Fire blight

Symptoms

Blossoms and young shoots suddenly turn brown and then black, looking as if scorched by fire — hence the name of the disease. The tips of infected shoots bend downwards into a characteristic hook resembling a shepherd's crook. In humid conditions a milky, sticky bacterial ooze seeps from the infected tissue, and the infection spreads rapidly down the shoot, killing whole branches. Infected leaves and fruit dry out but remain attached to the tree for a long time. Pear and quince are the most severely affected, followed by apple, and among ornamental and wild plants — hawthorn, cotoneaster, firethorn (Pyracantha) and rowan, which can be a dangerous reservoir of infection. On the bark of the branches and trunk, sharply delimited, sunken necroses (bark canker) form with cracking bark; when the bark is cut, the tissue beneath is discoloured reddish-brown, often with marbled streaks. The drop of bacterial ooze eventually turns amber and dries, and the presence of the bacterium can be confirmed by the so-called streaming test — from freshly cut infected tissue immersed in water, threads of a cloudy bacterial suspension flow out. Infected fruit turns black and mummifies, remaining on the tree, which distinguishes fire blight from ordinary wilting caused by water shortage.

Treatment

Fire blight is a quarantine disease — its occurrence must be reported to the regional plant health and seed inspectorate. Infected shoots are cut out well below the visible symptom margin (at least 30–50 cm into healthy wood), disinfecting the tools after every cut, while heavily infected trees are removed and destroyed. In amateur cultivation there are no effective chemical agents against the bacterium. Sanitary pruning is carried out in dry, cool weather with a sharp tool disinfected after each pass (e.g. 70% ethanol or a hypochlorite solution), and the removed material is burnt or buried deep — never composted. There is no systemic treatment that destroys the bacterium inside the plant, so removing all the infected tissue with a margin is crucial. Protective treatments (copper compounds, antagonistic biological products) make sense only preventively, during flowering, and will not replace cutting out. With severe, extensive infection, the only option is sometimes to grub out and destroy the whole tree in order to limit the source of infection for neighbouring plantings.

Prevention

Planting less-susceptible cultivars and avoiding excessive nitrogen feeding and pruning that stimulates vigorous, easily infected growth. Regular inspection of pome fruit trees during flowering and after hailstorms, when fresh wounds facilitate bacterial entry, is advisable. Healthy, certified nursery material and establishing orchards away from clusters of wild hosts (hawthorns, cotoneasters, firethorns) are of key importance. The bacteria are spread by pollinating insects, rain and wind, as well as by contaminated tools, so pruning hygiene and limiting wounds are just as important as cultivar choice. It is helpful to use weather-based models for forecasting the risk of blossom infection (based on temperature and humidity during flowering), which point to high-risk days and the optimal moment for a protective treatment. It is also worth removing secondary blossoms (late, summer flowering), because they create an additional entry point for the bacteria in the warm season.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.