Healthy and Wise – Garden Disease Scares

In health scares, careless talk costs livelihoods as well as lives. They seldom break upon us with their full epidemiology known or their likely impact predictable, so they provide fertile ground for misinformation, sensationalism and over-reaction. That is just as true of plant health scares, of which British gardeners have had more than their fair share of late.

There is, for example, the brace of non-avian viruses found in plants of hostas shipped here by our European partners. Their symptoms include faint, mosaic-like leaf mottling and brown spots that develop watery haloes. These incurable viruses disfigure, debilitate, and can ultimately kill the plants they afflict. They are transmitted by sap-sucking insects and by sloppy nursery procedure, such as failing to sterilise knives after trimming or dividing plants. It is the latter that seems to have caused a proliferation of Hosta Virus X and ringspot virus in the US and Europe. Now they stand at our threshold.

The only defence is prevention. Spurn any suspect garden centre imports. Buy instead from growers who maintain virus-free stock by hygienic methods. Adopt the same scrupulousness in the garden, and be prepared to destroy infected specimens. But, above all, and as ever with plant health scares, don’t panic – at least not until after you have sought expert advice. Many ordinary and curable hosta woes, from late frosts to fungal infections, produce symptoms that are easily mistaken for virus. It would be terrible if misplaced zeal and fear caused plants affected with such lesser ailments to be killed unnecessarily.

Which is exactly what many growers believe has happened in the case of Sudden Oak Death Disease. Although the Government seems to have left the hostas as, well, hostages to fortune, its policy on Sudden Oak Death Disease (Phytophthora ramorum) has come close to scorched earth. Acres of rhododendrons have been extirpated and various other possible host plants are being eyed up for the chop. This is understandable at the precautionary and emotional levels: might this not be a new plague on the scale of Dutch Elm Disease, and – worse still- one that strikes at the very heartwood of England, the oak? But the UK situation has yet to warrant this terror, and aspects of the response recall the last foot-and-mouth outbreak.

Meanwhile, plants that are carrying the disease continue to turn up from the Continent ready to infiltrate UK garden centres. Chief among them is the irreplaceable winter-flow-ering Viburnum x bodnantense, numerous specimens of which have shown tell-tale weepy bark lesions and sudden die-back. My advice is to adopt the same approach with the viburnum as one should with the hostas – a robust blend of circumspection and protectionism. Until immigration controls are tightened, buy vulnerable species only if they are British, bonny and from growers who know what they are doing and who are within the Department for Environment, Food and Rural Affair’s jurisdiction.

None of which really applies to the third of this year’s horticultural scares, a moth named Cameraria ohridella, alias the horse chestnut leaf miner. Its larvae eat their way into the leaves of members of the genus Aesculus with such gusto that the Forestry Commission reports as many as 700 tunnels being bored through a single horse chestnut leaf ‘under favour-able conditions’. Favourable for whom, I wonder. The riddled leaves discolour and wither, giving infested trees a late autumn look sometimes as early as midsummer.

Greater London is the area of worst infestation. The sight of blasted conker trees has been particularly alarming this year, their condition aggravated by drought. But even in acute cases, it seems the damage is chiefly cosmetic. Unless a secondary factor such as disease or prolonged drought complicates matters, most of these trees will make a full recovery.

Even so, can nothing be done? Well, spraying affected trees with insecticide is neither practicable nor desirable. There is something that might help, however-a twist in the tale of the horse chestnut leaf miner which supports my view that ‘green’ practice is not necessarily best practice. Its pupae spend the winter in leaf litter. Composting leaves, as we are all told to do, merely keeps the little blighters cosy and may even assist in their development and distribution. When it comes to fallen conker leaves and those who feast thereon, it seems the answer is to gather them up, look the local authority and the ozone layer in the eye, and light a bonfire.

Next week: In Yorkshire, Susan Cunliffe-Lister prepares for November in the kitchen garden, cutting down the perennials and ordering seeds for next year’s crops.