Minimizing Apple Tree Loss by Choosing a Fire Blight Tolerant Cultivar and Rootstock

Published: March 2, 2018

Fire blight is a devastating bacterial disease in Missouri. It commonly causes loss of apple trees under favorable conditions. Young apple trees are particularly susceptible to fire blight. In young tissue, bacteria can spread from the upper branches through the graft union and into the rootstock in 21 days. Infection during the first three years after planting can cause tree mortality. After this age, it is less likely that fire blight will kill the apple tree by progressing into the rootstock.

Erwinia amylovora, the pathogen that causes fire blight, overwinters in cankers (sunken diseased areas) on the bark. When humidity is high in the spring, the pathogen oozes out of cankers and insects carry the bacteria from the cankers to flowers. Bacteria multiply rapidly in the blossom nectar and infect the flowers through natural openings called nectaries. Infected blossoms turn brown and die, usually with the flower parts remaining attached.

Figure 1. A terminal shoot infected with fire blight has the typical "shepherd's crook" symptom with dead leaves remaining on twigs.

Bacterial infections on new shoots of apple trees occur in the spring and early summer when foliage is wet and temperatures are between 60 to 75°F. Infected leaves and terminal shoots turn black and often form a shepherd's-crook of dead tissue (Figure 1). Often the pathogen moves from the infected shoot tissue into older parts of the tree, including the trunk and rootstock, and forms cankers. The pathogen can also colonize natural openings in the bark, such as lenticels, and areas where the bark is wounded from pruning cuts, hail, or mechanical damage. During pruning, the disease can be spread from tree to tree on the blades of shears.

Because fire blight is one of the most difficult diseases to control, select a resistant apple cultivar for planting (Table 1). When possible, also choose a rootstock with high resistance to fire blight, such as G.16, G.30, G.202, or G.41. Avoid highly susceptible rootstocks such as M.9 and M.26. When highly or moderately susceptible apple cultivars and rootstocks are planted, fire blight control will be necessary when conditions are optimal for disease development. For more information regarding fire blight control options, refer to MU Extension Guide G6020.

Table 1. Fire blight susceptibility of selected apple cultivars.