First record of powdery mildew caused by Oidiopsis haplophylli on Tropaeolum majus in Brazil

M.F. Ribeiro¹, F.B. Rocha² and R.W. Barreto^2*

¹ Campus Universitário de Gurupi, Fundação Universidade Federal do Tocantins – UFT, TO, Brazil
² Departamento de Fitopatologia, Universidade Federal de Viçosa, 36570-000, Viçosa-MG, Brazil

^*rbarreto@ufv.br

Accepted for publication 19/07/06

Tropaeolum majus (nasturtium; local names in Brazil: chagas and capuchinho) is a herbaceous plant, native to South America.  In Brazil it is commonly grown as an ornamental (Lorenzi & Souza, 1995) but is also becoming increasingly popular as an edible flower used for decorating salads. 

Figure 1: Symptoms of attack of Oidiopsis haplophylli on Tropaeolum majus

In November 2005 plants showing symptoms of angular leaf spots with an abaxial whitish fungal cover, similar to those caused by Ramularia-like fungi, were collected in a garden at Embú das Artes, Sao Paulo.  The disease started as chlorotic leaf spots, which later became  necrotic and angular.  The whitish fungal structures turned out to belong to a powdery mildew, which had the following morphology: internal mycelium septate, branched, hyaline, 3.75-5.0 µm diameter; conidiophores arising from the internal mycelium and emerging through stomata, cylindrical, occasionally branched, 106-204 x 3.75-6.0 µm, septate, hyaline; conidia were dimorphic, with primary conidia lanceolate, 56–76 x 11–15 µm, mean length-to-width ratio 5.0, and secondary conidia cylindrical, 41–721 x 7.5–14.0 µm, mean length-to-width ratio 4.9; both kinds of conidia were hyaline and smooth.  A sample was deposited in the local herbarium (VIC 29442).

Figure 2: Branched conidiophores of Oidiopsis haplophylli arising from mycelium though stoma (arrow). Bar = 30 µm.	Figure 3: Primary (lanceolate) and secondary (cylindrical) conidia of Oidiopsis haplophylli from nasturtium. Bar = 30 µm

According to Braun et al. (2002), Erysiphales anamorphs that have branched conidiophores arising from internal mycelium and have dimorphic conidia belong to the form-genus Oidiopsis.  The morphology of the fungus in nasturtium corresponded well with the polyphagous species Oidiopsis haplophylli, the proper name for the fungus often referred to in the literature Oidiopsis taurica (a later synonym; Liberato & Barreto, 2006).  There are many records of this fungus attacking nasturtium worldwide (Farr et al. 1989) but this is its first record as a pathogen of T. majus in Brazil. 

References

Braun U, Cook RTA, Inman AJ, Shin H-D, 2002. The taxonomy of the powdery mildew fungi. In: Bélanger RR, Bushnell WR, Dik AJ, Carver TLW, eds. The Powdery Mildews: A Comprehensive Treatise. St. Paul, Mn, USA: APS Press, 13-55.

Farr DF, Bills GF, Chamuris GP, Rossman AY, 1989. Fungi on Plants and Plant Products in the St. Paul, Mn, USA: APS Press.

Liberato JR, Barreto RW, 2006. Powdery mildew of Ruta graveolens in Brazil caused by Oidiopsis haplophylli. Summa Phytopathologica 32, 80-81.

Lorenzi H, Souza HM, 1995. Plantas Ornamentais do Brasil. Nova Odessa, Brazil: Editora Plantarum.

The British Society for Plant Pathology