Source:P.W. Steiner, T. van der Zwet, and A. R. Biggs

Fire blight is a destructive bacterial disease of apples and pears that kills blossoms, shoots, limbs, and, sometimes, entire trees. The disease occurs in nearly all moderate to warm apple growing areas worldwide. Although outbreaks are typically very erratic, causing severe losses in some orchards in some years and little or no significant damage in others. This erratic occurrence is attributed to differences in the availability of overwintering inoculum, the specific requirements governing infection, variations in specific local weather conditions, and the stage of development of the cultivars available. The destructive potential and sporadic nature of fire blight, along with the fact that epidemics often develop in several different phases, make this disease difficult and costly to control.

Symptoms

Overwintering cankers harbouring the fire blight pathogen are often clearly visible on trunks and large limbs as slightly to deeply depressed areas of discolored bark, which are sometimes cracked about the margins. The largest number of cankers, however, are much smaller and not so easily distinguished. These occur on small limbs where blossom or shoot infections occurred the previous year and often around cuts made to remove blighted limbs. Since many of these cankers are established later in the season, they are not often strongly depressed and seldom show bark cracks at their margins. Also, they are often quite small, extending less than 2.5 cm, with reddish to purple bark that may be covered with tiny black fungus fruiting bodies (most notably Botryosphaeria obtusa, the black rot pathogen of apple).

Blossom blight symptoms most often appear within one to two weeks after bloom and usually involve the entire blossom cluster, which wilts and dies, turning brown on apple and quite black on pear. When weather is favorable for pathogen development, globules of bacterial ooze can be seen on the blossoms. The spur bearing the blossom cluster also dies and the infection may spread into and kill portions of the supporting limb. The tips of young infected shoots wilt, forming a very typical “shepherd’s crook” symptom. Older shoots that become infected after they develop about 20 leaves may not show this curling symptom at the tip. As the infection spreads down the shoot axis, the leaves first show dark streaks in the mid veins, then wilt and turn brown, remaining tightly attached to the shoot throughout the season. As with blossom infections, the pathogen often invades and kills a portion of the limb supporting the infected shoot. The first symptom on water sprouts and shoots that are invaded systemically from nearby active cankers is the development of a yellow to orange discoloration of the shoot tip before wilting occurs. In addition, the petioles and mid veins of the basal leaves on such sprouts usually become necrotic before those at the shoot tip.

Depending on the cultivar and its stage of development at the time infection occurs, a single blossom or shoot infection can result in the death of an entire limb, and where the central leader or trunk of the tree is invaded, a major portion of the tree can be killed in just one season. In general, infections of any type that occur between petal fall and terminal bud set usually lead to the greatest limb and tree loss. In addition, heavily structured trees tend to suffer less severe limb loss than those trained to weaker systems for high productivity. Where highly susceptible apple rootstocks (M.26, M.9) become infected, much of the scion trunk and major limbs above the graft union very typically remain symptomless, while a distinct dark brown canker develops around the rootstock. As this rootstock canker girdles the tree, the upper portion shows symptoms of general decline (poor foliage color, weak growth) by mid to late season. In some instances, the foliage of trees affected by rootstock blight develop early fall red color in late August to early September, not unlike that often associated with collar rot disease caused by a soil borne fungus. Some trees with rootstock infections may not show decline symptoms until the following spring, at which time cankers can be seen extending upward into the lower trunk.

Disease Cycle

The bacterial pathogen causing fire blight overwinters almost exclusively in cankers on limbs infected the previous season. The largest number of cankers and, hence, those most important in contributing inoculum, occur on limbs smaller than 38 mm in diameter, especially around cuts made the previous year to remove blighted limbs. During the early spring, in response to warmer temperatures and rapid bud development, the bacteria at canker margins begin multiplying rapidly and produce a thick yellowish to white ooze that is elaborated onto the bark surface up to several weeks before the bloom period. Many insect species (predominantly flies) are attracted to the ooze, and subsequently, disperse the bacteria throughout the orchard. Once the first few open blossoms are colonized by the bacteria, pollinating insects rapidly move the pathogen to other flowers, initiating more blossom blight. These colonized flowers are subject to infection within minutes after any wetting event caused by rain or heavy dew when the average daily temperatures are equal to or greater than 16 °C while the flower petals are intact (flower receptacles and young fruits are resistant after petal fall). Once blossom infections occur, early symptoms can be expected with the accumulation of at least 57 degree days (DD) greater than 13 °C which, depending upon daily temperatures, may require 5 to 30 calendar days.

With the appearance of blossom blight symptoms, the number, and distribution of inoculum sources in the orchard increase greatly. Inoculum from these sources is further spread by wind, rain, and many casual insect visitors to young shoot tips, increasing the likelihood of an outbreak of shoot blight. Recent research conducted in Pennsylvania indicates that aphid feeding does not contribute to shoot blight. More research is needed to determine whether or not leafhopper’s play a role in the incidence of shoot blight. Most shoot tip infections occur between the time that the shoots have about nine to ten leaves and terminal bud set, when sources of inoculum and insect vectors are available, and daily temperatures average 16 °C or more.

In years when blossom infections do not occur, the primary sources of inoculum for the shoot blight phase are the overwintering cankers and, in particular, young water sprouts near these cankers, which become infected as the bacteria move into them systemically from the canker margins. Such systemic shoot infections, called canker blight, are apparently initiated about 111 DD greater than 13 °C after green tip, although visible symptoms may not be apparent until the accumulation of at least 157 DD greater than 13 °C after green tip. In the absence of blossom infections, the development of shoot blight infections is often localized around areas with overwintering cankers.

Although mature shoot and limb tissues are generally resistant to infection by E. amylovora, injuries caused by hail, late frosts of -2 °C or lower, and high winds that damage the foliage can create a trauma blight situation in which the normal defense mechanisms in mature tissues are breached and infections occur. Instances of trauma blight are known to occur even on normally resistant cultivars like ‘Delicious’.

Rootstock blight, yet another phase of fire blight, has been recognized recently and is associated primarily with the highly susceptible M.26, M.9 and Mark rootstocks. On these trees, just a few blossom or shoot infections on the scion cultivar can supply bacteria that then move systemically into the rootstock where a canker often, but not always, develops and eventually girdles the tree. Trees affected by rootstock blight generally show symptoms of decline and early death by mid to late season, but may not be apparent until the following spring.

(c) Dr. Heinrich Denzer, Pessl Instruments GmbH, Weiz, 2007

Model Cougar Blight for Pears

Symptoms
The model requires the user to recognize specific and ever-changing local events and aspects of their orchard that may increase or decrease fire blight risk relative to other orchards in the region. The model requires the user to assume there is a risk of fire blight infection whenever blossoms are present on the trees, especially during the petal fall and “post-bloom” period, when scattered blossoms may remain on many apple and pear varieties. The model user is asked to carefully assess the situation on their specific site and to initiate control measures if blossoms are present, risk levels are “High” or “Extreme,” and blossom wetting is likely to occur sometime during the next 24 hours.

Model Structure
Temperatures and Wetness: The key Fire Blight process that must be modeled is the potential for bacterial growth on the stigmas of apple and pear flowers. This growth is temperature dependent, so dependable prediction of infection risk requires the use of a measurement method that most accurately reflects the growth of Erwinia amylovora colonies. The main disagreement among models is how this should be done.

Fire Blight Model Output in FieldClimate
The Cougarblight model estimates the bacterial growth rate with degree hours based on a specific growth rate curve. This growth curve is based on the growth rate of E. amylovora bacteria in laboratory tests. The degree hour values are accumulated each hour of the day that temperatures are over 15 °C. The hourly values increase as temperatures rise from 15 °C to 29°C, decline at higher temperatures, and reach zero for an hour with temperatures over 40°C.

Model Blossom Blight for Pears

1. Flower must be open with stigmas and petals intact, stigmas have to be exposed for colonization, flowers in petal fall are resistant;
2. Accumulation of at least 110 °C hours > 18.3°C within the last 66 °C days > 4.4 °C defines the epiphytic infection potential for the oldest open and hence most colonized flower in the orchard
3. A wetting event occurring as dew or 0.2 mm of rain or 2.5 mm of rain the previos day allows movement of bacteria from colonized stigmas to the nectarthodes
4. An average daily temperature of >= 15.6 °C: This may influence the rate at which the bacteria migrate into the nectarthodes as well as the multiplication of bacteria needed to establish infections.

When all four of these minimum requirements are met in the sequence shown, infections occur and the first early symptoms of blossom blight can be expected to appear with the accumulation of an additional 57 °C days > 12.7 °C. This can be 5 to 30 days after infection. When the orchard conditions are less than these minimum requirements, few or no symptoms occur and no significant epidemic develops. (STEINER P.W. 1996)

Graphical Presentation Fire Blight on Pears

In FieldClimate, the two fireblight models are displayed in one graph. The Cougar Blight model is named Fire Blight DIV and the Maryblight model is named Blossomblight. To interpret the Cougar Blight results the graph is underlaid in 5 different colors. The distribution of this colors is made on the base of the settings on the first blight history of the orchard. The 5 colors are indicating the risk class for the DIV values.

The opportunity of a Blossom Infection is indicated by a bar ranging from 0 – 1 (conditions are fulfilled or not) in the same graph. Settings about the history of the orchard are not integrated into this model. Each time a bar with blossom infection is calculated in FieldClimate.com is an infection with fireblight!

Practical Use Fire Blight on Pears

Aim of the Fire Blight Models is it to assess the probability of infections by Erwinia amyloflora in the orchard.

The Mary Blight model which is evaluating for blossom blight is very well indicating infection situations of high economic impact. For this reason it is quite frequently used to indicate the use of antibiotics against this pathogen.

Cougar Blight is giving information about the risk of fire blight infections do to the overall propagation possibilities of the pathogen. Its weighting done by the history of an orchard is very helpful to indicate to us how carefully we have to check the orchards for fire blight symptoms even in situations where Mary Blight will not indicate an infection.

(c) Dr. Heinrich Denzer, Pessl Instruments GmbH, Weiz, 2008