Damping Off and Root Rot of Alfalfa Caused by Phytophthora and Pythium

Carol Groves, Plant Pathology, UW-Madison, Damon L. Smith, Plant Pathology, UW-Madison/Extension

Wilting of an alfalfa plant as a result of Phytophthora root rot. Photo Credit: Craig Grau

Wilting of an alfalfa plant as a result of Phytophthora root rot. Photo Credit: Craig Grau.

Pythiums are probably the most common cause of seedling blight and seed rot in alfalfa, destroying seeds before germination or killing seedlings after emergence.  Phytophthora medicaginis can affect alfalfa plants at all stages of development and is probably the most common root rot pathogen of alfalfa.  Damping off and root rots caused by Pythium and Phytophthora are most problematic in acid, poorly drained soils with a high organic matter content, or when prolonged, wet weather follows planting.  It is not uncommon for complete stand losses to occur within a few days in low areas of an infected field.

Pythium ultimum and Pythium irregulare can infect alfalfa seeds, causing seed blight or rot (pre-emergence damping off), or they can cause water soaking and eventual death of the roots and stems of seedlings as they emerge (post-emergence damping off).  Pythiums also cause damage to mature plants and are often referred to as ‘root nibblers’ because they destroy the fine roots of plants without causing noticeable decay yet leading to measurable yield reduction.  Alfalfa plants infected by Phytophthora medicaginis appear stunted, yellow or reddish purple, and may be wilted.  Infected taproots have tan to brown lesions, especially where a lateral taproot emerges.  Lesions eventually turn black while the center of the root becomes yellow.  Affected taproots are often pinched or damped-off immediately below the crown region, resulting in a ‘pencil-point’ look.  When the roots are severely rotted the plants are easily pulled from the soil.  Infected seedlings die rapidly, making it difficult to distinguish death caused by Phytophthora from that caused by Pythium.  The taproot of Phytophthora-infected plants will be rotted and collapsed at any given point below the soil surface and only the small feeder roots of Pythium-infected plants rot.

To learn more about damping off and root rot of alfalfa, and management suggestions, download a full PDF of this article HERE.

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