How to recognise it
Groups of plants grow slowly with wilting and/or sparse foliage, or die back altogether.
When it infects woody plants the leaves yellow and fall early. This is accompanied by the growth of dense white or green/grey fungal mycelium over the roots, which later darkens to brown, then black, and becomes less dense. You may see the fungal growth on the soil around affected plants in moist conditions, possibly with tiny black dots of sclerotia amongst it.
Why it’s a problem
The disease will eventually kill the plant unless successfully treated, although the timescale will depend on the type of plant affected.
Where you are likely to find it
Occurs on almonds, apples, avocados, irises, ixias, grapevines, narcissi, pears, potatoes, privet and tulips. It’s most likely to occur in wet seasons and poorly drained soils, particularly when combined with high temperatures.
How to prevent it
Good garden hygiene, particularly in the removal of dead wood (including tree stumps) which could be a host to this disease, will reduce the risk of an infection. Fungi from the Rosellinia genus prefer acidic soil, so liming the soil can help to prevent an outbreak, or reduce an existing problem.
How to get rid of it
The fungus prefers moist conditions, so repeatedly digging over affected soil (or, preferably, turning it with a rotary cultivator) on a hot, sunny day can dry out the fungus.
Any affected plants should be isolated (by digging a trench around them to separate their soil from that of healthy plants) and, for woody perennials, root pruning of the worse affected areas can be helpful. However, most plants will not recover and should be removed and destroyed. The soil immediately around the plant should also be removed or regularly turned in hot weather to eliminate the fungus.
Chemical controls are only available for commercial growers, not for amateur gardeners.
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