Pythium aphanidermatum (Pythium root rot of Poinsettia)

Taxonomy

Domain	Eukarya
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Peronosporales
Family	Pythiaceae
Genus	Pythium

Scientific Name

Pythium aphanidermatum

Scientific Name Synonyms

Pythium butleri

Nematosporangium aphanidermatum

Common Name

Pythium root and stem rot

Author: Emma Lookabaugh and Garrett Ridge, North Carolina State University

Pathogen

Pythium aphanidermatum is a soilborne plant pathogen belonging to the fungus-like organisms Oomycetes. Pythium aphanidermatum is fast growing and produces white, cottony mycelium on potato dextrose agar. Main hyphae lack cross-walls and measure up to 10 µm wide. Pythium species are differentiated by the morphology of their asexual and sexual structures.

Asexual: P. aphanidermatum produces terminal, inflated, lobate sporangia with hyphal swellings up to 20 µm wide. Sporangia give rise to biflagellate zoospores, with tinsel and whiplash flagella. Encysted zoospores measure approximately 12 µm in diameter.

Sexual: P. aphanidermatum is homothallic, producing both male and female gametangia. Oogonia are terminal, globose, smooth, and 20 to 25 µm (av. 23 µm) in diameter. Antheridia are mostly intercalary, sometimes terminal, broadly sac-shaped, 10 to 14 µm x 10 to 14 µm. Oospores are produced after fertilization of 1 to 2 monoclinous antheridia per oogonium. Oospores are aplerotic, 18 to 22 µm (av. 20.2 µm) in diameter, with a 1 to 2 µm thick wall.

Hosts, Signs, and Symptoms

P. aphanidermatum causes root rot and, in severe cases, stem rot of commercially produced poinsettia (Euphorbia pulcherrima Willd.). Currently, it is not known if cultivars differ in susceptibility.

Infected cuttings show brown discoloration of the roots and callus tissue. As infection progresses, water soaked-lesions may be evident at the stem base, followed by a watery rot. Typically, infected cuttings wilt and die rapidly. Infected plants that survive through transplanting often are stunted and fail to produce healthy root systems (Figure 1).

Infected roots of mature plants are dark brown in color and the outer root tissues slough off, leaving behind a bare strand of inner vascular tissue (Figure 2). Due to the compromised root systems, plants in greenhouse production wilt during the heat of day and recover at night. As disease progresses, plants fail to recover and stay permanently wilted (Figure 3). Defoliation and plant mortality follow.

Spherical oospores and lobate sporangia may be present in root cortical cells. In severe infections, cottony mycelium may be observed at the stem base.

Ecology

Depending on the greenhouse and severity of the problem, Pythium root rot can result in significant losses to the grower. Pythium aphanidermatum is present throughout the production season and can be found on cuttings and mature plants.

Pythium aphanidermatum favors warm, moist conditions (30-40⁰C; optimum range) making it particularly problematic in greenhouses. Under favorable conditions, oospores of P. aphanidermatum germinate to produce sporangia. Sporangia develop a discharge tube through which cytoplasm flows to form a membrane-covered vesicle at the tip. Inside the membrane, the cytoplasm differentiates into zoospores, which are released at maturity by disruption of the membrane. Zoospores are chemotactically attracted to root exudates and move via flagella through soil and surface water. Upon contact with root tissue, zoospores encyst, form cell walls, and initiate infection. The pathogen penetrates root epidermal cells through the production of cell-wall-degrading enzymes and kills the tissue by producing enzymes and toxins. After colonization, P. aphanidermatum sporulates by producing sporangia and/or oogonia and antheridia. Oospores are produced through fertilization and serve as the primary survival structure.

Inoculum of P. aphanidermatum may enter the production system through a number of sources including infected cuttings and contaminated potting media. P. aphanidermatum can survive as oospores, sporangia, and/or mycelium in organic debris on benches, floors, pots, and tools. Contaminated irrigation water can serve as an additional source of inoculum, particularly in ebb and flow systems or flood irrigation in which drainage times are long and pots remain in standing water for extended periods. P. aphanidermatum can also be spread by shoreflies and fungus gnats.

Geographic Distribution

Pythium aphanidermatum is cosmopolitan in greenhouses and field systems. Globally, this pathogen is present in tropical and subtropical regions and is expected to be present in greenhouses that produce poinsettia.

Management

Cultural

Pythium root rot is difficult to control once infections are established. Sanitation and prevention are key elements in reducing disease development and pathogen spread. Benches, floors, pots, tools, and equipment can be disinfested between susceptible crops with a 10% bleach solution or other commercial disinfestants. Cuttings should be taken from disease-free stock plants or obtained from reputable sources. Inspect propagative material from outside sources for symptoms of root rot. Infected stock plants can harbor inoculum between crops and should be discarded (Figure 3). Grow plants in sterilized potting media and pots. Space plants adequately to reduce humidity and the possibility for pot-to-pot spread (Figure 4). Avoid excess irrigation and the accumulation of standing water (Figure 5). Irrigation water suspected of contamination by P. aphanidermatum should be tested and treated. Sterilization (heat, oxidizing chemicals, UV radiation) and membrane filtration methods are generally very effective in eliminating Pythium from water supplies.

For other species of Pythium, factors that favor Pythium root rot development in poinsettias include excessive fertilizer levels, high soil moisture levels, and soil pH above 5.5. Little is known about factors involved with infection by Pythium aphanidermatum and disease development on poinsettia.

It is not known if poinsettia cultivars differ in resistance to Pythium root rot and host resistance currently is not used to manage disease.

Chemical

Effective chemical control of Pythium root rot can be achieved through preventative fungicide applications. Because Pythium is not a true fungus, only certain fungicides are active against it. Fungicides used to manage Pythium root rot in greenhouse systems include mefenoxam, promocarb, cyazofamid, and etridiazole. Fungicides are applied as soil drenches and multiple applications are needed for effective control. Repeated use of selective fungicides with a single mode of action can result in selection of fungicide resistant strains and reduction or loss of efficacy. Fungicide rotation is recommended to prevent the build-up of resistant strains. Resistance to mefenoxam and promocarb has been documented in Pythium species in greenhouse systems.

Diagnostic procedures

Root rot symptoms caused by Pythium aphanidermatum are virtually the same as those caused by other species of Pythium and those caused by Phytophthora. Due to differences in efficacy of certain fungicides in controlling Pythium vs. Phytophthora, diagnosis to genus is recommended.

To isolate P. aphanidermatum from plant tissue, rinse roots or affected area in tap water, blot dry with a paper towel and plate small pieces on a selective medium such as PPP or PARP. Cultures are incubated in the dark at room temperature (25⁰C is optimal). P. aphanidermatum is fast growing and characteristic colonies can be observed after 24 to 48 hours. P. aphanidermatum has non-septate hyphae, lobate sporangia, and readily produces aplerotic oospores in culture. Sporangia of P. aphanidermatum are similar to other species with lobate sporangia so examination of oospores and antheridia is required for positive identification.

Due to the difficulty of differentiating Pythium species morphologically, molecular diagnostic tools often are used. Sequencing of the ITS region of ribosomal DNA or COX region of the mitochondrial DNA are commonly used.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)