Aphanomyces euteiches is a water mould, or oomycete, plant pathogen responsible for the disease Aphanomyces root rot. The species Aphanomyces euteiches can infect a variety of legumes. Symptoms of the disease can differ among hosts but generally include reduced root volume and function, leading to stunting and chlorotic foliage. Aphanomyces root rot is an important agricultural disease in the United States, Europe, Australia, New Zealand, and Japan. Management includes using resistant crop varieties and having good soil drainage, as well as testing soil for the pathogen to avoid infected fields.
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). Of particular concern is Aphanomyces euteiches f.sp. pisi, which is responsible for pea root rot and is the most economically damaging form of pea root rot. 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 roots 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.
Disease cycle
Aphanomyces root rot is an example of a monocyclic disease, causing only one infection cycle per season. This is in contrast to polycyclic diseases, which can infect new hosts and produce multiple disease cycles within a single season. A. euteiches is not usually a saprophyte in nature, but can be grown in culture in the lab.