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Symptoms and Signs
Aphanomyces root rot (ARR) is a disease that affects both annual and perennial crop species in the legume family. Symptoms of ARR are relatively common among both annual and perennial hosts, but timing and pattern of disease occurrence often differ. Because A. euteiches is a root-infecting pathogen, primary symptoms occur on roots and subterranean stem tissues. Initially, infected root tissue appears gray and water-soaked, becoming soft and honey-brown or blackish-brown in appearance (Figure 1). Eventually roots are reduced in volume and function (Figure 2). It is common for symptoms to advance from roots into the stems, which is often typified by chlorosis (yellowing) of the cotyledons and necrosis (death and discoloration) of epicotyls or hypocotyls (Figures 2, 3, 4). Primary symptoms of roots and stems will eventually lead to secondary symptoms of chlorosis, necrosis, and wilting of the foliage (Figures 2, 3, 5). Pre-emergence damping-off (sudden death/decay of seedling) is not commonly associated with ARR. Instead, seedlings are stunted (Figure 6) and less competitive against weeds (Figure 7). Roots infected by A. euteiches commonly have decreased nodulation, which contributes to symptoms of chlorosis. In the field, the pattern (locations) of symptomatic plants frequently corresponds to areas with poor soil drainage resulting from soils with a high clay content, compaction, and/or excessive irrigation.
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Many symptoms of ARR are common among hosts of A. euteiches while other symptoms are unique to an individual host. For pea and bean, lesions progress upwards on epicotyls and hypocotyls, respectively, and extend above the soil line (Figures 2, 3). Lesions on bean hypocotyls are water-soaked, gray-green in color and are firm. However, lesions on pea epicotyls eventually turn black and the tissues will collapse, resulting in a “pinched” appearance originating above the cotyledons. For alfalfa, hypocotyls seldom express noticeable symptoms, yet cotyledons are frequently chlorotic and will ultimately become necrotic (Figure 4).
Signs of A. euteiches are not visible to the unaided eye. Sexual reproductive structures, oogonia (female gametangia) and oospores (double walled spores resulting from sexual fertilization of a gametangium within the oogonium), can be readily seen in the cortical tissues of the root and hypocotyl with the aid of a compound microscope (Figure 8). One or more antheridia (male gametangia) may also be seen surrounding the oogonia, although they are ephemeral in the host tissue. Asexual reproductive structures, sporangia (specialized hyphae bearing asexual spores), primary spores, and zoospores (motile spores) of A. euteiches typically develop on, or near, the surface of host tissues (Figure 9). Formation of sporangia can be induced by placing pieces of infected root and hypocotyl tissues into a petri dish containing sterile water. After a 3- to 5-day period of incubation, sporangia bearing primary spores can be observed with either a stereoscope or compound microscope. Primary spores are spherical and are found loosely clustered, similar to a bunch of grapes, on the tips of a sporangium (Figure 9). The arrangement of primary spores on the sporangium is a characteristic feature of the genus Aphanomyces. Primary spores give rise to bi-flagellate (two flagella; whip-like structures that aid in locomotion) zoospores, which can be observed swimming in the water. Upon emergence, zoospores are attracted to roots in response to chemical signals released from roots.
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Copyright © 2007
by The American Phytopathological Society