Rust Disease of Oregano and Sweet Marjoram in California

In 1996 and 1997, a rust disease was detected on commercial, fieldgrown oregano (Origanum vulgare) and sweet marjoram (Origanum majorana) in coastal California. Symptoms on both plants were similar and mostly consisted of small (2 to 5 mm in diameter), circular, brown, necrotic leaf spots that developed cinnamon brown pustules in the center of the spot or in concentric groups on the spot periphery. Pustules sometimes developed without spots. On sweet marjoram, leaf spots were sometimes surrounded by a chlorotic halo. Teliospores were not detected on either host. Ellipsoidal urediniospores measured 22 to 25 μm by 19 to 22μm and contained 2 to 3 germ pores in an equatorial configuration. The rust was identified as Puccinia menthae. Pathogenicity was tested by depositing urediniospores onto leaves of healthy plants and then incubating plants in a humidity chamber for 48 h. Urediniospores from oregano infected Italian (Origanum× majoricum), Sicilian (Origanum × marjorana), trailing (O. prostrata), and Greek (O. heracleoticum) oregano, and sweet marjoram. Urediniospores from sweet marjoram infected sweet marjoram and the one oregano tested, Italian oregano. With all inoculations, both symptoms and fungal fruiting bodies were similar to those observed in the field. Neither the oregano nor the sweet marjoram isolates infected spearmint (Mentha spicata), which is consistent with previous studies (1,2). This is the first report of a rust disease of oregano and sweet marjoram in California. Rust significantly reduced the quality and yield of both crops. References: (1) J. T. Fletcher. Trans. Br. Mycol. Soc. 46:345, 1963. (2) M. Wilson and D. M. Henderson. 1966. British Rust Fungi, Cambridge University Press, Cambridge.