Greenhouse Diseases: Phytophthora
Phytophthora is a serious disease of floriculture crops, and it is especially difficult to control once it has become established. It is one of several organisms that cause root and crown rots. Phytophthora is closely related to Pythium, which also causes root rot.
While there are a wide variety of plants that are susceptible to Phytophthora, some are more likely to be infected than others. Poinsettia, rhododendron (azalea), fuchsia, vinca, gloxinia, lantana, African violet, begonia and ornamental peppers are among the more common hosts of the disease. There are several species of Phytophthora, and they are host-specific, meaning each species will only affect certain types of plants.
Symptoms To Watch For
Infected plants will display symptoms of wilting and overall decline, including dark lesions, known as cankers, at the crown of the plant. The cankers impede the flow of water and nutrients between the roots and leaves. When they become large enough to encircle the stem, the plant dies. Plants with soft stems, such as pansies, will pull off at the soil line when tugged gently. When the plant is removed from the pot, the roots will be brown and rotted, as opposed to the normal healthy white color. Symptoms can appear any time during the production cycle, from very young plants to finished, and the disease is also a problem in the landscape. It is important to obtain positive identification of the disease from a laboratory, since symptoms can be similar to those of other diseases.
Prevention of Phytophthora is especially important, because it is difficult to eradicate once infection begins. It produces several different types of spores that allow it to persist through a variety of environmental conditions. Oospores and chlamydospores can survive on pots, floors, media and other surfaces, while zoospores can actually swim for tiny distances (a few inches) on a film of water. Sporangia spread through the air. In addition, plants may be infected and infectious even before they show symptoms, which may not appear until the plant is stressed. High temperatures can cause a latent or mild problem to become severe. Warm, wet conditions are ideal for the development and spread of Phytophthora. Recirculating water in irrigation systems can also spread the disease.
Sanitation is of paramount importance. Remove plant debris and clean and disinfect potting surfaces, tools and pots. Sanitize the propagation area. Keep plants on benches away from contact with the ground and use potting mixes with excellent drainage and porosity. Cuttings should be inspected upon arrival for signs of disease. Remove diseased plants immediately and follow recommended protocols of preventative fungicidal sprays.
Daughtrey, Margery. Phytophthora Crown Rot on Pansy. Floricast. Greenhouse Grower. GreenhouseGrower.com/video/c:0/822/
Hausbeck, Mary and Blair Harlan. Greenhouse Disease Update for Phytophthora Root Rot. Michigan State University Extension, Department of Plant Pathology. msue.anr.msu.edu/news/greenhouse_disease_update_for_phytophthora_root_rot
Gillman, Dan. 2011. Seedling Root and Crown Rots. University of Massachussetts Amherst. http://extension.umass.edu/landscape/fact-sheets/seedling-root-and-crown-rots
Vincelli, Paul and Don Hershman. 2005. Plant Pathology Fact Sheet. Controlling Phytophthora Root Rot in Greenhouse Ornamentals. University of Kentucky Cooperative Extension Service. http://www.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-OR-H-9.pdf