Lupins (Lupinus polyphyllus) are an old fashioned and easy-to-grow plant, offering spikes of blue, purple, pink, yellow, or white blossoms throughout spring and summer. Lupins love moist, cool locations, full sun, and grow in nearly any type of soil. However, occasionally lupins become infected with disease, reducing bloom production, ruining their aesthetic appeal, and sometimes even killing the plant.
Oedema is a disease that strikes lupins when they consume more water than the plant can use. The water builds up in the plant, causing leaves to appear larger than normal. Very small areas that look like blisters appear on the leaves and may turn a brownish colour. According to the Cornell University Extension website, in especially bad cases the blisters also appear on the flowers and stems of lupins. As the disease progresses, leaves turn yellow and fall off and bloom production stops. Oedema commonly occurs during times of cloudy, cool weather--especially if the soil is warm and the air is moist. Lupins grown in greenhouses are more likely to develop oedema than those grown outdoors. To prevent oedema, avoid overwatering and make sure lupins are planted in full sun. Lupins with oedema will recover if given warmer, drier conditions.
Nematodes are a microscopic pest that look something like tiny eel-shaped roundworms. They sometimes feed on lupin roots, leading to the decline of the plant and susceptibility to additional diseases. Many nematodes reproduce rapidly, going from egg to adult in 28 days or less, according to the of California Agricultural and Natural Resources website (SOURCE #2). Most nematode eggs live in the soil all year round, making them especially difficult to get rid of. Symptoms of nematodes can include wilting (especially during the warmest time of day, although the lupin has sufficient moisture), yellow leaves, and general lack of vigour. Leaves and blossoms may also be smaller than plants that aren't affected. Badly affected plants may die. If nematodes are suspected, dig up plants, tap them free of soil, and look for swollen areas on the roots. To get rid of nematodes in the first 12 inches of earth, solarise the soil during the hot summer months. To do this, moisten the earth, cover it with a clear plastic tarp, and weigh down the tarp with stones. Remove the tarp after about six weeks.
Several different fungi cause powdery mildew, which appears as a white or grey powder on lupin leaves and stems. Powdery mildew usually starts in small patches, then spreads. In bad cases, the mildew covers all the leaves and stems. Eventually, the leaves turn yellowish or brown; they dry and fall off. Stems and flowers might also become disfigured. Although powdery mildew rarely kills plants, it can greatly reduce their vigour and bloom production. According to the Ohio State University Extension website, powdery mildew is most likely to occur when temperatures are between 15.5 and 26.6 degrees C. It does not necessarily require humid or wet conditions in order to appear. Crowded lupins, or overly-shaded plants are also more prone to powdery mildew. As soon as powdery mildew is seen on lupins, cut off the affected area of the plant and throw them away in the garbage. If this does not control the disease, fungicides are appropriate, but sprays are most effective if applied as soon as the disease is detected.