The New Killer Disease of Columbines

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By Carrie Thomas 

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A new disease is threatening all columbines (Aquilegia species) in the United Kingdom and beyond. The south of England is particularly badly hit, with purchased plants commonly infected. Aquilegia Downy Mildew (ADM) decimated my own national plant collections within the first year and killed them off in just two years, a loss of thousands of plants from my garden. The disease also has invaded Kew Gardens and the Royal Horticultural Society's garden at Wisley.

Like many new plagues--think Ebola--ADM is extremely virulent, killing plants after infecting others and leaving long-lived spores in the soil. Immediate and drastic action is needed. 

How can the spread be stopped? Early identification and removal of affected plants may be the only viable control for home gardeners. Prevention is better, so be careful where you buy columbines or their seeds, since many retail outlets and nurserymen do not yet know about ADM. 

Fortunately, the disease hasn't yet been reported in the United States. Having visited Florida recently, I see that Americans have much larger, open yards than our compact and crowded gardens here in England, so I have hope that the disease can be prevented in the U. S. or at least contained to a specific area. We also have a very mild, wet climate, especially in winter, and I've found spores forming in every month of the year. 

Should the disease reach the U. S., I hope that the harsh winters in many states would slow the formation of spores or might even kill them. But, although these physical considerations could mean that ADM may not spread in the same rapid way in America, I'm sure we all would like to prevent its occurring there at all. So U. S. gardeners should avoid purchasing columbine plants or seeds which have been imported from overseas. 

All gardeners should keep a close eye on their columbines when they emerge in the spring. As growth begins, plants may already be systemically infected Look for abnormal whitish-green foliage, as in photo 2 which compares a healthy columbine to an infected one, and plants which appear unnaturally erect, with longer leaf stems to leaf surface area than usual. The leaf edges may curl upwards or downwards and eventually die, crumbling away just to the main leaf veins. Plants may look sick, unhappy, blasted, blighted or frazzled.
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Slug slime (pictured in photo 3) may indicate infection, since slugs generally avoid columbine's toxic foliage, but like to graze on the disease mildew. If you leave such systemically infected plants in your garden, not only will they die, but they will efficiently infect other nearby plants and very likely contaminate the soil with resistant spores that may last decades. 
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Later in the season, when infection occurs from the spores, the signs are different. Look for yellowy patches on the leaves, as in photo 4, and angular mottling which follows the leaf veins, differing distinctly from the variegation seen in leaves of the Vervaneana group. You may also find dark ADM lesions on the flowering stems (photo 5), blasted or blighted flowers and buds (photo 6), and lesions on seed pods (photo 7). 
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In moist conditions the mildew grows out through the plants' pores and forms a downy covering on the underside of the leaves, as in photo 8, releasing millions of spores. Such downy growth isn't always easy to see because rain confuses its look. The yellowy patches are the easiest symptom to look for.
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How should you dispose of infected plants? Burning is best, or you can bury them in your garden at least 1 1/2 feet deep. Do not put them on your compost pile.

What can you do to help? I am compiling an online directory of where ADM is found and working with both UK and overseas plant pathologists to help eradicate the disease. If you discover any signs of it, please contact me through my Touchwood Plants web site, preferably with a photo for confirmation. This directory may become an important epidemiological tool, so please support it. Persons gardening where the disease already is present can also look for plants which seem resistant and inform me about those. What else? 

Even if you have no signs of ADM, you can tell other gardeners about it. Talk, email, and social media all help. If you want to continue to enjoy the wonderful flowers of columbines, such as those in photo 9, please act now to protect them.
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For more information on Aquilegia Downy Mildew, see the following web page:

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