ment. In most years, however, it is unlikely that the 

 model would call for more than three Streptomycin 

 applications, which would be within resistance man- 

 agement guidelines. 



5. The complexity of the fire blight disease cycle 

 and the way that symptoms manifest themselves 

 (several different phases showing up within a short 

 period of time) makes it at times difficult to deter- 

 mine what is happening in an epidemic situation. 

 An excellent feature of the model is that data files are 

 created and unusual or unexpected situations may 

 be studied at a later date. We have increased our 

 understanding of how this disease operates signifi- 

 cantly by reviewing these files over the years. 



6. Although the MARYBLYT model is excellent for 

 monitoring disease development and helpful in 

 cleaning up known problem sites, much of the de- 

 structiveness of fire blight is due to its erratic occur- 

 rence. If an orchard has had no history of fire blight, 

 there would be no incentive to implement an aggres- 

 sive control program including Streptomycin 

 sprays. Once a serious epidemic is in progress, it is 

 too late for the model or Streptomycin sprays to be of 

 much help. Repeatedly spraying Streptomycin on a 

 raging epidemic can only favor resistance develop- 

 ment and is of questionable value in stopping disease 

 progression. 



Where an epidemic of fireblight will occur each 

 year is still the overriding question. We have good 

 tools now available to aid in control decisions, par- 

 ticularly the MARYBLYT program, but where to 

 implement them if a site has no prior history contin- 

 ues to elude us. 



7. Our best strategies for fire blight management 

 are as follows: 



A. Keep nitrogen levels in check. Pushing young 

 trees with high nitrogen regimens favors lush 

 growth that is highly susceptible to infection. 



B. Watch vector populations, primarily aphids, 

 leafhoppers, and pear psylla. Keep them low. 



C. Implement a copper program annually in early 

 spring on all pears and susceptible cultivars of 

 apples. 



D. Avoid planting trees, if possible, where both 

 scion and rootstock are highly susceptible to 

 fire blight. 



E. Follow proper pruning techniques for winter 

 removal of overwintering cankers. Major epi- 

 demics are probably bestleflto nin their course 

 in summer infections; a few minor strikes 

 should be removed as soon as they are detected. 



F. Implement the MARYBLYT program as part of 

 your regular orchard recordkeeping activities. 

 If any stage of fireblight is detected in the 

 orchard or general vicinity, use the model to 

 time application of Streptomycin sprays in an 

 aggressive control program for at least two 

 successive years. 



In conclusion, with diligence and good manage- 

 ment techniques it seems possible to obtain satisfac- 

 tory control of fire blight in most growing seasons. 

 Many questions remain unanswered, however, such 

 as the role of systemically infected, asymptomatic 

 trees in the disease cycle, and where major epidem- 

 ics will strike from season to season. We are un- 

 doubtedly making progress in our understanding of 

 this complex disease. Hopefully, at some point we 

 will achieve the knowledge we need to be successful 

 consistently in its management. 



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16 



Fruit Notes, Spring, 1993 



