The caller had observed ‘rust’ patches on the stems of Kerria japonica ‘Pleniflora’ in a client’s garden which he suspected was a form of blight. The ends of some stems were wilted. He asked whether he should prune affected stems or remove the whole plant.
Kerria japonica ‘Pleniflora’ is a double-flowered cultivar of a suckering shrub from China and Japan, a member of the Rosacea family, ‘Pleniflora’ is a very vigorous grower and bears large, fluffy yellow flowers in spring. While kerria has no serious insect or disease problems, it is susceptible to leaf spots, cankers, blights such as the bacterial fire blight..
Blight is the term for various plant diseases whose symptoms include sudden and severe yellowing, browning, spotting, withering, or dying of leaves, flowers, fruit, stems, or the entire plant. Usually the shoots and other young, rapidly growing tissues of a plant are attacked. Most blights are caused by bacterial or fungal infestations,
Humidity, rain, overhead watering, insects and oozing wounds on the plants themselves all contribute to infestations. Most bacteria require moisture. Plant pathologists have coined the following phrase to help gardeners: if they dry, they die. Keeping foliage dry and never working with wet plants are the two most important steps to be taken.
Control measures include removing and destroying infected branches. Sick plants provide a source of infection that spreads the disease to all susceptible plants in the vicinity. Prune the affected limbs 30 cm below any visible lesion using sterilized tools.
If the shrub appears to be severely affected the bacteria may have reached the roots. Some of these bacteria can live in the soil or inside dead and dying roots for many years so remove and destroy the whole plant, removing as much of the root system as possible.
Discard all material removed by putting the material in garbage destined for a landfill or burning the material if legally allowed to do so. Do not discard in a municipal yard waste program.
References: D. Deardorrff and K. Wadsworth, What’s Wrong With My Plant, Timber Press, 2009