Scielo RSS <![CDATA[Tropical Plant Pathology]]> vol. 39 num. 3 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<b>Using dynamic diversity to achieve durable disease resistance in agricultural ecosystems</b>]]> <![CDATA[<b>Identification of <i>Fusarium oxysporum </i>f. sp. <i>lactucae </i>race 1 as the causal agent of lettuce wilt in Brazil</b>]]> The objective of this work was to assess the virulence to lettuce cultivars and host specificity and to identify races of Fusarium oxysporum f. sp. lactucae (FOLac) isolates associated with lettuce wilt in Brazil. Thirty-one isolates identified based on morphology were inoculated in a set of lettuce cultivars for race determination. Plantlets of 'Elisa', 'Vera', and 'Red Salad Bowl' cultivars were inoculated with all isolates using a root-dipping method. Isolates were also inoculated in species of Asteraceae and other botanical families. 'Elisa' and 'Vera' were susceptible to 27 isolates while 'Red Salad Bowl' was resistant to all isolates. Plants other than lettuce were not infected by any of the isolates, suggesting their specificity to lettuce. All pathogenic isolates were assigned to race 1 and used to evaluate a PCR protocol with primers targeting race 1. Only amplicons associated with race 1 pattern were observed for all pathogenic isolates but not for one non-pathogenic isolate, thus leading us to conclude that FOLac race 1 is so far the sole causal agent of lettuce wilt in Brazil. <![CDATA[<b>Mating type loci analysis indicates that <i>Villosiclava virens</i>, the casual agent of false smut disease of rice, is a homothallic fungus</b>]]> Villosiclava virens (anamorph: Ustilaginoidea virens) was isolated from infected rice plants, where it caused false smut. Similar to other ascomycetes, the sexual compatibility of V. virens is controlled by the mating type (MAT) locus. In this study, we applied a PCR-based approach to screen mating type genes in the single asexual spore isolate. Our results showed that V. virens possesses the two master genes required for mating (MAT1-1-1 and MAT1-2-1), suggesting that this fungus is likely to be homothallic. Mating type primer pairs were developed to genotype the single ascospore isolates from different places in China. These analyses provided further evidence that V. virens is a homothallic species and suggest novel genetic mechanisms of sexual reproduction in V. virens. <![CDATA[<b>First report and characterization of <i>Fusarium circinatum, </i>the causal agent of pitch canker in Brazil</b>]]> Pitch canker is one of the most important diseases of pine trees worldwide, including South America. The causal agent of this disease is Fusarium circinatum, a member of the Gibberella fujikuroi complex (GFC). In South America, the species is reported from Colombia, Uruguay and Chile, but is considered a quarantine organism in Brazil. In this study we characterized isolates obtained from symptomatic pine seedlings intercepted in a nursery in Santa Catarina State using phylogenetic analyses, crossings and morphological markers. The Brazilian isolates grouped with reference material in an unique clade and formed fertile perithecia when crossed with reference tester strains. The detailed characterization given here and the availability of tester strains will allow for reliable identification and support monitoring of this important plant pathogen. <![CDATA[<b>Fitness components of <i>Monosporascus cannonballus </i>isolates from northeastern Brazilian melon fields</b>]]> Monosporascus root rot and vine decline caused by Monosporascus cannonballus is one of the most important melon yield-limiting diseases in northeastern Brazil. This study investigated the fitness components of 57 isolates of M. cannonballus obtained from Brazilian melon fields by evaluating: i) their mycelial growth rate (MGR), and perithecia and ascospore production (PP and AP) on potato dextrose agar (PDA); ii) their sensitivity to the fungicide fluazinam; and iii) their virulence to melon seedlings. All M. cannonballus isolates showed variability in their MGR, PP and AP values. They were sensitive to the fungicide fluazinam, showing some degree of mycelial growth inhibition (MGI), and were pathogenic to melon seedlings, with a mean disease severity index (DSI) of 62.1%. By univariate analysis, the formation of groups of similarity amongst the isolates of M. cannonballus within each variable was not limited by the area of origin of each isolate, given that in most situations, different isolates of the same area were distributed into distinct groups of similarity. A multivariate cluster analysis allowed the separation of the 57 M. cannonballus isolates in 18 groups of similarity. The fitness variability among M. cannonballus isolates found in this study should be considered when possible sources of resistance are evaluated in melon breeding programs. <![CDATA[<b>Induction of resistance to <i>Pyricularia oryzae </i>in wheat by acibenzolar-S-methyl, ethylene and jasmonic acid</b>]]> This study investigated the effects of acibenzolar-S-methyl (ASM), ethylene (ET) and jasmonic acid (JA) on the resistance of two wheat cultivars (BRS-229 and BR-18) to infection by Pyricularia oryzae. The treatments included spraying plants with ASM (300 mg L-1), ET (0.5 mM), JA (0.1 mM) and distilled water (control) 48 h before inoculation with P. oryzae. Malondialdehyde concentration, an indicative of oxidative damage to the lipids in the leaf cells, was significantly higher for plants sprayed with ASM compared to plants sprayed with JA and ET. Plants sprayed with JA and ET showed reduced values for the number of lesions per cm² of leaf area and area under blast progress curve, but these hormones had no effect on the incubation period and lesion size (in mm). Plants sprayed with JA and ET showed reduce blast symptoms in comparison to plants sprayed with ASM due to greater peroxidase, polyphenoloxidase, chitinase and β-1,3-glucanase activities. <![CDATA[<b>First report of recombination in <i>Potato yellow vein virus </i>(PYVV) in Colombia</b>]]> Potato yellow vein virus (PYVV) is currently one of the most important viruses that infects potatoes in Colombia and other Andean countries, causing losses in the production of tubers ranging from 25% to 50%. This study analyzed the genetic variability of different viral isolates collected in the department of Nariño, Colombia, through bioinformatics analysis of the sequences of three genes encoding the capsid protein (CP), the heat-shock protein 70 (Hsp70) and the minor capsid protein (CPm). We found that CPm is the gene that shows greater diversity, with higher values of nucleotide substitution and evidence of recombination. Based on an analysis of the haplotype map using nucleotide sequences of the CPm, we propose a model of putative recombination in this genomic region. The non-recombinant segments are supported by the results of the program GARD (Genetic Algorithm for Recombination Detection), phylogenetic trees and the paired values of genetic distances of each non-recombinant segments. The model clearly shows that the amino region of the CPm is prone to recombination. To our knowledge, this is the first report of genetic recombination as an evolutionary strategy in the CPm of PYVV. <![CDATA[<b>Effects of powdery mildew infection on chloroplast and mitochondrial functions in rubber tree</b>]]> Powdery mildew (Oidium heveae Steinm.) is one of the most important leaf diseases in rubber tree (Hevea brasiliensis). However, physiological and molecular processes associated with the Hevea-powdery mildew interaction are still unclear. In this study, effects of powdery mildew infection on the mitochondrial and chloroplast functions in rubber tree were comprehensively investigated. Powdery mildew damaged the structure and function of mitochondria prior to chloroplasts, causing inner and outer membranes disruption. The intact rate of mitochondria membrane was reduced from 70% in control leaves to 23.1% in the leaves at 5 days after inoculation (dai). Significant decreases in the activities of cytochrome c oxidase, NADH oxidation and malate dehydrogenase (MDH) were observed in the powdery mildew-infected leaves. Tricarboxylic acid cycle (TCA) and electron transfer capacity were seriously impaired after powdery mildew invasion. Chlorophyll (Chl) contents, maximal photochemical efficiency (Fv/Fm), actual photochemical efficiency of photosystem II (ΦPSII), and electron transport rate (ETR) were dramatically decreased in the infected leaves from 10 dai. Our results provided new insights into understanding the mechanism of Hevea-powdery mildew interaction in rubber tree. <![CDATA[<b>Inheritance of soybean resistance to <i>Rotylenchulus reniformis</i></b>]]> Genetic inheritance of soybean resistance to Rotylenchulus reniformis was studied by evaluating the phenotypic reaction of soybean plants to the nematode. The resistant (Forrest and Custer) and susceptible (BR96-25619) soybean cultivars used as parents as well as the F1, F2 and F2:3 derived from their crosses were infested individually with 1,000 eggs and vermiform R. reniformis. About 70 days after the infestation, the nematodes were extracted from the roots and the reproduction factors and the numbers of nematodes per gram of roots were estimated, and data were adjusted for genetic models. Results suggested a predominance of additive genetic effects controlling the nematode resistance reaction. Based on mean and variance genetic models, further genetic gains are expected in the crossing Custer x BR96-25619. The effect of genetic dominance is towards susceptibility. The presence of significant epistasis indicates the existence of at least two genes controlling resistance and that they are interacting. The normal continuous distribution of frequencies of the number of individuals in different classes of resistance indicates that the resistance to the reniform nematode is inherited quantitatively. <![CDATA[<b>Assessment of som (<i>Persea bombycina</i>) clones for resistance against <i>Phyllosticta persae</i></b>]]> Som (Persea bombycina) is an economically important tree used as food source by the muga silkworm, Antheraea assamensis. Phyllactinia leaf spot (PLS), caused by Phyllosticta persae, affects all som cultivars during June to September (rainy season) and is responsible for foliage losses up to 26%. Information about the effect of this disease at the farmer level and host resistance is lacking. PLS severity was assessed in six major sericulture areas of Assam, India. Furthermore, eight clones were evaluated for resistance in experimental field trials. Disease incidence and severity index (DSI), area under disease progress curve (AUDPC), apparent infection rate, lesion diameter and frequency all indicated differences among clones. DSI and AUDPC values were 3.6 fold and 4.2 fold higher in the most susceptible accession M-8 compared to M-5. High broad sense heritability (h² = 0.80) of DSI suggested an additive nature of resistance. Correlations of DSI (r range: 0.55 to 0.91) with other parameters provided a good empirical evidence of obtained PLS responsiveness. M-5 showSed partial resistance on the basis of all measures and appeared as the most diverse accession based on Euclidean distance. These results suggested that clone M-5 may be a potential source of resistance for use in PLS breeding programs.