Fire blight is one of the most serious diseases affecting apple and pear trees (as well as related species such as Medlars, Quince, Cotoneaster, and Hawthorn). Whilst most diseases of fruit trees are caused by fungal infections, fireblight is a bacterial disease. Infection usually occurs in spring, and the bacteria are spread by rain and insects.
The most common method of infection by fireblight bacteria is through the blossom, with the bacteria being transported by pollinating bees. Fireblight can also attack the rapidly-growing tips of new young shoots. Older woody shoots are less likely to be affected.
It is unusual to see fireblight in very young trees since they will not have developed blossom, although the infection can also get in through aphid attacks, or even through holes created by heavy rainfall or hail.
Late-blossoming apple and pear varieties (including most European cider apple varieties) are often more at risk than earlier-blooming varieties - there will be more bacteria in the air, and the weather is likely to be warmer.
The infection progresses rapidly (up to 4cm per day in good conditions) and in a severe case leaves the tree looking as if it has been set on fire. The photo shows a mature apple tree killed by fireblight. Note the browned branches, and the attempts made by the owner to beat the infection by sawing off the main stem and the limb on the right.
Although fireblight can be a devastating problem for commercial apple and pear growers in affected areas, it is very dependent on warm weather and rain in early spring during the blossom season. It also tends to occur in cycles with heavy infections every 5-10 years.
History of fireblight
Fireblight is an interesting disease because it highlights the importance of international phytosanitary controls. It appears to have arisen spontaneously in the eastern USA in the 17th or 18th centuries, and progressed southwards. It is notable that some old local apple varieties in these areas appear to have been selected by the settlers for their fireblight resistance.
Because of poor phytosanitary controls between North America and Europe, infected plant material arrived in Europe in the 1950s, and it is now endemic in most of Europe. Some islands (e.g. Ireland, Isle of Man, Channel Islands, and Corsica) are still free of the disease.
Fireblight resistance
Most of the work on fireblight resistance in apples and pears has taken place in the USA, where the disease is particularly aggressive in orchards in the eastern and southern states. There are essentially two approaches, the development of disease resistant varieties, and the development of resistant rootstocks.
The development of fireblight resistant rootstocks has proved to be one of the most effective ways to limit the disease. Fireblight usually only becomes fatal once the disease gets into the roots, and resistant rootstocks prevent this phase of the disease, giving the tree owner more options to cut out the infected parts of the main tree. Fireblight resistance has been the central goal of the Geneva series of rootstocks developed by Cornell University. The first rootstocks - G.11 and G.16 - showed good resistance and more recent releases such as G.41, G.30, G.935 and G.202 have shown further improvements.
In contrast the Malling-series rootstocks were developed in England in the early 20th century, before fireblight existed there, so they are not resistant. The one exception is M7, which by chance has quite good resistance - and as a result is still widely used in the USA.
Resistant varieties are an eclectic mix of traditional varieties from the southern states such as Arkansas Black, and new disease-resistant varieties such as Honeycrisp. Contrary to the popular belief that supermarket apple varieties are not as disease-resistant as heirloom varieties, Red Delicious is well-known for its fireblight resistance. However note that "resistant" does not mean "immune".
Similar approaches have been followed with pear trees. It was found that two pear varieties, Old Home and Farthingdale were largely immune to fireblight, and most pear trees grown in the USA are now grafted on a combination rootstock developed from Old Home and Farthingdale - known as OHxF. This rootstock is rarely used in the UK, since fireblight is not a significant issue here.
If you live in an area where fireblight is active it is best to choose resistant varieties grafted on resistant rootstocks.
Treatment
Prompt action is the key with a fireblight strike. The approach is similar to dealing with canker - but fire blight is far more aggressive and spreads more rapidly. Cut back well behind the infected area of the shoot, and if in doubt cut back even further. You may feel this will ruin the attractive shape of your tree, but work on the basis that if left unchecked the infection will kill the tree, so a mis-shaped tree is better than no tree at all - and if the tree survives it will soon produce replacement branches.
As with canker, an infected tree is also a threat to all other nearby trees, so carefully remove debris from the orchard.
Copper-based sprays and bacteriacides are the main control method, applied in early spring. Insecticides are also effective in controlling the disease (since it is mainly spread by insects) but unfortunately this can include the bees needed to pollinate the blossom.
Fireblight is more likely to affect young fast-growing trees than older more mature ones, so avoid hard-pruning of young trees during the dormant season (particularly in the 2nd and 3rd years) and do not use fertilizers which might encourage excessive rapid growth. If you have applied too much fertilizer, hoe in sawdust (which will cause soil bacteria to use up the nitrogen in the fertilizer) and allow grass and weeds to grow in competition with the tree.
In short, whilst the disease can be controlled (particularly in the garden or backyard orchard), this is a case where prevention is better than cure, and it is best to plant fireblight-resistant varieties in areas where fireblight is known to occur.
More resources about Fireblight
US university extension services in areas where fireblight is common are a good source of insights into indentifying and managing the disease.
Washington State University - Fire Blight of Apple and Pear.