AZALEA - FLOWER BLIGHT, CAUSED BY OVULINIA AZALEAE WEISS

In the late summer of 1987, which was characterized by long periods of cool and rainy weather, a flower blight disease of azaleas caused severe losses in azalea nurseries of north-west Germany. In Germany this disease had scarcely been registered before 1987 except for a short disease incidence in the early 1970s. The disease, known as azalea flower blight, azalea flower spot or Ovulinia petal blight, is caused by Ovulinia azaleae Weiss (Discomycetes, Sclerotiniaceae), which was first observed in 1931 in the USA and described by Weiss(1940). The anamorphic state of the pathogene is described as Ovulitus azaleae Buchwald.

Nowadays the fungus is known to occur in many parts of the world. Reports are given from the USA, Australia, New Zealand, Great Britain, Japan, Switzerland, Belgium, France (Peterson 1986), The Netherlands (Miedema 1984) and Germany (Backhaus 1988).

Various species of Azalea and Rhododendron are susceptible to Ovulinia azaleae as well as Kalmia latifolia, Vaccinium corymbosum, Vaccinium fuscatum, Vaccinium tenellum, and Gaylussacia baccata (Peterson 1986).

Flowers are the only parts of the host plants which are infected. First symptoms at petals appear as small, round spots, which are about one millimeter in diameter and appear water-soaked. These spots may already occur before the flowers are completely open. As the spots enlarge, the infected petals become soft and slimy and develop a light-brown to dark-brown discoloration. Under moist conditions the fungus needs only 3 to 4 days to completely destroy infected flowers. The entire flower tissue becomes limp and is intensively occupied by mycelia. Infected flowers dry and may stay attached to the plant for several weeks.

The mycelia inside the petals tissue give rise to large amounts of macroconidia and microconidia (spores of the anamorphic state). Weiss (1940) counted up to 225 conidia per mm² of infected area. Peterson (1986) estimated that approximately 100 000 conidia may be formed at one single infected blossom. They are spread by wind, raindrops and insects from flower to flower. Under moist conditions the conidia germinate immidiately. The germ tubes form appressoria and colonize the petals by infection hyphae. Under dry conditions the macroconidia are able to remain viable for several months.

Between 4 and 8 weeks after the infection small, light-brown sclerotia appear at diseased petals. These sclerotia become dark brown and black as they mature and develop diameters between 2 and