164 • Impacts of Applied Genetics— Micro-Organisms, Plants, and Animals 
dangered Species Program are used for such activities 
as listing endangered species, purchasing habitats for 
protection, and law enforcement. 
16. The uses of pest-resistant wheat and corn cultivars on 
a large scale for both diseases and insects are classic 
success stories of host- plant resistance. However, re- 
cent trends in the Great Plains Wheat Belt are disturb- 
ing. The acreage of Hessian fly-resistant wheats in Kan- 
sas and Nebraska has decreased from about 66 percent 
in 1973 to about 42 percent in 1977. Hessian fly infesta- 
tions have increased where susceptible cultivars have 
been planted. In South Dakota in 1978, in an area not 
normally heavily infested, an estimated 1.25 million 
acres of spring wheat were infested resulting in losses 
of $25 million to $50 million. An even greater decrease 
in resistant wheat acreage is expected in the next 2 to 5 
years as a result of releases of cultivars that have im- 
proved agronomic traits and disease resistance but that 
are susceptible to the Hessian fly. Insect resistance has 
not been a significant component of commercial 
breeding programs. 
^'Office of Technology Assessment, U.S. Congress, Pest Manage- 
ment Strategies in Crop Protection (vol. 1, Washington, D C.; ll.S. 
(kjvernment Printing Office, October 1979), p. 73. 
17. Expressed in genetic terms, cases exist "where the in- 
troduction of novel sources of major gene resistance 
into commercial cultivars of crop plants has resulted in 
an increase in their frequency of corresponding \ iru- 
lence genes in the pathogen”.^® This has been reported 
in Australia with wheat stem rust, barley powdery mil- 
dew, tomato leaf mold, and lettuce downy mildew. E\ i- 
dence suggests that there is considerable gene flow in 
the various pathogen populations— e.g., asexual trans- 
fer can quickly alter the frequency of virulence genes. 
Furthermore, pressures brought about in the evolu- 
tionary process have developed such a high degree of 
complexity in both resistance and virulence mech- 
anisms, that breeding approaches, especially those only 
using single gene resistance, can be easily overcome. 
3“R. C. Shattock, B. D. Janssen, R. WhilInvacI, and D S. Shaw. 
"An Inlei-pretalion of the Freqiieneies of Host Speeifie Phenotypes 
of Phytophthora infestans in North Wales. " Ann. Appli. liiol. 86:249, 
1977, 
