Fire Blight

By John Donan

 

Fire blight is a disease of members of the rose family which is characterized by blossom, twig and fruit blight and also stem cankers. It is caused by the bacterium, Erwinia amylovora. Its name appeared around 1740-50 from the burnt appearance of the foliage, its exact origin is unknown. This oldest, most serious bacterial disease of apple and pear had, at that time, appeared in the Hudson River Valley in New York. The disease is indigenous to North America, and probably occurred on native American plants such as crabapple, hawthorn and mountain ash and then spread to susceptible cultivated apples, pears and woody ornamentals planted by the early American pioneers. As the settlers moved west, so did fire blight. By the early 1900s it became established as a serious threat wherever apples and pears were grown in North America. Today it affects every apple growing region of the world except Australia which is fire blight free. This disease is so prevalent and important, should one search for 'fire blight' on the Internet, data will be found from extension offices for every state in the country. For the University of California the corresponding URL is http://ucipm.ucdavis.edu/PMG/r603100211.html

Apple, crabapple, pear, cotoneaster, hawthorn, firethorn (pyracantha) and mountain ash are some of the principal hosts for fire blight. Resistant species and/or cultivars of most hosts are available. In addition to these seven important hosts, there are plants in 33 additional genera (a total of 75 species can be attacked) in the family Rosaceae that are susceptible to fire blight. Some of these others are flowering almond, flowering an common quince, raspberry, rose, spirea, serviceberry, loquat and toyon.. Rubus species include both red and black raspberry and thornless blackberry. Cherry, peach and other related species of stone fruits are not affected by fire blight

The bacteria enter the host plant through natural openings in blossoms and leaves (especially new shoots,) or through wounds in the bark. Through the winter, bacteria remain in a dormant state in diseased twigs and at the edge of cankers. During warm spring rains, or moist or humid weather the milky ooze that exudes from infected tissues contains millions of bacterial cells. This exudate attracts flies, ants, aphids, beetles and other insects. They in turn carry the bacteria to blossoms, foliage and twigs. Although honeybees are capable of spreading the bacteria from blossom to blossom, they apparently do not visit cankers. Ants, beetles and flies, that feed on the ooze in cankers, as they visit blossoms in search of nectar, carry the bacteria with them. Wounding by insects, hail, pruning , wind whip or other damaging events are necessary for infection of twigs. Spreading of the bacteria downward within the structure of a tree or shrub also occurs by rain splash. New infections continue to develop in a plant until the spring flush of growth stops or until about a month after flowering. The disease can kill blossoms, fruit, shoots, limbs, and tree trunks. It can even kill an entire tree.

When fire blight becomes serious in early spring, daily temperatures must average 60 degrees F or above. Symptoms appear quite rapidly. It is important to distinguish fire blight from other disorders by recognizing the presence of bacterial ooze characterized by droplets of red-brown sticky liquid that seeps from the surfaces of infected tissue. The bark in branch cankers and scaffold limbs becomes sunken, darker than normal and remains smooth. When the outer bark is removed the sapwood appears water-soaked with reddish streaks. These red streaks help distinguish this disease from low temperature injury. The cankers on large scaffold branches and the trunk eventually become cracked or creviced. When bacteria move through the pedicel to the fruit spur and out into the leaves, the leaf midrib and main veins soon darken after infection. The leaves wilt, turning brown on apple and quince and dark brown to black on pear. It is similar on other plants. The blighted leaves remain attached for much, if not all, of the growing season. A characteristic symptom of shoot blight is the bending in the shape of a shepherd's crook. In addition to occurrence on larger limbs, pearly or amber-colored droplets of bacterial ooze are often present on diseased blossoms, fruit, and leaf stems, on succulent shoot stems, and on the exterior of infected fruits. These droplets also contain bacteria in the millions, which may cause new infections. Tree vigor has a major influence on the extent of fire blight damage. Once established, the distance the pathogen moves relates directly to the rate of plant growth. Vigorously growing shoots are the most severely affected; therefore, conditions that favor rapid shoot growth, such as high soil fertility and abundant soil moisture, increase the severity of damage.

Experts disagree as to the effectiveness of sprays on this disease, however Bordeaux or Mancozeb are claimed to be controls for fireblight. Should one decide to try, it should be applied first at the pink stage of blossoms, 3 to 4 days before they open and then repeated every 5 to 7 days for about 3 weeks. Fire Blight cannot be controlled by spraying after this time. Remove and burn all twigs and branches having cankers. Any cutting out of the diseased wood when plants are dormant reduces the chance of spreading the bacteria on saws and cutting tools. All pruning tools should be disinfected after each cut by dipping them into a solution of 10% chlorox. (1 part chlorox to 10 parts water). Elimination of fire blight infections can be done by pruning out all diseased branches. Always cut an infected branch at least 8 to 12 inches below the visible injury or canker. A greater distance below infections may be required on major branches, scaffolds, or trunks in May or June, when blight bacteria are moving rapidly. The apearance of new infections below a pruning cut indicates that the cuts were not made far enough below the infection and the bacteria has spread beyomd.

After pruning,.one should be sure to remove all infected branches and leaves. Anything that falls to the ground should be picked up. It should be attempted to complete all pruning by late summer while dead or infected stems and leaves are visible. Fertilization infected plants should be done cautiously. Do not use excessive amounts of nitrogen. It is the young succulent tissue which is most susceptible to fire blight. After infection spraying is essential during the next flowering season. Commercial growers apply three antibiotic sprays the first application when 5 - 10 percent of the blooms are open and continue at 5-day intervals. Be sure to thoroughly cover all parts of the tree. If other susceptible plants are nearby, they should be sprayed as a preventative measure. For the home owner a certified nurseryman or licensed exterminator shoud be consulted before undertaking such action. Always follow label instructios to the letter before using pesticides. Explicit guidelines for the handling of garden chemials are listed below.



Warning on the Use of Chemicals
Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pets, and livestock. Confine chemicals to the property being treated. Avoid drift onto eighboring properties, especially gardens containing fruits Dispose of empty containers carefully. Follow label instructions for disposal. Never reuse containers. Make sure empty containers are not accessible to children or animals. Never dispose of containers where they may contaminate water supplies or natural waterways. Do not pour down sink or toilet. Consult your county agricultural Commissioner for correct ways of disposing of excess pesticides. Never burn pesticide containers.