Morus Leaves Curling
Bacterial blight and mulberry scale are the most common reasons morus leaves curl. This guide shows you how to identify each cause and keep your mulberry tree healthy and fruiting well.
1. Bacterial blight (Pseudomonas syringae pv. mori)
Bacterial blight is the most serious disease of mulberry in the UK. The bacterium enters through natural leaf openings or wounds and is spread by rain splash and infected pruning tools. Wet, cool spring weather immediately after bud burst creates ideal conditions for infection and rapid spread through the canopy.
What to look for
Small, water-soaked, angular spots appear on leaves in spring, often beginning at the margins or near veins. These spots rapidly turn dark brown or black and the surrounding leaf tissue turns yellow. Infected leaves curl, shrivel, and may hang on the tree for weeks before dropping. New shoots may also die back from the tip, showing brown streaking in the stem tissue just below the dead portion. Bacterial ooze may be visible in humid conditions.
What to do
Prune out all infected shoots promptly, cutting at least 20 cm below any visible discoloration and sterilising your tools with methylated spirits or a 10 percent bleach solution between each cut. Dispose of removed material in the bin rather than composting it. Mulberry is slow to seal pruning wounds, so make cuts in dry weather and consider sealing larger cuts with a wound paint to reduce re-entry of the bacterium. Apply a copper-based bactericide spray at bud burst the following spring as a preventive measure, particularly in years when a wet spring is forecast. Avoid overhead irrigation as wet foliage greatly increases the risk of infection.
2. Mulberry scale / white peach scale (Pseudaulacaspis pentagona)
White peach scale is an armoured scale insect that colonises a wide range of trees but is particularly troublesome on mulberry. It is established in parts of southern England and can build up to damaging levels on neglected trees within two or three seasons.
What to look for
Dense white or grey crusty encrustations appear on stems and branches, giving the bark a whitewashed appearance. On leaves, the round, greyish scale caps of individual insects can be seen on the underside. Affected leaves cup and curl, turn yellow, and drop early. Heavily infested branches die back progressively from the tips. The tree as a whole looks thin and poorly.
What to do
Scrub accessible bark with a stiff brush dipped in soapy water to physically dislodge scale colonies. Apply a plant oil-based spray (such as neem or a winter tar oil wash on dormant wood) in early spring before bud burst to smother overwintering scale. In late May and early June when the crawlers (mobile young scale) are active, spray with a horticultural soap or fatty acid insecticide to kill them before they settle and form their protective shell. Repeat the soap spray every two weeks for six weeks. Natural predators including parasitic wasps and ladybird beetles help to keep populations in check; avoid broad-spectrum insecticides that harm these beneficial insects.
3. Mulberry whitefly (Tetraleurodes mori)
Mulberry whitefly is a specialist sap-feeder found on mulberry, fig, and a few related plants. It is less widespread in the UK than some other whitefly species but is present in warmer, sheltered gardens in the south of England and spreading northward as summers warm.
What to look for
Clouds of tiny white-winged insects rise from the undersides of leaves when the tree is disturbed. The immobile scale-like larvae and pupae are pale green or yellow, flat, and oval on the underside of leaves. Like all whitefly, they excrete copious honeydew, which coats the leaves below with a sticky film. Black sooty mould then colonises this deposit, giving leaves a dirty, blackened appearance. Affected leaves curl and may drop early.
What to do
Yellow sticky traps hung in the canopy will catch adult whitefly and give an early warning of population build-up. Spray the undersides of leaves with a horticultural soap or fatty acid insecticide, targeting the nymph stages in spring and early summer. Several applications at seven-to-ten-day intervals are needed because the waxy pupal stage is resistant to sprays. Improve air circulation through the canopy by annual thinning pruning; whitefly prefer still, sheltered conditions. Wipe sooty mould from leaves with a damp cloth once the whitefly population is under control.
4. Drought stress
Mulberry is a surprisingly drought-tolerant tree once established but it suffers during prolonged dry spells, particularly when carrying a heavy fruit crop. Young trees planted within the last three years are especially vulnerable as their root systems have not yet extended far enough to access deep soil moisture.
What to look for
Leaves curl inward along the midrib during the hottest part of the day, often partially recovering overnight. The foliage takes on a dull, slightly grey tone rather than the usual rich mid-green. In more severe drought, leaf margins crisp and turn brown, beginning at the tips. Fruit may fall prematurely before it fully ripens. Young shoots may wilt at the tip.
What to do
Water young mulberry trees deeply once or twice a week during dry spells from May to September. Apply water at the drip line of the tree (the outer edge of the canopy) rather than right against the trunk to reach the actively growing root tips. Mulch the entire root zone with a 75 to 100 mm layer of wood chip or well-rotted compost to conserve soil moisture and keep roots cool. Established trees in the ground rarely need supplemental watering except during a heatwave, but monitor fruit development as a guide; small, dry, falling fruit indicates the tree is struggling for water.
5. Mulberry mosaic virus
Mulberry mosaic virus (also known as mulberry ringspot virus in some literature) causes a range of symptoms on morus that can include leaf distortion and curling. It is spread by eriophyid mites, by grafting infected propagation material, and possibly by some sap-sucking insects. Once a tree is infected there is no cure.
What to look for
Leaves emerge with a mosaic or mottled pattern of yellow and green, and may be puckered, blistered, or curled. The distortion is usually most obvious on young leaves in spring. Growth may be reduced or stunted. Fruit quality often declines on heavily infected trees. The symptoms vary in severity from year to year and can be more pronounced in some seasons than others.
What to do
There is no chemical treatment for mulberry mosaic virus. Manage mite populations with a miticide spray in spring to limit spread of the virus between plants. Do not take cuttings or grafts from infected trees for propagation. When buying new mulberry trees, choose certified virus-tested nursery stock where available. Keeping the tree in good health through appropriate feeding and watering allows it to carry the infection with minimal impact on fruit production for many years. Replace trees that show severe stunting or whose fruit quality has declined to an unacceptable level.
Frequently asked questions
Why are my mulberry tree leaves curling and turning brown?
Brown, curled leaves on mulberry in spring or after wet weather most commonly indicate bacterial blight (Pseudomonas syringae pv. mori). Look for water-soaked spots that turn dark brown, beginning at leaf edges and spreading inward. If curling appears later in summer with a sticky surface and black sooty coating, scale insects are the more likely cause.
How do I treat bacterial blight on morus?
There is no chemical cure for bacterial blight. Prune out infected shoots at least 20 cm below visible symptoms, sterilising tools between cuts. Apply a copper-based fungicide as a preventive spray at bud burst the following spring before wet weather sets in. Improve air circulation through the canopy with annual pruning to reduce the humid conditions the bacterium favours.
What are the white encrusted patches on my mulberry tree bark?
White or grey crusty patches on mulberry bark are almost certainly white peach scale (Pseudaulacaspis pentagona), also known as mulberry scale. This armoured scale insect colonises both bark and the undersides of leaves, causing yellowing, leaf curl, and branch dieback in heavy infestations.
Can mulberry mosaic virus kill the tree?
Mulberry mosaic virus rarely kills established trees outright but it reduces vigour, distorts leaves, and over many years can weaken the tree significantly. There is no cure; keep the tree as healthy as possible through good soil nutrition and watering, and remove and replace plants showing severe symptoms.
Why is my mulberry tree dropping fruit early?
Premature fruit drop on mulberry is most often caused by drought stress. Mulberries set a heavy crop and need consistent moisture to carry the fruit to maturity. Water deeply around the drip line of the tree during dry spells in June and July when fruit is swelling. Bacterial blight and mosaic virus can also cause early fruit drop in severe cases.
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