Sandy’s Garden

Downy Mildew

Just one week ago, I wrote, “1596 is recorded as the year in which the first Impatiens balsamina were brought to the British Isles from South-east Asia. Their botanical name means ‘the impatient plants which bear balsam’; and the reason for calling them ‘impatient’ plants refers to the explosive release of the seed when a ripe seed capsule is touched … we are, perhaps, more familiar with the explosive nature of the seed pods on broom bushes. … We are also more familiar with the plant under one of its common names, which include Touch-me-not and Jumping Betty … for fairly obvious reasons … or the name by which they are most often known in central Scotland, Busy Lizzies.”

I learned very recently that, while 1596 saw the introduction of Busy Lizzies into the United Kingdom, 2011 might just see their disappearance - perhaps a fairly unlikely circumstance, but nevertheless a distinct possibility. The reason is a plant problem with a fairly friendly name … downy mildew … but a very unfriendly nature. ‘Downy Mildew’ could just about be the name of a little girl’s pony in a 1950’s story of a courageous 10-year-old who dares all manner of risks with her faithful pony to uncover a gang of smugglers. But ‘downy mildew’ without initial capital letters is presenting a very real threat to Busy Lizzies, a threat which has been known about for quite a number of years but which has recently been transformed from an annoying problem for plant growers to an insuperable problem for ordinary gardeners. Let me explain.

Downy mildew or, more accurately, downy mildews, are, in the words of the Royal Horticultural Society, “specialised pathogens, requiring a living host on which to grow. Like most diseases of this type the downy mildews have restricted host ranges, so for example the species attacking pansies will not affect roses, and vice versa. Plants on which the disease is an important problem include Hebe, Impatiens, lisianthus, pansy, Nicotiana, rose, grapevine, brassicas, lettuce and onion.” The version that attacks Busy Lizzies (Impatiens) has, until quite recently, been controlled by the commercial nurseries which produce these plants for garden centres and, through them, for the ordinary gardener by the use of chemical sprays based on metalaxyl-M, a horticultural tool that is not available for amateur use. So far, so good, for metalaxyl-M used to inhibit any outbreaks of downy mildew so that all the young plants leaving the nurseries were disease-free. But a new and aggressive strain of downy mildew has appeared which is resistant to any chemicals currently in use within the horticultural trade, meaning that neither the trade nor the gardener can erect any defences against a very nasty enemy.

This aggressive infection most likely originated on imported cutting-raised material and then spread to seeded stock. Infected plants lose their foliage prematurely, being reduced to skeletons before they die. But, it might be argued, since Busy Lizzies are annual plants which are killed by the first frost, surely one winter will kill off the disease as well as the plant? Well, right now it is unclear whether the pathogen can survive a winter out of doors, either in the remains of infected plants or in the seed of such plants, lying dormant in the garden; and although most Busy Lizzies are grown from seed, some are raised from saved plants; and there is no possible weather inhibition on the survival of the pathogen in plants or seed protected from frost, as they will be in growers’ or gardeners’ heated greenhouses. In short, Busy Lizzies face a bleak future unless, or until, some means of combating downy mildew is developed … so don’t be surprised if they’re absent from garden centres in 2012.

Sandy Simpson, Polmont Horticultural Society