Aphanomyces cochlioides Drechsler, causal agent of root rot of spinach, root rot of feather cockscomb, and root rot and damping off of sugar beet, is an oomycete plant pathogen with a facultative necrotrophic growth habit. It was first identified as the cause of “black root” of sugar beet in Michigan in 1929 (Drechsler, 1929) and is an important concern as it can cause disease in storage facilities of susceptible host crops as well as in living field plants (Campbell and Klotz, 2006).
Taxonomy : Kingdom Chromista Phylum Oomycota Class Oomycetes Order Saprolegniales
Family Leptolegniaceae Genus Aphanomyces Species A. cochlioides
Host range and distribution:
Aphanomyces cochlioides infects the roots of sugar beet, spinach, cockscomb, and other species of Chenopodiaceae and Amaranthaceae, as well as some non-crop hosts such as lambsquarter, carpet weed, fireweed, and pigweed (Harveson, 2007). Although the host range of this pathogen is limited, it is well-recognized worldwide, especially in beet production areas in North America, Europe, and Chile. In the United States, it frequents southern Minnesota and the Red River Valley of North Dakota and Minnesota, and is less frequent in the more western states. In the last decade, Aphanomyces cochlioides has been found distributed throughout western Nebraska and other areas of the Central High Plains as a component of an important root disease complex including Rhizoctonia root rot and rhizomania (Harveson, 2007).
Common Crop Hosts of Aphanomyces cochlioides:
sugar beet (photo 1) spinach (photo 2) cockscomb (photo 3)
Alternative Weed Hosts of Aphanomyces cochlioides:
carpetweed (p4) lambsquarter (p5) fireweed (p6) pigweed (p7)
A selective medium has been developed for isolating Aphanomyces cochlioides directly from infected seedlings (Chikuo and Sugimoto, 1985). Small portions of tissue including symptomatic lesions should be excised from the hypocotyls and rinsed in sterilized water for 1 minute before plating on the media. The medium consists of the following ingredients per 1L water: 20g corn meal, 20g agar, 50mg metalaxyl, 50mg thiophanate-methyl, 50 mg chloramphenicol, and 5mg iprodione. This media is an excellent media to use for isolating the pathogen from host plant tissue because A. cochlioides exhibits good hyphal growth but there is no development of associating pathogens such as Pythium spp. or Rhizoctonia solani.
As a result of the limited host range of Aphanomyces cochliodes, pathogen identification is usually based on host specificity but microscopic characteristics can also be useful, especially in culture. The hyphae of A.cochlioides is 3-9 µm in diameter and is nonseptate and hyaline. Within and on media, the colony growth is delicate, sparse, and wandering in direction, characteristics useful in distinguishing it from other damping off pathogens (Windels, 2000).
Unlike seedling diseases caused by Rhizoctonia or Pythium spp., cotelydons infected with A. cochlioides rarely wilt prior to death. Another diagnostic symptom of A. cochlioides infection in contrast with Rhizoctonia root rot is non-permanent wilting (Harveson, 2007). Petioles in infected seedlings also exhibit a bowed condition, unique to infection by Aphanomyces cochlioides.
|Seedlings infected by A. cochlioides showing characteristic non-wilting and bowed condition. (Harveson, 2007).|
Seedlings attacked by the A. cochlioides exhibit symptoms commonly referred to as “black root”. Post-emergence damping off occurs and grayish, water-soaked lesions form on the stems at the soil line. As the disease progresses, the lesions turn from gray to black and the fragile, thready stem becomes highly susceptible to breakage due to high winds and other factors; some dessicated seedling can even blow away. Seedling infection occurs optimally when soil temperatures are below 60º F (Windels and Lamey, 1998).
|Young sugar beet host
plants infected with A. cochlioides showing thin, delicate stems in the
field in Nebraska.
Inset shows breakage as a result of high winds (Harveson, 2007).
If seedlings infected by the pathogen survive and the soil remains dry, adult roots will be malformed and scarred, resulting in stunted growth and yield loss ultimately (Windels and Lamey, 1998).
(A) Scarred and Deformed Adult Sugar Beets as Result of A. cochlioides.
(B) Yield Loss Due to A. cochlioides (Harveson, 2007).
When Aphanomyces cochlioides attacks older plants, the primary symptoms are foliar, consisting of stunted and yellow leaves with non-vigorous growth. Leaves may also become brittle and appear scorched, and while wilting occurs during the day, at night plants often recover. Root symptoms begin on the tap root with yellowish-brown water-soaked lesions that progress to become dry and necrotic resulting in scarring if infection halts. However, if the infection continues, the lesions extend into the interior root tissues, causing yellowish-brown discoloration. In severe disease infections, A. cochlioides can completely necrotize the taproot, leaving only the crown untouched (Windels and Lamey, 1998).
Foliar Symptoms Caused by A. cochlioides on Older Sugar Beet Plants in the Field in Nebraska (Harveson, 2007).
Ecology and life cycle:
Aphanomyces cochlioides is a “water-mold” oomycete pathogen and survives in the soil as oospores 16-24µm in size; large amounts of oospores are produced in root tissues. The pathogen is homothallic and a small number of these oospores produce sporangia at any time. Although oospores themselves can directly infect roots, most infections occur as the result of sporangia formation and the release of zoospores (Jacobsen, 2006).
|Sugar beet feeder root
infected with darkly stained oospores of Aphanomyces
Zoospores travel via water through saturated soils to infection courts on the host plant; exudates of the susceptible host in the rhizosphere serve as chemotaxic attractants for the zoospores (Tofazzal and Taharat, 2001). Warm, wet conditions are required for sporangia formation, zoospore release, and germination, with an optimal temperature of approximately 78º F. Once these conditions are met, oospores can germinate and produce hyphae that directly infect the host plant, or zoospores can swim through the soil to the roots of the host plant where they encyst, germinate, and infect. Zoospores produced by sporangia formed on diseased roots can also cause secondary infections (Jacobsen, 2006). The pathogen can also survive between crops of suitable host plants on weed hosts, which can also increase inoculum level in fields where crop rotation is practiced. Disease caused by Aphanomyces cochlioides occurs frequently in parts of fields that remain wet; near drainage ditches, low-lying spots, on hillsides, or in areas where the soil is highly compacted. Soil texture is also important in terms of disease development; while disease may develop in light-texture soils, it is favored in heavy-textured soils, as they tend to retain water (Windels, 2000).
(A) Aphanomyces cochlioides zoospores Chemotactically Attracted to Sugar Beet Root Tip.
(B) Aphanomyces cochlioides zoospores Aggregated on Root Tip (arrow). (Tofazzal and Taharat, 2001).
Campbell, L.G. and Klotz, K.L. 2006. Postharvest Storage Losses Associated with Aphanomyces Root Rot
in Sugarbeet. Online. Journal of Sugar Beet Research Oct-Dec.
Chikuo, Y. and Sugimoto, T. 1985. A selective medium for isolation of Aphanomyces cochlioides from
infected sugar beet seedlings. Annual Review of Phytopathological Society of Japan 51: 16-21.
Drechsler, C. 1929. The beet water mold and several related root parasites. Journal of Agricultural
Research 38: 309-361.
Harveson, R. M. 2007. Aphanomyces Root Rot of Sugar Beet.
Jacobsen, B.J. 2006. Root Rot Diseases in Sugar Beet. Zbornik Matice srpske za prirodne nauke 110:9-19.
Tofazzal, I.M. and Taharat, S. 2001. Chemotaxis of fungal zoospores, with special reference to
Aphanomyces cochlioides. Bioscience, Biotechnology, and Biochemistry 65: 1933-1948.
Windels, C.E. and Lamey, H.A. 1998. Identification and Control of Seedling Diseases , Root Rot, and
Rhizomania on Sugarbeet. http://www.ag.ndsu.edu/pubs/plantsci/rowcrops/pp1142-1.htm
Windels, C.E. 2000. Aphanomyces Root Rot on Sugar Beet. Online. Plant Health Progress 10.