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Disease Cycle and Epidemiology

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Disease Cycle

Overwintering

Erwinia amylovora overwinters in a small percentage of the annual cankers that were formed on branches diseased in the previous season. These overwintering sites are called “holdover cankers”. As temperatures warm in spring, the pathogen becomes active in the margins of holdover cankers. Free bacterial cells are released onto the bark surface, sometimes as visible ooze. Insects attracted to the ooze (e.g., flies) or rain disseminate the bacteria from the canker to blossoms.

Floral epiphytic phase

Stigmas, which are borne on the ends of the style, are the principal site of epiphytic colonization and growth by E. amylovora. During the floral epiphytic phase, the ultimate population size that the pathogen attains is influenced by temperature, which regulates the generation time of the pathogen, and by the number of blossoms in which the pathogen becomes established, which is facilitated by pollinating insects, honey bees in particular. Under ideal conditions, stigmas of each flower can support ~10⁶ cells of the pathogen.

Primary infection in flowers

Blossom blight is initiated when cells of E. amylovora are washed externally from the stigma to the hypanthium (floral cup). On the hypanthium, E. amylovora gains entry to the plant through secretory cells (nectarthodes) located on the surface. In pear, the importance of blossom blight is expanded further by the tendency of this species to produce nuisance, secondary or “rattail” blossoms during late spring and early summer, long after the period of primary bloom.

Secondary phases

This includes shoot, fruit, and rootstock blight. These phases are usually initiated by inoculum produced on tissues diseased as a result of blossom infection. Wounds are generally required by E. amylovora to initiate shoot and fruit blight. Insects, such as plant bugs and psylla, create wounds on succulent shoots during feeding. Strong winds, rain, and hail can create numerous, large wounds in host tissues. Infection events induced by severe weather are sometimes called “trauma blight.” Rootstock blight of apple can result from shoot blight on water sprouts or from internal translocation of E. amylovora from infections higher on the tree.

Canker expansion

Both primary and secondary infections can expand throughout the summer, with the ultimate severity of an infection being dependent on the host species, cultivar, environment, and age and nutritional status of the host tissues. Young, vigorous tissues and trees are more susceptible to fire blight than older, slower growing tissues or trees. Similarly, trees that have received an excess of nitrogen fertilizer, and therefore are growing rapidly, are more susceptible than trees growing under a balanced nutrient regime. Rates of canker expansion also can be enhanced by a high water status in a tree caused by excessive or frequent irrigation or poorly drained soils. Canker expansion slows in late summer as temperatures cool and growth rates of trees and shoots decline.

Epidemiological models

Blossom blight is sporadic from season to season owing to the requirement for warm temperatures to drive the development of large epiphytic populations. Several epidemiological models (e.g., COUGARBLIGHT, MARYBLYT) predict the likelihood of blossom blight epidemics based on observed climatic conditions (Figure 11). The models work by identifying the periods conducive for epiphytic growth of E. amylovora on blossoms before infection occurs, and thus are used widely to aid decisions on the need for and timing of chemical applications. Blossom blight risk models accumulate degree units above a threshold temperature of 15.5 (60°F) or 18°C (64°F). Data on rain or blossom wetness during periods of warm weather are also used in the models to indicate more precisely the timing and likelihood of blossom infection. Other temperature-based models predict the time to symptom expression after an infection event (i.e., the length of the incubation period) based on heat unit sums. These models are used to time orchard inspections and/or pruning activities.

Figure 11

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