Lupine is a grain legume, like beans, peas and lentils. Unlike these other legumes, however, lupine can be lethal if ingested by animals or humans if it is not first treated. Not many diseases affect the lupine plant, but some attack the roots of the plant, killing it from the bottom up.
Root rot occurs when fungi attack the roots. Rhizoctonia solani fungi thrive in well-drained soil, and Pythium fungi live in wet soil. Both types of fungi cause the roots to turn black and collapse, inhibiting the ability of the plant to take up water and nutrients. A different type of root rot occurs when Sclerotium or Fusarium attack. These fungi cause yellowing of the leaves and eventual death of the plant. Fungicides do not work effectively once visible symptoms appear, and you must remove the entire plant in order to get rid of the disease.
Female nematodes build webs in the root system of the lupine plant and lay eggs there. Once the eggs hatch, the young nematodes feed on the roots of the plant, causing galls or knots to form. The roots eventually decay, becoming unable to transfer water and nutrients to the plant. The lupine then wilts and eventually dies. Chemicals to control nematodes are toxic and expensive and are not often available to the homeowner. Complete removal of the plants and infected soil may be necessary.
Oedema occurs with overwatering, high humidity and low light conditions. With this disease, the lupine takes up water faster than it can use it. Water pressure builds up in the leaves, causing blistering and enlargement of the tissue. This disease is not life-threatening. Manage it by lowering the humidity and increasing air flow and sunlight around the plant.