A number of growers across the region have encountered problems with vines dying throughout some patches. There has been much conjecture about the likely source or sources of the problem, and thanks to widespread testing and surveys conducted by Ian Macrae from CCW, there is a clearer picture emerging of the problem. The main cause of vine death is a root and trunk disease known as ‘Blackfoot’, attributed to the ‘Cylindrocarpon’ fungus. This is a very common fungus in most soils and rarely becomes a problem for vines growing in well-aerated soils with good organic matter levels.
It now seems that this disease is largely a waterlogging issue, particularly where vines are over-irrigated early in the season with drip irrigation systems when vine water use is low. The anaerobic conditions caused by waterlogging may both favour the development of pathogenic fungi and also lead to weakening vines to the point where Blackfoot can start to take hold. Some drought–tolerant rootstocks (such as Ruggeri 140) appear to be vulnerable to waterlogging and later infection by various pathogenic fungi including Blackfoot. In addition, high levels of nematodes have been found on rootstocks thought to be resistant to nematode damage.
It appears that due to poor grape prices, many growers have understandably been increasing crop levels to reduce losses. This has led in many cases to excessive irrigation early in the season, to the point where vine root systems are weakened, and other pathogens can attack an otherwise resistant and healthy plant. What to do?
There is little that can be done with vines that are dying at this time of the season. However, it is a good idea to mark any vines that have shorter shoots, low vigour, or are clearly stressed, and observe these vines in the following seasons. Examining the trunk below the graft union and digging up roots from these vines will reveal the dark staining of the wood and blackened, rotten roots characteristic of this disease. Revisit irrigation regimes used in the past season and make sure that vines are not being excessively watered. A good guide is the CCW irrigation-scheduling tool, which CCW growers can find on the CCW website.
While there is no ‘silver bullet’ that can cure the problem instantly, there is more work being done to better understand potential control options. Advice from an experienced agronomist or viticulturist may be valuable in decision-making.