Viroses are very small and only visible under an electron microscope. They have no metabolism of their own and only multiply in the cells of their hosts.
Viroses become more and more important in worldwide orchid cultivation, in Germany alone about 20 different viroses on orchids have been recorded. The symptoms of infestation depend on the family, species and age of the infested plants and also on the conditions of cultivation. The viruses often occur in mixed infections which makes a visual diagnosis even more difficult. Phalaenopsis hybrids are highly susceptible to these three major viruses: the Cymbidium mosaic virus (CyMV), the Odontoglossum ring spot virus (ORV) and the Orchid fleck virus (KORV).
Cymbidium mosaic virus - CyMV
Odontoglossum ring spot virus - ORV
The CyMV belongs to the potex virus group, ORV to the tobamovirus group. Viroses are clearly visible, especially on younger leaves, by small or wide spots and often of a brighter tissue. On Phalaenopsis, after ripening of the leaf, these bright areas change to sunken black spots, in the beginning mainly on the undersides of the leaves, later also on the leaf surface. In cases of severe infestation, the infested leaves drop off. On blossoms of dark-coloured varieties, marked deformation and stripes or spotted areas are indicators of an infection with the Cymbidium mosaic virus, but similar colour changes also happen due to mutations, mainly in dark Phalaenopsis varieties. Symptoms on blossoms are relatively rare on Phalaenopsis. The young shoots of Cymbidia show small longish bright spots on the leaves, initially as longish stripes in the middle of the leaf, later on as marked spots on the leaf. With the disease progressing, older plants show black stripes on the upper and undersides of the leaf. Assimilation is considerably decreased, the leaves drop off. Blossom symptoms are frequent on Cymbidia. Cattleya shows sunk in, dark brown to black discolouring on fully grown leaves. These discolourations may be found over the entire leaf surface or as round spots. New shoots are usually very bright. Small round brown spots can be seen on the open blossoms that join later to form brown stripes. Cattleya, Cymbidium and .