Andressa C. Z. Machado

Avaliação de danos causados por Pratylenchus brachyurus em algodoeiro

Three greenhouse experiments and one field experiment were carried out to evaluate the damage caused by the lesion nematode Pratylenchus brachyurus on cotton (Gossypium hirsutum) plants. In the first experiment, three isolates of P. brachyurus (Pb20, Pb21, and Pb22) from different regions of Brazil were inoculated (12,000 nematodes/plant) in plantlets of cotton cv. Delta Opal and Fibermax 966. The initial populations of 3,000 and 12,000 nematodes/plant, and 12,000 and 30,000 nematodes/plant were tested in the second and third experiments, respectively, to evaluate the effect of isolate Pb20 on growth of cotton cv. Delta Opal. The results of experiments 1, 2 and 3 suggest that P. brachyurus is a mild pathogen for cotton, since nematode densities lower than 12,000 did not reduce plant growth. The experiment 4 was conducted in three cotton fields in in the State of Mato Grosso confirmed these results, since no correlation was observed between the root population of P. brachyurus and cotton yield.

Effect of population densities of Pratylenchus brachyurus on the growth of cotton plants

The effect of different population densities of Pratylenchus brachyurus on the growth of cotton (Gossypium hirsutum) plants was studied in three greenhouse experiments, using cultivars IAC 20 and IAC 22. Both cultivars proved to be good hosts to the nematode, which however did not affect plant height and affected only slightly fresh root weight and dry top weight of both cultivars, even at the highest population density (over 9,000 nematodes/plant).

Sucessão de cultura sob pivô central para controle de fitonematoides: variação populacional, patogenicidade e estimativa de perdas

A field study conducted over three consecutive years, on a farm using crop rotation system under center-pivot and infested with the nematodes Pratylenchus brachyurus, P. zeae, Meloidogyne incognita, Paratrichodorus minor, Helicotylenchus dihystera, Mesocriconema ornata and M. onoense, demonstrated that intensive crop systems provide conditions for the maintenance of high densities of polyphagous phytonematodes. Of the crops established on the farm (cotton, maize, soybean and cowpea), cotton and soybean suffered the most severe crop losses, caused respectively by M. incognita and P. brachyurus. Since maize is a good host for both nematodes, but tolerant of M. incognita, its exclusion from cropping system would be favorable to the performance of cotton, soybean and cowpea. Results from experiments carried out in controlled conditions confirmed the pathogenicity of P. brachyurus on cotton. Additional management with genetic resistance was useful in fields infested with M. incognita, although the soybean performance was affected by low resistance of the cultivars used for P. brachyurus. In conclusion, crop rotation must be carefully planned in areas infested with polyphagous nematodes, specifically in the case of occurrence of two or more major pathogenic nematodes.

Potencial de controle da erva-de-Santa-Maria sobre Pratylenchus brachyurus

Antagonist plants have been used as an alternative to control plant-parasitic nematodes. One in vitro assay and two greenhouse experiments were carried out to evaluate the effects of Mexican-tea (Chenopodium ambrosioides) on the control of Pratylenchus brachyurus. In the in vitro test an extract of Mexican-tea was tested at four concentrations (20; 2; 0.2 and 0.02%), in suspensions containing 500 exemplars of P. brachyurus. After 48 h, living nematodes were collected and counted. A nematicidal action of Mexican-tea was observed by higher juvenile mortality in comparison with chemical control (aldicarb). In greenhouse experiments, 1 and 2, the Mexican-tea and soybean plants were inoculated with 1,500 and 5,000 P. brachyurus, respectively. Forty-five days later, plant shoots were incorporated in soil, as follows: shoots of Mexican-tea, were incorporated; shoots of soybean were incorporated; and plant shoots were not incorporated. One month later, each pot received one soybean plant, to act as a biological indicator of parasitism. At the end of experimental period (45 days later) the final nematode population was estimated by counting the nematodes extracted from soybean roots and soil, and data were collected of fresh root weight and shoot dry weight. Mexican-tea reduced the nematode population, although soybean plants had been affected by phytotoxicity.

Development of Colletotrichum gloeosporioides isolated from green pepper in different culture media, temperatures, and light regimes

Control of anthracnose in green pepper involves the use of resistant varieties and/or fungicides. The selection of varieties and efficient products demands great amounts of conidia as inoculum. It is thus necessary to optimize the production of Colletotrichum gloeosporioides conidia in the laboratory, establishing the best conditions for fungus development. The present study aimed at determining the most favorable culture media, temperature, and light conditions for the production of fungus inoculum. The fungus was isolated from green pepper fruits (Capsicum annuum L.) and transferred to four culture media (PDA, oat, filtered pepper extract, and autoclaved pepper extract), under different temperatures (15, 20, 25, 30, and 35oC) and light conditions (24h dark, and 24h light). Colony growth was evaluated after 7 and 12 days of incubation. No differences were found between the culture media. However, the greatest number of conidia was obtained from colonies grown in oat medium at 25oC. Temperatures of 20 and 25oC were the most favorable for colony growth and sporulation. Higher sporulation was obtained under incubation in constant light. Cultivation of C. gloeosporioides in oat medium, at 25oC, and constant light is recommended.

Host suitability of oats for Pratylenchus brachyurus

Black oat (Avena strigosa), white oat (A. sativa) and Algerian oat (A. byzantina) are extensively cultivated in the south of Brazil for grain, forage, hay and silage production, or as cover crop in no-tillage and crop-pasture integration systems. In both systems, the genotypes of oat used as cover crop must be nonhosts or poor hosts of damaging nematodes for summer cash crops. Taking into account the relevance of Pratylenchus brachyurus as a pathogen for many cash crops in Brazil, two experiments were carried out in a glasshouse in order to evaluate the host suitability of selected oat cultivars to this nematode. The initial population inoculated (Pi) were 92 specimens/plot in experiment 1, and 270 in experiment 2. At the end of experimental periods (86 days after inoculation in experiment 1 and 67 days in experiment 2), the final population (Pf) of P. brachyurus was estimated and the reproductive factor (RF = Pf/Pi) was calculated. The results demonstrated that black oat (RF = 0.04-1.03) is more valuable than Algerian oat (RF = 2.63-2.88) or white oat (RF = 1.37-1.93) for the management of P. brachyurus.

Control of Meloidogyne paranaensis in coffee plants mediated by silicon

Coffee plants are considerably affected by the presence of Meloidogyne spp. and management is based in the use of resistant rootstocks. Systemic induced resistance could constitute an option to manage Meloidogyne paranaensis in coffee-growing areas. Coffee seedlings cv. Mundo Novo were treated with silicate before the inoculation of M. paranaensis to evaluate the potential control of this pathogen. We found, 120 days after the inoculation of 2000 eggs, that the application of silicate was effective to reduce the nematode population, based on reproduction factor and number of nematodes per g roots. Based on the present results, we suggest that the application of silicon constitutes an important alternative for the management of M. paranaensis in infested coffee-growing areas. Further studies are necessary to clarify the biochemical and molecular relationships between M. paranaensis and coffee in the presence of silicon.

Broad-sense heritability and variance component estimates for Pratylenchus brachyurus resistance in Brazilian soybean genotypes

Pratylenchus brachyurus has become one of the most serious problems in Brazilian soybean fields. The use of resistant cultivars has long been considered the most practical and effective means of controling nematodes. Nonetheless, to our knowledge, there is no study on the genetic parameters associated with resistance of soybean to P. brachyurus in Brazil. This study was undertaken to estimate resistance genetic parameters under glasshouse conditions. In both trials, we found high phenotypic and genotypic variability, either for reproductive factor or number of nematodes per fresh gram of roots. Furthermore, high coefficients of genotypic determination and variation index were obtained. These results suggest that breeding for this feature can be relatively easy. The next logical step will be to conduct inheritance genetic studies.

Host status and phenotypic diversity of rice genotypes in relation to Pratylenchus brachyurus resistance

The aim of this study was to assess the susceptibility of rice genotypes to Pratylenchus brachyurus and investigate the inheritance of resistance in the crop. Two experiments were conducted under controlled conditions, using naturally infested soil. Twenty-six rice genotypes were assessed, with maize used to show the susceptibility pattern. The maize was cultivated for 90 days and the initial nematode population was determined. Then, rice genotypes were cultivated, and the final nematode population and the reproduction factor (RF) were estimated 90 days after germination. All genotypes were susceptible to P. brachyurus in the two experiments but only one showed statistical differences, indicating variation in susceptibility, and genotypes Linhagem L03-107 and Cateto Amarelo scored higher than the control genotype, with RF of 8.80 and 9.48, respectively. Inheritance of resistance was poorly influenced by genotype genetics. Cluster analysis allowed the identification of five different groups of genotypes but there was low genetic variability among the genotypes.