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Fire blight

Erwinia amylovora (Burrill) Winslow et al.

Blossom blight occurs in the spring. Infected blossoms first exhibit a water soaking, followed by wilting and their eventually turning brown on apple and nearly black on pear (A). Individual flowers or the entire cluster may be affected. Typically, infected blossoms do not fall and bacteria progress into the tender shoot growth. In the shoot, the bacteria travel along the midvein of the leaves and they soon wilt, shrivel, and turn brownish black, killing the entire shoot. Bacterial ooze may be observed under warm and humid conditions (B). Flowers will cling to the infected stem and often remain attached throughout the season. Infected fruit appear black and shriveled and usually remain attached to the tree.

Shoot blight develops in late spring or early summer on actively growing terminal shoots, including suckers and water sprouts. Infected shoots first have an oily appearance and turn a dark green. On apple, the infected shoot becomes light to dark brown, in contrast to pear, in which the shoot becomes black. Blighted shoots often form a characteristic "shepherd's crook" at their tip and the dead leaves remain clinging to the affected twigs (C). The disease can progress to whole limbs and, when infection is severe, the whole tree appears to be scorched by fire, hence the name fire blight.

Rootstock blight is the result of infections that travel from infected blossoms or shoots into the rootstock, although direct infection of the rootstock through sucker growth is possible. Infected rootstocks are discolored and there is usually a sharp demarcation between infected rootstock tissue and scion tissue when the bark is peeled back (D). Under favorable conditions, the bacteria may ooze from the rootstock, leaving streaks down the rootstock.

Distribution

Occurs throughout the US and Canada but is most problematic in warmer production regions, such as the mid-Atlantic states.

Similar Species

Blossom and shoot blight can be confused with Nectria twig blight, but can differentiated by noting that fire blight infections progress from infected blossom clusters or the tips of shoots downwards, where Nectria infections progress from the base of shoots upwards. Rootstock blight can be confused with Phytophthora crown rot, but rootstocks infected with Erwinia do not exhibit the reddening that is characteristic of Phytophthora-infected tissue, particularly below the soil line.

Management

Managing blossom blight is achieved through well-timed bactericide sprays during bloom, usually streptomycin. After bloom, management focuses on minimizing shoot blight and the development of cankers that overwinter and serve as next year's inoculum source. It is important to prune out infected limbs as soon as symptoms are detected and before extensive necrosis develops. Pruning cuts should be made at least 30 cm below symptoms. Rootstock blight is best managed through the use of fire blight-resistant rootstock. Several of the newer dwarfing rootstocks have been selected for fire blight resistance.

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