This year for the first time the boxwood topiary across the garden have dead branches. In most cases these are small branches but some instances are more serious and in one case it is a substantial part of a plant that is part of the o-karikomi and is highly visible.
We have diagnosed that this is most probably a disease called Box Blight. The early stages of Box Blight infection are easily missed, because when you see dead leafs on the outer edge, the inside will have died for long. We are not always alarmed when we see a dead leaf. Commonly it is not detected until parts of the plant die and pronounced leaf fall occurs, which in our garden is/was the case in two instances.
Advanced infections can be readily recognized as often a central section of the top foliage will appear to be dead whilst the side foliage retains its green. On a topiary piece, such as balls, cones and spirals, commonly a small area comprised of individual stems will die first, however the fungus will spread throughout the plant eventually killing it.
We read that box blight is actually caused by two separate fungi, sometimes acting together.
Cylindrocladium buxicola can be recognized from the spots it causes on leaves which become larger areas of dead (necrotic) tissue. As the disease advances leaves are lost from the plant. Cylindrocladium buxicloa has only recently been identified (the first research paper on it was only published in 2000) and it is currently being studied. This is an airborne disease and there is no guaranteed means of prevention, however there are measures which significantly reduce the possibility of severe infection.
The second box blight fungus is Volutella buxi. An infected plant typically develops pink pustules on underneath the leaves. These are most likely to develop in humid weather. The leaves yellow and fall off the plant and in severe cases cankers can develop on the branches and trunk.
Both diseases are believed to lie dormant on dead leaves on the ground, producing spores in spring as temperatures rise. Dead leaves should be removed (and not re-used). All other infected material such as branches should also be cut off and disposed. Don’t forget to disinfect tools afterwards.
The conditions in which the fungus proliferates are damp, shade and poor ventilation, so avoidance of these will help prevent firm establishment of the disease. There are no garden chemicals available specifically to deal with either blight, but copper fungicides and those for instance containing penconazole, triticonazole and the like, seem to have some effect. This is something I will try in the coming weeks.
And here goes our irrigation-system strategy.
It is most important to avoid overhead irrigation as the spores are carried and activated in water droplets and damp leaves provide ideal conditions for the fungus. Water the roots if required, possibly by a ‘leaky hose’, Buxus do not need foliage irrigation.
Clearly, if this gets hold in a garden where box hedging or topiary is a crucial aspect of the layout (and yes, that is in our case, see The main garden o-karikomi ) it is nothing short of a disaster. The fungus needs humid, damp conditions to thrive, but a hot dry summer (33°C will kill the fungal spores) will restrict its spread. Certainly the worst thing you could do to box is to put them anywhere near the arc of a sprinkler. Keep all your watering to the base of the plants and avoid it in warm weather. The spores are sticky, so they adhere to insects, birds or even humans, and are then spread; they can also be splashed from plant to plant.
I also saw an advise to never clip a box hedge in warm, damp weather. Volutella enters the plant through open wounds and it is most active in humid conditions. But then, how feasible is this ?
Waterlogging and compaction can create ideal conditions for diseases such as phytopthora and other fungal attacks. For example, box is prone to box blight in poorly-drained sites.
Most of the garden is well drained. This however is a general measure and does not change the fact that we have a lot of clay in the underground that results in different conditions for every plant.
Always ensure that all garden tools, particularly shears and clippers, are clean. Do not infect healthy plants with dirty shears. Shears may be cleaned by dipping in bleach or disinfectant mixed in the dilutions indicated on the label for domestic/kitchen use.
Improving ventilation may be problematical; by its very nature Box is often tightly clipped and hence poorly ventilated. With new plantings it is worth bearing in mind ventilation and shade implications.
Removal of dead leaves, plant debris and foliage will reduce the availability of spore releasing material and may reduce any ‘resting spores’. In addition it will take away one of the means by which the plant can remain damp and moist for a longer period. Although we do this every year, this time it is done more rigorous and precise.
The photo above-left shows some of the brown tips that we regarded innocent (and still hope they are).Now we know better and we hope and pray that this in our case is any other (local) fungus than blight.
Sources: We used half a dozen of websites to compose this post. Most of them used the same unknown source.