Downy Mildew
Sclerospora graminicola (Sacc.) Schroet.
Symptoms: Symptoms often vary as a result of systemic
infection. Leaf symptoms begin as chlorosis at the base, and successively
higher leaves show progressively greater chlorosis. Infected chlorotic
leaf areas can support abundant white asexual sporulation on the
lower leaf surface. Severely infected plants are generally stunted
and do not produce panicles. Green ear symptoms result from transformation
of floral parts into leafy structures.
Pathogen and disease characteristics: Asexual sporangia
are produced during the night under moderate temperatures and
high humidity. Optimum sporangium production occurs at 20 °C.
No sporulation below 70 percent relative humidity. Sporangia germinate
to liberate 1 to 12 zoospores, which encyst and germinate by germtube.
Sporangia generally do not remain viable very long after daybreak.
Sexual oospores are thick-walled, spherical, brownish yellow,
and 22 to 35 µm in diameter. Oospores form in colonized
plant tissue and can survive from 8 months to 13 years under laboratory
conditions.
Host range: Pearl millet. Host specificity is important
in determing host range for this pathogen. S. graminicola
has been reported from maize, sorghum, Echinochloa crus-galli,
Panicum miliaceum, Pennisetum leonis, P. spicatum,
Setaria italica, S. lutescens, S. verticillata,
S. viridis, S. magna, Euchlaena maxicana,
and Agrostis alba. Cross-inoculation studies to different
hosts have usually been unsuccessful when attempted.
Geographic distribution: On page 2 of the primary citation,
a reference to a 1884 publication by Farlow indicates that S.
graminicola has been identified in the United States on "other
millets." On page 3, the continental U.S. is included in
the geographic distribution of the pathogen. Despite this information,
THIS PATHOGEN HAS NOT BEEN REPORTED ON PEARLY MILLET
IN THE UNITED STATES, AND ALL EFFORTS TO RESTRICT ITS ENTRY SHOULD
BE CONTINUED.
Sclerospora graminicola has been reported on pearl millet
in the countries listed below:
Africa: Chad, Egypt, Gambia, Malawi, Mozambique, Niger, Nigeria,
Senegal, South Africa, Zimbabwe. Also Burkina Faso, Ghana, Ivory
Coast, Kenya, Mali, Sudan, Tanzania, Togo, Uganda, Zambia (S.B.
King, personal communication, 1995).
Asia: India, Pakistan. Also Israel (S.B. King, personal communication,
1995).
Nomenclature discrepancies: Synonym: Scleropthora
macrospora (for example: Manivasakam
et al. 1986, Mangath 1986).
Alternative disease name: Green ear disease.
Seed transmission: Evidence for transmission by seed
is inconsistent and controversial. It has been suggested that
this disease can be transmitted by oospores on the seed surface.
To prevent introduction of S. graminicola, seed treatment
is recommended.
Primary citation: Singh et al.
1993.
United States
Department of Agriculture
Agricultural
Research Service
The material on this page is in the public
domain.
Original posting: June 5, 1999.
|
previous page