Exploring Fire Blight Management, Part 2: Infection Sources
The most serious disease of pear and apple worldwide is fire blight, caused by the bacterium Erwinia amylovora. Infections can occur through various routes and the bacteria attack all parts of a tree, so symptoms are referred to by the plant part affected: blossom blight, shoot or twig blight, fruit blight, limb and trunk blight, and collar or rootstock blight. The main reservoir of the bacteria is in cankers of infected trees. When the trees break dormancy in the spring, bacteria in cankers begin to multiply and can be spread to susceptible tissues, especially open blossoms and to a lesser extent into new shoots. Secondary infections can occur throughout the growing season, both from infected shoots and from oozing cankers. Rain, wind and insects can spread the disease. Unfortunately, there are sources of bacteria and routes into susceptible tissues than cannot be controlled. Researchers from Germany have observed that some blossom infections can actually come from within a previously infected tree (Moltmann and Viehrig 2007). In such situations, systemically infected trees showed blossom blight before it was warm enough in the spring for fire blight cankers to be a source of bacteria. In other words, the blossom infections occurred from bacteria moving systemically within the shoots rather than being moved into open blossoms from outside the tree. Antonet Svircev from AAFC in Vineland supports this concept based on observations that pear shoots cut in the winter (no chance of bacteria from cankers elsewhere in the orchard in winter) and forced to blossom in the lab do sometimes have E. amylovora present. These findings are a reminder that systemically infected trees are best removed entirely from the orchard so as not to be a continual source of secondary infections, early and late in the season. It is also unfortunate that one of the primary insect vectors of the bacteria is one which is needed for pollination. Honey bees regularly pick up the bacteria from infected blossoms and unwittingly move it to other blossoms in their quest for nectar and pollen in the orchard. Austrian researchers have been working to overcome this problem by having honeybees deliver microbial fire blight antagonists (microbes that out-compete fire blight bacteria in the blossoms). Although more work is needed to fine tune dispenser construction and product formulation, the work has so far shown that honeybees can easily be manipulated to carry effective loads of strains of Aureobasidium pullulans to blossoms. A. pullulans is a fungus that competes with the fire blight bacterium but the product ("Blossom Protect") is not yet available in Ontario. Other insects such as pear psylla are also implicated in the spread of fire blight, so insect pest management is directly related to fire blight management. Rain can spread E. amylovora from tree to tree or blossom to blossom. In fact, the bacteria can survive in water for some time. Researchers form Spain have demonstrated that the bacteria survive best in cooler water (5° C versus 26° C) and that they can survive in nutrient poor water and retain the ability to infect plants for at least 45 days and as long as three years (Biosca et al. 2007). The mere presence of E. amylovora is nothing to be alarmed at though; there are bacteria everywhere, and E. amylovora in particular has been found in cherry and dandelion blossoms near pear orchards (Moltmann and Viehrig). Infected tissues (especially oozing cankers) remain as the biggest source of secondary infections. A lively discussion on the appropriate time and methods of pruning fire blight strikes took place at the International Fire Blight Workshop this year. There is still a variety of opinions on pruning and especially on whether to remove prunings immediately or let them dry down in row middles before mowing. Part of the disagreement comes from the large number of prunings some areas have to deal with during the growing season as well as the uncertainty of whether workers can be careful enough to avoid spreading fire blight from actively oozing cankers. A few prunings in the row middles are likely not a significant source for fire blight spread but improperly handling those prunings on the way out of the orchard may be. Most participants agreed that sterilization of pruning tools was not necessary to avoid spreading the bacteria as long as shoots and wood were dry (remember the bacteria live quite well in water). Some felt that occasionally cleaning pruning tools with straight Lysol was prudent. Philion et al. (2007) found no benefit from sterilizing pruning tools when working with a young orchard newly infected with fire blight. They also showed that pruning reduced the severity of the disease but not disease incidence. When considering the question of when and if to prune out fire blight strikes, it is a good idea to question how much chance there is of continued spread through the year. If high winds, heavy rains, or lots of insect activity (including pear psylla) are forecast, natural spread could be significant if infections are not removed. References:Biosca, E G., R.D, Santander, E. Marco-Noales, M. Ordax, B. Aguila and M.M. Lopez. 2007. Erwinia amylovora survives in natural water: influence of temperature. 11th International Workshop on Fire Blight, Paper P9. Moltmann, E. and M. Viehrig. 2007. Blossoms of infected pear trees may be sources of primary infection for fire blight. 11th International Workshop on Fire Blight, Paper O2. Philion, V., J. Charest and V. Toussaint. 2007. Natural epidemic of fire blight in a newly planted orchard and effect of pruning on disease development. 11th International Workshop on Fire Blight, Paper O49. Learn more
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