The disease is characterized by elongated, irregularly shaped, firm black swellings (knots), up to 30 cm long, on the woody parts of the plant, mainly twigs. Infected branches exhibit small hyperplasia and hypertrophy and soon rupture of the branch epidermis occurs. There is a growth of a mass composed by fungal cells, at first olive green, later black and hard and brittle, which often develops on one side of a shoot. Adjacent knots may become confluent. Fungal conidial fruiting may cover the knot and are later replaced by brown to black ascostroma, which are produced about one year after the inoculation in the field. Sometimes knots may girdle and kill the tips of smaller shoots. Numerous knots may cause general decline of the plant (Wainwright & Lewis 1970, Hickey 1995, Crop Protection Compendium 2005).
The fungus:
Stromata black, hard, in branches. Ascostromata black, globose, sometimes turbinate, up to 500 µm broad, with a slightly flattened apex and central ostiole. Asci clavate, sessile, bitunicate, 8-spored, 50—75 x 12—15 µm. Ascospores clavate, apex obtusely rounded, tapered towards the base, 1-septate near the base, smooth-walled, olivaceous, 13—18 x 4—7 µm. Conidiophores erect, pale brown, flexuous, geniculate with thickened scars on small denticles, 40—70 µm x 5—7 µm. Conidia pale brown, 0—1 septate, smooth, singly or in short chains, 6—13 x 3—5 µm (Sivanesan 1984). Additional information is given by Sutton & Waterston (1970).
