Published Date: 1997-04-29 23:50:00
Subject: PRO/PL> Sorghum ergot (03)
Archive Number: 19970429.0868

SORGHUM ERGOT (03)
==================
[see: Sorghum ergot - Brazil, Argentina, Bolivia, Colombia,
Australia 970228151822
Date: 1996
From: Sorghum Improvement Conference of North America (SICNA)

Introduction
------------
Ergot is a serious disease of sorghum that affects the production
of F1 hybrid seeds, particularly if 'nicking' is poor or seed-set
is delayed in male sterile lines. Damage in commercial grain
crops is often significant in pollen-limiting environments and in
forage sorghums. A disease of the ovary, ergot reduces grain
yield because infected flowers do not produce grain. The disease
lowers grain/seed quality making threshing difficult, reduces
germination and seedling emergence, and predisposes seedlings to
other diseases. Ergot has implications for quarantine and the
seed trade because seed harvested from infected fields is often
rejected. The pathogen has extraordinary capacity to spread
rapidly. In Brazil, an epidemic of the disease in 1995 covered
800,000 km2 in a week, and in Queensland it spread over 60,000
km2 in 3 weeks in 1996.

Distribution
------------
First reported in India in 1917 and in Kenya in 1924, sorghum
ergot was initially restricted to Asia and Africa. In 1995, a
widespread ergot epidemic caused serious losses in Brazil, and it
is now known to occur in Argentina, Bolivia, Colombia, and
Paraguay. In spite of strict quarantine precautions, in April
1996, the disease was first noticed in Australia where it is now
endemic throughout Queensland. The disease has not yet been
reported in the USA.

Symptoms
--------
Ergot only attacks unfertilized ovaries. Few or all flowers in
an inflorescence may be infected. The most obvious external sign
of the disease is the exudation from the infected flowers, of
honeydew, a thin-to-viscous, sweet, sticky fluid that gives the
name 'sugary' or 'honeydew' disease to the malady.
The ergot pathogen is a fungus that infects the ovary, following
nearly the same path as the pollen takes for fertilization.
Normally, pollen requires only a few hours for fertilization
while it takes 2-3 days for the fungus to colonize the ovary.
Once fertilized, an ovary can usually resist infection. Thus,
flowers are susceptible when their stigmas become receptive, and
not after their ovaries are fertilized.
The ovary is infected much before the initiation of honeydew
exudation. In fact, the earliest symptoms of infection can be
seen on the ovary if flowers are dissected 3-4 days after
infection. The infected ovary appears dull green and smaller or
larger than the healthy, fertilized ovary which is dark green and
round.
Superficial, white mycelial growth initially appears at the basal
end of the ovary and extends upward as the pathogen colonizes
ovary tissues both internally and externally. Finally, the
complete ovary is converted into a white, fungal mass, or
sphacelium, that is visible between the glumes. Honeydew
exudation then begins.
Newly formed honeydew droplets are colorless and transparent, and
become progressively opaque. Honeydew can be uniformly yellow-
brown to pink, or superficially matt white. Continued production
of honeydew causes droplets to lengthen, smearing seeds and
leaves, and falling to the ground. When infection is severe,
affected panicles can be recognized from a distance. They may be
white with fresh honeydew, or black if the honeydew is
saprophytically colonized.
During wet or humid periods at relative humidities above 90%, the
ergot fungus produces secondary conidia on the surface of the
honeydew which appear as a white scum or powdery growth. This
white growth covers the surface of the honeydew wherever it is
present including the panicle, leaves, and soil. If moist
conditions persist, several saprophytes grow on the honeydew. One
of these, _Cerebella_ sp. produces a large, black convoluted
matted mass that conceals the sphacelium. If conditions are warm
and dry after the honeydew is formed, it desiccates, forming a
brittle, hard, white crust on panicles and leaf surfaces.
Under warm conditions, sphacelia gradually harden to form solid
dense sclerotia. But in moist conditions, the sphacelia shrivel,
become fibrous, and fail to develop into sclerotia.

Causal organisms
----------------
Ergot can be caused by two fungal pathogens: Indian and African.
The sexual stage of the Indian pathogen is _Claviceps sorghi_
while that of the African pathogen is _Claviceps africana_. The
asexual stage of both fungi is _Sphacelia sorghi_. The alkaloid
content, and to a less extent, the morphology, of the sclerotia
determine the identity of the two species.
Evidence to date suggest that the sclerotia-contaminated sorghum
grain has little, if any, implication for animal health. On the
contrary, honeydew-smeared leaves are more palatable to animals,
but further studies are required to confirm these observations.
Caution! Ergot is not the only cause of honeydew exudation in
sorghum. Insects, such as aphids, also secrete sticky honeydew
that is often interspersed with the white molts of the insects.
Leaves can also exude honeydew at temperatures below 20C due to a
physiological disorder called 'leaf sugary disease'. Honeydew
from insects and leaf sugary disease do not contain spores of the
ergot fungus.

Disease cycle
-------------
Primary infection in the field is possibly established by
ascospores from germinating sclerotia, sphacelial conidia, and
conidia from collateral grass weed hosts (e.g., _Cenchrus_ spp.,
_Panicum_ spp. _Ischaemum pilosum_, _Dicanthium_ spp.), wild
sorghums, volunteer plants, and infected panicle debris in soil.
After infection, millions of conidia are released in the
honeydew, and are spread by rain splash and probably by insects.
Secondary conidia are spread by wind and cause new infections
both near to and far from the initial infections. The pathogen
can also be dispersed on clothing, footwear, and farm implements.
Several cycles of infection can occur in the same growing season
if susceptible sorghum hosts are available.
Survival between seasons can be via infected panicles left in the
field after harvesting, or as sclerotia mixed with seed during
threshing and seed processing. Conidia remain viable in panicle
debris for at least 9 months, whilst sclerotia can survive for
several years.

Beware of conditions that favor the disease
-------------------------------------------
Factors that ensure rapid fertilization reduce the chances of
ergot attack. Factors that increase the time between stigma
emergence and fertilization enhances the risk and severity of
ergot. Such factors include:
* Poor 'nicking' in seed-production plots (the better hybrid seed
parents 'nick', the lower their chances of being infected)
* Low night temperatures (<12C) during the 3-4 weeks before
flowering, and 5 days after flowering (induces pollen sterility,
and slows pollen tube growth)
* High relative humidity (>90%), cloudiness, and/or wetness
following stigma emergence
These factors not only favor infection, they also reduce anther
emergence, anther dehiscence, and pollen deposition.
* After honeydew production, high relative humidity (>90%) and/or
wetness favor the production of secondary conidia which are
efficiently spread by wind.
Control
-------
Several options can be integrated:
* Enforce strict quarantine measures (effective in keeping the
disease out of the USA to date)
* Sow seed produced in ergot-free areas (to reduce the risk of
infection by eliminating primary inoculum)
* Alter sowing dates to allow flowering when environmental
conditions do not favor disease development (make it possible for
crops to escape the disease)
* Protect seed production plots with a triazole fungicide sprayed
3-4 times at 5-7 day intervals, starting before stigma emergence
What should you do if you observe ergot?
----------------------------------------
* If sorghum ergot is suspected in a region where the disease has
not yet been reported, contact a sorghum worker who can have the
disease confirmed by a plant pathologist (e.g., in the USA, a
county extension agent or G. Odvody, 512-265-9201), or contact a
plant quarantine official (e.g., USDA-APHIS, Q. Kubicek, 301-734-
7601 or R. Bech, 301-734-5215).
* Send specimens to be identified in closed containers. This
probably does not pose a danger of transmission to new areas, but
check with recipients before sending specimens through the mail.
* After leaving a suspect field, launder or change clothes and
shoes before entering other fields. Dried honeydew sticking to
such apparel could provide for long-term survival and spread of
the conidia contained within it.
--
Sorghum Improvement Conference of North America (SICNA)
P.O. Box 530
Abernathy, Texas 79311
USA
International Crops Research Institute for the Semi-Arid Tropics
(ICRISAT)
Patancheru 502 324
Andhra Pradesh
INDIA
The International Sorghum and Millets Newsletter (ISMN) is
published annually by the Sorghum Improvement Conference of North
America (SICNA) and the International Crops Research Institue for
the Semi-Arid Tropics (ICRISAT). It is intended as a worldwide
communication link for all those who are interested in the
research and development of sorghum (_Sorghum bicolor_ (L.)
Moench), pearl millet (_Pennisetum glaucum_ (L.) R. Br.), and
finger millet (_Eleusine coracana_ (L.) Gaertn.), and their wild
relatives.
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