First record of leaf-spot caused by Acroconidiella tropaeoli on Tropaeolum majus in Brazil

Tropaeolum majus (nasturium - local name chagas or capuchinho) is a member of the Tropaeolaceae native from Peru and Brazil. In Brazil it is commonly used as an ornamental (Lorenzi & Souza, 1995) but it is also being increasingly grown as a vegetable for its edible and showy red, orange or yellow flowers used in decorating salads. Damaging leaf-spots were found affecting a group of plants in Viçosa and also at Catas Altas - state of Minas Gerais, Brazil. No record of a similar disease on this host in Brazil was found in the literature.

Leaf lesions initially were yellow and punctiform becoming circular with a brown centre, 2-25 mm, necrotic tissue often cracking or falling resulting in shot-hole symptoms coalescing and leading to extensive necrosis of the leaves. Severely affected areas or whole leaves became yellow and died (Fig 1).

The fungus had conidiophores arising from the hyphae that are cylindrical, straight to slightly flexuose, branched, 64-301 x 4-8 µm, pluriseptate, smooth and pale brown. Conidia were clavate, ellipsoidal to ovoid, 15-50 x 6-19 µm, mostly two septate, echinulate and brown (Figs 2 a, b and c). The morphology of the fungus associated with the symptoms was equivalent to that described for Acroconidiella tropaeoli (Bond) Lindquist & Alippi (Ellis, 1968). Some relevant differences such as a less pronounced to absent constriction of conidial septae and a smaller size of conidia and conidiophores in the specimen from Viçosa are here interpreted to represent only a geographical variation of the taxon, not requiring the proposal of a separate species. A previously undescribed feature was the occasional production of short acropetal conidial chains (Fig 2 b). The specimen was deposited in the herbarium as: VIC 22163 (specimen from Viçosa) and VIC 22192 (specimen from Catas Altas).

Typical symptoms of the disease developed on five healthy plants five days after they were sprayed with a mycelial suspension of A. tropaeoli containing 9 x 10⁴ mycelial fragments/ml . Plants sprayed with water did not develop any symptoms, confirming the results of Baker & Davis (1950). Acroconidiella tropaeoli was reisolated from the necrotic tissue. It has a wide distribution in tropical and subtropical regions of Australasia, Africa, West Indies, and North and South America (Farr et al. 1989) but Mendes et al. (1998) did not mention its occurrence in Brazil. Therefore this is the first record of A. tropaeoli in Brazil.