Melampsora euphorbiae, a new rust disease found on Euphorbia rigida in Turkey

Euphorbia rigida occurs widely in Turkey and is a weed of gardens and uncultivated areas. It is found throughout the Mediterranean basin and the Middle East, occuring from Turkey and Iran through to Portugal and Morocco. During surveys performed in early summer 2003, a severe rust disease was found on E. rigida, in the Malatya province of eastern Anatolia, where the plant is common. The severity of the disease attack had notably increased by mid-August when many plants were covered with rust pustules (uredinia). Many leaves had died before normal flowering was due to occur. Numerous uredinia occurred, mostly on the lower surface of leaves (occasionally above) and on stems. Initially scattered, these later occur at much higher densities, measuring 0.5-2 mm across, becoming a dark colour when mature and 0.2-0.4 mm in height. The urediniospores had a wall of uneven thickness and varied in shape, including spherical, ellipsoidal, pyriform and other irregular forms, 19-23 µm x 24-33 µm. Many of the urediniospores had a characteristic, colourless paraphysis. These varied in length from 7-10 µm to 30-38 µm and also have a cell wall of uneven thickness. Based on these characteristics and measurements, the fungus was identified as Melampsora euphorbiae (Kaneko & Hiratsuka, 1984).

To my knowledge this is the first record of M. euphorbiae on natural populations of E. rigida in Turkey. E. rigida is also planted in Europe as a garden ornamental but there are no known records of rust infection. M. euphorbiae has been recorded previously on E. lagascae in Spain (Villalobos et al., 1992) and E. esula in northern China (Wang et al., 1994). The fungus has been screened for use as a biological control agent against the weeds E. esula and E. cyparissias in the USA (Bruckart & Dowler, 1986). There is little indication that E. rigida is seen as an invasive plant anywhere and while M euphorbiae has clear potential as a biological control agent against other Euphorbia species, the rust disease poses more of a threat to the horticultural trade of E. rigida at present.