Impacts on Plant Pathogens - Climate Resilient Agriculture for Ensuring Food Security - page 150

Plant diseases are considered an important com-
ponent of plant and environmental health and can
be caused by infectious or biotic pathogens. Biotic
plant diseases are caused by organisms such as
fungi, bacteria, viruses, and phytoplasmas.

fact that the sociopolitical repercussions of major
epidemics go far beyond simple economic
impacts (Padmanabhan 1973 ).
Almost 100 years after the potato famine in
Ireland, another fungus, Helminthosporium ory-
zae , the cause of brown spot of rice, precipitated
another catastrophe in Bengal (now part of India
and Bangladesh). In 1943 the weather conditions
were exactly right to encourage an epidemic of
the disease. Losses were extreme, often rising to
90 % or causing total destruction of the rice crop.
Malnutrition and starvation caused the death of
over 2 million people.
The southern corn leaf blight epidemic of 1970
and 1971 was the most dramatic epidemic in the
history of agriculture in the USA. Just as genetic
uniformity of the potato crop in Ireland, together
with the spread of a virulent pathogen, led to the
Irish potato famine in the last century, a similar
combination of events brought about the southern
corn leaf blight epidemics of 1970 and 1971. Crop
production losses were even greater, but, since they
occurred in the USA where the agricultural indus-
try is highly diversifi ed, human suffering occurred.
The grayish black rot caused by a fungus
( Helminthosporium maydis ) was found in
October 1969 on corn ears and stalks samples
from a seed fi eld in Iowa. The following year, the
epidemic struck. The disease fi rst occurred in
Mississippi in May of 1970 and rapidly spread
northward through the Midwest on the air cur-
rents of a tropical storm in the Gulf of Mexico.
Because 85 % of the corn was susceptible to the
pathogen and the weather conditions were favor-
able for pathogen reproduction and dispersal, a
dramatic epidemic occurred across the Corn Belt
within 2 months, causing a 15 % decrease in
national corn yields. The disease was most severe
in the Midwest and south of the USA, with some
areas reporting 50-100 % losses. For the nation
as a whole, losses were offi cially estimated at the
time as $1.09 billion. Although genetic unifor-
mity in the corn crop contributed to the wide-
spread occurrence of this disease, favorable
meteorological conditions allowed it to occur.
Afl atoxin, a compound that lowers corn
quality, is related to drought conditions. The
concentration of afl atoxin is raised during crop-

8.1 Crop Losses

Agricultural trends are infl uencing the incidence
and importance of plant pathogens. First, the
expansion of worldwide trade in food and plant
products is spreading the impact of diseases.
Second, changes in cultural techniques, particu-
larly intensifi cation of cropping, reduction in
crop rotations, and increase in monocultures,
encourages the activity of pathogens.
In extreme cases, pathogen damage can lead
to severe impacts on society. In such cases, the
climate conditions are conducive to widespread
pathogen epidemics. The late blight of potato,
caused by the fungus Phytophthora infestans ,
was a major factor in the Irish famine of the
1840s. Genetic uniformity of the potatoes was
also a contributing factor. Late blight is still one
of the most important diseases of potato and its
epidemics continue to be highly correlated to
weather conditions during sporulation. This dis-
ease presents a threat in the USA today.
Plant diseases are signifi cant constraints to the
production of some 25 crops that stand between
the rapidly expanding world population and star-
vation (Wittwer 1995 ). Worldwide losses from
diseases range from 9 to 16 % in rice, wheat, bar-
ley, maize, potato, soybean, cotton, and coffee,
and in the USA alone, fungicides worth over
US$5 billion are used to control diseases (Oerke
et al. 1994 ). In Australia, diseases cost an esti-
mated Au$1.3 billion annually in the six major
agricultural commodities which are worth over
Au$10.9 billion (Chakraborty et al. 1998 ). The
economic impact of disease stems from losses in
productivity, the cost of disease management,
and the economic penalty paid for having to grow
less profi table alternative crops. Diseases such as
Panama wilt have resulted in the abandonment of
entire banana plantations in Central America.
The Irish potato blight (1845-1846) and the