Garrett Owen, PhD Student and Floriculture Technician, Purdue University and
Roberto G. Lopez, Ph.D., Associate Professor & Floriculture Extension Specialist, Purdue University
Poinsettia scab (Sphaceloma poinsettiae) or spot anthracnose is a fungal pathogen that can infect poinsettia crops. It is rarely encountered in poinsettia production, however poor sanitation practices and wet conditions favor development of the disease.
On the stem and leaf petioles, symptoms include scab-like or raised lesions (Figure 1). Lesions are often tan in color and can be surrounded by white, red, or purple margins (Figure 2) with a velvety, grayish-brown layer of fungal growth growing from the depressed stem lesion. Lesions may encircle or girdle the stem, causing stem dieback above the infection site. Heavily infected stems may become abnormally elongated exhibiting a gibberellin-like appearance (Figure 3), which will aid growers in scouting for plants infected by scab.
On leaves, symptoms of spot anthracnose are brown, round or angular spots with purple margins that develop on the leaf or leaf margins (Figure 4). Yellowing or chlorosis will develop around leaf spots and leaves with spots are often distorted or puckered. Severely affected leaves may drop prematurely.
The degree of visual symptoms may vary and plants may only exhibit stem or leaf symptoms. Other symptoms similar to poinsettia scab include leaf scarring, odema, Alternaria leaf spot, and environmental disorders.
Diagnosis and Control
Cultural and chemical control options have been developed for poinsettia scab. In general, cultural control options are the first line of defense. Adopting cultural practices that minimize leaf wetness and water splash, remove excessive foliage whenever possible, sanitation, crop scouting, and the removal and isolation of suspected plants will limit the impact of poinsettia scab. If poinsettia scab is present, a protective fungicide program should be implemented. It is important to follow all fungicide labels as directed by the manufacturer and most fungicides are labeled for use on poinsettias until bract formation.
Click image to enlarge
Figure 1. Poinsettia scab symptoms on a stem appear to be raised and elongated, scab-like lesions.
Figure 2. Poinsettia scab symptoms include depressed, tan lesions with white margins on the stem of a poinsettia plant.
Figure 3. Abnormally elongated poinsettia stem caused by infection of poinsettia scab on the stem.
Figure 4. Poinsettia scab leaf symptoms with round to angular brown spots with purple margins developing as the disease progresses on the leaf margin resulting in leaf distortion.