Fire Blight - When to Prune and When Not to Prune

Fire blight is one of the most serious diseases of apples and pears worldwide. Several orchards in Ontario became infected with fire blight during the 2003 season. The pathogenic bacteria Erwinia amylovora that causes fire blight can infect all plant parts of susceptible apple and pear trees including blooms, fruit, leaves, shoots limbs, trunks and rootstocks. Once the disease gets into the trunk or rootstock of a tree, the tree usually dies.

When fire blight infects an orchard, sometimes a few strikes are noticed, where as in other cases, several fire blight strikes are observed. Growers often ask what they should do? Should the fire blight strikes be pruned out immediately or should they be left until the dormant season? On one hand, excessive pruning during the summer may stimulate trees to produce succulent shoots that are very susceptible to fire blight infection, which could make the situation worse. On the other hand, if the fire blight strikes are left in the orchard, they will serve as a source of inoculum should conditions for pathogen dissemination and infection occur again in the season. Researchers in Israel studied the timing of pruning on the effectiveness of controlling fire blight in infected pears. Their results and conclusions are probably just as relevant in North America and may be applied to apples.

In replicated trials, researchers pruned out fire blight strike at different times during the season and recorded the growth habits of the infected tree before and after pruning. They also recorded whether the disease was eradicated or if became more severe after pruning. They concluded that the decision to prune out strikes during the growing season depends upon the time of season the fire blight is noticed (spring or fall), the growing condition of the tree (growing vigorously or growing normal and limited), and the tissue exhibiting symptoms (limb canker or blighted shoot).

Pruning out shoot and blossom strikes from trees with many fire blight strikes early in the growing season stimulated the trees to produce succulent shoots, which made the tree more susceptible to fire blight infections. Strikes on low vigor trees that were not pruned appeared to only colonize the infected spur and did not move into the major limbs or trunks. They concluded that trees with many strikes and low vigor growth should not be pruned until the dormant season.

Strikes in trees regardless of vigor that became infected late in the season either due to injury caused by hail (Trauma blight) or shoot blight should be pruned out immediately. Late season infections tend to be more dangerous in Isreal than early spring infections.

Pruning out cankers on major limbs and trunks early in the season did not eradicate the cankers from the trees. In fact, many more active and larger cankers were observed on trees that had been pruned early in the season than on trees that were not pruned. Less disease occurred the following season when pruning out cankers was delayed to mid-summer and even less when delayed until autumn. Best results were obtained when cankers on limbs and trunks were marked with bright paint and pruned out during the dormant winter season.

Based on the results of this study, pruning out fire blight strikes or cankers observed early in the season should be delayed until the dormant season unless there are few strikes to prune and the pruning will not stimulate the tree to produce succulent shoots. Late season infections should be pruned out immediately.

When pruning out fire blight, the cuts should be made about 30 cm (12 inches) or more below the visible symptom, into 2-year old wood or older. It is advisable to disinfect tools between cuts by dipping them into a 70-75% denatured ethyl alcohol solution (one part denatured alcohol to three parts water) or a 6% sodium hypochlorite solution (one part laundry bleach to four parts water) particularly if pruning is done during the growing season.