Susceptibility of Some Apple 

 Cultivars to Fire Blight: Implications 

 for Massachusetts Growers 



Daniel R. Cooley 



Department of Plant Pathology, University of Massachusetts 



Parts of this article were excerpted from an 

 article to be published in Plant Disease by T. M. 

 Thomas and A. L. Jones of Michigan State 

 University, who generously agreed to share the 

 information so that it may be used in a timely 

 way. In 1991, there was an unusually severe 

 epidemic of fire blight on apples in Michigan. In 

 trying to make the best out of a disastrous 

 situation, Michigan researchers evaluated 84 

 apple cultivars being grown at the Southwest 

 Michigan Research and Extension Center in 

 Benton Harbor. They concluded the evaluation 

 with a warning: many of the new cultivars being 

 planted are particularly susceptible to fire 

 blight. In addition, more trees are being planted 

 on dwarf rootstocks, including M.26, that are 

 highly susceptible to fire blight. Finally, strep- 

 tomycin-resistant strains of the bacteria that 

 cause fire blight (Erwinia amylovora)have been 

 foundin many apple production areas, including 

 Michigan. Without streptomycin, susceptible 

 cultivars are in serious danger from fire blight in 

 Michigan. 



While streptomycin-resistant Erwinia 

 amylovora have never been found in Massachu- 

 setts, the bacteria could easily mutate or be 

 transported here. In some years, isolated Mas- 

 sachusetts orchards have had serious problems 

 with fire blight. Still, fire blight pressure in our 

 region has never reached epidemic proportions. 



Should Massachusetts growers be con- 

 cerned with fire blight susceptibility when 

 planting apple cultivars? Cultivar selection 

 usually is driven by market forces rather than 

 disease susceptibility; however, we generally 

 have methods, chemical or cultural, which can 

 manage serious diseases. How would the situa- 



tion change, however, if streptomycin were not 

 effective against fire blight? 



New York rated fire blight susceptibility of 

 several cultivars in the early 1970's, and these 

 ratings correlated well with the ratings ob- 

 tained in Michigan in 1991. Older, well-known 

 cultivars generally were rated the same in each 

 test. Many new cultivars, however, such as 

 Gala, Braeburn, and Fuji, not grown in 1974 

 were present only in the Michigan test. Right 

 now, most of our trees in New England are 

 Mcintosh. On a five-step scale (l=very resis- 

 tant; 2=moderately resistant; 3=intermediate; 

 4=moderately susceptible; 5=very susceptible), 

 Mcintosh is rated intermediate. Interestingly, 

 Marshall Mcintosh was not more susceptible 

 than Macspur, Pioneer Mac, or Red Max in the 

 Michigan test. 



According to these tests, our major cultivar 

 is not particularly sensitive to fire blight. From 

 experience, we know that in some situations, 

 natural occurrence of fire blight in Massachu- 

 setts can damage Mcintosh trees significantly. 

 We could conclude that fire blight pressure is 

 variable in the state, and on some occasions in a 

 few orchards it can be severe enough to damage 

 Mcintosh. In the Michigan test, other cultivars 

 such as Delicious and Liberty were more fire- 

 blight resistant than Mcintosh. Empire and 

 Mcintosh were both rated "intermediate". 

 Golden Delicious, Jonagold, Mutsu, and most 

 Gala strains were somewhat more susceptible 

 than Mcintosh. Idared, Braeburn, and Fuji 

 strains were much more susceptible than Mcin- 

 tosh. (Table 1). The ratings are not terribly 

 surprising, if we consider that fire blight is not 



10 



Fruit Notes, Summer, 1992 



