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Kerria, once considered a disease free, spring flowering shrub, is now affected by a fungal infection that causes lesions and defoliation.
Kerria twig and leaf blight (Blumeriella kerriae) on Kerria japonica
Since 2014 RHS Gardening Advice has received a number of reports and samples from across the UK of Kerria japonica exhibiting severe defoliation, spots on the leaves and stem lesions. Upon closer examination it was determined that the symptoms were caused by the fungus Blumeriella kerriae. This fungus has been recorded on Kerria japonica in America but has not previously been recorded on any plant in the UK. It is not known how this fungus arrived in the UK. Unfortunately the fungus appears to be very difficult to control and produces large quantities of spores on the lesions and leaf spots. The good news is that this fungus Blumeriella kerriae appears to be specific to Kerria, so other plants in the garden should not be at risk.
No effective control measures for Kerria twig and leaf blight have yet been identified. The disease is best managed by removing all infected plant material and either burning it or disposing of it at your local council composting facility. Home compost heaps rarely reach the temperatures achieved by large-scale facilities required to kill fungal spores. Clearing fallen leaves should reduce fungal inoculum in the following year.
There are no fungicides available to amateur gardeners with specific recommendations for use against Blumeriella kerriae. However, the fungicide tebuconazole with trifloxystrobin (Bayer Fungus Fighter Plus) carries a label recommendation for use against 'leaf spots' on ornamental plants. Tebuconazole (Bayer Fungus Fighter Concentrate) and triticonazole (Scotts Fungus Clear Ultra and Scotts Fungus Clear Ultra Gun) are labelled for the control of a number of other diseases on ornamental plants and could therefore be used legally on Kerria (at the owner’s risk) to try and control leaf spot.
There is no specific information available as to the efficacy of these products against Kerria twig and leaf blight. It is likely that repeated sprays will be required where the disease is present, particularly during warm wet weather. Success is more likely if the plant is cut back hard to remove affected material, and then sprays applied to protect the new growth. It would be prudent to apply a small amount of the chosen fungicide first to ensure that the product will not cause plant damage.Inclusion of a fungicide product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
The cause of twig and leaf blight on Kerria japonica is the fungus Blumeriella kerria. This fungus is closely related to Blumeriella jappii, a fungus which can cause leaf spot on cherry trees.The development of Kerria twig and leaf blight is likely to be favoured by wet weather conditions. In damp conditions Blumeriella kerriae produces large numbers of asexual spores from each spot/lesion. Spores are likely to be transferred between plants via rain splash, wind, and transfer on contaminated tools. The fungus overwinters on fallen leaves and lesions on the stems then releases spores to reinfect new growth in spring. Spore production has been observed throughout the year on Kerria plants in the UK. The sexual stage of the fungus has not yet been observed to occur in the UK.
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