Fire blight disease: Effects and symptoms

The fire blight is a disease that primarily affects Rosaceae (pear, apple, quince, medlar) and wild ornamental addition. As its name suggests, this disease is caused by bacteria ( Erwinia amylovora). We are going to know some of the symptoms that this disease causes.



Fire blight has the particularity of being the first disease caused by a bacterium to be detected and registered. Its effects are devastating on the crop for several reasons. It has a great capacity for migration towards other trees, a limited range of hosts that causes a specialization in the infection of economically important crops, its great capacity for survival in the tissues of the host plants and worst of all, there is no treatment.

The insemination of the disease is amazing. Rain (the pathogen can live for 6 months in water), wind (transport of bacteria exudates), insects (up to 77 species), tools, birds (long-distance transport) and plant material carrying  Erwinia amylovora.

Currently, the disease is spread throughout Spain, but there are only isolated cases, due to the great eradication process that was carried out by eliminating the infected trees.


As we have commented previously, the range of affected species belongs to the Rosaceae ( Rosaceae family ). Within this family, around 200 species of plants of 40 genera that are susceptible to being infected by fire blight have been described. Let’s also look at the affected subfamilies:

  • Spiraeoideae
  • Rosoideae
  • Amygdaloideae (Prunoideae)
  • Maloideae (Pomoideae)
The genus in bold is the most affected, which corresponds to pome fruit trees.


As fire blight is considered a quarantine disease, if the infection is positively identified in a tree, it should be reported to the authorities.
The bacteria infect the plant regardless of age. Cases of affected fruit trees have been studied in the nursery, and consequently eliminated. The first symptoms are born in spring, when the budding and flowering of the crop occurs.

The symptoms of the disease, which we will see below, are in most cases the same among all affected subfamilies and the disease is initially located in the lower-middle part of the tree. The disease progresses rapidly in all directions, affecting main and secondary branches (crown) and roots. The best known and most studied symptoms of fire blight are on the pear tree, because the appearance of the affected pear tree is much more spectacular than in other trees such as apple, medlar or other rosaceae. The pear tree turns black (flowers, leaves, buds and fruits. In fact, the name “fire” is due to the fact that, when the disease is advanced, the appearance is similar to if the pear tree had burned.

  • Leaf symptoms : leaves become infected either because the shoot is infected or because the disease has progressed to the leaves. At the beginning of the disease, the leaves wilt, and necrotic spots may appear on the margins. The disease can also be identified by observing that a dark-colored, wet-looking spot appears on the petiole. Besides another clear identification, the leaves do not fall off. The normal thing is that when the leaves wither they fall. In this disease the leaves remain dry on the tree.
  • Symptoms on branches and trunk: cankers form on the trunk that appear in spring, summer or autumn. These cankers usually measure between 3 and 8 mm on shoots, and on branches or trunks they increase to 15-20 cm. They are not easily identified. They are discolored areas on the trunk, brown in color, sometimes cracked.
  • Symptoms in flowers: In the initial stages, the flowers appear damp, withering over time. They take on a brown or blackish appearance, wither and die. Like the leaves, the necrotic flowers remain on the tree and do not fall.
  • Symptoms in fruits: from the beginning of the bacterial fire the fruits can be affected. If the fruit is already affected by wounds or cracks caused by conditions unrelated to this disease, the bacteria progress more quickly. The affected fruits acquire a moist appearance that over time turns blackish and necrotic. Some fruits remain mummified on the tree (such as leaves and flowers) and others fall due to their weight.
  • Symptoms in roots: they are not observable, since the roots are not visible, and it is also not the most common. But if an infection occurs on the roots, the death of the host is rapid. Cankers similar to those on the trunk or branches are formed.

A very peculiar symptom of the disease are exudates. The affected organs (flowers, trunk, shoots, branches and fruits) emit or exude drops that are white or yellowish and viscous. These drops contain a lot of bacteria protected by mucopolysaccharides, and it is a great way of spreading the disease.


Many times we have observed that before the appearance of one of the symptoms described above, panic runs and we directly diagnose as bacterial fire and the solution is to uproot the tree. Let’s look at different pests and diseases that can cause confusion from Erwinia amylovora- like symptoms  .

Some fungi cause fire-like cankers on trees. Nectris spp. , Phomopsis spp.  and Phytophtora spp. The difference is that fungal cankers are superficial and well limited.
Janus comppresus known as “shoot twister  ” can produce symptoms on young shoots similar to those of  Erwinia amylovora . In addition to  Zeuzera pyrina  , it similarly injures apple and pear shoots.

Pseudomonas syringae pv. syringae  is common in Spain and produces similar symptoms with bud, bud, or flower necrosis in spring. It differs from fire blight because the symptoms are noticeable in spring and do not progress in summer or fall.

Pear tree varieties most susceptible to infection

Very sensitive

  • Alexandrine Douillard
  • Dean of the Office
  • Packam’s Triumph
  • Passe Crassane


  • Slay Fetel
  • Water from Aranjuez (Blanquilla)
  • Conference
  • Devoe

Medium sensitivity

  • William’s (Bartlett)
  • Santa Maria Morettini
  • Lemon tree (Dr. Jules Guyot)
  • Grand Champion
  • General Leclerc
  • Morettini Precocious Butter
  • Buttery Hardy
  • Buttery Bosc (Kaiser)
  • Good Luisa of Avranches

Little sensitive

  • Rome
  • Magallón (Leonardeta)
  • Ercolini (Coscia)

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