The lupin is a stalwart of the cottage garden, available in a huge range of colour combinations. Unfortunately, it is susceptible to a fungal disease called lupin anthracnose, which in wet weather can cause severe dieback.
What is lupin anthracnose?
Lupin anthracnose is a fungal disease of the leaves and stems. It is spread from plant to plant by rain-splashed spores, and is therefore particularly damaging in wet weather. Affected plants are not usually killed, but can become very unsightly as a result of severe leaf-spotting and dieback.
Anthracnose first became a problem on ornamental lupins in the 1980’s, and is now the most damaging disease affecting them. It is also a problem on commercial field crops of lupins, which are grown for their seeds, rich in protein and oil.
Lupins suffering from lupin anthracnose will look unsightly;
- Brown areas of dead tissue (necrotic lesions) on leaf blades, leaf stalks, stems, flower stalks and seed pods
- Under wet conditions the fungus produces a slimy mass of orange-coloured spores on the surface of the affected areas
- Affected leaf stalks will sometimes coil several times to give a ‘corkscrew’ appearance, which is very characteristic of anthracnose
- Severely affected leaves may shrivel and turn brown
- The growth of stems and flower stalks may become distorted, or they may collapse completely, due to points of weakness caused by the lesions
- Pick off and destroy affected leaves if only one or two show symptoms. Monitor the plants closely for any further development of symptoms
- Severely affected plants should be removed and disposed of as soon as possible. Ensure that no debris remains on the soil surface
- Avoid dense plantings, so that air can circulate freely around the plants
- Avoid overhead watering, particularly where the disease is present
- Do not save seed from affected plants
- If repeated infection occurs, consider replacing lupins with another herbaceous plant
There are no fungicides available to gardeners with specific recommendations for use against lupin anthracnose. However, the fungicides difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Doff Systemic Fungus Control and other products), tebuconazole (Bayer Garden Multirose concentrate2) and triticonazole (Scotts Fungus Clear Ultra) are labelled for the control of some other diseases on ornamental plants, and could therefore be used legally on lupins (at the owner’s risk) to try and control anthracnose.
There is no specific information available as to the efficacy of these products against lupin anthracnose, however. It is likely that repeated sprays will be required where the disease is present, particularly during unsettled weather. It would be prudent to apply a small amount of the chosen fungicide first, at a solution suggested on the packet for other problems, to ensure that the product will not cause plant damage.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
Lupin anthracnose is caused by the fungus Colletotrichum. A number of species have been associated with the problem, including Colletotrichum lupini, C. acutatum and C. gloeosporioides. The disease is favoured by periods of warm, wet weather. The orange spore masses that develop on the surface of the lesions contain huge numbers of microscopic spores, and these are splashed around by rain droplets (or by overhead watering). They will germinate and infect the plant if the leaf or stem surface stays wet for a few hours, eventually producing new lesions.
The fungus can be found at the very base of affected stems, and sometimes on the roots, so the problem can develop again even if affected plants are cut back hard. Seed can also become contaminated by the anthracnose pathogen.