 μm long (66.9 ± 19.5 μm) and 4.4 to 6.4 μm wide (5.3 ± 0.8 μm) followed by two to four cells. Conidia were produced singly or in short chains (two to three), without distinct fibrosin bodies, ellipsoid to ovoid and measuring 24 to 37 μm long (29.5 ± 3.2 μm) and 12 to 19 μm wide (15.2 ± 1.4 μm), often with a wrinkled appearance. Primary conidia had truncate bases and rounded apex while both base and apex were truncated in secondary conidia. Germ tubes were produced apically (pseudoidium type). Chasmothecia were not observed. Genomic DNA was used to amplify the internal transcribed spacer (ITS) region using the ITS1 and ITS4 primers. The resulting sequence (602 bp) was deposited (Accession No. KF499270) in GenBank. BLASTn searches revealed similarity of 100 and 99% with Erysiphe platani from P. orientalis L. (Accession No. JQ365943.1) and P. occidentalis L. (Accession No. JX997805.1), respectively. Phylogenetic analysis placed our sequence in a clade (99% bootstrap support) which included only other E. plantani sequences. In short, morphological and molecular approaches allowed us to identify the infecting fungus as E. platani. For Kochs postulates, 10 detached leaves were inoculated (10 to 15 conidia cm2) on their adaxial surface using an eyelash brush. Non-inoculated leaves served as control. All leaves were kept inside trays with petiole immersed in humidified cotton and maintained at 25 ± 1°C. Symptoms identical to those of the original leaves were observed 6 to 8 days after inoculation, whereas the control leaves remained symptomless. Although E. platani has been previously reported on P. × acerifolia in the city of Poços de Calda, state of Minas Gerais, Brazil (1) and on P. occidentalis in Korea (2), to our knowledge, this is the first record of E. platani on P. × acerifolia in Rio Grande do Sul, Brazil. References: (1) E. M. Inokuti et al. New Dis. Rep. 15:38, 2007. (2) Y. J. La and H. D. Shin. Plant Dis. 97:843, 2013.