Spalding Guardian Gardening 25 August 2011
I am getting inundated with questions at the moment about troublesome busy Lizzies. Once so bombproof, in many gardens, including mine, they are showing signs of flower drop, leaf loss and eventual death.
This is, in fact, caused by a fungal disease with the grand name of Plasmopara obducens, and the much easier one to remember of impatiens downy mildew. It is a fungal disease causing yellowing leaves, defoliation, and death of bedding impatiens. It was noticed for the first time in the UK in 2003, thought to have arrived on imported commercial seed or cuttings, and has become widespread since then, with the worst outbreak so far occurring this year.
It is at its most rife in damp conditions Ė wet weather and on plants which are regularly watered overhead Ė although infection can be spread by airborne spores. Infected plants may often be bought accidentally, although they may look healthy at the time, as there is a long period of development. Home raised plants from seed are generally free of the disease at the beginning, but can be affected once planted out in the garden.
Impatiens downy mildew is easy to spot. Leaves turn yellow and quickly drop and rot. Flowers and buds are also shed, but a small tuft of sick-looking leaves and tiny buds may remain at the ends of the branches. Badly diseased plants usually die, but those that donít are so unsightly that they need to be pulled up. However, to prevent the spread of disease, all plants showing symptoms should be removed and disposed of immediately noticed Ė preferably by burning, or, if this isnít possible, burying at least 20in deep, but definitely not on the compost heap, although the heat produced in commercial composting processes will usually destroy the fungus. There are no effective chemical controls.
Itís best not to grow busy Lizzies in the same part of the garden for at least a year after an attack. Containerised plants should be removed immediately, the compost buried deeply or, if this is not possible, taken to the recycling centre for landfill. Sterilise the containers with before reusing.
This is the second time I have noticed the disease in my own garden. The first was about three years ago during a prolonged wet spell; until now, because most of mine are grown in containers so infection doesnít carry over from year to year, my busy Lizzies have shown no signs of the problem, and now it is only those that are not on our automatic irrigation system that are diseased, so have been watered overhead rather than at the roots.
So, what now?
Well, tempted as I may be when it comes to ordering my plug plants next spring, I shall give busy Lizzies a miss, at least for next summer, and possible the following one, to break the chain of infection, and when I do start growing them myself, I shall raise my own plants from seed to avoid bringing in infected plugs. If youíve had the same problem this season, as I know many of my readers have, I would strongly recommend you do the same. After all, there are so many other beautiful bedding plants, impatiens should hardly be missed for a year or two.
Editorís note. 2013, and impatiens downy mildew is still a problem, so busy lizzies difficult to buy this year and best avoided.
This piece originally appeared in the Spalding Guardian