DISEASES OF ALFALFA
SEEDLING BLIGHTS

The first disease typically encountered by alfalfa producers is seedling blight. Seedling blights of alfalfa are caused by the several soil-borne fungi, the most common of which are Phytophthora megasperma and Aphanomyces euteiches. Often, determining the pathogen (i.e., disease-causing organism) responsible for seedling blight is difficult, and many times both pathogens occur in the same field. P. megasperma and A. euteiches are both members of a group of fungi called the water molds, an apt name because fungi in this group love wet conditions. Thus alfalfa seedling blights tend to be more of a problem in wet soils.

In general, P. megasperma tends to be the more aggressive of the two pathogens, often attacking seeds and seedlings before they emerge from the soil. This leads to poor stands. P. megasperma can also infect after emergence, leading to seedling death (see black arrow in the photo).

In contrast, alfalfa seedling stands affected by A. euteiches tend to emerge, but plants remain stunted and are often yellow or red (see white arrow in photo). These plants tend to survive, but remain stunted throughout production, thus leading to seriously reduced yields.