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Fire blight's basic symptom is necrosis or death of tissues. Droplets of ooze on infected tissues are also an important symptom; they are the visible indication of the presence of fire blight bacteria. Except for minor differences, the symptoms of fire blight are basically the same on all host plants.
Infected blossoms initially become water-soaked and of a darker green as the bacteria invade new tissues. Within 5-30 plus days (commonly 5-10 days), the spurs begin to collapse, turning brown to black. Initial symptoms are often coincident with the accumulation of about 57 degree days, base 12.7°C, from the infection date (Steiner, 2000).
Infected shoots turn brown to black from the tip; shoots often bend near the tip to form a so-called 'shepherd-crook' shape. Shoots invaded from their base exhibit necrosis of basal leaves and the stem. Leaves and fruits may be invaded through petioles or stems or infected through wounds, resulting in discoloration followed by collapse of the leaves and fruit. During wet, humid weather, infected leaves and particularly the fruit often exude a milky, sticky liquid, or ooze containing bacteria.
From infected flowers and shoots, the bacteria may invade progressively larger branches, the trunk and even the rootstock. Infected bark on branches, scaffold limbs, trunk and rootstock turns darker than normal. When the outer bark is peeled away, the inner tissues are water-soaked often with reddish streaks when first invaded; later the tissues are dark brown to black. As disease progression slows, lesions become sunken and sometimes cracked at the margins, forming a canker.
Trees with rootstock blight may exhibit liquid bleeding from the crown at or just below the graft union in early summer. Water-soaked, reddish and necrotic tissues are visible when the outer bark is removed. Trees with infected rootstocks often exhibit yellow to red foliage about a month before normal autumn coloration. Rootstocks such as M.26, M.9 and relatives of M.9 often show these symptoms without evidence of infection in the trunk of the scion. Infection of M.7 and a few other rootstocks occurs following infection of suckers arising from the rootstocks; the infected suckers exhibit typical shoot blight symptoms. Many trees with rootstock blight will die in the first year after infection; the remaining rootstock-infected trees often die within 2-3 years.
Any plant tissues invaded by the bacteria can show ooze production on their surface. This exudate is a specific symptom of fire blight. Depending on weather conditions and on the time of the day, ooze may or may not be produced. It is most frequently observed early in the morning when the host water potential is positive. It may appear in different ways: droplets, threads or film on the plant's surface.
Legislative Control (Exclusion)
Fire blight is a quarantine disease in most countries and, therefore, shipments of plants, or parts of plants that can be host to fire blight, are under strict regulation. This regulation requires that only healthy plants produced in healthy environments are shipped. At the European level (EU), the genera relevant to quarantine regulation for fire blight are the following: Chaenomeles, Cotoneaster, Crataegus, Cydonia, Eriobotrya, Malus, Mespilus, Pyracantha, Pyrus, Sorbus (other than S. intermedia) and Stranvaesia.
In countries where fire blight is not yet detected, but exposed to permanent threat by nearby foci, a network for monitoring may be preventatively organized (Mazzucchi, 1994; Santos, 1995).
In the EU, a list (map) of so-called 'protected zones' in which fire blight is considered as absent is periodically published. In such protected zones, the import of host plants of fire blight from a contaminated country is forbidden (except from 'protected areas'). In non protected zones, where fire blight is likely to be endemic, specific 'protected areas' are settled (minimal surface: 50 km²) in which special surveys and official control guarantee the absence of fire blight on plants grown in nurseries. From these areas plants are allowed to be shipped (Petter and de Guenin, 1993). Heat treatment of plant propagation material has been proposed (Keck et al., 1995).
In some countries the production and commercialization of the most susceptible cultivars may be banned, or discouraged, particularly for certain cultivars of Cotoneaster, Pyrus, Malus and Crataegus.
As is the case with most bacterial diseases, cultural practices are very important to control fire blight. These practices will tend to reduce the frequency of infections, by decreasing the potential entry of bacteria into the plant: suppression of blossoms by severe trimming of Crataegus hedges has been recommended in the Netherlands (Meijneke, 1984b); suppression of secondary blossoms in pear orchards is a proposed control measure in France (Lecomte and Paulin, 1992).
A complementary strategy for reducing the severity of infection is to follow growing practices aimed at reducing tree vigour and the duration of shoot growth (also see Chemical Control/prohexadione calcium). Restricting nitrogen and water supply to the trees is the most common advice in this respect, together with a regular pruning of the trees.
Insect control is no longer believed to be a key factor in the limitation of movement of bacteria from tree to tree. Nevertheless, care should be taken with transportation of beehives to avoid movement from an infected to a healthy orchard. Similarly, overhead irrigation should be avoided in an orchard with a history of fire blight.
Cultural methods include the sanitation of trees, obtained by a prompt pruning out of symptoms as soon as they are detected in an orchard or a plantation (Steiner, 2000). The disinfection of tools (pruning shears) with chlorine or alcohol is probably useful (Teviotdale et al., 1991) during the growing season but not in winter when trees are dormant (Lecomte and Paulin, 1991).
The early detection of symptoms is important to the success of sanitation programmes. Surveys in orchards and nurseries are recommended in spring just before bloom (active cankers), after bloom (new flower infection), in summer after hailstorms and near the end of the period of shoot elongation (shoot infections and cankers). These surveys must be followed by the removal (cutting out) of all visible infections. In most cases, warning systems will provide an indication of the most suitable period when these surveys are useful (Billing, 2000).
Risking catastrophic tree losses from rootstock blight in high-density apple orchards can be avoided only by selecting trees propagated on resistant rootstocks for new orchards. Several promising highly resistant rootstocks have been released or will soon be released from rootstock-breeding programmes (Cline et al., 2001; Norelli et al., 2003). Some of these are dwarfing rootstocks suitable for high-density orchard systems; avoiding M.9 and M.26 rootstocks in favour of resistant rootstocks is the best control for rootstock blight. Rootstock blight has not been a problem on trees propagated on Budagovsky (B.) 9 and on some Japanese rootstocks (Bessho et al., 2001; Ferree et al., 2002).
Susceptible cultivars (and rootstocks) should be avoided when establishing new orchards and ornamental planting in regions with significant fire blight problems; unfortunately, this advice is seldom followed in practice. For example, many of the most commercially successful apple cultivars introduced in recent years (Braeburn, Fuji, Gala, Ginger Gold, Jonagold, and Pink Lady) are much more susceptible to fire blight than many older cultivars and planting of these cultivars, particularly when propagated on highly susceptible rootstocks, has resulted in devastating financial losses (due to fire blight) to individual apple growers and entire apple industries (Longstroth, 2000).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Fire blight is a serious disease of plants in the subfamily Maloideae, especially apple, pear, quince and loquat. Epidemics, although sporadic, are often devastating depending on the occurrence of favourable climatic conditions, the amount of initial inoculum and virulence of the pathogen, and the susceptibility of the host species. Therefore, in any given site where fire blight is present, the disease can either be devastating or of secondary importance, according to the year and the varieties grown.
In addition, fire blight is a quarantine disease in most countries and, therefore, a single introduction, even of very limited importance in the field, may have a considerable economic impact in a newly infected country, due to the possible limitations in the international trade of plants.
It is generally considered that fire blight - with exceptions for particular years and cultivars - is unlikely to cause severe damage in Northern Europe (UK, Sweden, Norway and Denmark). In contrast, the threat is very serious for susceptible cultivars (pear, but also a number of recently released apple varieties and a number of ornamentals) in Southern and Central Europe.
Conversely, fire blight may be considered a disease of usually minor direct influence for a number of apple cultivars (such as Golden Delicious) and ornamentals in most areas. But the risk of unusual climatic conditions conducive to disease activity remains permanently present. This is illustrated, for example, by unexpected infections in UK cider apples in 1980 and 1982 (Gwynne, 1984), in pears and apples in Aquitaine and Anjou and Paris in France, in 1978 and 1984, respectively (Lecomte and Paulin, 1989) and in apple and pear flowers in Switzerland in 1995 (Mani et al., 1996).
In the Crimea, regional pear varieties are susceptible to E. amylovora, which can cause losses of 60-90% of flowers and buds in some years and reduce yields by a factor of 8-10 (Kalinichenko and Kalinichenko, 1983).
In Egypt, the first fire blight outbreaks since 1962 were recorded in 1982 and were associated with heavy rainfall during bloom. In 1983 and 1984, outbreaks also occurred and were associated with rainfall combined with wind storms during bloom and one 2-day rain during bloom, respectively. The severe occurrence of the disease was expressed mainly as flower blight and caused a loss of 10-75% flowers/tree (van der Zwet, 1986).
Fire blight is said to be at the origin of decreasing pear production in the eastern USA (van der Zwet and Keil, 1979). Similarly it caused, directly or indirectly, (following restrictive regulations) the progressive suppression of a number of cultivars in Europe: Laxton's Superb, Beurré Durondeau, Passe Crassane for pears, James Grieves and several cider-varieties for apples, and the ornamentals Cotoneaster salicifolius and Pyracantha atalantioïdes (Paulin, 1996).
In New York state, the potential economic loss of the rootstock phase in fire blight was estimated. Economic losses were estimated based on a 10% tree loss in high density apple orchards and were $8818/ha (Momol et al., 1999).
An epidemic in south-west Michigan apple orchards in 2000 was particularly severe and followed unusually warm, humid, wet weather in May. It was estimated that between 350,000 and 450,000 apple trees will be killed and 1500 to 2300 acres of apple orchards will be lost; the development costs of these orchards was over $9 million. Apple yields will be reduced by 35% over the region and some growers will experience losses of 100%. Out of the normal 4.5-7 million bushels produced in the region, the expected crop loss is 2.7 million bushels worth an estimated $10 million. It is estimated that the cumulative loss of yield will be $36 million since 5 years will be required for the region to recover. This will bring the total economic loss in the region to ca $42 million (Longstroth, 2000). The total loss might have surpassed those estimates that were made in 2000, as growers found out that partly infected orchards were not economical to run.