EVALUATION OF SAPROPHYTIC CAPACITY AND PATHOGENICITY OF RHIZOCTONIA SPP. IN THE CLONAL PROPAGATION SYSTEM OF EUCALYPTUS
EA SANFUENTES and AC ALFENAS
Departament of Plant Pathology. Federal Višosa University. 36571-000, Višosa, MG, Brazil
Background and objectives
Mass propagation of eucalypt by cuttings, currently applied by most Brazilian forest companies, allows the establishment of homogeneus plantations from selected genotypes. However, the ideal environmental conditions for mass propagation in hedges and for the rooting of cuttings in greenhouses or shadehouses are highly favorable to the incidence of plant pathogens. Rhizoctonia spp. is found in all phases of mass propagation through cuttings in the sub-tropical Eucalypt-producing areas, being not only the most important pathogen ocurring in this system but also the most difficult to control . Epidemiological and pathogen-controlling studies have recently in this area, as well as the need to obtain fast and reliable methodologies that colud allow the evaluation of pathogen and diseases characteristics under field conditions. The objectives of these studies were to test different biological baits to estimate the saprophytic competence ability (SCA) of Rhizoctonia isolates and to develop a methodology to test the pathogenicity of these isolates in eucalypts.
Materials and methods
To evaluate SCA, forest and agricultural soils were used, infested in different proportions (0:1, 1:1, 1:4, 1:16, 1:64, 1:256 and 1:1024) with autoclaved soil containing R. solani inoculum. The method used for the pathogen inoculum production was the chopped potato-soil. Eucalypt bark compost infested with the pathogen and treated with fungal antagonist was also used. The baits used were eucalypt stem segments, potato and beet segments (12x5 mm) and heat-trated bean seed. The baits were placed in soil or substrate, incubated for 48 hours at 26oC, following washed in tap water, superficial desinfestation (1000 ppm Cl), placed in acidified water-agar and incubated for 24-36 hours, at 26oC.The pathogenicity tests of the Rhizoctonia isolates were conducted in eucalypt shoots originated from mass propagation. The shoots were 45-50 cm long with 10-12 leaves. The pathogen inoculum used was ground mycelium obtained from a semi synthetic liquid medium (500 mg mycelium/ 20 ml water) for 4 shoots/isolate. All leaves were trated with the inoculum and incubated for 72 hours at 25oC in a growth chamber with a 12 hour photeriod and intermittent nebulization. The severity of the leaf blight was evaluated by use of a diagramatic scale.
Results and Conclusions
The results showed that the eucalypt stem segment was the most efficient bait for estimating SCA, measured as percentage of segments colonized by the pathogen and presenting the highest sensibility for detecting the pathogen at low inoculum density. In addition, this bait does not require autoclavage, presenting lower contaminating indices, mainly bacteria, as compared to potato and bean seeds. Hence, the eucalypt bait was rather selective, consequently limiting the saprophytic activity of the other soil-fungi. A correlation between the incidence of leaf blight and SCA, as evaluated with eucalypt baits was observed in eucalypt bark compost infested with the pathogen and trated with antagonists. In others cultures, positive correlations between disease under greenhouse and field conditions have been already using different baits . The use of SCA as an estimate of saprophytic activity and, indirectly, as an estimate of Rhizoctonia inoculum density, is an important tool in epidemiological studies as well as for evaluating the efficiency of control treatments.The methodology used in pathogenicity tests for the Rhizoctonia isolates in eucalypts has swon to be efficient, fast and easy to apply. Different degrees of severity of the isolates tested were determined, confirming the presence of higly virulent isoltes as well as of non pathogenic ones (5%).
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