Many of the apple and pear cultivars grown today are susceptible or highly susceptible to fireblight.
Fireblight, caused by the bacterium Erwinia amylovora, seriously limits apple and pear production in most fruit-growing regions of the United States. During epidemics, financial losses due to lost yield and tree death, can exceed tens of millions of dollars. Fireblight disease is particularly difficult for growers to manage, and the situation is exacerbated by three major problems:
- Many of the popular cultivars grown are either rated as susceptible or highly susceptible to fireblight;
- Many of the popular dwarfing apple rootstocks utilized are also highly susceptible to fireblight; and
- The few chemical control options available are further limited by the development of streptomycin resistance in areas including the Pacific Northwest, California, Utah, and Michigan.
The diversity of host tissues, combined with the limited number of management tools available to control the disease, have made it difficult to stop or slow the progress of fireblight epidemics.
Since the introduction of streptomycin as a tool for blossom blight control in the 1950s, this antibiotic has been and continues to be a highly effective bactericide in the absence of resistance of the pathogen.
How does streptomycin resistance arise in bacteria? Chromosomal resistance occurs when the bacteria mutate, changing the streptomycin target protein in the cell so that it no longer binds the antibiotic. These mutant strains will only increase in frequency in orchard populations through the continued use of streptomycin which kills off other streptomycin-sensitive strains of the pathogen. A similar scenario exists with fungal pathogens and the development of fungicide resistance.
The prevailing opinion of plant pathologists is that overuse of chemical controls contributes to resistance development through continuous exposure and selection for resistance. This is probably true with streptomycin as well, and a good policy for growers is to use a maximum of two to four streptomycin applications per year and also to limit the timing of application to bloom.
In Michigan, streptomycin resistance was first observed by Dr. Alan Jones in 1992. The mechanism of resistance was a new type of gene-based resistance that was known to be capable of being transferred to streptomycin-sensitive strains of the pathogen. With this knowledge, the probability of rapid spread of streptomycin resistance and streptomycin-resistant strains would have been predicted to be high. However, 15 years later, although we have observed the spread of the streptomycin resistance problem in Michigan, streptomycin-resistant strains are not endemic throughout the state.
Based on our results from yearly surveys conducted in Michigan from 2002 to 2007 and genetic studies in the laboratory, we have found that two predominant streptomycin-resistant strains, distinguished by the unique placement of the streptomycin-resistant genes in the bacterial genome, are responsible for the spread of streptomycin resistance in Michigan. These results discount the possibility that transfer of streptomycin-resistant genes occurs readily in E. amylovora. Transfer of streptomycin-resistant genes into E. amylovora has occurred at least twice in Michigan; but we would have predicted many more streptomycin-resistant gene acquisitions based on what is known about antibiotic resistance in other bacteria.
These results also imply that the evolution of new streptomycin-resistant E. amylovora strains is a rare event. Thus, in orchards that do not harbor streptomycin-resistant E. amylovora, growers should continue to use, but not overuse, streptomycin for blossom blight control. Streptomycin provides the best alternative for control (in the absence of resistance) because the antibiotic is bactericidal and kills pathogen cells. Alternative materials such as oxytetracycline that allow more pathogen survival and more disease (which leads to increased pathogen growth and population size) would likewise increase the remote possibility of the further new development of streptomycin resistance. In summary, as with any resistance management strategy for plant pathogens, killing the pathogen is critically important as the development of resistance requires living cells.
What are the alternatives in situations where streptomycin resistance is a problem? Oxytetracycline is the first antibiotic alternative; this material is bacteriostatic and inhibits pathogen growth but does not kill cells. Inhibition of E. amylovora growth on blossoms can result in good to excellent control of blossom blight, especially under low to moderate disease pressure. However, the inoculum remains, and trees treated with oxytetracycline can exhibit abnormally high levels of shoot blight even though blossom blight counts were low. Another alternative is the biological control Serenade MAX (Bacillus subtilis) which provides good blossom blight control under low to moderate disease pressure and fair to good control under higher disease pressure. Efficacy data for Serenade MAX are generally better in the eastern United States than in the West.
Two bacterial biological control agents, BlightBan (Pantoea agglomerans strain C9-1) and Bloomtime Biological (Pantoea agglomerans strain E325), are also registered.
These bacteria must be applied initially at about 10 percent bloom and to provide disease control must colonize blossoms before the pathogen arrives. In contrast to Serenade MAX, efficacy data for the bacterial agents are typically better in the western United States, and control with these materials has been highly variable in the East.
All growers with streptomycin-resistance problems should consider additional management strategies, including an early season application of copper prior to half-inch green tip. A copper "blanket" on trees can reduce fireblight inoculum emerging from overwintering cankers. Apogee (prohexadione calcium) is a growth inhibitor that provides excellent control of shoot blight. Apogee should be applied at a timing of petal fall of the king bloom, and a single application at the high rate provides better shoot blight control under high disease pressure than multiple applications at lower rates. Do not apply Apogee to Empire or Winesap, as fruit cracking may occur under some environmental conditions.
Finally, the problems we are seeing with streptomycin resistance and the lack of many efficacious alternatives signifies a need to return to more active physical methods of disease control, i.e., pruning out infected tissue. This is a proven method of inoculum reduction that continues to be important as we lose other methods of reducing pathogen numbers.
The fireblight pathogen is one of the most virulent bacterial plant pathogens known. A very small number of pathogen cells is all that is required to initiate infections. Our current prospects of reducing cell numbers in orchards below infection thresholds is limited if faced with highly susceptible varieties and streptomycin resistance.
Planting less susceptible apple varieties and using resistant rootstocks are two of the easiest management practices a grower can use to combat fireblight.