It’s less than 10 years since fungal leaf spot* started playing havoc with Escallonia plants in the UK. Now it affects Escallonias the length and breadth of the country.
You might first notice the disease as a small dark spot on an otherwise seemingly healthy leaf.
Given the right conditions (a stressed plant and damp weather) a few small black dots on a handful of leaves rapidly develop into a full blown infestation.
Once badly infected, the plant will quite rapidly lose its leaves. The more mature leaves are almost always the first to be affected, consequently the symptoms most usually start at the bottom of the bush.
There is no effective and complete cure for this problem.
This year, I experimented by keeping one Escallonia bush well fed and watered through the summer and leaving another to look after itself. Keeping the plant fed and watered certainly and significantly reduced the infestation, but it didn’t stop it completely.
Just like humans, any plant that is under stress because it is lacking in nutrients or water is less resilient to any kind of ailment. However, too much food and water can also be a bad thing. In particular, avoid too much nitrogen which can cause overly soft and floppy leaves (that’s for the plants, not the humans:). Also try and include micronutrients in the feed, not just nitrogen, phosphorous and potassium – the NPK of most standard fertilisers.
Ultimately though, this disease will probably continue to result in the demise of many Escallonia shrubs throughout the UK. Gardeners and the horticultural industry have become so reliant on vegetative propagation (through cuttings and tissue culture) that we’ve wiped out much of the natural genetic diversity that would have helped improve the disease resistance of some of our favourite plants.
*A species of the Septoria genus of fungi is the most usual culprit with Escallonia leaf spot, but it is not the only one.