Mulberry leaf spot

Mulberry trees bear attractive leaves and tasty fruit but can be affected by a number of diseases, such as fungal leaf spot.  This disease can look unsightly, and in wet summers can cause significant early leaf fall and reduce the fruit crop.

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Mulberry leaf spot (Mycosphaerella mori). Image credit: Julie Skinner

Quick facts

Common name: Mulberry leaf spot
Scientific name: Mycosphaerella mori (syn. Phloeospora mori)
Plants affected: Black Mulberry (Morus nigra) White Mulberry (Morus alba)
Main causes: Fungus
Timing: late spring, summer to early autumn, particularly in wet conditions

What is mulberry leaf spot?

Mulberry leaf spot is a fungal disease favoured by wet weather. The older wood of the tree is not affected, but the fruit crop yield can be reduced.

Severely affected trees can suffer premature defoliation, but the disease has little long-term effect on the vigour of a mature tree. Repeated infection may delay the establishment of young trees, however.

Symptoms

You may see the following symptoms:
  • Small, dark brown and irregular leaf spots with pale-green yellow haloes appear in the spring
  • The spots increase in size, the centres becoming whitish and the haloes developing brown necrotic margins (dead edges)
  • Tiny, brown–black, spore-producing fruiting bodies (acervuli) occur on the lesions and defoliation can occur after severe attacks

Control

Non-chemical control

Ensure that any leaves falling prematurely from an affected tree are collected and disposed of (by burning, burial or consigning to the council green waste), together with all the rest of the leaves when they fall in autumn. Good hygiene may prevent a noticeable level of the disease recurring in the future. The disease is favoured by wet weather so may also be less of a problem in drier summers.

Vigorous, healthy trees tend to be more resistant to attack.  Young trees will benefit from feeding in spring and watering regularly in dry spells.

Some cultivars of Morus alba appear to be less susceptible to mulberry leaf spot.  Morus alba  ‘Pakistan’ is a variety with long black fruits and could be considered an unusual alternative.  It requires a warm and sheltered site to perform well in the UK.

Chemical control

Once the fungus is in the leaf, fungicides are not particularly effective against this disease.  Fungicides can only be used on mulberries if you have no intention of eating the fruit since there are currently no fungicides available for use by home gardeners on fruit crops for consumption.

The fungicide tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect) has a recommendation for the control of leaf spots on ornamental plants. Additonally, the fungicides tebuconazole (Provanto Fungus Fighter Concentrate) and triticonazole (Fungus Clear Ultra) are labelled for the control of other diseases on ornamental plants. These fungicides may give some control of the disease on trees being grown as ornamental specimens, from which the fruit will not be consumed.

Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: storing and disposing safely

Other disease symptoms on mulberry leaves

Mulberry leaves can be affected by a number of other diseases but none is as common or as damaging in the UK as Mycosphaerella mori.

Bacterial leaf spot is caused by the bacterium Pseudomonas syringae pv. mori, producing numerous small angular black leaf spots, often with a yellow halo (see photo 3 in the gallery above).  Whole leaves may turn yellow and the vigour of the tree may be compromised.  Elongated black lesions may occur on the stems.  Oozing bacterial slime may be visible in wet weather causing dieback of the shoots.  The affected shoots should be cut out and burned.  Fallen leaves should be collected and disposed of (by burning, burial or consigning to the council green waste), together with the rest of the leaves when they fall in the autumn.

Powdery mildew is not seen commonly on mulberries but where it occurs it affects both black and white mulberries. It is a fungal disease of the foliage initially covering the leaf surface, visible as a white powdery coating particularly on the underside of the leaf. Destroying fallen leaves in autumn should reduce the amount of infectious spores produced the following spring.  Pruning out any infected shoots will reduce subsequent re-infection.  Managing the environment around susceptible plants can help to reduce the occurrence of powdery mildew. Plant in full sun where possible and aim for an open environment to increase air circulation and reduce humidity.  Providing good drainage and watering regularly in dry periods will reduce the likelihood of powdery mildew infection.

Further details on powdery mildews and their management can be found in our advice profile on this link. Remember that any fungicides mentioned can only be used on mulberry trees grown as ornamental specimens.

Biology

Mulberry leaf spot is caused by the fungus Mycosphaerella mori and fluctuates in severity from year to year, being most damaging in wet summers.

The fungus can overwinter in young parts of branches, and in fallen leaves lying on the ground. In spring, sexual spores (ascospores) are produced from tiny fruiting bodies produced on the fallen leaves, and these spores are carried upwards in the air to infect the new spring growth. Spores can also be produced in spring from the lesions on the branches.  Once leaf spots have developed, splash-dispersed asexual spores (conidia) are produced from them to spread the disease further. Transmission is therefore favoured by wet weather, both to disperse the spores and to provide the several hours of leaf wetness required for spore germination and infection.

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