Agriculture Reference
In-Depth Information
Table 8.3
Major crop diseases that appeared after 1970 in the USA
Crop
Disease causal agent
Expansion
Soybean
Sudden death syndrome,
Fusarium
solani
f sp.
glycines
Appeared in Arkansas in 1971 and has spread to the
northern soybean region as far as Ontario
Southern stem canker,
Diaporthe
phaseolorum
First observed in 1973, has developed into a devastating
disease in the southern production region
Sclerotinia stem rot,
Sclerotinia
sclerotiorum
Reemerged as a leading disease in 1990 in the north central
soybean regions
Soybean cyst nematode,
Heterodera
glycines
Expanded to northern soybean regions
Corn
Gray leaf spot,
Cercospora zeae-maydis
First reported in the 1940s, became a concern in the 1970s in
the eastern states, and now is a major concern in the Corn Belt
Potato
Late blight,
Phytophthora infestans
Reemerged in 1990 as a new threat to potato production
after a new mating type was found in Mexico
Powdery scab,
Spongospora subterranea
Increased damage in Washington and Oregon
Rice
Sheath blight,
Rhizoctonia solani
Major rice disease worldwide since the 1970s
Wheat
Wheat scab,
Fusarium
spp.
Reemerged after 1990 as a leading wheat disease in the
central and north regions
Listed by 14 wheat production states as a recently emerging
disease
Source: Plant Pathology Department, North Carolina State University
Barley yellow dwarf, Barley yellow
dwarf virus
The gray leaf blight of corn caused by the fun-
gus
Cercospora zeae
-
maydis
ranks number one
in causing yield losses of corn in recent years. It
is also a disease whose range expansion was fi rst
noticed in the 1970s; in the last two decades, the
disease has gradually developed into a major pro-
duction problem in the Corn Belt. Although the
increase in the abundance and epidemics of this
disease may be due, in part, to the increase in the
use of conservation tillage, the observed trends in
minimum temperatures and precipitation in the
region may also have contributed.
Among the earliest attempts to relate historic
records of meteorological conditions and crop
pest damage were the studies of potato leaf roll
outbreaks in North America and Europe. Analysis
of the historic records from 1930 to 1991 sug-
gests that the outbreaks of this aphid-borne viral
disease were related to drought and sunspot
cycles. In the USA, the frequency of the reported
outbreaks seems to have increased since 1970
(Table
8.3
).
Another example of the linkage between
meteorological variables and pests is the wheat
stem rust disease in the US Great Plains. The epi-
demics of the disease from 1921 to 1962 seem to
be related to the conditions during El Niño epi-
sodes (Yang and Scherm
1997
). In contrast,
wheat stripe rust epidemics in the US Northwest
may be more severe during La Niña years
(Scherm and Yang
1995
).
Warming will generally cause a pole-ward
shift of the risk of damage from late blight of
potato which would increase in all regions and
potato nematodes may become a serious problem
with additional generations per year. Similar pre-
dictions of northward migration and increased
severity in areas of current distribution have been
made for the oak decline pathogen,
Phytophthora
cinnamomi
(Brasier and Scott
1994
; Brasier
1996
). Pathogens will follow migrating host
plants and their dispersal and survival between
seasons, and changes in host physiology and
ecology in the new environment would largely
determine how rapidly the pathogens establish in
the new environment.
Pathogens, in particular unspecialized necro-
trophs, may extend their host range to cause new
disease problems in migrating crops. There are at
least two well-known examples of an indigenous
pathogen attacking an introduced plant when
grown in close proximity. In its native habitat in
the USA, the fi re blight bacterium,
Erwinia amy-
lovora
, attacks indigenous plants of the family
Rosaceae without causing signifi cant damage.
When European settlers grew apples and pears in
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