Xanthomonas leaf spot. Xanthomonas leaf spot on poinsettias was originally reported in 1951 from India (54). It was later described as causing commercial losses in outdoor production of poinsettias in Florida in 1960 (49). The cause is Xanthomonas campestris pv. poinsettiicola. Pinpoint spots start as dull gray to brown slightly water-soaked areas. As the lesions mature they turn yellow to tan and are scattered across the leaf surface. Sometimes they enlarge and become angular in shape due to limited movement across leaf veins (Fig 20). This disease can be confused with early symptoms of scab. Severe infections can cause distortion of new leaves as well as complete chlorosis and finally abscission of older leaves.
When the disease was discovered in Florida, researchers found that many, if not all, popular cultivars of the day were highly susceptible to Xanthomonas leaf spot. No new work has been conducted to evaluate cultivar resistance. Control must be based on elimination of all stock plants with Xanthomonas leaf spot. In this day of mass production of rooted poinsettia cuttings it is most important for the propagator to identify a bacterial leaf spot outbreak and eradicate it at the source. Use of copper bactericides may be partially effective in controlling this disease but are rarely effective in stopping an outbreak once infection has occurred. The disease is nearly impossible to control unless plants are produced without overhead watering or exposure to rainfall.
An examination of the root cortex cells often reveals the presence of spherical oospores (Fig. 24). However, oospores may be absent and only sparse hyphae are observed in the roots. When such roots are plated onto water agar or a selective medium (21,43), Pythium grows out within 24 hour Pythium irregulare and P. ultimum are common species found in poinsettia roots (29,57) but a recent examination of isolates obtained from clinic samples in Pennsylvania from 1996 to 2001 indicated that Pythium aphanidermatum accounted for 76% of the Pythium root rot cases (Moorman, unpublished). At this time it is not known whether poinsettias are more susceptible to P. aphanidermatum than other species, whether P. aphanidermatum is infecting cuttings during propagation and is being inadvertently shipped to other growers or whether the greenhouse temperatures and other cultural conditions used in poinsettia production make it more likely that P. aphanidermatum will successfully attack poinsettias than other species.
Pythium inoculum may enter the production system from a number of sources. If the propagator of cuttings allows Pythium to infect plants during the callusing and rooting process and infections go unobserved, then Pythium-infected cuttings are sold to customers. Other sources of the organism include contaminated soil from under and between benches or outside the greenhouse that is then moved into pots or irrigation water reservoirs or onto benches or flood floors. If the grower uses contaminated field soil as a component of the potting mix or if the properly treated potting soil is contaminated with untreated soil, it is not unusual for a very high percentage of the crop to be affected. Growers employing potting mixes composed of mixtures of peat moss, bark, vermiculite, coir, peanut or rice hulls, perlite, and other non-soil materials sometimes allow the mix to become contaminated with Pythium-infested soil from the sources noted above. It is known that peat moss can harbor Pythium (45) and it has been found that commercial soilless potting media sometimes contain the pathogen. Surface water supplies such as streams and ponds may bear Pythium. When used as sources of irrigation water, plants will be inoculated regardless of whether the water is applied via sprinkler irrigation, trickle, or by ebb and flow systems. In subirrigation systems, severe cases of waterborne Pythium root rot occur in operations employing long flooding times (more than 30 to 40 minutes) or where flooded floors or benches do not drain completely and pots remain in puddles for extended periods. While it is known that fungus gnats and shoreflies are vectors of Pythium in some systems (31,32,42), the importance of this in potted plant production has not been studied. Factors that favor Pythium root rot development in poinsettias include excessive fertilizer levels (51), high soil moisture levels (2), and soil pHs above 5.5 (1) when P. ultimum is involved. Little work has been done on factors affecting other species known to attack poinsettias.
Pythium root rot is difficult to control once it has begun. Every effort should be directed toward preventing the disease before it begins by eliminating the pathogen from the production system. Pathogen-free potting mixes are essential in this effort and general sanitation practices in the greenhouse should target the removal or treatment of soil that may harbor Pythium. Disinfesting bench surfaces, flood floor systems, potting benches, tools, and equipment that will contact the potting mix is important. If pond or stream water is used for irrigation, the intake pipe should be well above the bottom so that sediment is not drawn in. If the water supply is suspected of being a source of Pythium, it may be necessary to treat the water (22). Irrigation systems in which unused water is recycled, once contaminated with a pathogen, becomes an ongoing source of Pythium. For that reason, ebb and flow system reservoirs should be covered in order to prevent contaminated soil or debris from entering them. To remove plant debris that may harbor the pathogen from the water, the water should be passed over a coarse screen before it is returned to the reservoir.
In a greenhouse operation with a history of Pythium root rot, fungicides or biological control agents should be applied as early in the cropping cycle as possible. Biological agents including species and strains of Trichoderma, Bacillus, Gliocladium, and Streptomyces, can be applied to the potting mix before, during or immediately after transplant. In general it is recommended that chemical pesticides not be applied to the potting mix during the period from 10 days before to 10 days after applying the biological control agent. Biological control agents and fungicides may have to be applied more than once in order to maintain adequate protection throughout the season, particularly on stock plants. Several fungicides, including mefenoxam/metalaxyl, propamocarb, and etridiazole are registered for use on poinsettias in the U.S. However, some populations of P. aphanidermatum and P. irregulare have resistance to mefenoxam/metalaxyl.
Phytophthora root, crown, leaf, bract, and flower blight. Two species of Phytophthora, P. nicotianae Breda de Haan (=P. parasitica Dastur) and P. drechsleri have been recovered from poinsettias in greenhouses at various locations in the United States. Root, crown, and stem rot caused by P. nicotianae was first described by Engelhard and Ploetz in 1979 (27), and foliar infection caused by both P. nicotianae and P. drechsleri was first described by Yoshimura et al. in 1985 (58). Infected roots are brown and depending on the environmental conditions and the age of the plant, infection may be present for varying lengths of time before wilt or stunting is noticed. Infections at and above the soil line are characterized by purple-black lesions that may expand rapidly from stems to brack petioles causing the bracts to wilt (Fig. 25). Leaf lesions are paper-like and dry in texture, grayish brown at first, turning brown to black. Blight symptoms produced by these two species are virtually identical (58).
Both P. nicotianae and P. drechsleri are heterothallic, requiring A1 and A2 mating types to complete the sexual stage, and form thick-walled oospores (28). Single isolates readily produce sporangia on diseased tissue and sporangia can quickly release swimming zoospores as the crop is watered. Recent epidemics of Phytophthora blight in floriculture production facilities throughout the U.S. were caused by the spread of a single clonal lineage (e.g., sporangia and zoospores from a single Phytophthora isolate) including one epidemic in poinsettia caused by P. drechsleri (Lamour and Hausbeck, unpublished). Many poinsettia production facilities utilize ebb and flood watering strategies which provide frequent opportunities for the spread of sporangia and zoospores to other plants as the crop is watered. The incubation period between the initial infection of a plant and subsequent visible symptoms provides ample time for spread of inoculum throughout a facility.
Control strategies that prevent initial infections are most effective in limiting the development of Phytophthora epidemics in poinsettia. These include the use of pathogen-free potting mix and strict sanitation in the production facility. The fungicides metalaxyl (Subdue) and more recently mefenoxam (Subdue Maxx) have been shown to be very effective in controlling Phytophthora blight of poinsettia (58). However, isolates of P. nicotianae resistant to the highest rate of application of metalaxyl or mefenoxam have been reported from floriculture production facilities (30, Lamour and Hausbeck, unpublished). Applying fungicide to known infected plants will increase the chance of selecting for insensitive isolates. The most effective way to halt an epidemic is to remove all plants from a production area in a facility that have been irrigated with water from the reservoir that collects recycled water from known infected plants. Even healthy looking plants may be infected so strict sanitation following an epidemic is mandatory to prevent re-occurrences.
Rhizoctonia stem rot. Stem rot caused by Rhizoctonia solani Kuhn AG-4 is the most important disease that affects poinsettia during propagation. The pathogen has a very wide host range, as well as a high competitive saprophytic ability. This combination of characteristics provides an ideal opportunity for both infection of host tissue and colonization of plant debris followed by subsequent long-term survival either as mycelium or sclerotia associated with crop debris. Growers may practice strict sanitation but often times R. solani becomes an indigenous pathogen in greenhouse production facilities as the fungus survives between crops in infested debris left behind on the bench, equipment, floor, walkways, and even in crevasses on wooden benches.
Poinsettia may be propagated in 10-unit polyfoam rooting strips, rockwool cubes, or stuck directly in a soilless potting mix in pots. Propagation of poinsettia typically occurs in the hot months of July and August in the greenhouse under conditions of almost constant leaf wetness from mist systems designed to prevent the cutting from wilting. Under these ideal conditions, Rhizoctonia stem rot develops when debris colonized by R. solani is dispersed to the rooting strip by splashing water or other means of contact. The initial symptoms of disease are small lesions that often develop at the point on the stem even with the top surface of the rooting cube. Lesions have a dry appearance with a dark border and tan center (Fig. 26). Multiple lesions may develop on a single cutting. If newly made cuttings become infected, lesions expand rapidly girdling the stem and collapsing the cutting within 5 to 7 days (Fig. 27). Cuttings that have been in propagation a week or more are more resistant to R. solani. Drooping leaves on cuttings that contact the bench surface also can be infected by Rhizoctonia harbored in residual debris. Infected areas on leaves are brown and beads of white latex from the host often appear on the infected tissue under conditions of high humidity. Infected leaves are quickly colonized and provide an avenue of entry to stems for further stem rot development. Commercial ELISA kits are available for growers and diagnosticians to detect R. solani in infected poinsettia tissue (5). Once one cutting in a rooting strip becomes infected, R. solani can use that diseased tissue as a food base to grow both on the surface and through the strip to infect other cuttings in the 10-unit strip. Sclerotia and hyphae may be visible on the surface of the rooting cube particularly on the sides exposed when the protective cover holding the strip is removed (Fig. 28).
Rhizoctonia crown and root rot. Root rot may develop either in the rooting cube or on rooted cuttings transplanted to pots as the crop is finished for retail. Infected roots become water-soaked then brown. Both root tips and sections of the root away from the tip may develop symptoms. Crown rot can develop on the stem as lesions expand from stem infections occurring during propagation. However, stem lesions may develop at a much slower pace on rooted plants, since this tissue is more hardened off and thus more resistant than stems of newly made cuttings. Foliar symptoms of crown and root rot include chlorosis, leaf necrosis, wilting, defoliation, and plant death, but often the most common symptom is stunting. Root rot infections may be initiated from lesions on stems or from inoculum introduced to the potting mix from debris surviving in the greenhouse. Generally, moist but not wet conditions in the potting mix favor development of Rhizoctonia crown and root rot on potted plants. Spacing plants with a full canopy too close together can result in moisture and soil temperatures favorable for development of disease due to shading of the container surface.
Control of stem and root rot begins with thorough removal of all crop debris from the production facility at the end of a cropping cycle. Sanitation of work area and bench surfaces with surface disinfectants is important. During propagation, misting cycles should be monitored closely to avoid over wetting foliage of cuttings once newly made cuttings become turgid. Daily scouting of propagation areas is needed so that rooting strips with any cutting showing symptoms or signs of stem rot can be removed. No rooted cuttings from a strip with an infected cutting should be transplanted since the pathogen may have grown through the foam cubes resulting in apparently healthy but infected cuttings that will not produce saleable plants.
In greenhouse production facilities with a history of Rhizoctonia stem and root rot, soaking dry rooting strips in a fungicide solution can protect cuttings from disease (4). Fungicides such as flutolanil, iprodione, and chlorothalonil actually prevented R. solani from colonizing the rooting cube (4). Fludioxonil and the new strobilurins such as azoxystrobin also are effective for stem rot control (7). Generally one application of fungicide is sufficient to protect the crop during the propagation cycle. Since some fungicides affect the number of roots formed on cuttings, growers should test specific fungicide by cultivar combinations for safety before using a product on the entire crop (6). After transplanting, fungicide drenches may be needed at regular intervals to prevent crown and root rot.
In addition to traditional fungicides, several biocontrol agents such as Burkholderia cepacia, Paecilomyces lilacinus, and binucleate Rhizoctonia spp. (BNR) have shown potential for control of Rhizoctonia stem and root rot (11). Commercial biocontrol agents mentioned above also are available, although those based on Gliocladium virens are phytotoxic to unrooted poinsettia cuttings (Benson, unpublished). In a unique study, a sequential application of B. cepacia to rooting strips followed by incorporation of a Pesta formulation of BNR into potting mix at transplanting protected poinsettia from stem rot in propagation and root and stem rot for 52 days after transplanting (38).
 Fig. 20. Angular leaf spots on poinsettia caused by Xanthomonas (click image for larger view). |
Root Diseases
Pythium root rot. Every season, some poinsettia growers encounter crop losses as a result of Pythium root rot. Depending upon the circumstances in the particular greenhouse, a few plants may be affected or a very high percentage of the crop can be lost. Pythium usually attacks early in the season (3), soon after cuttings have been potted. Severely affected rooted cuttings wilt and die rapidly. The base of the cutting is brown and has a water-soaked appearance. The callus and any new roots at the base of the cutting also turn brown. The growth of infected plants that survive is stunted (Fig. 21) and these plants often wilt (Fig. 22) during the heat of the day and recover at night later in the season. Infected roots of established plants are dark brown in color and the outer layers of root tissue strip off leaving a bare strand of inner vascular tissue exposed (Fig. 23). Infected plants that survive until flowering usually flower prematurely and defoliate. |  | |
| Fig. 21. Stunting of poinsettia growth (left) due to Pythium root rot compared to healthy plant (right) (click image for larger view). | Fig. 22. Wilted foliage of a poinsettia due to Pythium root rot. Wilted plants may recover at night (click image for larger view). |
| Fig. 23. Roots of poinsettia with dark brown discoloration and sloughed off cortical tissues (click image for larger view). |
 Fig. 24. Spherical oospores of a Pythium sp. in the cortical cells of a poinsettia root (click image for larger view). |
Pythium inoculum may enter the production system from a number of sources. If the propagator of cuttings allows Pythium to infect plants during the callusing and rooting process and infections go unobserved, then Pythium-infected cuttings are sold to customers. Other sources of the organism include contaminated soil from under and between benches or outside the greenhouse that is then moved into pots or irrigation water reservoirs or onto benches or flood floors. If the grower uses contaminated field soil as a component of the potting mix or if the properly treated potting soil is contaminated with untreated soil, it is not unusual for a very high percentage of the crop to be affected. Growers employing potting mixes composed of mixtures of peat moss, bark, vermiculite, coir, peanut or rice hulls, perlite, and other non-soil materials sometimes allow the mix to become contaminated with Pythium-infested soil from the sources noted above. It is known that peat moss can harbor Pythium (45) and it has been found that commercial soilless potting media sometimes contain the pathogen. Surface water supplies such as streams and ponds may bear Pythium. When used as sources of irrigation water, plants will be inoculated regardless of whether the water is applied via sprinkler irrigation, trickle, or by ebb and flow systems. In subirrigation systems, severe cases of waterborne Pythium root rot occur in operations employing long flooding times (more than 30 to 40 minutes) or where flooded floors or benches do not drain completely and pots remain in puddles for extended periods. While it is known that fungus gnats and shoreflies are vectors of Pythium in some systems (31,32,42), the importance of this in potted plant production has not been studied. Factors that favor Pythium root rot development in poinsettias include excessive fertilizer levels (51), high soil moisture levels (2), and soil pHs above 5.5 (1) when P. ultimum is involved. Little work has been done on factors affecting other species known to attack poinsettias.
Pythium root rot is difficult to control once it has begun. Every effort should be directed toward preventing the disease before it begins by eliminating the pathogen from the production system. Pathogen-free potting mixes are essential in this effort and general sanitation practices in the greenhouse should target the removal or treatment of soil that may harbor Pythium. Disinfesting bench surfaces, flood floor systems, potting benches, tools, and equipment that will contact the potting mix is important. If pond or stream water is used for irrigation, the intake pipe should be well above the bottom so that sediment is not drawn in. If the water supply is suspected of being a source of Pythium, it may be necessary to treat the water (22). Irrigation systems in which unused water is recycled, once contaminated with a pathogen, becomes an ongoing source of Pythium. For that reason, ebb and flow system reservoirs should be covered in order to prevent contaminated soil or debris from entering them. To remove plant debris that may harbor the pathogen from the water, the water should be passed over a coarse screen before it is returned to the reservoir.
In a greenhouse operation with a history of Pythium root rot, fungicides or biological control agents should be applied as early in the cropping cycle as possible. Biological agents including species and strains of Trichoderma, Bacillus, Gliocladium, and Streptomyces, can be applied to the potting mix before, during or immediately after transplant. In general it is recommended that chemical pesticides not be applied to the potting mix during the period from 10 days before to 10 days after applying the biological control agent. Biological control agents and fungicides may have to be applied more than once in order to maintain adequate protection throughout the season, particularly on stock plants. Several fungicides, including mefenoxam/metalaxyl, propamocarb, and etridiazole are registered for use on poinsettias in the U.S. However, some populations of P. aphanidermatum and P. irregulare have resistance to mefenoxam/metalaxyl.
Phytophthora root, crown, leaf, bract, and flower blight. Two species of Phytophthora, P. nicotianae Breda de Haan (=P. parasitica Dastur) and P. drechsleri have been recovered from poinsettias in greenhouses at various locations in the United States. Root, crown, and stem rot caused by P. nicotianae was first described by Engelhard and Ploetz in 1979 (27), and foliar infection caused by both P. nicotianae and P. drechsleri was first described by Yoshimura et al. in 1985 (58). Infected roots are brown and depending on the environmental conditions and the age of the plant, infection may be present for varying lengths of time before wilt or stunting is noticed. Infections at and above the soil line are characterized by purple-black lesions that may expand rapidly from stems to brack petioles causing the bracts to wilt (Fig. 25). Leaf lesions are paper-like and dry in texture, grayish brown at first, turning brown to black. Blight symptoms produced by these two species are virtually identical (58).
 Fig. 25. Wilting of flower bracts and foliage due to stem lesions caused by a Phytophthora sp. (click image for larger view). |
Control strategies that prevent initial infections are most effective in limiting the development of Phytophthora epidemics in poinsettia. These include the use of pathogen-free potting mix and strict sanitation in the production facility. The fungicides metalaxyl (Subdue) and more recently mefenoxam (Subdue Maxx) have been shown to be very effective in controlling Phytophthora blight of poinsettia (58). However, isolates of P. nicotianae resistant to the highest rate of application of metalaxyl or mefenoxam have been reported from floriculture production facilities (30, Lamour and Hausbeck, unpublished). Applying fungicide to known infected plants will increase the chance of selecting for insensitive isolates. The most effective way to halt an epidemic is to remove all plants from a production area in a facility that have been irrigated with water from the reservoir that collects recycled water from known infected plants. Even healthy looking plants may be infected so strict sanitation following an epidemic is mandatory to prevent re-occurrences.
Rhizoctonia stem rot. Stem rot caused by Rhizoctonia solani Kuhn AG-4 is the most important disease that affects poinsettia during propagation. The pathogen has a very wide host range, as well as a high competitive saprophytic ability. This combination of characteristics provides an ideal opportunity for both infection of host tissue and colonization of plant debris followed by subsequent long-term survival either as mycelium or sclerotia associated with crop debris. Growers may practice strict sanitation but often times R. solani becomes an indigenous pathogen in greenhouse production facilities as the fungus survives between crops in infested debris left behind on the bench, equipment, floor, walkways, and even in crevasses on wooden benches.
Poinsettia may be propagated in 10-unit polyfoam rooting strips, rockwool cubes, or stuck directly in a soilless potting mix in pots. Propagation of poinsettia typically occurs in the hot months of July and August in the greenhouse under conditions of almost constant leaf wetness from mist systems designed to prevent the cutting from wilting. Under these ideal conditions, Rhizoctonia stem rot develops when debris colonized by R. solani is dispersed to the rooting strip by splashing water or other means of contact. The initial symptoms of disease are small lesions that often develop at the point on the stem even with the top surface of the rooting cube. Lesions have a dry appearance with a dark border and tan center (Fig. 26). Multiple lesions may develop on a single cutting. If newly made cuttings become infected, lesions expand rapidly girdling the stem and collapsing the cutting within 5 to 7 days (Fig. 27). Cuttings that have been in propagation a week or more are more resistant to R. solani. Drooping leaves on cuttings that contact the bench surface also can be infected by Rhizoctonia harbored in residual debris. Infected areas on leaves are brown and beads of white latex from the host often appear on the infected tissue under conditions of high humidity. Infected leaves are quickly colonized and provide an avenue of entry to stems for further stem rot development. Commercial ELISA kits are available for growers and diagnosticians to detect R. solani in infected poinsettia tissue (5). Once one cutting in a rooting strip becomes infected, R. solani can use that diseased tissue as a food base to grow both on the surface and through the strip to infect other cuttings in the 10-unit strip. Sclerotia and hyphae may be visible on the surface of the rooting cube particularly on the sides exposed when the protective cover holding the strip is removed (Fig. 28).
 |  | |
| Fig. 26. Poinsettia cutting with stem lesion in leaf abscission area (left) and close up of Rhizoctonia hyphae growing out into rooting cube (right) (click image for larger view). | Fig. 27. Collapse of a poinsettia cutting with stem rot in a rooting strip. Other cuttings in strip have visible stem lesions as well (click image for larger view). |
 Fig. 28. Rooting strip removed from protective plastic cover to reveal Rhizoctonia hyphae growing through strip (click image for larger view). |
Control of stem and root rot begins with thorough removal of all crop debris from the production facility at the end of a cropping cycle. Sanitation of work area and bench surfaces with surface disinfectants is important. During propagation, misting cycles should be monitored closely to avoid over wetting foliage of cuttings once newly made cuttings become turgid. Daily scouting of propagation areas is needed so that rooting strips with any cutting showing symptoms or signs of stem rot can be removed. No rooted cuttings from a strip with an infected cutting should be transplanted since the pathogen may have grown through the foam cubes resulting in apparently healthy but infected cuttings that will not produce saleable plants.
In greenhouse production facilities with a history of Rhizoctonia stem and root rot, soaking dry rooting strips in a fungicide solution can protect cuttings from disease (4). Fungicides such as flutolanil, iprodione, and chlorothalonil actually prevented R. solani from colonizing the rooting cube (4). Fludioxonil and the new strobilurins such as azoxystrobin also are effective for stem rot control (7). Generally one application of fungicide is sufficient to protect the crop during the propagation cycle. Since some fungicides affect the number of roots formed on cuttings, growers should test specific fungicide by cultivar combinations for safety before using a product on the entire crop (6). After transplanting, fungicide drenches may be needed at regular intervals to prevent crown and root rot.
In addition to traditional fungicides, several biocontrol agents such as Burkholderia cepacia, Paecilomyces lilacinus, and binucleate Rhizoctonia spp. (BNR) have shown potential for control of Rhizoctonia stem and root rot (11). Commercial biocontrol agents mentioned above also are available, although those based on Gliocladium virens are phytotoxic to unrooted poinsettia cuttings (Benson, unpublished). In a unique study, a sequential application of B. cepacia to rooting strips followed by incorporation of a Pesta formulation of BNR into potting mix at transplanting protected poinsettia from stem rot in propagation and root and stem rot for 52 days after transplanting (38).
Conclusion
The long production season for poinsettias (from propagation in the hot months of summer to vegetative growth and then flower bract development in the shorter days and cooler months of fall and early winter) provides a wide range of environmental conditions that can foster a series of diseases. We have described the major poinsettia diseases that are widespread in the industry. A number of other less common diseases can cause significant problems for individual growers when favorable environmental conditions prevail. In addition to biotic agents, improper fertilization practices can cause symptoms in poinsettias. A convenient table summarizing the management of poinsettia diseases and nutrient deficiencies is available online from Penn State University. A description including images of physiological disorders in poinsettia due to improper fertilization can be found online from North Carolina State University. Plant pathologists will continue research and extension efforts to provide growers with the best possible disease management strategies that will help to preserve the poinsettia as the Christmas flower.Electronic Resources for Further Information
Diseases of poinsettia: Common Names of Plant Diseases
Diseases and Disorders of Poinsettia
Plant Disease Facts: Poinsettia Diseases
Paul Ecke Ranch, Technical Information Bulletin: Poinsettia Scab
Poinsettia Scab (Spot Anthracnose)
Texas Poinsettia Producers Guide Diseases and Disorders of Poinsettia
Plant Disease Facts: Poinsettia Diseases
Paul Ecke Ranch, Technical Information Bulletin: Poinsettia Scab
Poinsettia Scab (Spot Anthracnose)
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