Anyone involved with the craft of gardening at anything beyond a rudimentary level will sooner or later encounter pests and diseases. Most are just the standard ongoing issues we all have to a greater or lesser degree- slugs and snails, rusts and mildews, sometimes mice, rabbits or deer- but some diseases are rather more sinister and require a more robust response.
The early 2000s saw the arrival of Sudden Oak Death, Phytophthora ramorum, to the UK. Initially regarded as a disease of rhododendrons, camellias and magnolias, Phytophthora ramorum spread like wildfire among so many popular woody garden plants. Woodland gardens were hit particularly hard thanks to a combination of factors: many big old woodland gardens were densely packed with susceptible species, and the humidity and lack of air flow allowed the pathogen to propagate and spread. Now Sudden Oak Death is managed through careful cultivation to encourage airflow and reduce humidity; plants aren't quite so tightly packed in as they once were.
There's a newer phytophthora species to be aware of: Phytophthora ilicis, also known as Holly Blight. The pathogen isn't actually particularly new but its effects have been minimal and fairly isolated until comparatively recently. This is a disease of hollies, notably our native Ilex aquifolium and its hybrids, but some exotic hollies are showing some signs of susceptibility too. The disease enters the plant via the leaves and green growth before spreading through woody branches and into the trunk. Once the disease has spread through the trunk the holly is dead.
Our native hollies are an important part of our landscape and both cultural and natural history. This does not diminish the impacts of pathogens that impact non-native plants in gardens, but given how widely hollies grow in our woodlands and hedges, let alone our gardens, it's easy to see how this disease could have a lasting impact on our landscape.
Officially there is no way to treat or manage this disease; plants should be felled and burnt. However...
As with any living thing, the success of phytophthoras relies on the conditions being right for the organism to thrive. For leaf phytophthoras (which is to say the species that invade the plant through its leaves and not its roots) a combination of high humidity and reduced airflow is perfect. Phytophthora ilicis has previously primarily been a problem in dense, mature gardens and woodlands- places where air flow is usually reduced- but with recent patterns of prolonged rainfall and persistent humid air from the west I'm now seeing a lot more cases of Phytophthora ilicis in open conditions too. The naturally dense nature of hollies means that once the pathogen infects a small part of the plant it can spread rapidly from leaf to leaf.
Seeing black markings on diseased foliage is easy when you're dealing with big light green leaves. Spotting diseases on naturally dark leaves, those of hollies and camellias for example, require much closer attention to detail. Leaf loss after a major storm can be a big clue; it's easy to assume that the storm itself caused the damage, while actually the storm blew away leaves already weakened by disease. After all we'd all know if hollies dropped their leaves suddenly after a storm because they would do so more frequently.
There are no effective chemical controls for phytophthoras, either synthetic or organic, so physical action is the only way to control the disease. So far I've not been able to ascertain whether hollies that are regularly trimmed are more susceptible to this disease or less so. I'm not even going to speculate about it. Indeed even if I came across a hedge showing symptoms I couldn't say whether it was hedges in general that were susceptible or just that specific hedge in that specific place.
Healthy hollies have an extraordinary ability to regenerate from bare wood and I hope that this might provide gardeners with a way to rescue, at least partially, infected hollies. By hard-pruning or even hat-racking larger hollies it should be possible to get them to re-shoot. I should cover what is meant by 'hat-racking'; this is a rejuvenation technique where a tired old holly tree is pruned back to its framework. All twiggy outer growth and foliage is removed completely, and all major branches are cut back to around 30cm or a foot in length. It's usually practice to shorten the trunk as desired. In the first year the new growth is usually minimal, the trunk standing like a giant hat stand or hat rack, but by the second year you would typically expect plenty of new growth. By the third year that new growth should be looking quite respectable.
Stopping the disease spreading from the leaves into the main plant cannot not work; it's removing the source of infection before it hits the core of the plant, much in the way we might remove signs of infection from other plants in our gardens. We know that hollies will nearly always grow from epicormic buds, dormant buds hidden in the bark of the plant that become new growth if the plant gets damaged; logically we could do a 'hard reset' on the plant, as it were.
Whether this rejuvenation will actually work long term is another matter.
The big issue will come from reinfection. Hollies, particularly our native Ilex aquifolium, will regrow to make nice new dense specimens, but what if that regrowth gets hit by disease before it has the chance to mature? Even if the plant makes impressive growth and becomes a new tree, will it tolerate being cut back hard again within a few years from its previous severe pruning if the disease strikes again? Would it be prudent for the diligent gardener to thin the new shoots as they appear to make a tree of a more open habit, improving air flow? Indeed should gardeners be proactively thinning their hollies in the hope of providing greater airflow and reducing humidity around the plant, in anticipation of the disease's arrival?
I honestly can't say for certain that any of this will work. Phytophthoras can be persistent and aggressive diseases, but whether it's possible to thwart the pathogen and keep your hollies is a big question. There will be those who simply grub out their hollies and replant with something else, and there is a sound logic behind doing this. Others will try to fight the disease any way we can out of respect for this noble genus. Some hollies will develop resistance while others will prove incredibly susceptible.
When we're dealing with these particularly troublesome diseases we must keep calm and stay well-informed. When Sudden Oak Death arrived in the UK there were those who said it was the end for certain plants in cultivation; while the negative publicity did extraordinary harm to the image of certain plants and misinformation still abounds in the horticultural and mainstream media (a large number of people cannot differentiate between the troublesome 'wild' Rhododendron ponticum and the rest of the genus, for example), I'm not aware of any genus disappearing from cultivation because it can be susceptible to Sudden Oak Death. Hollies might well take a pasting from Phytophthora ilicis, and there may be some years the disease is out of control, but we must think tactically and proactively do whatever we can to save our hollies.
Hi Ben,
Its good that you have highlighted this little known but devastating blight. It has denuded and killed a lot of holly in our two acre wood and affected several but not all of the varieties of holly that we have in the garden. I was particularly distressed at it attacking 4 standard plants of JC Van Tol which I had trained into mop heads on 6 feet high trunks. Here I did a lot of cutting out of diseased stems and sprayed with Signum, which we use to try and control Box Blight. The plants seem healthy again so there may be some beneficial effect of the spray, though I wouldn't jump to the conclusion that it is helpful until we use it more.
Thank you Ben for this compassionate point of view - it seems to me the big public forums mostly focus on the damage caused by climate change as it affects humans and to some extent endangers animal species, few consider the plight of plants...