Q: Some of the trees on the boundary of our small block have dead bits in them, some worse than others. Can you tell what causes this?
Question from A. Smith, Te Kowhai, Waikato
A: Your trees are Leyland cypress and this looks like typical symptoms of cypress canker, a relatively common fungal infection especially in humid parts of the country where there’s reasonably high rainfall. The disease spreads by spores that enter the plant through fissures in the bark or through wounds, such as those caused by pruning and trimming or wind rub. You may be able to spot the spore-producing organs of the fungus; they look like black dots on the diseased stems. Spores spread by drifting in the air or being washed down or splashed around in rain, and on pruning tools. The fungus interferes with the sap system, leading to death of the stems. Some conifers are quite susceptible to the disease, while others seem quite resistant. The older the tree the more susceptible it may become, and stressed trees can succumb regardless of their age. The disease can eventually kill some trees, while others struggle on for years in a state of gradual decline.
It’s virtually impossible to control this disease, so, in areas where it’s prevalent it pays not to plant cypresses at all, or to search out the odd species and cultivars that may show some resistance. Things you could do to help reduce its spread among your trees include cutting out infected branches as soon as they appear and burning them to stop the spread of spores, or totally removing any trees that seem particularly badly diseased. Copper-based sprays are reported to help provide some protection but their use is simply not feasible on large trees.
As cypress species are important for both farm shelter and timber production some research work is being done in an effort to select cypress varieties showing resistance to this disease.
Answer by Andrew Maloy (The Plant Doctor)