Plant diseases and busy Lizzies

Posted: Monday 18 March 2013
by Adam Pasco

Every so often in gardening, something happens that is a real game changer. I’ve seen some devastating tree diseases in my lifetime.

Busy Lizzies

Every so often in gardening, something happens that is a real game changer. I’ve seen some devastating tree diseases in my lifetime, including Dutch elm disease in the 70's and 80's, and now there’s the impending threat from ash dieback.

These big environmental problems really are beyond our immediate control, but at least when it comes to our own gardens, we're in a position to make decisions. In particular, we have the seasonal choice of which ornamental and edible crops to grow, and preventing or controlling their diseases informs our decision making.

A few years ago, a devastating viral infection spread through many growers' petunia stocks. Immediate measures were taken to change practices and control it before it reached the gardener. Their responsible actions gave gardeners confidence that the plants they bought were healthy, vigorous and free from virus. Unfortunately, this isn’t always the case, as some other plants we buy can be badly infected with virus. I've had problems with canna lilies in particular.

Fungal diseases, on the other hand, are very different to viral ones. Spores travel wide distances through the air and infect plants wherever they're growing. On an annual basis, I think about how I can avoid fungal problems - blight on potatoes and tomatoes, powdery mildew on my greenhouse cucumbers, rust on my roses ... I think I'll stop now before I get depressed!

But this is what gardening is all about. Many plants can succumb to disease and infection. By understanding these problems, we can learn either how to cope with them, or whether to try growing something different so problems don’t arise in the first place.

This is now the case with busy Lizzies. Back in 2010 and 2011, gardeners found their busy Lizzies had become infected by a new strain of downy mildew. The problem got so bad that last year Gardeners' World Magazine recommended people not to grow Impatiens walleriana, and most seed companies and retailers stopped selling them too.

Thankfully, there's a rich range of other bedding plants to choose from. For the people who love the colours and performance of busy Lizzies, New Guinea impatiens (Impatiens x hawkeri) provide a close alternative, and are resistant to downy mildew. Check seed and young plant catalogues for New Guinea 'Divine', which promises flowers from June until autumn. Also, look out for SunPatiens®, with a strong growing habit and large flowers. If the catalogue descriptions can be believed, a single plant can cover an area of about one square metre!

I love growing New Guinea impatiens in large pots in a fully shaded position, and actually prefer them to the traditional busy Lizzies we can no longer grow. So, all is not lost. As the saying goes ... Out of disaster comes opportunity!

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flowering rose 19/03/2013 at 20:27

is it because we are inporting more plants and the movement of people round the world.we are no longer confined to area and this makes for viruses to spread.

Adam Pasco 19/03/2013 at 20:34

Hi Flowering Rose. Many plant diseases have been 'imported' or have spread to the British Isles in the, including devastating tree diseases.
Downy mildew is a fungus disease, but virus ones can also spread easily in different ways.
I'm not sure where this particular disease came from, but it certainly spread quickly! All we can do is avoid growing problem plants and choose resistant ones instead.