Amaryllis have large trumpet-shaped flowers that grow on thick stems in clusters of four or five to a single stem. The leaves of the amaryllis do not grow on the stems, but grow from the base of the plant. The colors can be solid red, white, pink, salmon and orange or they can be multicolored or striped, mostly in pink or red combined with white. The flowers are so large that the stalks cannot support them and the plant needs to be tied to a stake. Amaryllis are susceptible to several serious, and sometimes fatal, diseases.
Red Leaf Spot Disease
Red leaf spot disease is caused by a fungus and appears as spots on the leaves. They will develop into cankers with red borders at the base of the stalks and the base of the leave. Eventually the leaves will turn yellow and drop off. The flower stalks may become weak and break off, but it is not known to be a fatal disease. There is a chance to prevent the disease by using only sterile potting soil. If it does occur, there are effective fungicides.
Mosaic is a viral disease. The leaves will develop a yellow mottling pattern, which will grow and develop small spots or streaks of yellow and green. The plant will become stunted and it will get worse year after year. If the plant is able to bloom, the leaves, the stalks and the flowers themselves will be smaller than they are supposed to be. Unfortunately, there is no cure or way to control the disease and the infected plants should be destroyed.
Root and Bulb Rot
Root and bulb rot are caused by fungi and attack young seedlings. A black lesion may appear at the soil line, which will cause the stems to fall over. The plant will become pale, fail to grow, wither and die. There is no cure, but there are ways to prevent the disease from getting established. Wet the pot with a fungicide before planting and pot in sterilized soil. Do not reuse a pot or soil from an infected plant.
Botrytis blight is a fungal disease that strikes when the temperatures are too cool-60s to low 70s--and the plant does not get proper air circulation. Damp conditions, humidity that reaches at least 92 percent and moving infected plants can cause the spores to spread. It can infect the plant in just 14 hours. It will appear as small brown spots on the flowers. In the worst cases, the spots will merge and a gray web-looking growth will appear. Pick up any fallen debris as it acts as a breeding ground for the fungus. Keep water off of the plant itself. Once it hits, there is no stopping it.