Page 212 - Encyclopedia Of World History

562 berkshire encyclopedia of world history

Ants and Black Pod
Disease in Africa

Black pod disease (Phytophthora pod rot) is a
serious disease in Africa where it damages the eco- Dwarf Virus (MCDV). The initial confusion among sci-
nomically valuable cocoa plant.The role of ants in entists slowed their response, opening the entire Midwest
spreading the disease was first noted in 1927 and and South to the danger of an epidemic.
described in detail in 1970. The method of virus transmission saved corn growers.
A species each of aphid and leafhopper transmits MDMV
A small black ant also has been implicated as an
and MCDV respectively by bite. Both feed primarily on
agent in bringing Phytophthora from the soil to
Johnsongrass that grows along the Ohio and Mississippi
pods on which it attends to scale insects. Two
rivers. Both viruses inhabit Johnsongrass, as the ergot fun-
trees with no black pod, at the margins of a
gus inhabits the barberry bush, without signs of disease.
cocoa plot had pods above the ground with
But neither insect is a strong flier, and, unlike the leafhop-
scales on the pod stalk being cared for by the
per that carries Rice DwarfVirus, neither aphid nor leaf-
ants.Two sporulating diseased pods were placed
hopper retains MDMV and MCDV in virulent form more
at the base of each tree and covered with litter.
than forty-five minutes, limiting the range of both viruses.
Two weeks later all seven pods (three on one tree,
Once scientists had fingered the aphid, leafhopper, and
four on the other tree) had the black-pod disease
Johnsongrass as culprits, the U.S. Department of Agri-
starting at the petiole side where the ants had
culture in fall 1964 launched a campaign to kill aphids
built tents made of soil and debris to protect the
and leafhoppers by insecticide and Johnsongrass by her-
scales from the rain. Pods at the same height but
bicide (chemicals unavailable to save the Japanese from
without the ant on three adjacent trees remained
famine in 1733). The expenditure of chemicals and
healthy and free of the disease.
money ended the threat of these viruses and led scientists
Source: Gorenz,A. M. (1969). Spread of disease from the tree base to pods in the to believe they held the upper hand against corn diseases.
canopy. Annual Report of the Cocoa Research Institute of Nigeria (p. 53).
Success prevented all but a few scientists from question-
ing the wisdom of growing genetically uniform corn
throughout the Midwest and South.
The Corn Viruses
Since the 1920s, corn breeders have reduced the genetic Southern Corn Leaf Blight
diversity by breeding a small number of high-yielding Catastrophe struck in 1970 as Southern Corn Leaf
corn plants of roughly uniform genotypes, making corn Blight, a fungal disease, swept the United States, destroy-
vulnerable to epidemics. An outbreak of corn stunt dis- ing 710 million bushels of corn, 15 percent of the corn
ease along the lower Mississippi valley in 1945, reminis- crop that year. From Texas to Georgia and Florida, farm-
cent of the stunt disease that ravaged rice in Asia, ers lost half their corn crop.These losses cost farmers $1
presaged corn’s vulnerability to an epidemic.As is true of billion and the collapse of farm commodity prices cost
rice stunt, a virus causes corn stunt disease and is spread investors billions more. In a single summer, one corn fun-
by an insect, in this case a species of aphid. gus threatened financial ruin.
Worse was to follow. A few stunted corn plants in Plant pathologists identified a single female parent of
Portsmouth, Ohio, erupted in an epidemic that engulfed corn (a type of corn that produced no pollen and so was
the Ohio and Mississippi valleys in 1963 and 1964, cost- male sterile) as susceptible to Southern Corn Leaf Blight.
ing farmers who had planted on some lands along these By dropping it from the corn pedigree, agronomists bred
rivers their entire corn crop.The culprit was not the corn- new varieties of corn resistant to South Corn Leaf Blight,
stunt virus as scientists first thought but two viruses: but corn remains as genetically uniform today as it was
Maize Dwarf Mosaic Virus (MDMV) and Maize Chlorotic in 1970.