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today from the viewpoint of a plant pathologist. 
Each year plant diseases -- the cost of plant 
diseases in this country is very high in terms of crop 
losses and the expense of control measures. Due to 
concerns for our environment and the public safety, 
many pesticides have been taken off the market, and 
relatively few new ones are taking their place. This 
situation increases our need for basic research in 
plant pathology, so that new control measures can be 
devised. At present, it is not known exactly what 
properties enable only certain specialized pathogens 
to cause disease, while closely related species cannot. 
And likewise, we do not know how plants recognize 
potential parasites. 
Disease physiologists, studying plant pathogens 
and disease resistance, have been handicapped in their 
approaches to these problems due to the lack of good 
genetic systems for their use in identifying genes for 
virulence and constructing strains to use in biochemical 
studies. Plant pathogenic bacteria make excellent model 
systems for basic studies on host- parasite relationships 
since the genetics of the host and the parasite can be 
studied in parallel. Defining that the members of 
the genera Pseudomonas , Erwinia , Agrobacterium naturally 
exchange DNA with E. col i , and Pseudomonas aerug inosa 
[271] 
