Phytophthora Root and Stem Rot
Phytophthora Root and Stem Rot
Phytophthora root and stem rot is caused by the fungus Phytophthora
megasperma Drechs. f. sp. glycinea T. Kuan & D.C. Erwin.
The fungus naturally infects soybean and three members of Lupinus
(lupines) in the U.S. P. megasperma f. sp. glycinea exists in
races with race 2 and 3 (?) being common in Nebraska. Soybean
cultivars vary greatly in their susceptibility of the fungus based
on race. The causal organism over seasons as oospores in plant
residue in the soil. It may survive in this state for several
years, even in the absence of a soybean crop. The disease is
most common in saturated soils, especially those with a heavy
clay content. Oospores germinate, producing zoospores, when soil
becomes saturated. The optimum soil temperature for disease development
is 15oC. Zoospores are attracted to the roots where
they encyst, germinate and infect host tissue. Sporangia form
on infected roots and serve as inoculum for secondary infections.
Leaves may also become infected if soil particles containing
inoculum are windblown or splashed onto their surface and weather
conditions are humid and cloudy. Root rot may occur at any stage
of plant development but is common when flooding rains occur within
one week of planting. Cultural conditions which generally increase
the incidence and/or severity of the disease are reduced tillage,
monocropping of soybeans and excessive fertilizer application
before planting.
Symptoms
Plants may become infected at any stage of
development so a wide range of symptoms can occur with this disease.
The earliest symptoms consist of seed rot and preemergent damping-off.
Soybean cultivars range from low-tolerant to high-tolerant in
their reaction to infection by P. megasperma f. sp. glycinea.
Symptom development varies depending on reaction type of the
host. Low-tolerant seedlings yellow, wilt and die when infected.
High-tolerant seedlings exhibit stunting and root discoloration.
Older low-tolerant plants show a gradually progression of symptoms.
Plants initially show yellowing between veins on the leaves and
chlorosis of upper leaves. Affected plants then wilt with leaves
remaining attached. Taproots and lateral roots are usually destroyed
on these plants. One of the most common symptoms found on low-tolerant
plants is a brown discoloration of the stem extending from the
soil line up the plant as high as 20 cm or more (10 nodes). Older
high-tolerant plants may be stunted and chlorotic when infected.
Root may be discolored. When stem lesions occur on these plants
they are long, narrow, brown in color and occur on one side of
the plant. These lesions can be confused with those produced
by the disease "stem canker". When infections occur,
a blight which is characterized by light brown lesions yellow
margins is visible on leaflets of young plants. Lesions on older
plants are greatly restricted, a condition known as age-related
resistance. Field symptoms often develop in irregular patterns
which reflects areas of poor drainage.
Plant Health Management
Resistant Cultivars
Race-specific resistant cultivars are widely available; however, because many cultivars
are race specific it is essential that one knows the races prevalent in one's area. If one
is unsure, one should contact the local county extension office or University Extension Service.
Seed Treatment
Available fungicides labeled for seed treatment control of Phytophthora
are metalaxyl and oxadixyl (1997).
Plant in warm, well dried soil
Do not apply excessive rates of fertilizer
prior to planting
References
Nyvall, R.F. 1989. Handbook of field crop diseases,
2nd ed. Van Nostrand Reinhold. New York, NY. 817 pp.
Sinclair and P.A. Backman. 1989. Compendium
of soybean diseases. American Phytopathological Society. St. Paul,
MN. 106 pp.
Useful Links
Caveat
This description is presented for information only and no endorsement is intented
for products listed, nor criticism meant for products not mentioned. Always
consult the product
label before buying and using any fungicide.
Material contained on the Links from the page are the responsibility of the linked page's author(s).
Questions, Comments, Complaints and Complements?
This page was drafted by:
Jane Christensen,
Department of Plant Pathology, University of Nebraska-Lincoln
Disease images were provided by:
Dr. David Wysong,
Department of Plant Pathology, University of Nebraska-Lincoln
This page is maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln
jpartridge1@unl.edu
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Copyright (C)2000 J.E. Partridge, University of Nebraska-Lincoln. All Rights Reserved.
