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