Scielo RSS <![CDATA[Tropical Plant Pathology]]> vol. 39 num. 2 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Algal polysaccharides as source of plant resistance inducers</b>]]> Algal compounds exhibit great potential to enhance plant growth and resistance to abiotic and biotic stresses. This review focuses on aspects concerning the physical-chemical properties, function and biological activity of macroalgae polysaccharides. Updated results of the main poly- and oligosaccharides studied for the control of plant diseases are discussed and summarized. The carrageenans from red algae have a well-established obtaining system, but its high market value discourages its use in plant protection. The fucans found in the cell walls of brown algae are present in several fertilizers and accounted for the benefits of such commercial products on plant physiology. The laminarans, from brown algae mainly Laminaria digitata, are currently the main algal polysaccharides on the phytosanitary market. The ulvans, from cell the walls of Ulva spp., open new ways to obtain polysaccharides able to induce resistance due to its abundance worldwide. All these algal polysaccharides show ability to activate multiple plant defense mechanisms against a broad spectrum of plant pathogens. Taking into account the promising results reported in the literature and the enormous biochemical diversity of these biopolymers, it is likely that they will provide new types of resistance inducers in a near future. <![CDATA[<b>Genetic variation among <i>Phyllosticta </i>strains isolated from citrus in Florida that are pathogenic or nonpathogenic to citrus</b>]]> Citrus black spot is an emerging disease in Florida since 2010. The causal agent is Phyllosticta citricarpa (teleomorph Guignardia citricarpa), but non-pathogenic P. capitalensis (teleomorph often referred to as G. mangiferae) is often isolated from black spot lesions. Florida isolates of P. citricarpa and P. capitalensis from citrus have not been characterized in detail. In this study, Phyllosticta species isolated from Florida citrus were compared with worldwide isolates using multi-locus sequencing of four conserved loci (rDNA ITS, TEF1, ACT, and GPDH genes). Moreover, the diversity within the two Phyllosticta species was compared based on the same four loci. DNA sequences of P. citricarpa and P. capitalensis were clearly distinct, coinciding with other P. citricarpa and P. capitalensis sequences from different continents. The species showed different population structures in Florida. P. citricarpa isolates did not exhibit genetic variation and were similar to strains from other continents. In contrast, Florida P. capitalensis isolates were distributed over five sequence groups. This study did not point to the potential origin of P. citricarpa and P. capitalensis in Florida. More variable genetic markers and isolates from various continents would be required to track the possible movement of these Phyllosticta species. <![CDATA[<b>Managing soybean rust with fungicides and varieties of the early/semi-early and intermediate maturity groups</b>]]> The objectives of this study were to evaluate the influence of soybean cultivar of two maturity groups, early/semi-early (E) or intermediate (I), on the management of Asian soybean rust (ASR) with fungicides and yield. Field trials were conducted during the 2006/07 and 2007/08 growing seasons. Seven cultivars of the two groups were tested in the first season and eight in the second season. All cultivars had plots that were treated (T) or non-treated (NT) with a commercial mixture of pyraclostrobin + epoxiconazole. ASR severity (%) was visually assessed several times during the crop cycle and yield (kg ha-1) was determined at harvest. Values of the standardized area under the disease progress curve (AUDPC) calculated from the severity assessments was higher in 2007/08 than in 2006/07, but no differences were found between cultivars of the E and I maturity groups. Differences in yield between between T and NT plots were lower in cultivars of the E group than those of the I group in both the 2006/07 (37.6% and 52.8% respectively) and the 2007/08 season (56.9% and 85.0%, respectively). A higher stability in yield was found for cultivars of the E maturity group compared to those of the I group. <![CDATA[<b>Detection and cellular localization of <i>Xanthomonas campestris </i>pv. <i>viticola</i> in seeds of commercial 'Red Globe' grapes</b>]]> Commercial grapevine fruit (Vitis vinifera) of the Red Globe variety were collected in vineyards from Vale do São Francisco lower basin, an area of occurrence of grapevine bacterial canker. Seeds were extracted, classified as symptomatic or asymptomatic and processed in order to be observed under light (LM) and scanning electron microscopy (SEM) along with silver-enhanced immunogold labeling, to allow bacterial detection using a policlonal antibody against Xanthomonas campestris pv. viticola (Xcvi), etiological agent of the disease. The seed samples showed bacterial aggregates associated to the tegument surface and to the first parenchymal layer beneath the seed tegument. Bacterial identity was confirmed by immunogold labeling. This appears to be the first report of Xcvi associated to asymptomatic seeds and berries, suggesting a systemic mechanism to spread and colonize different tissues and sites, driving attention to seeds, presenting them as an important niche for survival and dissemination of this pathogen. These results point towards the need of including seed-bearing fruit in studies regarding Xcvi epidemiology. <![CDATA[<b>Species of <i>Uromyces </i>(Pucciniales, Basidiomycota) on Loranthaceae</b>]]> Two new species of Uromyces with reticulated teliospores are compared with nine species of this genus known from Loranthaceae. The new species Uromyces bahiensis from Brazil has smaller spores than all known species of Uromyces with reticulate teliospores on Loranthaceae. Uromyces struthanthi from Panama is characterized by long teliospore pedicels and spinose-echinulate aecidiospores. In addition, new details of ornamentation of aecidiospores and teliospores of known species are presented. <![CDATA[<b>Comparing <i>Acidovorax citrulli </i>strains from melon and watermelon</b>: <b>Phenotypic characteristics, pathogenicity and genetic diversity</b>]]> Melon and watermelon bacterial fruit blotch, incited by Acidovorax citrulli, is limited to some areas in Brazil but causes important losses, mainly in melon-producing regions. Although genetic diversity has been observed among strains belonging to the species, they are considered a homogeneous group based on the fact that they show only slight physiological or nutritional differences. The objective of this study was to compare Brazilian strains from melon and watermelon by means of biochemical, pathogenicity, serological and molecular assays. Fifteen biochemical tests, cross inoculation between strains and hosts, ELISA and repetitive sequence analysis (rep-PCR) with the primers REP, ERIC and BOX were conducted. No differences were revealed by nutritional characterization or serology, but cross inoculation showed different pathogenicity groups, which could explain high aggressiveness of the bacteria to melon crops in some regions. Molecular analysis by BOX-PCR clustered strains according to their geographical origin, while ERIC- and REP-PCR, analyzed together, indicated genetic diversity, but without geographical or host origin relationships. One test that could be used to verify the pathogenicity of strains by inoculating detached leaf petioles, showing results in 36 h, is proposed here. <![CDATA[<b>Genetic variability of <i>Bipolaris sorokiniana </i>isolates using URP-PCR</b>]]> Spot blotch disease, caused by Bipolaris sorokiniana, is one of the major diseases of wheat and is responsible for large losses of wheat crops worldwide. We used polymerase chain reaction (PCR) with universal rice primers (URP) for molecular characterization of 60 monosporic B. sorokiniana isolates from Brazil and other countries, and evaluated the diversity of the samples. PCR amplification generated 232 different DNA fragments ranging in size from 100 to 2018 bp. The primers URP-4R, URP-2R, and URP-1F generated greater numbers of amplified fragments (36, 30, and 25, respectively) from the single-spore isolates, and some diversity was observed among the isolates generated using these primers. The primers URP-2F, URP-6R, URP-17R, URP-30F, and URP-38F produced a pattern of monomorphic fragments and 73% of the isolates showed an average of 44 different DNA-amplified fragments. Primer URP-2F generated a 578-bp fragment that was common to 83.7% of the isolates; primer URP-6R generated a 548-bp fragment and primer URP-38F generated a 650bp product that was common to 89.1% and 80% of the isolates, respectively. The URP-PCR primers provided important information about the genetic profiles of the monosporic cultures, which showed intraspecific variability among the monoconidial isolates and among the monosporic cultures that originated from the same polysporic strain. Our results indicate that URP's are sensitive and give reproducible results for assaying the genetic variability of B. sorokiniana. <![CDATA[<b>Baseline sensitivity of Brazilian <i>Mycosphaerella fijiensis </i>isolates to protectant and systemic fungicides</b>]]> Black Sigatoka caused by Mycosphaerella fijiensis is a foliar disease that affects banana plants and large amounts of fungicides are required to prevent crop losses. Intensive applications of single-site fungicides can select for fungicide-resistant isolates. The objective of this study was to assess the sensitivity of 60 isolates of M. fijiensis to commonly used fungicides. Using two different protocols, microtiter and Petri plate tests, the effective concentration at which mycelium growth is reduced by 50% (EC50) was determined for thiophanatemethyl, tebuconazole, chlorothalonil and mancozeb. Additionally, partial sequences of the cytochrome b gene were obtained for 46 isolates to detect the G143A mutation, commonly associated with strobilurin resistance. The EC50 values for tebuconazole and thiophanate-methyl ranged from 0.02 to 1.39 and from 0.008 to 8.22 µg mL-1, respectively. For chlorothalonil, the lowest and highest EC50 values were 0.39 µg mL-1 and 53.7 µg mL-1, respectively. For mancozeb, approximately 50% of the isolates had EC50 values greater than 1000 µg mL-1. No mutation was found in the isolates assayed for strobilurin resistance. There was no correlation between sensitivity levels to any fungicide and geographic region. Low EC50 values were estimated for most fungicides but, some isolates had high EC50 values for mancozeb. <![CDATA[<b>Postharvest rot and mummification of strawberry fruits caused by <i>Neofusicoccum parvum </i>and <i>N. kwambonambiense </i>in Brazil</b>]]> In addition to the rots that are commonly found on strawberries, a new disease was found in 7% of stored fruits during a survey of strawberry diseases at the postharvest stage. Koch's postulates were satisfied, and the fungi were identified as Neofusicoccum kwambonambiense and N. parvum based on morphology and phylogenetic analysis of the internal transcribed spacers, β-tubulin, RNA polymerase subunit II and transcription elongation factor 1-α regions. This is the first report of Neofusicoccum kwambonambiense in Brazil and the first report of Neofusicoccum spp. causing mummification and postharvest rot of strawberry. <![CDATA[<b>Analyses of the 16S-23S intergenic region of the phytoplasma causing the sugarcane white leaf disease in Yunnan Province, China</b>]]> Sugarcane white leaf (SCWL) is an important disease caused by a phytoplasma. In Baoshan, Yunnan, China, SCWL was firstly observed in 2012, and has extended its area of occurrence to 600 hm² . Up to 52% of the plants may become diseased in a field and even complete loss of cane yield may result in the heavily infected fields, posing a serious threat to Yunnan sugar industry. To ascertain the causal agent of suspected SCWL disease in Yunnan, nested PCR using two sets of phytoplasma primer pairs (MOLX/MLOY and P1/ P2) was used to successfully amplify a genomic region of the 16S ribosomal DNA (16S rDNA) from 36 suspected SCWL samples. On the basis of sequencing, phylogenetic analysis and nucleotide alignments of 17 nested PCR products randomly selected from positive samples, identical fragments of 210bp in length were obtained that could be clustered into the 6Sr group XI (Rice Yellow Dwarf group) and shared 100% identity with the 16S-23S intergenic spacer region (ISR) of a member of this group, the SCWL phytoplasma (GenBank: HQ917068), and 99.52% with Sugarcane grassy shoot phytoplasma, of the same group. These results indicate that the SCWL disease in Baoshan is caused by a phytoplasma of the 6Sr group XI.