Fire Blight: How Does This Simple Bacterium Outsmart Us Every Time ---
Research 1999

Sherman Thomson
Department of Biology, 5305 Old Main Hill, Utah State University
Logan, UT 84322-5305; e-mail: shermt@ext.usu.edu

The bacteria that cause fire blight are very simple yet cause millions of dollars of damage every year and threaten entire fruit industries. The bacteria are only composed of a single cell with a single circular piece of DNA. They do not have a brain. They must get all their energy from living trees and do not grow as saprophytes in nature. They can only utilize a limited range of sugars and amino acids for food. They do not have a mouth. They are very sensitive to sunlight and dry conditions. They do not have skin. They do not have any way of moving by themselves for distances over a fraction of a millimeter. They do not have feet. They have no way of determining where the next susceptible tree is located. They do not have eyes. They cannot plan ahead or think of their next move yet despite all these deficiencies they cause one of the most damaging bacterial plant diseases known to man.

Fire blight has been a mental and physical challenge for me for over 25 years. I am constantly amazed about the information that I continue to learn about this simple pathogen. No doubt it will provide many more years of challenge because controlling the disease is still less than satisfactory. The following information describes some of the research we have been involved in over the last few years.

Chemical control and evaluation of new products for control of fire blight.

Bactericides were evaluated for fire blight control in apple and pear orchards in Kaysville, Payson and Santaquin, Utah, New Zealand and France. We have demonstrated that Actigard provides significant control of fire blight when applied early and using repeated applications. It is a unique chemical because it has no direct effect on the pathogen but induces resistance in the trees. It is more effective when combined with streptomycin. Actigard is not yet registered for any crops in the USA. Cit 30, A new experimental copper based compound with citric acid was also significantly better than the check in our trials. Apogee is an experimental plant growth regulator that regulates shoot growth and has been shown to reduce shoot blight. Our trials showed a small reduction on shoot blight of Rome apples. However we could not evaluate its effect on pears because no shoot blight occurred in the Kaysville orchard. Streptomycin is still the best registered compound for fire blight control.

Stigma imprinting to predict fire blight risk.

We have demonstrated that Erwinia amylovora is present on a variable percentage of flowers in orchards prior to fire blight. This incidence of colonized flowers can be determined easily by doing stigma imprints on a large number of flowers in the orchards. We propose that the stigma imprint technique could be used to monitor Erwinia amylovora populations in orchards and be used as a guide for growers about when to spray bactericides. Stigma imprints could be done by growers or trained personnel but reading the plates requires a trained technician. Stigma imprinting can also be used as a powerful research tool to determine what factors are responsible for colonization of flowers.

Understanding shoot blight.

We have been studying the events and epidemiology that lead to shoot blight. There are currently no chemicals or methods recommended for shoot blight control and yet it is probably the most destructive aspect of fire blight. We have developed a leaf imprint technique that allows us to monitor leaves for the presence and location of the pathogen. Erwinia amylovora is not an epiphyte and does not multiply or even survive very long on leaves. It is only present on leaves within a few feet of fire blight infections and then usually only for a short time after rain. It is usually not present on the youngest leaves and more common on leaf 6 from the tip. We hypothesize that Erwinia amylovora enters these older leaves intermittently after rains and causes shoot blight by moving systemically to the shoot tip. This information aids growers in knowing more about when to prune and how to manage fire blight.

We have studied the survival of E. amylovora on Gala apples leaves by inoculating with known populations of the bacteria and monitoring the populations over time. We have compared laboratory cultured bacteria with naturally produced bacteria in ooze and found no significant differences. High populations of bacteria survived for over a week on leaves but low populations declined.

Leaf injury.

Shoot blight occurs when bacteria enter leaves and move systemically to the shoot tips. We know that injury such as hail allows entry but we are uncertain how the bacteria enter healthy leaves when there is no apparent injury. We inoculated injured and non-injured Gala apple and pear leaves and discovered that shoot blight occurred a high percentage of the time on injured or uninjured pear shoots. However uninjured apple leaves were only infected 25% of the time whereas injured leaves were infected 100% of the time. This suggests a major morphological difference between pear and apple leaves.

Antibiotic injections for controlling fire blight

The fire blight bacteria enter flowers and leaves from external entry points or injury. Bactericides are normally sprayed on the surface to prevent bacteria from entering plants. We studied the injection of streptomycin and oxytetracycline to determine if the internal presence of the bactericide would reduce blossom blight and shoot blight. Oxytetracycline is currently registered on pears for control of fire blight by injection (Tree Tech, Williston, Florida) .

Both streptomycin and oxytetracycline were highly phytotoxic to pears and apples and could not be used at the rates tested. Pear flowers were killed by streptomycin. However blossom blight on apples was reduced 60% and shoot blight was reduced by about 60% on both pear and apple.