Botryosphaeria stevensii (Botryosphaeria disease, grapevine)

Symptoms do not vary substantially on the main tree hosts. On oak, B. stevensii was first reported as pathogenic on Quercus prinus in the USA by Schmidt and Fergus (1965) and on Q. petraea in Europe (Hungary) by Vajna (1986). Infections produce bark necroses and cankers on twigs with a diameter of 2-6 cm, especially on apical twigs, and are often characterized by mucilaginous exudates. When a necrotic lesion girdles the stem, the upper portion is immediately affected by wilting and subsequent dieback. Leaves become chlorotic and finally wilt, often remaining attached to the dying stems. During the vegetative growing season, erumpent pycnidia, which look like black pustules, may be observed on necrotic areas after weeks or months. The necrotic bark can be easily removed, revealing black phloem, cambium and outer sapwood. Single infections are not able to kill an adult tree, but repeated attacks over consecutive years may compromise large sections of the crown and impair its tolerance to environmental stresses. However, symptoms caused by B. stevensii often merge with those of the syndrome known as oak decline or, more recently, oak puzzle disease; these syndromes are triggered by various soil and environmental factors.

On other broadleaved trees, B. stevensii induces quite similar symptoms: for example, on young shoots of Fraxinus excelsior, dark-brown, necrotic lesions of the bark are sometimes associated with brownish discoloration and wilting of leaves (Przybyl, 2002). 5-7-year-old suckers and stems of adult trees of Fraxinus ornus in Sicily, Italy, were found to be infected by B. stevensii, and suffered necrotic areas that subsequently split, epicormic shoots under the cankers, and crown dieback (Sidoti and Granata, 2004).

On juniper (Juniperus spp.), infections cause elongated cankers at the base of shoots, associated with sudden yellowing and browning of large portions of the crown.

External symptoms on grapevine are not substantially different from those previously described for tree hosts. In Hungary, where the attacks by B. stevensii have been known since 1974, the syndrome is known as 'black dead arm disease' and consists of leaf chlorosis and wilting, browning and subsequent drying of the berries. The xylem of the trunk is streaked black under areas of necrotic bark tissue (Lehoczky, 1974).

Cultural Control and Sanitary Methods

Cultural control of attacks by B. stevensii is only possible for isolated trees, as it is not feasible to perform systematic sanitary prunings in the forest. Infected twigs must be cut at least 15 cm under the visible lower edge of infection and carried out the forest to reduce the inoculum potential. In the nursery, plants must be grown in optimal conditions, especially with regard to water supply, to avoid stress conditions that promote attack by the pathogen.

Host-Plant Resistance

Studies for resistance of oaks to B. stevensii are still at an early stage. Trials performed on several cultivars of Quercus suber have shown the existence of resistant genotypes in all tested Mediterranean countries of origin (Bakry et al., 1999).

Biological Control

Possible attempts of biological control must be based on antagonistic or hyperparasitic organisms already found in nature (see Notes on Natural Enemies), because they have to find a suitable environment for growing and reproduction when introduced to the natural habitat of B. stevensii.

Worthy of further studies is the activity of some species of Pseudomonas (P. fluorescens, P. corrugata, P. tolaasii) isolated from Tuber borchii (an ascomycete often living in natural oak forests), which have been found to produce in vitro biocontrol substances with a weak effect against some pathogenic fungi, including B. stevensii (Bedini et al., 1999).

Attacks by B. stevensii have an economic impact in oak forests, reducing the annual increment of wood as a consequence of the wilting and the death of large sections of crown. Unfortunately, quantitative valuations of damage are lacking and, the specific damage caused by this pathogen often merges with the heavier damage as a result of "oak puzzle disease". As regards the grapevine, except for the case of "black dead arm disease" which is geographically restricted, the role of B. stevensii in the context of various decline syndromes is too controversial and/or uncertain to ascribe to it a specific damage.