1Departamento de Fitopatologia, Universidade Federal de Viçosa, Viçosa, 36571-000 MG, Brazil

Background and objectives
Aquatic weeds considered to have areas in Brazil as their centre of origin have played a major role as biological invaders throughout the tropics. Eichhornia crassipes is the most famous of such species, being widely regarded as the world's worst aquatic weed. Few accounts of systematic surveys of the pathogenic mycobiota of this weed undertaken in the Neotropics have been published. Also, these surveys covered only a limited area of the native range of this weed. The Amazon Basin, for instance, indicated by many authors as the centre of origin of this species, remained unstudied in this regard. Other aquatic weeds native from Brazil and also of world relevance such as Alternanthera phylloxeroides and Myriophyllum aquaticum remain practically unknown in pathological terms. Native aquatic macrophytes which are still restricted to their native range, such as Eichhornia azurea and Paspalum repens, cause problems in Brazil under certain circumstances and are regarded as a potential threat if introduced into new habitats. During 1988 and 1989 a systematic survey for fungal pathogens of aquatic weeds in a series of 10 freshwater sites, representing the major watersheds and the main climatic types in the state of Rio de Janeiro, was undertaken [1]. The purpose of the present work was to continue and expand this study and, based on damage observed in the field and some laboratory and greenhouse tests, to carry out a preliminary evaluation of the potential of these Brazilian fungi as biocontrol agents for the above-mentioned plant species.

Materials and methods
Collections of fungal pathogens associated with the target weeds were made in the states of Minas Gerais (MG), Bahia (BH), Rio de Janeiro (RJ) and Amazonas (AM). During July-August 1997 a survey concentrated on sites along some of the main 'white water' rivers of the Amazon Basin (Amazonas, Madeira and Solimões) was undertaken. Short survey expeditions were started from the cities of Benjamin Constant, Coarí, Tefé, Nova Olinda, Manaus, Itacoatiara and Parintins, covering neighbouring areas. Notes were taken on the effect of the diseases on plant populations in the field, and samples of diseased materials were collected and preserved. Isolations, taxonomic studies and pathogenicity tests were undertaken later for selected fungi.

Results and conclusions
The mycobiota of E. crassipes in AM, as shown by the survey, is surprisingly poor. Only the widespread and well known leaf-spot fungus Cercospora piaropii and some clearly opportunistic fungal pathogens were found. It may be premature at this stage to draw conclusions, but if complementary surveys confirm this low diversity of the pathogenic mycobiota, this could indicate that AM is not in the centre of origin of E. crassipes. This information would be particularly relevant for the planning of future surveys. Acremonium zonatum and C. piaropii were commonly found in BA and MG associated to this weed. Uredo eichhorniae was found only at a high locality (ca 1400 m above sea level) at Poços de Caldas, MG. The latter is being considered as a candidate for future introductions (but conditioned to the elucidation of its life cycle and other tests).

The fungi Cercospora alternantherae and Nimbya alternantherae were recorded in Brazil associated to A. phylloxeroides for the first time. The latter is a damaging pathogen of this weed that may have potential for the development of a mycoherbicide. Preliminary results of tests carried out with this fungus will be discussed. A series of four different fungal pathogens was found associated to M. aquaticum, three of which are regarded as new to science: Cercospora sp., Mycosphaerella sp. and Pyrenochaeta sp. All of these are damaging pathogens that appear to have biocontrol potential. No additions to the known mycobiota of E. azurea resulted from the present survey, but a previously unknown Phyllachora was found commonly associated to P. repens in AM.

1. Barreto RW, 1991. PhD thesis, University of Reading, UK.