Violet Fungi
Violets are liable to the attacks of several species of Fungi, which grow on the stems and leaves, and on the sepals and petals of the flowers.
Urocystis Violae causes the stems and leaves of Viola odorata, and of other Violets, to become greatly thickened and distorted. After a time, the epiderm on the swellings is torn here and there, and displays the dark masses of spores. Each spore consists of a large central cell (which, on germinating, forms a slender mycelium tube), and an outer layer or coat of smaller cells. Puccinia Violae is of very frequent occurrence on the leaves and petioles of many Violets, including the common wild species and most of the cultivated ones. It appears, in early summer, in the form of Cluster Cups (AEcidium Violae), and is then easily recognisable by the small, yellow cups thickly dotted over reddish-orange, swollen patches. Each cup has a nearly white margin of little teeth, formed by the torn edges of the burst outer coat (peridium), which incloses a multitude of minute, orange-yellow spores. Afterwards, the cups are replaced by small, brown, powdery masses, irregularly scattered, or in circles, composed of rounded or oval, prickly, brown, one-celled, stalked spores, and of the true Puccinia spores, each on a rather long stalk, brown, and made up of two cells end to end. Another Puccinia that grows on Violets is P. aegra. This species greatly weakens or kills V. cornuta, and has also been found on V. lutea. Its AEcidium has been named AE. depauperans, from its weakening (or depauperating) effect on the host-plants. It may be distinguished from AE. Violae by the cups being scattered, and not grouped on thickened spots. The two also differ in microscopic peculiarities of their spores.
Puccinia Fergussoni grows on the leaf-stalks and leaves of V. palustris, producing thickenings of the tissues, over-grown by the brown, two-celled spores. This Fungus is not known to have more than this single form of spore. Not unfrequently, the leaves of Violets show discoloured spots, which, on examination with the microscope, are found to be due to the presence of species of Ramularia (R. agrestis and R. lactea), or of Cercospora (C. Violae), &c. These are minute Moulds, and bear elongated, multicellular, transparent or brown spores, on the tips of slender stalks. Or such spots may be dotted over with the small, dark perithecia of Laestadia Violae, one of the Sphaeriaceae, which has eight oval, unicellular spores in each ascus. Of all the Fungi that occur on Violets, the most destructive are Puccinia aegra, P. Violae, and Urocystis Violae; though the others also may cause considerable disfigurement. These all grow within the tissues of the plants, the only parts pushed to the surface being the organs of reproduction. It is therefore of little use to attempt to cure diseased plants, the interior of which must be full of the Fungi; and it is advisable to remove and burn the plants, to prevent the spread of disease to previously healthy specimens.