Control of Fire Blight on Fruit Trees

GardenLine | Trees and Shrubs | Control of Fire Blight on Fruit Trees

Russell Friesen

Humid summers for the past few years have led to an increased number of fire blight reports in some areas of Saskatchewan. Fire blight is a serious and potentially lethal bacterial disease which affects a wide range of trees in the rose family. The trees most commonly affected are: apple, crabapple, pear, mountain ash, and cotoneaster. While the disease may also be found on plum, raspberry and saskatoon, cases on these species are rare.

Symptoms

Fire blight is named for the red, fire-scorched appearance of diseased leaves. Blighted leaves eventually brown and die, but remain attached to the tree. Other symptoms include a dramatic downward wilting at the tips of new growth, resulting in a distinct "shepherd's crook" on the ends of infected twigs. Clear amber liquid may be found oozing from diseased twigs. This liquid is highly charged with the bacterium which causes the disease, and transfer of even minute traces of it to healthy trees can generate new infections. On badly infected trees, bark cankers may develop on the trunk and larger branches. Such cankers appear as indented, discoloured areas which may also exhibit bacterial ooze.

Transmission

Any agent capable of transporting bacteria can lead to spread of fire blight. Wind or rain splash may be all that is needed to carry minute traces of the bacteria from diseased to healthy branches, leading to infection. Insects and infected pruning tools also serve as effective means of disease transmission. Because bees and other pollinating insects travel to many different trees as they forage, they can quickly spread the disease from a single infected tree to numerous healthy ones. Drying does not seriously reduce the viability of the bacterium, as it will dry and lay dormant for long periods of time on pruning equipment which was previously used on infected trees. When contaminated tools are later used on healthy trees, the disease is spread as the bacteria gain direct entry to new wounds on healthy trees.

Control

Removal pruning of all diseased wood is the primary means of control. However, since the act of pruning can also serve as a means of disease transmission, it is critical that correct procedures be followed. All cuts should be made a minimum of 25 cm (10 inches) into healthy wood. This helps assure that no traces of diseased wood are left. After making each cut, the cutting blade must be sanitized. Dipping, wiping or spraying the blade of you pruning tool with a solution of household bleach and water at a concentration of one part bleach to nine parts water will kill any bacteria on the cutting blade, and prevent the further transmission of the disease.

Increased growth of cankers on major branches can be arrested by cleaning the area with a sharp knife, and removing a strip of bark around the infected area. This will help to isolate each canker, and prevent its spread. Again, the knife blade must be sterilized each time a new cut is made.

Following pruning, all diseased branches should be burned or buried to prevent re-infection. Branches which are not properly disposed of, will continue to harbour the disease, and serve as a source of potential re-infection. Cotoneaster hedges also serve as reservoirs of this disease. Since it is essentially impossible to sanitize hedge shears between each cut made during shearing, the disease is often spread during the act of shearing a hedge. For this reason, it is best to inspect cotoneaster hedges for any signs of infection prior to shearing. Where disease is suspected, branches should be fully removed well below the point of infection. While this may leave a gap in your hedge, it will help prevent even greater losses to disease.

An effective chemical cure for fire blight is unknown. Those sprays which are presently used, function more as preventatives than curatives. Most of the registered chemicals are fungicides which use elemental sulphur or copper as the control agent. The antibiotic streptomycin is also registered for use on apple and pear trees.

Prevention

Succulent new growth is highly vulnerable to fire blight invasion. For that reason, suckers and water sprouts should be removed when they appear in apple trees. Clean cultivation below trees, application of excessive water or high nitrogen fertilizers near susceptible trees also increase the succulence of the growth, increasing the chance of disease invasion.

Planting resistant cultivars is another recommended means of fire blight prevention. A number of apple and crabapple cultivars demonstrate good resistance to fire blight. `Adanac,' `Breaky,' `Goodland,' `Harcourt,' `Heyer 12,' `Heyer 20,' `Kerr,' `McLean' and `Patterson' are all listed with good resistance to fire blight. Of these, `Goodland' is perhaps the tastiest, `Heyer 12' is the most cold hardy, and `Patterson' combines both good flavour and hardiness. The popular apple cultivars `Westland' and `Battleford' are only moderately resistant, while `Norland' apple is considered susceptible.

Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.