Greenhouse Disease Management
Pythium Root Rot
Pythium crown and root rot are a common and persistent disease in the greenhouse industry. Pythium is a water mold that can “nibble” the feeding roots of plants, resulting in stunted growth. Pythium favors wet, saturated soil conditions, such as overwatered media. The persistence of this pathogen can be traced to its ability to “hibernate” on dirty plant containers, benches, hoses, and greenhouse walkways, ready to become activated by the right crop and weather conditions. Although Pythium can be a problem on almost any greenhouse crop, plants like geranium, poinsettia, and snapdragons are often the most affected. The three most commonly encountered species of root-rotting Pythium species in the greenhouse industry are Pythium irregulare, Pythium ultimum, and Pythium aphanidermatum.
Disease Cycle
The pathogen, Pythium, lives in water, where it will produce sporangia, the fruiting body. The sporangia will form zoospores which will swim around roots, infect, and colonize them. Zoospores are the main means of dissemination of this pathogen. Pythium will also form resting structures (chlamydospores and oospores) that will allow them to survive in pores of float trays and serve as the primary inoculum for new infection in float systems.
Symptoms
Pythium attacks juvenile tissues such as the root tip (See Figure 28.8). After gaining entrance to the root the fungus may cause a rapid, black rot of the entire primary root and may even move up into the stem tissue. As the soil dries, new roots may be produced and the plant may recover or never show symptoms of disease. Under wet conditions brought about by poor soil drainage or excess irrigation, more and more roots are killed and the plant may wilt, stop growing, or even collapse and die. Bulbs of susceptible plants turn black, gradually desiccate, and form a hard mummy.
Cultural Management Strategies
Scouting is an important first step in controlling Pythium root rot. Sanitation is especially important in limiting root rot. Conditions that favor good plant growth and minimize stress make the plant less vulnerable to attack by a root rot. Use a pressure washer with soap and water when cleaning walkways, benches, etc. and follow with a disinfestant to remove any remaining Pythium. Overwatering and over-fertilization increases rates of infection by Pythium. In addition to overwatering, the use of poorly draining medium, or the placement of pots or flats in standing water, will also affect drainage and predispose plants to infection by Pythium.
Chemical Management Strategies
If Pythium is diagnosed as the problem, choosing a fungicide that is specific for Pythium is important. However, if Pythium has a significant head start, the root system of some plants will be too rotted and the fungicide drench won’t be able to rescue them. Products effective against Pythium include Subdue MAXX (mefenoxam), FenStop (fenamidone) and Truban and Terrazole (etridiazole). If you have Pythium resistant to Subdue MAXX in your greenhouse it is recommended that Truban or Terrazole be used as they have been shown to be the most effective products in our greenhouse trials. Since Truban and Terrazole have the same active ingredient, rotating between these two fungicides is not recommended.
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