CYLINDROCLADIUM SCOPARIUM CAUSING WILT DISEASE IN RHODODENDRON AND AZALEA

Cylindrocladium scoparium Morgan (Deuteromycotina) has become a widespread and major pathogene in Rhododendron spp. and Azalea spp. as well as in Erica gracilis, Erica carnea, Calluna vulgaris and Gaultheria procumbens. Although inoculation experiments showed that Pieris japonica is also susceptible to C. scoparium, the disease rarely can be found on these plants in nurseries. Especially in forcing cultures of Rh. simsii and Erica spp. in greenhouses and in containerized productions the soilborne fungus is able to cause severe losses year after year.

Typical disease symptoms are root- and stem rots of infected plants. Leaf spots and necrotic flecks on flower petals, as reported in the literature (e.g. Linderman 1986), have so far not been registed in field stands of north-west Germany.

Cylindrocladium spp. show a saprophytic ability which is highly competitive with other soilborne microorganisms. In addition, the fungus forms microsclerotia, which are able to survive longer periods of time in the soil or in infected plant debris without a host plant.

Infections take place at the stem's tissue, mainly in or above the upper layer of the soil. The first infections by mycelia result in the death of cortical cells. This may however not immidiately results in disease symptoms, since the growth of the pathogene and the spread of the disease is sensitive to the cultivation practices in nurseries. Particularly temperature and moisture are factors which influence the outbreak of the disease (Stegmann and Böhmer 1989), but also overfertilization with nitrogenous fertilizers (Backhaus and Bettin, unpubl. data) and any other stress factors at the beginning of forcing favor the fungal development inside the plant's tissue. When plants are produced under optimal growth conditions the pathogene can stay latent for several weeks.

The growth of the fungus across the vascular systems and the collapse of the cortical tissue result in steunting of the infected plants and in dramatic wilt symptoms. A closer look at the inside of stems from collapsed plants shows a distinct brown discoloration. In Rh. simsii the