Fire Blight (Erwinia amylovora Bacterium)

Growers and scientists alike would agree that fire blight is like no other plant disease. For growers fire blight is this capricious bacterial disease which can decimate apple and pear orchards in a single season, thus limiting areas where most susceptible apple and pear varieties can be grown. The economic impact of fire blight is difficult to determine, as losses are not recorded when they are low (a few flower clusters on a few trees killed within a season) and a single severe outbreak can disrupt orchard production for several years. However, over the last few years, fire blight has caused serious losses around the world.

In 1998 alone, losses for the north-west USA were estimated to be in excess of US$68 million, and those for the Hawke's Bay region of New Zealand were estimated to be at least NZ$10 million. Fire blight also caused severe outbreaks in Lebanon and Emilia-Romagnia in Italy, where the previous year 500,000 fruit trees were destroyed due to fire blight.

Furthermore, the economic importance of this disease is likely to increase, for several reasons. Firstly fire blight is still spreading geographically into new apple- and pear-growing areas. In Europe, for example, it is spreading west to east and around the Mediterranean Sea and, in 1997, fire blight was detected for the first time in Australia in the Royal Botanical Gardens of Melbourne. Secondly, except for streptomycin, there is still no registered product that can effectively control fire blight. Furthermore, the development of streptomycin-resistant strains of Erwinia amylovora is threatening the future use of this antibiotic in countries where its use is still permitted. Finally, the new production methods, such as high-density planting, the use of rootstocks, such as M.26 and M.9, and of new cultivars, such as 'Gala' and 'Braeburn', which are susceptible to fire blight, will also have an impact on the future economic importance of fire blight.

So far, mountains, such as the European Alps, and oceans, such as the Pacific or Atlantic, have not been able to keep fire blight at bay. If it continues to expand geographically as it has done over the last 100 years, it seems inevitable that fire blight will sooner or later reach Asia. Interestingly, apple tree grow wild in some parts of Asia and it is believed that the domestic apple tree actually originated in Central Asia, probably around Kazakhstan and Kirghiztan. If fire blight were to spread into this part of the world, it could lead to the loss of numerous genotypes that have never been in contact with the pathogen before, or it could lead to the discovery of resistance genes to E. amylovora among wild apple trees.

To limit the incidence of fire blight, some integrated orchard and nursery management practices have been developed that can significantly reduce the risk of severe outbreaks. These practices rely on orchard sanitation and the timing of chemical sprays to coincide with potential infection events during the blooming period. Such timing is made possible thanks to the development of several risk assessment models. Today only heavy metals and antibiotic-type compounds are registered for control of fire blight; the best antibiotic, streptomycin, is not registered in every country where fire blight occurs and, as pointed out earlier, development of streptomycin-resistant strains jeopardizes its effectiveness. In the future, novel compounds that do not directly affect the pathogen but interfere with the physiology of the plant, either regulating plant growth or inducing plant defence reactions, such as prohexadione-calcium (ApogeeTM, BASF), harpin (MessengerTM, Eden Bioscience) or BHT (1,2,3-benzothiadiazole-7-carbothioic acid S-methyl ester) (sold as BionTM or ActigardTM by Novartis), might be registered for control of fire blight. In addition to these chemicals, a strain of Pseudomonas fluorescens (A506) has recently been commercialized in the USA under the name BlightBan A506TM (Plant Health Technologies) as a biocontrol agent for fire blight. Other bacterial strains that could also be used as biocontrol agents are being considered for commercialization, such as Erwinia herbicola C9-1 in the USA and E. herbicola P10C in New Zealand. These biological control agents might offer an alternative to chemical treatments while waiting for the development of cultivars and rootstocks of apple and pear bred for their natural resistance to fire blight or being engineered using genes that code for antibacterial compounds.

For scientists, several characteristics set fire blight apart from other bacterial diseases. The development of fire blight symptoms, for example, allows us to distinguish this disease from others. E. amylovora, in contrast to most plant-pathogenic bacteria that induce necrosis, can travel rapidly and extensively from the point of infection. On susceptible cultivars, if conditions (including climate and the physiology of the tree) are favourable, the disease can migrate from one infected flower down to the rootstock, killing the tree in a season. Fire blight is an evolutive, necrogenic disease. This ability to migrate long distances through the cortical parenchyma, without producing any enzyme that would help to dissolve the tissues, is quite remarkable. Also remarkable is this ability of the pathogen to spread and to survive within host tissues, while unable to survive as an epiphyte. The overwintering in sometimes inconspicuous cankers and the existence of symptomless carriers are two very significant stages of the pathogen's life cycle, which can probably explain some of the sudden outbreaks of fire blight.

The limited host range of E. amylovora, with no recognized pathovars, is also worthy of interest. The host range is limited to some plant species that belong to the Rosaceae, with the majority of them belonging to the subfamily called Maloideae or Pomoideae. These plants, grouped together on the basis of their flower morphology, must have in common a factor that makes them susceptible to fire blight. However, to date there is no indication about the nature and the function of this putative factor, or whether there is only one factor or a family of functionally similar factors.

E. amylovora: a unique pathogen

E. amylovora is the only bacterium capable of inducing fire blight. It is a Gram-negative bacterium belonging to the Enterobacteriaceae and its anatomy, physiology and serology have been well described. Microbiologists can, with a few tests, easily separate isolates of E. amylovorac from those belonging to other species. However, what distinguishes E. amylovora from other Enterobacteriaceae is a series of relatively minor differences. It is, for example, able to grow, although weakly, under anaerobic conditions and it is unable to reduce nitrate to nitrite. Until recently, one of the most amazing characteristics of E. amylovora was the great homogeneity of this species. However, with the advent of new molecular tools that allow detection of the most minute differences in a bacterial genome and with the finding of isolates that can induce symptoms on selected host ranges, E. amylovora does not look like the monolithic species it once was thought to be. It is still, however, quite a homogeneous species.