First report of a phytoplasma-associated witches' broom disease of Miconia calvescensin Brazil

Velvet tree (Miconia calvescens DC), a medium size, evergreen tree in the melastome family (Melastomataceae), is native to rain forests of South and Central America. Following its introduction into tropical oceanic island habitats, this species has invaded forested areas. Today, it is regarded as the worst invasive plant species in French Polynesia (Meyer, 1996) and Hawaii (Medeiros et al., 1997) and threatens the biodiversity of both archipelagos. Since 1995, surveys for diseases have been conducted to identify potential biocontrol agents of velvet tree for introduction into Hawaii. One such pathogen has been introduced (Killgore et al., 1999) and others are being evaluated in Brazil and Hawaii.

A 'rosette stunting' was observed affecting young velvet trees growing at Cachoeiras do Macacu, Ilha Grande (State of Rio de Janeiro) and Viçosa (State of Minas Gerais). Plants exhibited shortened internodes, small pink-coloured leaves with long marginal hairs and witches'-brooms (Figs. 1-2). Mealybugs and leafhoppers were also found in association with these plants (Fig. 3). Symptomatology and failure to culture fungi or bacteria from affected plant tissues suggested that M. calvescens witches'-broom might be a phytoplasma disease.

Fresh tissue samples (petioles and main leaf vein) collected from six symptomatic plants and one symptomless plant were prepared for transmission electron microscopy (TEM) as described by Matsuoka & Carvalho (1987). Phytoplasmas, appearing as spherical to ovoid structures of variable sizes, were consistently observed in phloem sieve tubes in diseased plant samples but not in symptomless plant samples by TEM (Fig. 4). These observations are the first to implicate phytoplasma as the likely etiological agent of M. calvescens witches'-broom disease in Brazil.

In July 2000, a mature velvet tree with initial symptoms of witches' broom disease was found on the campus of the Universidade Federal de Viçosa. Within one year it became moribund and a few remaining shoots were completely broomed. Although the disease is extremely damaging, practical difficulties related to the biotrophic nature of phytoplasmas and their spread by insect vectors are likely to limit its potential use for biocontrol of velvet tree.