Bacterial Fireblight Pathogen (Erwinia amylovora)

Origins and history: The fireblight pathogen is a little different invasive species to consider for several reasons. First, it is not a plant or an insect that we can see, but a tiny, invisible-to-the- naked-eye microorganism. Second, this bacterium is not an exotic invasive species that came from abroad, but vice versa: it has spread in the past century from the U.S. to Europe and other areas. A third additional twist is that, though native to North America, Erwinia amylovora became a major problem only after highly susceptible fruit trees from Europe were brought to the New World and planted into orchards by colonists.

The first epidemics of this disease on apples and pears in the U.S. were reported from New York in 1780. By the early 1900s, the pear industry in Ohio was so devastated by fireblight that commercial pear orchards were largely eliminated in favor of Pacific Northwest locations which did not have warm, wet weather during bloom. Then the pathogen crossed the seas and showed up in New Zealand by 1919, in England by 1957, and in the Netherlands by 1966. From 1974 to 1998, the pathogen and fireblight disease has showed up anew in an additional 20 countries overseas.

Plants fireblight affects: Fireblight occurs only on plants in the rose family (the Rosaceae) and nothing but the rose family. A few of the most common hosts are apples and crabapples (Malus), eating and ornamental pears (Pyrus), firethorn (Pyracantha), and mountainash (Sorbus).

Conditions conducive to fireblight: The key environmental conditions for fireblight epidemics are extended periods of warm (over 62 degrees C) and wet weather during bloom. Wet weather later in the growing season can also lead to fireblight on shoots and leaves, but early blossom infections are often the key to a bad fireblight year. The Erwinia amylovora bacterium overwinters on stem and trunk cankers and is spread each spring by pollinating insects and splashing water.

Above: Stem cankers.

The damage fireblight causes: Effects of fireblight are blossom blight, leaf discoloration, shoot blight, stem dieback, trunk dieback and in some cases plant death. The bacterium moves from blossoms to shoot tissue, along leaf stalks (petioles) to leaf blades and along shoots into the main stem, resulting in fireblight "strikes" in which browned and blackened leaves on blighted shoots stand out on the tree in the classic shape of a "shepherd's crook" symptom. Highly susceptible plants such as certain apple and pear cultivars tend to have longer strikes (up to several feet) with the bacterium eventually moving through the stem into the trunk. Less susceptible plants such as the Callery pears may "tolerate" fireblight by delimiting strikes to several inches in length.

Control: Pruning out fireblight strikes a foot or more back of the obviously affected plant tissue is the main approach to fireblight in the landscape. If this pruning is done during the growing season, disinfest pruners between cuts with a Clorox solution; dormant pruning is sometimes safer relative to bacterium spread. Avoid highly susceptible cultivars.

Comments: Incidence of fireblight on crabapples and Callery pears was high in Ohio and much of the Midwest in the past few years (2001-2003), presumably due to unusually warm (several days in the 70s and 80s), wet weather during mid to late April in those years. The nursery industry is learning from these experiences and has improved screening techniques and informational resources relative to less fireblight susceptible cultivars of crabapples and ornamental pears.