Sorghum Diseases in India

annual regional meetings have been held so that
local scientists could report to others the disease
situations in their countries. In many cases, the
reporting scientists did not have time to identify
the pathogen with certainty. Confirmation of a
number of the reported diseases is required. In
many cases, there is no information as to the
presence of a disease (Table 2). Listing the sorghum
diseases present in southern Africa
should be completed as soon as possible. Mapping
disease distribution in relation to the
agroecological regions should be undertaken
promptly in all areas, as is now being done in
Zimbabwe. Beginning with the 1984/85 season,
a team of Zimbabwe plant-protection scientists
has surveyed and recorded pest and disease incidence
in each of its five agroecological regions,
with the view of obtaining a historic picture of
crop-protection problems. The incidence of each
disease on sorghum is assessed in each agroecological
region. This is an exemplary effort,
and the experience in conducting this survey
and learning to use its data could be of great use
to other countries. However, with a single sampling
period, the strong variations in disease incidences
between years, the limited knowledge
of the relation between disease scores and yield
losses, and possible disease interactive effects
(related perhaps to changing situational variables),
it may be difficult for me to draw, after
only 3 years, conclusions useful for extrapolation
to the future.
Importance of Plant Diseases
Plant diseases reduce grain and dry matter
yields. Certain diseases, such as grain molds and
ergot, reduce grain quality and cause health hazards
to people and cattle as they produce toxic
fungal structures and toxic exudates. In some
areas of southern Africa, it is difficult to collect
statistics on yield, but it is even more difficult to
collect loss data, and this has not been done in
the last 10 years. In some cases, therefore, I must
present only "qualified guesses.'' It is believed
that:
1. Not one of the 12 southern African countries
had to take areas out of production because of
a specific disease (Table 1).
2. The national grain yields did not show drastic
changes as a result of a specific disease.
12
3. Loss of dry matter is of some importance, because
common sorghums are not absolutely
resistant to the common leaf diseases.
4. Quality loss was limited because of the photoperiod
sensitivity; in photoperiod-sensitive
sorghums, the heads ripen during a drier part
of the season.
5. There is little data to assess the importance of
plant-disease hazards to man and animals.
King (1975) and Frederiksen (1986) estimated
sorghum yield losses in Africa due to diseases to
be from 10 to 15%. These estimates may be applicable
to sorghum-yield losses in southern Africa.
As long as agriculture is taking place in a
closed system, it is to be expected that diseases
and their hosts have a natural balance, because
of elimination of high virulence and high host
susceptibility But the agricultural systems have
opened up during the last 10 years, and the new
situation may serve to increase the importance
of diseases in the region and cause yield losses
exceeding 10 to 15%. Characteristics of the new
system include:
1. Increased communication and exchange of
germplasm between countries and within
countries. Large numbers of germplasm
lines are now available. Most national programs
have only limited facilities and few
staff members to test this germplasm. New
material tends to differ from the local germplasm,
and often gives distinctively higher
yields. It was observed, e.g., by Doggett (in
Williams et al. 1980) that new introductions
may become severely affected by diseases,
such as charcoal rot, that are unimportant on
local germplasm. ZSV1 became affected by a
sorghum virus previously not noticed in
Zambia.
Because of the superior yields of introduced
germplasm, expansion programs were
accelerated and the time until release became
less than 5 years. National facilities in many
of the countries were inadequate to test
the new material in all the country's ecological
zones before the material reached the
farmers. At the same time, farmers were finding
it easier to get "improved material."
There was improved communication throughout
much of the land, and drought relief and
other assistance programs were making it a
point to "get better seed to the farmer."