PYTHIUM ROT OF SAINTPAULIA, THE AFRICAN VIOLET

Abstract

Many fungi and bacteria were isolated from African violets affected with root and crown rot, but, of these, Pythium ultimum Trow was the only organism that proved to be pathogenic to this host. P. ultimum readily infected leaf cuttings, rooted cuttings, and the petioles and leaves of plants when these were in contact with moist infested soil. Inoculum placed against the crown of a healthy plant also caused infection, but the lesion developed slowly and usually only a small amount of tissue was decayed. On the other hand, when the crowns and roots of healthy plants were set in infested soil they remained healthy; but they became predisposed to attack when they were subjected to prolonged exposure to light of very high intensity or when the roots were infested with nematodes (Meloidogyne sp.). Alternating periods of overwatering and drying did not increase the susceptibility of these healthy plants. When young plants, large enough to be planted singly in soil, were allowed to remain in vermiculite, a nonnutritive substrate, until they yellowed and were then planted in infested soil, they became infected and completely decayed. However, plants at least a year old that had been placed in vermiculite until they yellowed and were then planted in infested soil did not become infected.Infection and decay of leaf or rooted cuttings caused by P. ultimum occurred readily throughout the range of 10°–30 °C. The disease also occurred over a wide range of soil moisture, but was favored by the higher levels.When leaf cuttings and rooted cuttings of 30 varieties of African violets were planted in infested soil, all varieties proved susceptible.