Phytophthora diseases of ornamentals

The plant pathogen Phytophthora attacks all parts of the host plants, causing root and crown rots, and foliar and stem blights. Many Phytopthora species are pathogens on hundreds of hosts, including many important ornamentals. Phytophthera infestans caused the great Irish potato famine in the 1840's, which resulted in the death of 1.5 million people, and the immigration of another million to the United States. Once established in a nursery or landscape, it can be difficult to manage.

Common diseases of ornamentals caused by Phytophthora spp. in south Florida include stem and leaf blight of English ivy; root, stem and leaf blight of pothos and peperomia; root, crown and leaf blight of spathiphyllum; stem rot of dieffenbachia; foliar blight and root rot on anthurium and mandevilla; and root and crown rot of liriope.

Symptoms and Signs
Foliar symptoms of root and crown rots resemble those of nutritional deficiencies or overwatering, and include chlorosis, wilting, and defoliation. The plant canopy fails to thrive, indicating a problem with the root system of the plant. The root system itself may be stunted, with areas of brown and reddish necrosis, especially on feeder roots. The root cortex sloughs off easily, leaving the root stele.

Phytophthora also causes stem and foliar blights. These are irregular areas of necrosis on the stems and leaves that often are black and can cover large parts of the canopy. Phytophthora foliar blight can be distinguished from Rhizoctonia blight by the absence of mycelia, although heavy production of sporangia can be seen in necrotic areas in white patches. Symptoms of stem blights and crown rots can be seen on the right. Phytophthora also causes cankers and root rots on several large tree species.

Causal Agent
Although Phytophthora resembles a fungus, producing hyphae and spores, it is actually an oomycete and not in the Kingdom Fungi at all. It belongs to the stramenopiles, a group of protists that also contains diatoms and brown algae. Other plant pathogenic oomycetes are Pythium and the downy mildews (Peronospora, Plasmopora).

Like fungi, Phytophthora has an asexual and sexual stage. The genus produces lemon-shaped asexual sporangia that can wither serve as new inoculum or produce zoospores. These zoospores have a tail, called a flagellum, that they use like a whip to propel it through a film of water. Zoospores spread disease by swimming from infected to healthy tissue. See below left for pictures of an intact sporangium and one that is expelling its zoospores.
Under certain conditions, Phytophthora will undergo sexual reproduction and produce a round, thick-walled oospore. These spores can survive harsh weather conditions and so allow pathogen survival through unfavorable seasons, making Phytophthora diseases a perennial problem.

Disease cycle and epidemiology
Phytophthora survives dry, unfavorable weather in infected tissue or soil as oospores or another thick-walled spore called a chlamydospore. These spores germinate when weather is favorable for disease development and produce sporangia that in turn produce zoospores. The zoospores swim through films of water on the host and soil and infect susceptible hosts. In general, cool, wet weather favors disease development, although some species are more active at warmer temperatures.

Disease management
Sanitation is important in managing Phytophthora disease, because once a soil is infested with the pathogen, it can be very difficult to get rid of. All nursery stock should be inspected for symptoms before planting and pathogen-free soil should be used. Disinfect used pots and tools with a 10% bleach solution. Alternatively, soil, tools and pots can be sterilized with steam treatments.

Do not over water plants, as the disease is favored by wet conditions. Overhead irrigation encourages development of foliar blight by splashing spores within water droplets, and providing films of water on the foliage that the zoospores can swim through. Keeping foliage dry can limit the development of foliar blights.

If Phytophthora is a recurring problem, plants should be regularly scouted for disease symptoms. Diseased plants should be rogued and destroyed so as not to increase inoculum loads. In areas of the landscape infested with Phytophthora, only species that are not susceptible should be planted.

Several fungicides are available for control of Phytophthora. These may be applied according to the manufacturer label as part of an integrated approach that also follows the cultural practices outlined above.