Damping off is caused by numerous pathogen species, predominately Pythium sp. as well as Phytophthora, Fusarium, Rhizoctonia, and other bacteria and fungi. These pathogens cause damping off diseases of seeds and seedlings and root and crown rots of older plants. Damping off is found in greenhouse plants, grain, vegetable, and fruit crops, and seedling trees in a wide range of environments. Seeds planted into infested soil and infected with a damping off pathogen become soft and rot. Pre-emergence damping off also occurs when germinating seedlings are infected with a damping off pathogen which colonizes the plant below-ground causing cell collapse and death. Post-emergence damping off occurs after seedlings have emerged. The roots or even stems at or below the soil line are infected by the fungi causing cell collapse and the plant falls over at the soil line. Significant stand losses can be attributed to damping off seedling blights in many crop species. Older plants are also infected with damping off pathogens which cause lesions on the roots and stems, but death is dependent on the susceptibility of the plant to the pathogen. Vegetables touching the soil which become infected during long wet periods may show mycelial growth on the outside of the vegetable while the inside become soft and watery.
 Forest Pests |
 Oregon State University |
 Purdue University |
 Pythium zoosporangium |
Pythium and Phytophthora species are known as ‘water molds’. They are protists meaning they do not have crosswalls in their hyphae like true fungi do, have cell walls containing cellulose instead of chitin like true fungi, and produce motile zoospores which swim in free water and infect plants. Pythium and Phytophthora species survive the winter as sporangia, oospores, or mycelia in infected roots and stems or in the soil. During prolonged cool, wet conditions, sporangia and oospores germinate forming sporangium which produce zoospores. Zoospores are able to swim from plant to plant in free water and penetrate plants directly. The number of infection cycles per season is dependent on the type of pathogen and the host. For example, damping off of soybeans caused by many Pythium species is monocyclic meaning one infection cycle per year.
Rhizoctonia and Fusarium are true fungi. Rhizoctonia survives in the soil as sclerotia or as mycelia in plant debris. Unlike the water molds, infection occurs during warm temperatures in unsaturated soils. Sandy soils and herbicides favor seedling infection in field crops. There is only one infection cycle per year. Fusarium survives in the soil, infected plant debris, or in the seed as chlamydospores, mycelia, or conidia depending on the species. Optimal temperature for germination varies with the pathogen. Root rots later in the season are favored by herbicides and mechanical and insect damage to roots.
|
Damping off symptoms include rotted seeds and seedlings, lesions on roots and stems, collapse of seedling stems near ground, plant wilt, death of young seedlings, and white mycelial growth on vegetables while rotted and watery inside.
Material contained on the Links from the page are the responsibility of the linked page's author(s).
Questions, Comments, Complaints and Complements?
This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln
E-Mail
Home Page
Copyright (C) 2008 J.E. Partridge, University of Nebraska-Lincoln. All Rights Reserved.