This disease is caused by species of the fungus Botryosphaeria which are commonly associated with diseases of woody plants. In grapevines:
• Cankers form on wood.
Declining vine showing short stunted shoots and dead cordons Botryosphaeria canker originating from a large pruning wound Cross-section of trunk showing characteristic wedge-shaped dead wood
• There are no symptoms on leaves or green shoots.
• Rot occurs on ripe berries (especially exposed bunches).
Botryosphaeria spp. are found in most grape growing regions of Australia. The fungus is known to infect a wide range of hosts including native Acacia and Eucalyptus trees and shrubs, and members of the protea family.
Common names for the condition include
Macrophoma bunch rot, Black dead arm, Botryosphaeria (‘Bot’) canker and dieback, Excoriose, Grapevine decline syndrome, Diplodia cane dieback and bunch rot.
BOTRYOSPHAERIA ROT IS REPORTED TO COMMONLY CAUSE 5-10% CROP LOSS IN VINES BUT THERE IS NO KNOWN EFFECTIVE TREATMENT
Disease cycle and infection
• The fungus over-winters as small dark ‘pimplelike’ structures (pycnidia) on diseased wood.
• These structures release spores whenever they are wetted, throughout the growing season.
• Spores are spread by wind and water splash.
• Infection occurs through fresh wounds in wood, eg. pruning cuts
• The fungus can germinate at temperatures between 15-37ºC and grows between 5-37ºC.
AVOID PRUNING OR OTHER DAMAGE TO WOOD DURING WARM WET WEATHER WHEN SPORES ARE BEING PRODUCED
Infection by Botryosphaeria spp. is also favoured by conditions that reduce vine vigour, including drought, frost, hail, high summer temperatures, poor nutrition and poor pruning practices.
Varieties or individual vines which produce tight bunches with thin skins are the most prone to infection by a range of bunch rotting agents, including Botryosphaeria. Generally, Chardonnay and Semillon appear to be more susceptible to this kind of wood infection leading to vine decline.
Symptoms
Infection by species of this fungus results in fruit and wood rots. The specific symptoms caused by each species are currently under investigation, however in general, those listed below have been observed to be associated with Botryosphaeria in grapevines.
Wood and canes
• Cankers form around pruning wounds.
• Wounded cordons and trunks become infected and the infection moves towards the ground from the wound site.
• Cordons and trunks show a wedge of necrotic tissue when cut in cross-section (this symptom is often confused with that caused by Eutypa lata).
• There can be a loss of spur positions and bud mortality.
• Pycnidia form on dormant canes.
Bunches
Symptoms can be seen on young vines, but the impact of the disease is often more serious as vines mature:
• 1-4mm flat lesions with pycnidia forming on berries.
• Infected berries of white grape varieties turn light brown (colour change is less noticeable in red varieties).
• Berries blacken and shrivel, and become sticky with black spores on the surface.
• Infected berries can drop from the vine.
Co-occurrence with other rots
Botryosphaeria spp. are often isolated along with other fungi from discoloured wood associated with dieback symptoms. This co-occurrence can confuse identification further as many symptoms of Botryosphaeria infection are also similar to those of Eutypa dieback and other problems causing wood discolouration, eg. petri disease.
It can be very difficult to determine the role of all these different fungi. In many cases it is suspected that Botryosphaeria spp. are secondary invaders of wood damaged by other more aggressive organisms. In these cases they may hasten the decline by invading and killing wood that has merely been weakened by the primary agent, however in some cases it is probable that Botryosphaeria is the primary disease-causing organism.
It is often difficult to differentiate symptoms caused by Botryosphaeria both in the wood and on bunches from those caused by Diaporthe, Phomopsis, Eutypa, Colletotrichum and Greenaria.
