Plant Pathology
Factsheet

Several common types of alfalfa seedling problems are described below. If you cannot establish the cause of the problem with a field visit, we recommend collecting a sample for disease diagnosis. For diagnosis, collect samples containing both plants and soil. Collect plants that are not yet completely dead, as a postmortem on dead plants is often impossible.

Seedling diseases. Seedling diseases sometimes occur during periods of high rainfall in fields with soil compaction or other causes of poor drainage. Seedling diseases of alfalfa in Kentucky are usually caused by Pythium, Phytophthora, Aphanomyces, or Rhizoctonia fungi. Aboveground, plants appear stunted and yellowed, and may be wilted. In the case of Phytophthora infection, leaves sometimes also exhibit reddening. Belowground, roots and nodules appear decayed. Be sure to determine whether Apron fungicide was used on the seed, as it provides protection against Pythium and Phytophthora. Samples of dying seedlings should be sent to one of the UK Diagnostic Labs for accurate determination of the fungi involved.

Loose Seedbed. A properly prepared seedbed should be fine but firm. If the seedbed soil is too loose, the soil around the seed can fail to provide a uniform supply of moisture to the developing seedling. A loose seedbed can be recognized by better emergence in wheel tracks.

Seeding too deeply. Seed placed lower than a half inch often emerges poorly and grows slowly. Check the depth of seed placement while on field visits.

Failure to close furrow in no-till seeding. If a no-till drill was used on a wet soil with a high clay content, the press wheel may not close the furrow completely. This open seed furrow allows seedlings to dry out very quickly during a dry spell, leading to poor stands.

Poor nodulation. Poor root nodulation may result from poor viability of inoculant or poor inoculation technique. Usually sufficient Rhizobium inoculant is present in the soil if the field has a recent history of alfalfa production. Carefully dig roots and examine whether healthy nodules (plump, with a reddish color inside) are present. If healthy nodules are lacking, emergency inoculation of the field should be considered. An easy way to do this is to use a boom sprayer to apply one pound of inoculant per acre to the soil surface. Best results can be expected if the inoculant is sprayed prior to a rain.