Aphanomyces Euteiches F.sp. Pisi - Hosts and Symptoms

Hosts and Symptoms

Hosts of Aphanomyces euteiches can be annuals or perennials in the legume family, including: pea (Pisum sativum), alfalfa (Medicago sativa), snap bean and red kidney bean (Phaseolus vulgaris), faba bean (Vicia faba), red clover (Trifolium pratense), and white clover (Trifolium repens). In North America, genetically distinct populations of A. euteiches demonstrate host specificity, but such specificity has not been observed in Europe.

Because A. euteiches is a root-infecting pathogen, primary symptoms occur on roots and stem tissue below the soil line. Infected root often appear gray and water-soaked, eventually becoming soft and honey-brown or blackish-brown in appearance. Infection causes a reduction in root volume and function, including reduced nodulation, leading to decreased water and nutrient up-take, which lead to above-ground secondary symptoms. Symptoms in the above-ground plant tissue can include chlorosis of the cotyledons and necrosis of the epicotyls and/or hypocotyls, stunting, and wilting of foliage. Some symptoms can differ among hosts. Infected plants and patterns of infection in the field often correspond to areas in the field with poor soil drainage, which can be the result of soil compaction, soil texture (high clay content), or excessive wetness due to irrigation or rainfall. Symptoms are generally similar among infected legumes, however timing and pattern of disease can differ among hosts and between annuals and perennials. In both peas and beans, lesions tend to progress up the plant tissue, starting with the epicotyls and moving to the hypocotyls, eventually extending above the soil. Lesions on pea epicotyls turn black, eventually creating a pinched region above the cotyledons as the result of pinched tissue. Lesions on beans, on the other hand, have a characteristic water-soaked appearance, are grayish-green in color, and are firm to the touch. In alfalfa symptoms include chlorotic cotyledons which may eventually become necrotic.

Although the symptoms caused by A. euteiches can be difficult to distinguish from symptoms caused by other root-infecting plant pathogens (such as Pythium, Rhizoctonia, and Fusarium), there are some distinct differences. Aphanomyces root rot rarely results in seed rot and/or pre-emergent damping-off. The characteristic lesions caused by the different pathogens also differ. Fusarium infection results in black or reddish vascular tissue, and Rhizoctonia infection results in sunken, cankerous lesions.

A. euteiches exhibits no macroscopic signs, but oogonia and oospores can be seen in root tissue with a compound microscope.