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            image of pink root of onionOnion pink root disease symptoms. (image courtesy of Tom Isakeit, Texas A&M University Plant Pathology & Microbiology)                                               

   Phoma terrestris  E.M. Hans.

(syn. = Pyrenochaeta terrestris)

By Jeffrey Frye

A class project for

PP728 Soilborne Plant Pathogens
North Carolina State University
Department of Plant Pathology

                                               

 

 

 

                                        

                 symtoms of red root rot on cornRoot and basal stem symptoms of red root rot of corn.  (image courtesy of S.R. Koenning, NC State University)                                                     
 

Introduction:

Pink root of onion (Allium cepa) and red root rot of corn (Zea mays) are caused by the soilborne pathogen Phoma terrestris.  Historically the greatest economical impact has been with pink rot of onion worldwide, but since the mid 1980’s there has been an increase in prevalence of the red root rot disease in the Delmarva (Deleware, Maryland and Virginia peninsula) corn producing areas (3).  Campbell et. al. first reported red root rot in the Delmarva Peninsula in 1991 where yield losses of 15-20% were observed (3).  The first report of the pathogen causing disease on maize in North Carolina occurred in a field in Hyde County in 2007 (5).

Host Range and Distribution:

Phoma terrestris is reported as a widespread saprophyte and a weak parasite of many hosts.  The most devastating losses from this pathogen in many cases are caused from a disease complex with species of Fusarium, Pithium, Rhizoctonia and Helminthosporium (7).  P. terrestris has been successfully isolated from hosts of 45 genera which include many of the cereals, vegetables and grasses.  In 1941 it was reported to survive on soybean, pea, cane, millet, oats, barley, wheat, corn, squash, cucumber, cantaloupe, muskmelon, tomato, pepper eggplant, cauliflower, carrot, spinach, and onion with reports of high disease to no disease damage (6).  The pathogen is well adapted to sub-temperate, temperate and tropical climates due to its ability to survive well in many soil types and a wide range of temperature and pH.  Its presence in the United States covers many differing environments.

Isolation:

P. terrestris is readily isolated from host tissue.  Symptomatic tissue can cut into 2-3mm pieces and surface sterilized with a 70% ethanol solution for 30 seconds.  The host tissue can then be plated onto PDA (potato dextrose agar) and allowed to grow for 5 days at ambient temperature in the dark.  A pinkish-red coloration is indicative of the pathogen.  Mycelial tips can then be removed and transferred to a sterile petri-dish containing PDA.

Identification:

As noted by the name of the diseases caused by P. terrestris it forms a pink to red pigment (phomazarin) that can be seen on infected tissue.  Mycelium are hyaline, septate and anastomosing.  Pycnidia are dark brown to black, subglobose, ostiolate, and occur singly.  Conidia are continuous, oblong to ovoid and sessile in pycnidia.  The pathogen presumably overwinters as dark, thick walled, multicellular microsclerotia in diseased roots of the host that initiate from intercalary hyphal swellings (4,2,1). 

Symptoms:

Below ground symptoms associated with most hosts are a pink to deep carmine root and basal stalk rot, leading to necrosis and a reduction in total root mass.  Foliar symptoms on corn are associated with a grayish green discoloration of the stalks and leaves which rapidly wilt and die.  Red root rot of corn is generally a late-season disease that occurs during the ear-fill stage that results in rapid premature senescence and severe yield losses.  Foliar symptoms on onion may occur as a wilting of the leaves  but  the plant is rarely killed, rather the bulb becomes never matures and becomes soft and undesirable.

Ecology and Life Cycle:

It is presumed that the pathogen overwinters as microscleriotia in the soil for many years in the absence of a host.  These structures also serve as the primary inoculum, importance of the pycnidial stage has not been investigated thoroughly and no sexual stage has been found.  Hyphal infection occurs most vigorously at 25-28C.  Root epidermal and cortical tissues are invaded and hyphal penetration occurs through a release of enzymes to break down host tissue.  The disease is enhanced in the case of red root rot of corn, by other fungal pathogens such as Pythium spp. which attack the plant first and weaken the root system making it easier for P. terrestris to invade the roots more effectively (4).

Links to other sites:

Invasive Species 

Widely Prevalent Fungi of the United States

Report on Plant Disease. Onion Pink Root.

References:

1.  
Biles, C. L., Holland, M., Ulloa-Godinez, M., Clason, D., and Corgan, J. 1992. Pyrenochaeta terrestris microsclerotia production and pigmentation on onion roots. HortScience. 27:1213-1216.

2.
 Brown, W. M. 1999. Red root rot more common on corn this fall. Pest Alert. 16(22).

3.
 Campbell, K. W., Carroll, R. B., Veitenheimer, E. E., Hawk, J. A., and Whittington, D. P. 1991. Red root rot, a new disease of maize in the Delmarva Peninsula. Plant Dis. 75:1186.

4.
 Carroll, R. B. 1999. Compendium of Corn Diseases. 3rd Edition. APS Press. p.14.

5.
 Koenning, S. R., Frye, J. W., Patakay, J. K., Gibbs, M., and Cotton, D.  2007. First report of Phoma terrestris causing red root rot on corn (Zea mays) in North Carolina. Plant Dis. 91:1054.

6.  Kreutzer, W. A. 1941. Host-parasite relationships in pink root of Allium cepa L. II. The action of Phoma terrestris on Allium cepa and other hosts. Phytopathology 31:907-915

7.
 Mao, W., Carroll, R. B., and Whittington D. P. 1998. Association of  Phoma terrestris, Pythium irregulare and Fusarium acuminatum in causing red root rot of corn. Plant Dis. 82:337-342.