Monday, August 11, 2008

Greenhouse - Watch Out for Phytophthora in New Poinsettia Plantings

Phytophthora root and crown rot caused by P. nicotianae is a problem you may encounter with recently transplanted poinsettia cuttings. The following is more information on this disease.

The optimum conditions for this disease are saturated soil and high temperatures. The Phytophthora pathogen does not travel easily through the air for long distances. It is possible that contaminated irrigation water can introduce the fungus to new sites but it is very difficult to detect Phytophthora in irrigation water.

Prevention is the key to managing Phytophthora because the disease is difficult to suppress with fungicides once it develops. Phytophthora like Pythium is a lower fungus favored by excess moisture and excess nitrogen fertility. Unlike Pythium, species of Phytophthora are more aggressive, more likely to be host specific.

The most likely source of origin is plant material. Start with soil-less growing media and avoid contaminating growth medium with soiled hands, tools, or flats. Promptly remove diseased plants, avoid splashing water when irrigating, and keep hose ends off the floor.

The best means for controlling Phytophthora crown rot and stem canker is with drenches of systemic fungicides which will move up into crown area such as mefenoxam (Subdue Maxx), foestyl-Al (Aliette) azoxystrobin (Heritage), zoxamide plus mancozeb (Gavel), mancozeb plus copper (ManKocide), and dimethomorph (Acrobat), Check labels for crop appropriateness. These fungicides should be rotated by chemical class to prevent resistance development.

The most effective way to halt an epidemic is to remove all affected plants. Even healthy appearing plants may be infected, so strict sanitation following an outbreak is advised.

Information from the August 8, 2008 posting of the New England Greenhouse Update.

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