|
 Onion
pink root disease symptoms. (image courtesy of Tom Isakeit, Texas A&M University Plant
Pathology & Microbiology)
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Phoma terrestris E.M. Hans. (syn. = Pyrenochaeta
terrestris)
By
Jeffrey Frye
A
class project for
PP728
Soilborne
Plant Pathogens
|
 Root and basal stem symptoms of red root rot of corn.
(image courtesy of S.R. Koenning, NC
State University)
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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).
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.
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.
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).
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.
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:
Widely
Prevalent Fungi of the United States
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.
