Pests AND environmental PROBLEMS of High Altitude Landscapes

Jessica Hendryx B.S. in Horticulture Science with a Landscape Design Option ~ Montana State University, Bozeman

Fire Blight (Erwinia)

Susceptible Plants to Fireblight: Makamik Crab (Malus 'Makamik'); Red Splendor Crab  (Malus 'Red Splendor'); Sargent Crab (Malus sargentii)

Less Susceptible Plants: Professor Spenger Crab (Malus 'Professor Sprenger'); Thunderchild Crab (Malus 'Thunderchild')

Alternative plant in the landscape: Ohio Buckeye (Aesculus glabra)

What is Fire Blight?

Fireblight is the most devastating bacterial disease affecting apple, pear, and rosaceous plants. It is caused by the bacteria Erwinia amylovora. The disease is very common among the Flowering Crabapples (Malus baccata) planted in higher altitudes. It is also common on mountain ash, hawthorn, cotoneaster, and apple trees all of which grow well in cooler climates. There are three factors that allow the infection to occur. The bacteria needs a susceptible host, a virulent pathogen, and favorable weather conditions. 

How is it spread?

The bacteria overwinters in the cankers formed from previous years. In the spring an orange-colored ooze of bacteria flows from the cankers and is transported by water, insects, and humans. An opening into the plant is needed for infection. Open flowers, young leaves, and vigorously growing shoot tips contain very small openings that allow the bacteria to enter. The bacteria can also enter in any open wound caused by environmental conditions, insects, and mechanical damage. Storms high in condensation can be the perfect tool to help spread the bacteria. The wind will often damage the plant creating an opening and the moisture will aid in the transportation.

The most susceptible time for infection is in the spring to early summer when moisture levels are high or high humidity (>65%) and the temperature is moderate (>65F). Insects, particularly sucking insects, such as aphids, can cause a large amount of damage because not only do they create an opening for the bacteria to enter, but they can also carry the disease from an infected plant, spreading it to every plant they come in contact with. Bees can also spread the bacteria from flower to flower.    

What does the bacteria do to the plant?

After the bacteria enters the plant it begins to kill plant cells, giving rise to typical symptoms, which are similar to drought. Eventually the bacteria kills enough plant cells around a limb that water to the upper portion is restricted. The branch turns brown to black and wilts quickly. If the plant is severely infected it will, over time, kill the plant.

Symptoms:

 The most common symptom of fire blight is the fire scorched appearance of shoots (see Figure A). Leaves, stems, flowers, and fruit can all show signs of blight. Infected leaves will begin to show signs of marginal necrosis (localized death). This will start along the edges and work inward giving the appearance of burnt edges (see Figure B). Eventually the entire stem will wilt causing the tip to curl creating a 'crook or 'u' shape, giving this symptom the name, shepard's-crook. Shepard's-crook is usually followed by a gray-green discoloration of the stem and ooze. Cankers will eventually form on the branches where the bacteria is present. The bark in these areas is darker and sunken (See Figure D).  When the inner tissue is exposed it is water-soaked with red to brown streaks. The flowers, after being infected, will appear water-soaked and gray-green as well. A milky ooze, which carries the bacteria may also be present. The entire flower will then turn brown to black and along with the leaves will remain on the tree through the winter (See Figure E). The fruit can also be infected at any stage of development. Fruit infected early will remain small, shriveled, dark, and attached to the tree. Hail or insect injury will often result in red, brown, and black lesions. A milky ooze may be present, which later becomes red to brown, and glassy when dry (See Figure F).

Overall: Look for entire branches of dead leaves that are persistent through the winter. Check for cankers and the orange-colored ooze. Cut away the bark where the cankers are present and look for the red and brown streaks.

What can you do about it?

Check the plant periodically for cankers and ooze and remove the lesions no less than 20cm below the area of infection. Sterilize pruning tools between each cut to help prevent spreading the disease from one plant to the next, even from one branch to the next. A liquid solution of 1 part bleach to 9 parts water or rubbing alcohol can be used to sterilize the pruning tools. Remove all prunings from the plant and dispose of them by burning or bagging and properly discarding. Insects play a large role in the spread of the disease. Insecticides can be used to control the insects however pollination will be limited resulting in loss of fruit.

The most successful approach to controlling this disease is by using less susceptible varieties. Some examples include Professor Spenger Crab - Malus 'Professor Sprenger' and Thunderchild Crab - Malus 'Thunderchild'. Keep in mind that no plant is ever resistant when in the same family. If Fireblight is a problem in the area where it is to be planted try using a substitute plant such as Ohio Buckeye - Aesculus glabra. This tree is slow growing, has beautiful fall color, and has an ornamental form.  When Crabapples are desired avoid planting these varieties: Makamik Crab - Malus 'Makamik', Red Splendor Crab - Malus 'Red Splendor', and Sargent Crab - Malus sargentii, compared to other varieties they are more susceptible to fireblight. 

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Reference:

Jones, A.L. Aldwick, H.S. Compendium of Apple and Pear Disease. American Phythopathological Society Press: 1990 Pgs 61-63

Montana State University Extension Service 

Designed and researched by Jessica Hendryx - B.S. in Horticulture with a Landscape Design Option
For problems or questions regarding this web contact [martha@montana.edu].
Last updated: May 08, 2002.