INTRODUCTION
Corn (Zea mays) is the most widely produced
grain in the United States. Approximately 80
million acres of land are planted to corn. In India,
where the Brown Stripe Downy Mildew (BSDM)
pathogen was first discovered, yield losses from
BSDM range from 20-70%. If BSDM were to cause
comparable damage in the U.S., it would translate
to a $4.6-16.1 billion financial loss
1.
Because of this potential impact, BSDM has been
designated a Select Agent. Early diagnosis and
expedited reporting are necessary to minimize
damage should BSDM be found in the United
States. The National Plant Diagnostic Network’s
Diagnostic Subcommittee plays an essential role
in early detection of diseases by offering Standard
Operating Procedures (SOPs) for the diagnosis,
identification, and reporting of Select Agents. This
poster summarizes the pathogen and disease
information in the SOP for brown stripe downy
mildew.
THE PATHOGEN
BSDM is caused bySclerophthora rayssiaevar. zeae,
a member of an economically important group of
organisms which includes plant pathogens such as
Phytophthora, Pythium, and other downy mildews. S.
rayssiaevar. zeaeis an obligate parasite and will not
grow on artificial media. Diagnosis depends on
microscopic examination of diseased tissue; the
pathogen is identified on the basis of its morphology.
There are no published serological or molecular
methods of detection.
SYMPTOMS
Sclerophthora rayssiaevar. zeaecauses only leaf
lesions; the pathogen is not systemic within the plant.
•In early stages of infection the leaves will show narrow
chlorotic or yellowish stripes 3-7 mm wide.
•At later stages of infection, the chlorotic stripes take on
a reddish or purple color. Chlorotic striping of maize
leaves may be due to both abiotic and biotic factors, but
few agents produce the purplish-red vein-limited striping
of leaves.
•The disease first appears on the lowermost leaves.
Severely affected leaves may be shed prematurely, but
affected leaves remain intact and do not shred, even
after severe storms.
• When disease occurs prior to flowering, seed
development is adversely affected, and early plant death
may result.
Brown Stripe Downy Mildew of Corn
1 based on 2004 annual estimated earnings of $23 billion. (2005 Corn
Annual http://www.corn.org/CRAR2005.PDF
).
The mycelium of S. rayssiaevar. zeaeis found in the leaf
mesophyll; the hyphae are irregular in shape and are
lobulate rather than tubular.
Asexual spores
•Sporangia are produced sympodially in groups of
between two and six in a basipetal succession on
sporangiophores which arise from hyphae in substomatal
cavities; sporangial production occurs superstomatally.
Sporangia measure 29-66.5 x 18.5-26 µm, are hyaline,
ovate to elliptic or cylindrical, smooth-walled, have a
projecting apex and a persistent, straight or wedge-
shaped peduncle.
•Four to eight zoospores are formed within a sporangium,
which may encyst within or outside the sporangium.
Sexual spores
• Oogonia are found scattered in the leaf mesophyll or the
substomatal cavities, but never in the vascular bundles. Melodie Putnam, Oregon State University Plant Clinic, Corvallis Oregon
EPIDEMIOLOGY
Primary inoculum comes from oospores that overwinter
in soil or plant debris, or from infected seed. Oospores
in air dried leaf tissue remain viable for four years.
Oospores or mycelium may also be present in or on the
seed, however, infected seed dried to 14 % moisture or
less and stored for four or more weeks will not be
capable of transmitting the disease.
• Secondary spread occurs after oospores produce
sporangia that are dispersed in wind and water splash.
The sporangia contain 4-8 zoospores that may initiate
new infections.
• In a favorable environment, one generation of
sporangia requires no more than 10 days to mature,
leading to rapid spread of the disease throughout a
crop.
• Moisture is the most important environmental factor
affecting disease development. Sporangia require at
least a 12-hour moisture period. Sporangia are
produced most abundantly at moderate temperatures of
22°-25° C. Losses due to S. rayssiae var. zeae are
greatest in regions of high rainfall (100-200 cm
annually) where susceptible cultivars are grown.
•Warm soil temperatures (28–32.5° C) are required for
disease development.
•Young plants are most susceptible to infection, with
susceptibility decreasing as the plants age.
Sporangia of Sclerophthora rayssiaevar. zeae. Used
with permission of Phytopathology. From: Payak, M.M.,
and B.L. Renfro. 1967. A New Downy Mildew Disease of
Maize, Phytopathology57:394-397. Sporangia (left) and
sporangium (above). Bar above = 20 microns.
Advanced symptoms of Brown Stripe Downy Mildew. Image courtesy C. De Leon. Reproduced,
with permission, from Compendium of Corn Diseases, 3
rded., 1999, American Phytopathological
Society, St. Paul, MN, USA.
DETECTION
ƒSporangia production may be stimulated by placing
chlorotic symptomatic tissue into a moist chamber and
incubating at 22°-25°C. Sporangia should be produced
within three to nine hours.
ƒNecrotic tissue will not produce sporangia.
ƒOogonia and oospores are produced in necrotic tissue
and may be visualized by clearing the leaf tissues in
2% sodium or potassium hydroxide solution at 45°-50°
C, washing in several changes of distilled water, then
staining with 0.1 % methyl blue in 50% glycerin for up
to 20 min at 45°-50°C. Oospores measure 30-37 µm in
diameter.
Sclerophthora rayssiaevar. zeaediffers from other
downy mildew pathogens of corn in the following:
ƒUnlike the BSDM organism,Sclerophthora
macrosporacauses leaf or floral malformation or
distortion. Oospores are significantly larger (45-75 µm
dia.), and develop mainly in vascular bundles or in their
parenchymatous sheath cells. Sporangia do not form
on intact leaves.
ƒPeronosclerospora phillippinensiscauses long
chlorotic streaks of a lighter color on maize, and causes
stunting when infection is early. Oospores are 15-22
µm in diameter (average ~19 µm).
ƒPeronsoclerospora sorghi systemically infects plants.
Affected leaves always have chlorosis of the base of
the leaf blade. No purpling or reddish color to affected
tissues. Oospores are 25-43 µm in diameter (average
~36 µm).
