3.7.28
CONTROL STRATEGIES OF DISEASES ASSOCIATED TO CLONAL PROPAGATION OF EUCALYPTUS IN BRAZIL

A ALFENAS, S SILVEIRA and E VON STOWASSER

Federal University of Viçosa, Viçosa, Brazil

Background and objectives
Mass propagation of eucalypt by cuttings is currently applied by most Brazilian forest companies. The area covered with Eucalyptus in Brazil is presently estimated in 5 million ha. Although cutting propagation makes possible clonal multiplication of genotypes resistant to diseases, the management conditions normally used during the different stages of rooting are favorable to the attack of plant pathogenic fungi [1]. This paper aims to present a brief review of the control methods of the main diseases that occur during the clonal propagation of Eucalyptus in Brazil.

Results and conclusions
Eucalyptus canker (Cryphonectria cubensis)
Canker-resistant clones selected for conventional planting have recently been attacked by the pathogen under hedge conditions. Results of tests carried out by a forest company in the northeastern part of the state of Espírito Santo have shown that selective and continuous shoot harvesting reduces canker incidence and enhances stump survival and cutting rooting [1].

Eucalyptus rust (Puccinia psidii)
This disease is highly dependent on the existence of young organs and on the predominance of mild temperatures (20-25°C) as well as prolonged leaf wetness periods (ca 8 h). Clear cut in conventional planting and successive prunings in hedges are believed to induce shoot rejuvenation, making them more susceptible to rust. Use of resistant clones and fortnightly spraying of triadimenol at 0.15 g i.a./L are recommended for rust control in hedges [1].

Leaf and cutting blight (Rhizoctonia solani AG1-1B, Rhizoctonia sp. and Cylindrocladium spp.)
Under favorable conditions, in which prolonged leaf wetness (UR 100%) periods and temperatures in the range of 20-30°C predominate, the incidence of cutting blight in the greenhouse is correlated with the incidence of leaf blight in hedges. Losses up to 100% of cuttings caused by blight can occur due to the use of infected shoots or reuse of infested substrate. Infested substrate or reused and inadequately desinfested trays and tubes as well as the greenhouse floor are sources of inoculum (mycelium and sclerotia) for cuttings during rooting. The incidence of gray mold, caused by Botrytis cinerea, has also been found in a few nurseries. In case of rot incidence, associated with management practices, a weekly spraying program with the following fungicide mixtures may be used: against Cylindrocladium and Rhizoctonia, benomyl (0.5 g i.a./L) + captan (1.0 g i.a./L) or thiram (1.2 g i.a./L), alternated by benomyl (0.5 g i.a./L) + iprodione (1.0 g i.a./L). For Botrytis control, benomyl may be replaced by vinclozolin (2.0 g i.a./L). Hot water (70°C/60 s and 80°C/30 s) washing of trays and tubes is highly effective in eradicating rotting fungus inoculum. Besides pathogen eradication, hot water washing of containers significantly enhances rooting [1]. During acclimation, cuttings must be subjected to pruning, classified by height and placed in trays in a manner to facilitate airing in order to avoid a microclimate favorable to infection. In case of incidence of disease, the same spraying program used during the rooting phase can be employed. The use of antagonistic microorganisms is a viable alternative for suppression of plant pathogens in Eucalyptus cutting nurseries, since the rooting substrate and the humidity and temperature conditions required for the emission and development of the roots favor the biocontrol agents. On-going research in our laboratory aims to render viable the incorporation of such microorganisms in rooting substrates and in hedges so as to suppress, mainly, Cylindrocladium and Rhizoctonia.

Reference
1. Alfenas, A. C.; Silveira, S. F. & Stowasser, E. A. S. V. 1997. In: IUFRO Conference on Silviculture and Improvement of Eucalypts, vol. 3, pp. 106-111.