Resumo em Inglês:

Web blight is one of the main diseases that affects bean (Phaseolus vulgaris) cultivation. It infects diverse organs at any growth stage of the plant and can be present at different altitudes in a humid tropical climate. The causal agent of this disease is Thanatephorus cucumeris in its sexual stage and Rhizoctonia solani in the anamorph. The objective of this investigation was to characterize molecular isolates of R. solani obtained from bean plants from diverse production regions in Costa Rica and determine their virulence. Fifty-one samples of symptomatic bean plants were collected using a global positioning system. Virulence was evaluated using the detached leaf technique. Isolates were identified using AG 1-IA, AG 1-IB, AG 1-IC, AG 1-ID, AG 2-2, AG 2-2IIIB, AG 2-2IV and AG 4 molecular markers. ITS sequences were obtained and analyzed with BLAST, aligned, and a phylogenetic tree was constructed. A high degree of virulence and genetic variability between isolates was identified and the anastomosis subgroups of isolates were independent of their geographical origin.

Resumo em Inglês:

Blast, caused by Pyricularia oryzae, has become one of the most important diseases on wheat in Brazil. Knowing the beneficial effects of silicon (Si) in reducing rice blast severity, this study aimed to examine the cytological events occurring during the infectious process of P. oryzae on the leaves of wheat plants non-supplied (-Si) or supplied (+Si) with Si in an experiment arranged in a completely randomized design. The high foliar Si concentration for the +Si plants contributed to decrease the blast symptoms. Hyphae of P. oryzae grew successfully and formed an extensively branched mycelium in the first-invaded epidermal cell and invaded several neighboring cells on the -Si plants, while on the +Si plants fungal hyphae was restricted to the first-invaded epidermal cell. The number of brown adaxial epidermal cells and their intensity of browning were significantly lower for the +Si plants in comparison to the -Si plants. The frequency of appressorial sites showing type B reaction (infection hyphae within the epidermal cell and absence of cytoplasm granulation) was lower for the +Si plants than for the -Si plants from 72 to 96 hours after inoculation because the frequency of appressorial sites showing type A reaction (unsuccessful penetration) was kept higher in comparison to the -Si plants. This study provides novel cytological evidence of the negative effect of Si on the infection process of P. oryzae on leaves of wheat plants contributing, therefore, to reduce the blast symptoms.

Resumo em Inglês:

This study investigated the importance of silicon (Si) in rice grain husk in association with fungicide treatment for preventing Bipolaris oryzae transmission from seed to seedling and the improvement in seedling emergence. Plants from cv. Oochikara and its mutant (lsi1 mutant) defective in active Si uptake were grown in nutrient solution without (-Si) or with (+Si) Si and their panicles were inoculated with B. oryzae. Seeds were evaluated for brown spot severity (BSS) and husk Si concentration. The Si concentration in the husks of cv. Oochikara plants was up to four times higher than the lsi1 mutant plants. BSS was significantly reduced in the husks of the seeds obtained from +Si plants, particularly for cv. Oochikara. A higher percentage of seedling emergence occurred for +Si plant seeds, particularly for cv. Oochikara, and a lower percentage of infected seedlings was observed for the +Si cv. Oochikara treatment, particularly for the fungicide treated seeds. Fungicide was not efficient in preventing seedling infection by B. oryzae for seeds with BSS greater than 50%. High Si concentration in the husk resulted in lower BSS and the fungicide efficiency was greater with a low BSS on seeds from +Si plants, particularly for cv. Oochikara.

Resumo em Inglês:

The purpose of this study was to evaluate the resistance to coffee leaf rust (CLR) caused by Hemileia vastatrix and to Ceratocystis canker (Cc) in coffee genotypes derived from crosses of Coffea arabica var. Caturra with accessions of C. canephora backcrossed to Caturra. Twenty-three F3BC1 progenies including C. arabica var. Caturra and var. Colombia as controls were established in a field experiment. CLR evaluations were made during five years of natural infection, using an incidence rating scale. For Cc, artificial stem inoculations were made with an isolate of Ceratocystis colombiana and the results were assessed after one year. The selection process also included agronomic aspects such as plant height, canopy diameter, number of branch pairs, yield and grain characteristics. Twenty progenies showed >70% of rust resistance. Twelve progenies exhibited >80% of Cc resistance, while no resistance was observed in either of the controls. Only three progenies performed well for all criteria, including resistance to both pathogens and agronomic characteristics.

Resumo em Inglês:

A collection of Gaeumannomyces graminis var. graminis isolates obtained from symptomatic rice and grass plants in central and northern Brazil were studied in regard to pathogenicity on rice and cultural characteristics. For all isolates, only one type of lobed hyphopodia was observed both in the field and artificially inoculated plants. Perithecia were formed on artificial media and inoculated leaf sheaths. Hyphopodia were formed from ascosporic germ tubes. The hyphae under moist stress conditions produced dark brown chlamydospores that were initially hyaline. Large variation in the number of chlamydospores produced and duration of perithecia formation was observed among the isolates. Pathogenicity assays showed that 60-day old rice plants were more susceptible than 35-day old plants. The isolates from rice and grasses varied significantly in regard to disease severity on both rice seedlings and adult rice plants. In general, the isolates from rice were more aggressive than the isolates from grasses. Spontaneous infection of rice plants by ascosporic inoculum from perithecia on rice stubbles was observed in the greenhouse, suggesting their role as a source of primary inoculum in the field, which deserves further investigation.

Resumo em Inglês:

Isolates of Trichoderma spp. (ACB-14, ACB-33, ACB-37, and ACB-40) and Bacillus subtilis (ACB-66, ACB-69, ACB-77, and ACB-83) were tested separately or in mixtures for suppression of postbloom fruit drop in citrus, caused by Colletotrichum acutatum. This work aimed at: (i) determining the incubation time and temperature for production of cells of biocontrol agents; (ii) determining the effect of the isolates, separately or in mixture on the germination conidia of C. acutatum; (iii) evaluating the efficiency of antagonistic isolates on detached citrus flowers and under field conditions. The results of the interactions in vitro showed that there was little differentiation in cell production among the species, and the optimum temperature was 27ºC. The best time for multiplication of bacterial cells was 36 hours, whereas for Trichoderma, the production of conidia continued to increase up to 120 hours of incubation. The mixtures of the ACB-77 plus ACB-66, ACB-33, or ACB-37 inhibited pathogen germination from 84% to 89%. Studies with detached citrus flowers showed that ACB69 alone gave 99% control. The use of mixture ACB-69 plus ACB-37 proved to be viable in the control of disease under field conditions, but the efficiency of the control was lowest than the obtained by applications of ACB-69 alone.

Resumo em Inglês:

Several species of Calonectria cause diseases on a wide range of forest tree species that are propagated either via seedlings or rooted cuttings. In nurseries these fungi cause damping-off, cutting and root rots, stem lesions, and leaf blights. Recently a Calonectria sp. was isolated from rooted cuttings of Anadenanthera peregrina (Fabaceae), Piptadenia gonoacantha (Fabaceae), and Azadirachta indica (Meliaceae) exhibiting leaf blight and cutting rot in a forest nursery at the Universidade Federal de Viçosa, Brazil. Morphological comparisons and DNA sequences of three loci containing partial gene sequences of β-tubulin (TUB2), calmodulin (CAL), and elongation factor (TEF-1α) indicated that these isolates represent an unnamed species of Calonectria, described here as C. hodgesii sp. nov. Sprayinoculated plants of all three hosts with a suspension at 1x10(4) conidia mL-1 induced leaf lesions, cutting rot, and intense defoliation as observed under natural conditions. Calonectria hodgesii was re-isolated from infected tissue, which fulfilled Koch's postulates and confirmed its status as a pathogen with a wide host range.

Resumo em Inglês:

Another form of Marchitez manifestation affecting oil palm (Elaeis guineensis) has been observed since 2002 in the Alto Huallaga area from Peru. Externally, the disease appears as chlorosis, drying and breaking of the lower leaves, rot in fruit bunches, accumulation and rot of spears. Internally, the meristem presents brown lesions with a yellowish halo around its base. A high proportion (94%) of affected plants showed trypanosomatids in their roots. After follow up asymptomatic plants for 22 months, 85% develop symptoms of slow wilt. After the appearing of the symptoms the affected plants die between 5 and 8 months. Lincus spurcus population present in affected plants was evaluated and 44% were positive for trypanosomatids. In addition, trypanosomatids were observed in wild plants such as Acalypha cuneata, Urera caracasana, Trema micrantha, Cecropia spp., present in areas with incidence of the disease. The chlorosis, the long period of incubation and the time evolution of the disease are different in comparison with sudden wilt, suggesting a new disease manifestation affecting oil palms. The presence of Lincus positive to trypanosomatids leads us to propose an association of these parasites with slow wilt as reported for sudden wilt.

Resumo em Inglês:

Cercospora leaf spot (Cercospora beticola) is the most important disease of table beet in the world. In this study, the preventive application of chitosan or acibenzolar-S-methyl (ASM) to control the disease was evaluated, and the involvement of pathogenesis-related proteins in the plant-pathogen interaction was verified. Beet plants cultivar Early Wonder were sprayed with distilled water, HCl 0.05 N, chitosan at 2 mg/mL or 4 mg/mL and ASM at 25 mg/L, and 24 h or three days later, inoculated with the pathogen. Leaf samples were collected at different times after spraying and enzymatic activities were quantified spectrophotometrically. It was observed that chitosan and ASM reduced disease severity in table beet leaves by 76% and 68%, respectively, on the average of two experiments, and ASM promoted the accumulation of peroxidases and β-1,3 glucanases in beet leaves. Thus, both inducers reduce cercospora leaf spot in table beet, and the effect of ASM is associated with the activation of PR-proteins, whereas chitosan act by a still unknown mechanism.

Resumo em Inglês:

Sesame (Sesamum indicum L.) production in Paraguay has been severely affected by infection with Cowpea aphid-borne mosaic virus (CABMV), which causes a disease known locally as ka'are. Because very little is known about the epidemiology of this disease, a survey was performed in fields surrounding affected sesame plantations to identify CABMV-infected plants that may be acting as sources of inoculum. Samples from 48 plant species (symptomatic or asymptomatic, mostly spontaneous and a few cultivated) belonging to 17 families were evaluated by biological and serological assays. In a few select cases, confirmation of the infection was achieved by RT-PCR. The following species were found to be naturally infected by CABMV: Amaranthus hybridus, Arachis hypogaea, Crotalaria incana, Crotalaria juncea, Crotalaria spectabilis, and Vigna unguiculata. The absence of resistant/tolerant sesame cultivars along with the ineffectiveness of disease control through the chemical control of aphid vectors indicates that the only alternative available for disease management at present is the elimination and/or reduction of the sources of inoculum immediately before starting new plantations.

Resumo em Inglês:

In this study the susceptibility of different genotypes of gladiolus (Gladiolus callianthus and the genotypes of G. grandiflorum T-704, Red Beauty, Tradehorn, Verônica, Amsterdam, Yester Gold, and Rose Friendship) was evaluated and correlated with the development of Curvularia gladioli infective structures. The plants were inoculated with a 3 x 10(4) conidial suspension of the pathogen, and the severity of infection was assessed eight days after inoculation. Leaf samples of G. callianthus and G. grandiflorum vars. Amsterdam, Red Beauty, and T-704 were collected 24 hours after inoculation. They were cleared and the rates of spore germination and appressorium formation of C. gladioli were quantified. G. grandiflorum vars. T-704 and Red Beauty were the most susceptible genotypes, followed by G. callianthus and G. grandiflorum var. Tradehorn. The remaining genotypes did not differentiate among them. No differences related to the conidial germination (93.5%) were found for the genotypes. However, the highest values for appressorium formation were observed for G. callianthus (66.5%) and G. grandiflorus var. Amsterdam (55.7%), and the lowest values were observed for T-704 (32.4%). Thus, appressorium formation of C. gladioli was higher on leaf surfaces of resistant genotypes.

Resumo em Inglês:

In order to evaluate the effect of potassium silicate spray on the control of coffee leaf rust and coffee yield, an experiment was conducted during the 2005/2006 and 2006/2007 growing seasons on Coffea arabica cv. Catuaí Vermelho IAC 144, in a location situated at an elevation of 850 m in Coimbra, Minas Gerais, Brazil. The treatments consisted of: 1 - potassium silicate; 2 - potassium silicate + copper hydroxide; 3 - copper hydroxide; 4 - epoxiconazole formulated with pyraclostrobin and 5 - control treatment. Six applications of potassium silicate and copper hydroxide were made twice per month from December to March of each year. Two applications of the systemic fungicide were done in December and March of each year. Potassium silicate alone or in combination with copper hydroxide was not efficient in reducing coffee leaf rust incidence and increasing yield under high incidence of coffee leaf rust.