The Christmas Flower III

Botrytis cinerea produces massive numbers of spores that are readily detached by air movement or rapid changes in relative humidity within the crop (39). Depending upon the plant infected, 104 to 107 spores/cm² have been observed on leaf tissue (37). Peak spore concentrations in greenhouses can occur at midmorning and midafternoon (33,34). Spores can also be dispersed by splashing (40), in addition to wind dispersal. Sprinkler irrigation splashes spores from place to place and provides the moisture they need in order to germinate and invade plant surfaces. Although Botrytis thrives most under cool temperatures (20 to 25°C), it can in fact survive, sporulate, and continue to cause disease between 4 and 30°C (41).

It is important to manage Botrytis in the entire greenhouse because many different crops are susceptible to gray mold (37). If geraniums, primulas, impatiens, or other highly susceptible crops are grown in the same greenhouse as poinsettias, the fungus may form large numbers of spores on those very susceptible species and spread to the poinsettias. In general, fading flower tissue of almost any crop becomes a food source for Botrytis. In order to inhibit Botrytis development and minimize gray mold damage, the maintenance of low relative humidity (below 93%) within the crop canopy is crucial. Plants should be spaced and heating, venting, and air circulation regimes adjusted to can keep the humidity low throughout the poinsettia production season. This usually means that pots must be separated once or twice during production so that the canopy does not close. Irrigation systems that do not apply moisture to the above ground portions of the plants help to maintain an environment inhibitory to Botrytis growth. By maintaining low humidity, the need for fungicides is greatly reduced or eliminated. In operations where humidity can be consistently minimized within the canopy, fungicides are not needed.
Because the fungus readily attacks damaged, fading, or dead tissue, it is important to avoid damaging plants and, when possible, to remove damaged tissues. All plant debris should be promptly removed from the greenhouse or disposed of in covered containers to prevent the spores formed on this material from being dispersed to the crop. If these practices are followed, then fungicides can help in management. Fungicides including chlorothalonil, fenhexamid, fludioxonil, and copper and copper plus mancozeb can be used to protect foliage from Botrytis. Great care must be exercised in selecting and applying fungicides to bracts because phytotoxicity, in the form of yellow spots or bleaching of the bract color, can be caused by some materials. Furthermore, some fungicides leave a very visible residue that is unacceptable, particularly on darkly colored foliage and bracts. Botrytis populations in most greenhouses in the U.S. are resistant to the benzimidazole class of fungicides (thiophanate methyl). Some populations have multiple resistance to benzimidazoles and dicarboximides (iprodione and vinclozolin) (23,46,52,53,60) and other classes of fungicides must be employed.
Powdery mildew. A powdery mildew disease on poinsettias was first seen in Mexico and Puerto Rico in the late 1980s, and in Pennsylvania and the Pacific Northwest in 1990. Over a hundred growers across the U.S. were affected by powdery mildew in 1992 (16). Since that time, the disease has appeared sporadically in North American greenhouses and has been seen in rare instances in Europe. Since there were no previous problems with powdery mildew in North America, many of the ornamental fungicides used to control powdery mildew on other plants were not at the time labeled to allow application to poinsettias. Growers were extremely concerned that application of fungicides to poinsettias late in the growing season would be injurious to the bracts. There was also a tendency at first to underestimate the impact that powdery mildew would have on the poinsettia crop. Not only did white colonies mar the leaves (Fig. 11), but the fungus formed dramatic white colonies on colored bracts, rendering plants unsalable (Fig. 12).

Fig. 11. Colonies of powdery mildew on a leaf (click image for larger view).		Fig. 12. Colonies of powdery mildew on flower bracts (click image for larger view).

The pathogen is an Oidium sp. (only the anamorph stage has been observed). It has no known hosts other than the poinsettia, which means that unless growers keep poinsettias over in the greenhouse from year to year, it is not likely to survive except in frost-free climates where poinsettias live year-round as landscape ornamentals. The pathogen is moved from greenhouse to greenhouse on infected cuttings or plants. Any grower activity in the crop such as spacing, or even spraying can disperse airborne conidia (10). All cultivars of poinsettia tested have shown susceptibility to the disease (12).
Environmental studies at Michigan State University have demonstrated that powdery mildew will not develop during the summer because of the deleterious effects of high temperature on the infection process (8,9,12,13). Only when greenhouse temperatures cease to reach 86°F (30°C) during the day will the powdery mildew epidemic begin to gain momentum. If growers are scouting carefully, they can respond with fungicide treatments if and when they find the first colonies (16,35,36). Preventive treatments when the pathogen is not present are purposeless and costly.
The most effective fungicides for control of powdery mildew on poinsettia appear to be the DMIs triadimefon, triflumizole, and myclobutanil (17). The strobilurins kresoxim-methyl and trifloxystrobin are also very effective, as is piperalin (18,19). A number of other materials provide some disease suppression. Since fungicides are available which give excellent control of the disease, only powdery mildew infestations that are detected too late, after the disease has had numerous cycles of infection, would be expected to result in significant crop loss.
Poinsettia scab. Poinsettia scab is a spot anthracnose disease caused by the fungus Sphaceloma poinsettiae Jenk. & Ruehle (44,56). Until recently, it had been known to be a production problem only for poinsettias in Florida, where inoculum presumably survives from year to year on landscape poinsettias. In the past few years, poinsettia cuttings produced in Central America have been the source of outbreaks in greenhouses across the United States. The host range for scab includes, in addition to Euphorbia pulcherrima Willd. (poinsettia), two other euphorbs, E. heterophylla L. (Mexican fire plant) and E. prunifolia L. (painted euphorbia). Weed species of Euphorbia are likely to be the source of spores for occasional infections in ornamental stock produced in subtropical and tropical America. For the most part, crop losses in North American production greenhouses have been minor, but the economic impact has been significant in a few cases.

Fig. 13. Scab lesions on a leaf. Note multiple lesions along top of midrib due to channeling of spores in water (click image for larger view).

The disease affects both leaves and stems of the plant (15). Small, round lesions (1 to 5 mm in diameter) form on the leaf blade, and commonly occur on the midvein or lateral veins where they frequently coalesce (Fig. 13). The spots develop whitish to brown centers, have a dark red to purple rim, and often show a diffuse yellow halo. The most distinctive feature of the spots is that they are buckled out from the upper leaf surface. With time the leaf spots develop a coating of sporulation, causing them to change from white to a velvety brown coloring (Fig. 14). The fungus makes single-celled, ovoid, hyaline conidia (3 to 7 x 1.5 to 4 µm) and also produces larger (7 to 25 x 2.5 to 7 µm), pigmented, 1 to 2 celled conidia, constricted at the septation. The latter spore type, called the “fawcetti conidium” has been thought to be the form most likely to be involved in long-distance spread of the fungus by wind (Fig. 15).

Fig. 14. Sporulating scab lesion on midrib with velvety brown spores (click image for larger view).

Fig. 15. One to two-celled ‘fawcetti’ conidia of Sphaceloma poinsettiae (click image for larger view).

Fig. 16. Scab stem lesions with red pigmentation at margin (click image for larger view).

The stems of poinsettias show oval to elongated raised cankers, with dimensions of 3 to 10 x 2 to 6 mm. The stem lesions are whitish in color, and there may be red pigmentation around them (Fig. 16). One of the most bizarre features of the disease is the result of a growth regulating chemical produced by the fungus in which a shoot with only a single canker can show “superelongation.” Thus, the internodes are lengthened so that the shoot rises six inches or more above the rest of the crop (Fig. 17). Cassava is affected by a similar fungus, Sphaceloma manihoticola, and that pathogen has been shown to produce gibberellin A₄ (62). Even if the lesions on leaves and stems are overlooked, growers will notice the affected shoots that stretch high above the rest of the crop.

Fig. 17. Superelongation of poinsettia stems in the vegetative stage (left) and later in the flower bract stage (right) due to scab (click image for larger view).

S. poinsettiae is favored by high humidity and wet growing conditions, which are not uncommon during poinsettia propagation. Splashing water will spread the spores easily, and insects might also move spores from plant to plant. Details of the epidemiology of S. poinsettiae have not been determined.
Control studies by Engelhard in Florida two decades ago showed benomyl, chlorothalonil, mancozeb, mancozeb plus thiophanate-methyl and copper hydroxide to be effective against poinsettia scab, whereas dicarboximides were ineffective (25). Trials conducted in 2001 by Chase and Daughtrey (14) indicated that triazoles, strobilurins, materials containing mancozeb, chlorothalonil, and a chlorothalonil plus thiophanate-methyl combination were very effective as protectants against poinsettia scab. Bicarbonates and copper sulfate pentahydrate were less effective but provided some control.
Poinsettia growers are encouraged to scout for scab symptoms on leaves and stems. If scab is detected, infected plants should be removed, and the remainder treated with an effective fungicide. Splashing during irrigation and lengthy periods of leaf wetness should be avoided in order to reduce the opportunity for new scab infections.

Fig. 18. Alternaria leaf spot on poinsettia (click image for larger view).

Alternaria leaf spot and blight. Alternaria leaf spot in poinsettia (caused by Alternaria euphorbiicola) was first reported causing commercial losses from Florida in about 1984. In 1986, Yoshimura et al. (59) described what appears to be the same disease caused by Alternaria euphorbiae (=Macrosporium euphorbiae). The disease started in Hawaii in late 1983. Alternaria blight of poinsettias is characterized by small (less than 1 mm in diameter) lesions that are initially water-soaked. The roughly circular lesions turn reddish-brown to dark brown and reach 20 mm in diameter (Fig. 18). Lesions may or may not have a halo. Lesions also form on bracts reaching 40 mm across with a black-purple margin. Stem lesions can result in girdling and subsequent loss of the stem. Cultivar resistance was examined in the 1980s but no work has been done with the multitude of new cultivars introduced since that time. Elimination of water on leaves is important to completely control Alternaria leaf spot of poinsettias. In 1985, iprodione and the combination product of thiophanate methyl and mancozeb were reported effective in controlling Alternaria leaf spot (26). Although both products are still available, McGovern (50) identified azoxystrobin and fludioxonil as giving superior disease control in studies conducted in the late 1990s.
Erwinia blight and cutting rot. Soft rot of poinsettia cuttings was originally described in 1959 from Missouri (55). The pathogen was identified as Erwinia carotovora, the cause of many ornamental soft rot diseases. The onset of symptoms is very rapid with soft rot evident as much as 7 to 10 cm above the cut end of a cutting within 24 hours of infection. The rot starts as a watery area at the cut end or anywhere on the cutting, causing disintegration (Fig. 19). A characteristic rotten fishy odor often is present in the propagation house when this disease is present. Infected cuttings parts that have dried remain sources of active bacteria for at least 6 weeks. Erwinia carotovora is widely distributed throughout most ornamental production areas. It has even been found in irrigation water in some of the southern states. Growers have reported that preventative water treatment with 0.5 to 1 ppm bromine or chlorine has been effective. Use of bactericides is rarely helpful with soft rot diseases of cuttings.

Fig. 19. Erwinia blight on poinsettia cuttings in propagation (click image for larger view).