Gray Mold (fungus – Botrytis cinerea): This disease is especially serious on double varieties. The inflorescence, leaves and stems are attacked. A gray mold occurs on the inflorescence causing a blasting and browning of the flower clusters and colored bracts. Water-soaked lesions will appear on leaves and stems. In the later stages of the leaf and stem disorder, the lesions become dry and crisp.
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Cotton root rot is restricted to extreme South Texas where Poinsettias are planted in landscapes. In most areas of Texas, Poinsettias will not survive winter temperatures. (See the section on Cotton Root Rot)
Scab (fungus – Sphaceloma poinsettia): A very common disease of poinsettia, particularly in South Texas. The fungus attacks leaves and stems, starting out as conspicuous, raised lesions or cankers on diseased stems. The cankers are at first white, later turning gray. Frequently, these cankers unite to cover irregular raised areas which give the plants an unsightly appearance. The disease cannot be controlled once it is well established on plants. Severely diseased plants should be cut back and the new growth should be protected with a fungicide.
Black Leg (fungus – Rhizoctonia solani): This disease can affect poinsettias at any stage of growth, but it is usually more prevalent right after the plants are potted. Stems affected by Rhizoctonia become dark-brown, water-soaked, with lesions that extend above and below the soil line. The lesion may encircle the stem, causing the stem to become weak. As a result of the black leg condition, the foliage becomes pale yellow and some of the leaves fall off. If roots and stems become severely infected, the plants die suddenly. Drenching the soil with a fungicide around the base of the plants may help in checking this disease.
Pythium Root Rot (fungus – Pythium sp.): Roots and basal portions of the stem turn brown and soft. Foliage may wilt rapidly. Pythium root rot is a major problem in commercial poinsettia production. Control measures consist of fungicide treatment of soil mix, fungicide drenches, acquisition of clean planting stock, and using a potting medium that drains well.