Reports on Plant Diseases

Fire blight, caused by the bacterium Erwinia amylovora, is a common and very serious bacterial disease. The disease is also referred to as blossom blight, spur blight, fruit blight, twig blight, or rootstock blight – depending on the plant part that is attacked. Erwinia amylovora infects approximately 75 different species of plants, all in the family Rosaceae. The hosts for this bacterium include apple, blackberry, cotoneaster, crabapple, firethorn (Pyracantha), hawthorn, Japanese or flowering quince, mountain-ash, pear, quince, raspberry, serviceberry, and spiraea. The cultivated apple, pear, and quince are the most seriously affected species, but many ornamentals serve as overwintering hosts for the bacterium and are important sources of new infections each year.

Click on image for larger version Figure 1. Infected blossoms wilt, turn light to dark brown on apple, black on pear.

Back to Top Disease Cycle The fire blight bacteria overwinter in living tissue at the margins of trunk and branch cankers that were formed by infections initiated in previous years, and possibly in buds. The bacteria resume growth in the spring when temperatures are above 65° F (18° C) with survival favored by rain, heavy dews, and high humidity. By the time trees are blossoming, ooze containing bacteria are present on the surface of cankers. Relatively few cankers survive winter, become active, and produce bacteria in the spring. However, a single active canker will produce millions of bacteria, enough to infect an entire orchard. The cankers most likely to produce bacteria in the spring are those with smooth margins between healthy and infected tissue, and those formed in older wood. Cankers produce bacteria in droplets of ooze that are spread by splashing rain or insects (mostly bees, flies, and ants) to open blossoms. The bacteria multiply rapidly on the blossom and invade the tissue through the nectaries (non-cutinized or flower parts). The bacteria then spread from blossom to blossom by rain or pollinating insects. The optimum temperature range for blossom blight infection is 65° to 86° F (18° to 30° C).

Click on image for larger version Figure 6. Fire blight on the M.9 root stock of an apple tree.

Succulent shoot tips are frequently infected by bacteria that have been spread from cankers and infected blossoms. The invasion of shoot tips can occur through natural openings, such as lenticels and stomata, but more commonly through wounds created by sucking insects such as aphids, leafhoppers, and tarnished plant bugs, by wind whipping, or by hail. The fire blight bacteria reproduce rapidly within an infected shoot. Droplets of ooze form on the shoots within three days. This ooze serves as a source of inoculum for the further spread of the disease. Shoots remain highly susceptible to infection until vegetative growth ceases and the terminal bud forms.

Warm (optimum temperature 76° F or 24° C) and moist weather is favorable for infection, and rapid growth encourages disease development. Nitrogen fertilization, late fertilizer application, poor soil drainage, and other factors that promote succulent growth or delay the hardening of the tissues from midsummer into autumn tend to increase the severity of this disease.

Dried bacterial ooze remains infectious for more than a year if it is not subjected to alternate wetting and drying. Contaminated boxes or other containers that are taken into orchards for fruit picking may serve as a potential source of infection.

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Control

No single method is adequate to effectively control fire blight. A combination of practices is needed to reduce the severity of the disease.

1. Choose the proper cultivars. Apple cultivars differ widely in their susceptibility to fire blight (Table 1 and Table 2). During warm and rainy weather, cultivars rated moderately susceptible or moderately resistant will develop shoot infections; however, the extent to which shoot infections progress will be less in resistant cultivars than in susceptible cultivars. Commercial growers should select rootstocks that are less susceptible to fire blight. Bartlett pears are extremely susceptible to fire blight and are not recommended for planting in Illinois.

2. Select planting sites with good soil drainage. Trees are more susceptible to fire blight in poorly drained sites than in well-drained ones. Tree productivity will also be lower on such sites. Drainage can often be improved by tiling.

3. Follow proper pruning and fertilization practices. Using nitrogen containing fertilizer and/or doing heavy pruning promotes vigorous growth and increases susceptibility. Fertilization and pruning practices on susceptible cultivars should be adjusted to limit excessive growth. For bearing trees, moderate shoot growth is 6 to 12 inches (15 to 30 centimeters) per year. If the growth is more than 12 inches, do not apply fertilizer until shoot growth is reduced to less than 6 inches.
   
   Apply fertilizer in the early spring (6 weeks before bloom) or apply in late fall after growth has ceased. Applications in midseason prolong the time during which shoots are susceptible to infection and increase the likelihood of winter injury to tender wood.

4. Prune out fire blight cankers during the dormant season. Delay the removal of infected shoots until the dormant season in order to avoid spreading infection to healthy shoots. Make pruning cuts at least 6 inches (15 centimeters) below the last point of visible infection. After each pruning cut, sterilize the pruning shears by dipping them in a freshly made solution of 1 part liquid bleach (Clorox, Purex, Saniclor, Sunny Sol) added to 4 parts of water. Examine the larger branches and trunks carefully for cankers, since these are likely to overwinter and produce new infections in the spring.
   
   Root suckers and watersprouts should also be removed because infection of these parts can lead to infection and death of entire trees. Certain dwarfing rootstocks used for apples are prone to suckering. Commercial growers should select rootstocks that are resistant to fire blight or that show little tendency to produce root suckers.
   
   Examine wild, neglected, ornamental hosts of the fire blight bacterium growing in the vicinity of home or commercial orchards for cankers. In addition to seedling apples, crabapples, pears, and quince, check hawthorns (Crataegus spp), firethorns (Pyracantha spp), cotoneasters, mountain-ashes (Sorbus spp), and spiraea. Remove the cankers when found or destroy the entire plant where feasible.

5. Follow a bactericide spray program. Like most bacterial diseases of plants, fire blight is very difficult to control; however, it can be reduced by spraying. Commercial orchardists should follow the spray schedule outlined in the annual Midwest Tree Fruit Pest Management Handbook. The antibiotic streptomycin is the most effective material for controlling fire blight; timely sprays will reduce the incidence of fire blight but must be applied before the appearance of symptoms.
   
   Temperatures at the prebloom and bloom stages are important in determining whether fire blight will occur in any given year. The bacteria reproduce only when the temperature is warmer than 65° F (18° C).
   
   The following concept was developed for predicting outbreaks of blossom blight in Illinois. The first idea to understand is that of a "heating degree-day." A "degree-day" occurs when the maximum daily temperature reaches 66° F (19° C). Start counting degree days after each spring frost. A freeze greatly reduces the number of fire blight bacteria in holdover cankers and on tree surfaces. Bacteria reach dangerous population levels ONLY after 30 degree-days have elapsed since the last frost. Degree-days may be accumulated in a variety of ways; for example, 2 days with a maximum daily temperature of 80° F (27° C), 3 days of 75° F (24° C), or 6 days of 70° F (21° C) following a freeze will provide enough accumulated warmth to allow bacterial populations to increase greatly in number and present a serious fire blight threat to blossoms. When 30 degree-days have occurred and when blossoms are still present (including secondary bloom), apply the first streptomycin spray. Repeat the spray at 4-day intervals through the bloom period. At temperatures above 86° F (30° C), bacteria will not multiply. Therefore, it is not necessary to apply streptomycin when the temperatures average below 65° F or above 86° F.
   
   Dr. Paul Steiner at the University of Maryland has developed a computer software package which very accurately predicts the potential for fire blight occurrence. The prediction is based upon temperature and moisture during bloom. This computer software is called MARYBLYT and is available from the University of Maryland, Department of Botany, College Park, MD 20742.
   
   Streptomycin can effectively protect the susceptible apple and pear flowers, but for maximum effect it must be applied the day of, or the day before infection event occurs. Missing the critical window of effectiveness by even 24 hours can result in plant infection and buildup of a significant amount of bacteria for later infections. If the blossom blight is well controlled, the subsequent increase of fire blight in summer is often prevented. To prevent development of streptomycin-resistant strains of the pathogen, no more than 4 applications of streptomycin per season is recommended. Streptomycin is more effective in preventing blossom infection and the management of the shoot blight phase of fire blight should not be attempted with streptomycin. However, application of streptomycin immediately following hail storms is highly recommended. Streptomycin is most effective when applied alone, as a dilute spray, under slow drying conditions (generally between 10 p.m. and 3 a.m.), and when daytime temperatures reach 65° F or above. Apply 100 parts per million (ppm) of streptomycin if the temperature is below 65° F, and 50 ppm if the temperature is above 65° F.
   
   Bordeaux mixture (6-6-100), made by mixing 6 pounds of crystalline copper sulfate (bluestone or blue vitriol) and 6 pounds of fresh hydrated spray lime in 100 gallons of water, will help control fire blight but may cause russeting of the fruit. Bordeaux mixture is recommended for use by growers who had a severe epidemic the previous year. Bordeaux mixture should be applied at the green tip to quarter inch green stage of flower bud growth. Do not mix bordeaux with other chemicals, and use it as soon as it is prepared. Do not follow bordeaux mixture with streptomycin, and do not concentrate bordeaux mixture greater than 2 times. Bordeaux is not as effective as streptomycin in controlling fire blight.
   
   Copper sulfate (4 lb/100 gal or 2kg/400 liters), applied when trees are dormant in early spring also helps reduce the number of bacteria present in ooze on cankers and thus slows the buildup of bacteria in the orchard prior to bloom.

6. Control sucking insects. Good control of aphids, leafhoppers, plant bugs, and psylla on pears helps prevent shoot infection. Commercial orchardists should follow a spray program outlined in Midwest Tree Fruit Pest Management Handbook. [Copies of the circular mentioned are available at your nearest Extension Office or from Information Technology and Communication Services (ITCS), 1401 S. Maryland Dr., Urbana, IL 61801 (217/333-2007).]

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Table 1. Relative Susceptibility of Common Apple Cultivars and Rootstocks to Fire Blight.

Table 2. Modern Crabapples not suggested for Illinois but highly resistant or immune to Rusts,
Scab, Fire Blight, Powdery Mildew and Frogeye Leaf Spot

For further information concerning diseases of crucifers and other vegetables, contact Mohammad Babadoost, Extension Specialist in Fruit and Vegetable Pathology, Department of Crop Sciences, University of Illinois at Urbana-Champaign.

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