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By Peter Coles

It’s been a bad year for our black mulberries (Morus nigra) in the UK, especially in London and the south-east. Over the past few weeks, I’ve received distressed emails from Chiswick to Charlton, from Belsize Park to Barnes, lamenting that their beloved old black mulberry tree looks as though it’s at death’s door and asking me to have a look, offer some advice, or recommend an arboriculturist. 

Indeed, some of London’s old mulberry trees do look wretched. Leaves are scarred with dark spots and brittle, black edges, falling prematurely and leaving the crown in August looking more like it usually would in November.  “There was almost no fruit this year,” said David Percy, who looks after a veteran black mulberry in Belsize Park, as we contemplated his sorry-looking tree, which still stands on the site of the long-gone Belsize House. The tree, with its tailor-made gap in the stucco wall running beside the pavement, is much loved by local residents, who have always enjoyed the overhanging fruit in previous years and are wondering what’s happened. 

In recent weeks I have come across similar stories from all over London. “The tree has always been healthy, giving enormous crops of luscious fruit, which we bottle, turn into jam, sorbet, etc.,” wrote a resident of Barnes. “This year, for the first time in the 55 years that I have lived in or around this house, there has been a minimal harvest of fruit and the tree is clearly suffering.”  A number of black mulberries in Chiswick that I visited in August have the same issues, as does the Charlton House mulberry on the other side of the city.

The culprits?

The main issue seems to be leaf spot caused by a fungus, either Cercospora moricola, or Phyleospora maculans. Both are quite common on mulberries (the Cercospora in particular). The infection usually happens in May, manifested by tiny brown spots on the leaves, and only reveals the bigger spots in late summer, often after fruiting. The spots are dark brown, surrounded by a light-coloured halo and then a darker ring. Sometimes blobs of tiny white spores can be seen on the central spots, notably for Phyleospora maculans. 

These fungi can cause defoliation (leaf drop) and can undermine the vigour of the tree, but they aren’t usually fatal, though they could indicate the start of a slow decline.  It is a disease of the leaf, not the roots or cambium (living cells just under the bark). All being well, the trees should spring back next year. In any case, no need to panic, and no need to start hefting the chainsaw.

The first line of treatment is to rake up fallen leaves and bag them, disposing of them away from the tree (and other trees). Try to maintain hygiene to avoid spreading the spores again on garden tools or clothing. There may be fungicide treatments, but best to get further advice from a tree specialist first.  Always be wary of treating fruit trees with any kind of toxic chemical, as this could ultimately end up in birds, insects – or you and yours.

Summer deluge

While leaf spot is fairly common, the severe extent of the present infection – both within a given tree and across a large geographic area – points the finger at the exceptional weather in and around London this summer. The capital received twice its usual rainfall in July, with a month’s worth falling in a single day, causing widespread flooding. The leaves almost never dried out during what should be the peak of the growing season. These fungi love warm, damp conditions, and spores would be spread by wind and water splashing or dripping from infected leaves.  Furthermore, trees on clay or poorly drained soils could have suffered from waterlogging, which can be harmful to the roots and cause leaching of key minerals, making the leaves even more vulnerable to infection.  

Bacterial blight

The black, brittle edges of some leaves in some of the more serious cases are rather more worrying. This could be bacterial blight (Pseudomonas syringae), which can be present at the same time as the leaf spot infections. This is more commonly seen in spring on new leaves and shoots and is less easy to treat. Twigs and branches can have cankers that ooze.  I don’t think this is happening to our mulberries, but be vigilant.  The first line of treatment is similar to that for leaf spot, as well as pruning and destroying infected twigs and shoots.

There are plenty of useful resources online, a good UK-based one being: 

The Royal Horticultural Society (RHS) page on mulberries.

And a good first stop in your search for a qualified tree consultant or tree surgeon is the Arboricultural Association.