Fire Blight

The fire blight pathogen can invade all parts of the pear tree. Shoots, flowers, and fruits wilt, blacken, and die. Shoot tips exhibit the typical shephards crook. If infections are not removed, the entire tree may be killed as the disease spreads into the main scaffolds, trunk, and roots.

Sticky, amber-colored droplets, containing millions of bacterial cells, exude from freshly blighted tissue. This can be used to distinguish fire blight from blossom blast infections.

Erwinia amylovora bacteria overwinter in limb cankers and are spread by rain splash and insects. Infection occurs mostly through blossoms and less often through succulent shoots. Growth of the bacteria is favored by warm, humid weather in spring or fall.

Fire blight development is influenced primarily by seasonal weather. Warm spring weather, accompanied by intermittent rain and hail, is ideal for disease development. Other influences on disease development are the varieties and rootstocks used in the orchard, location of the orchard, application of too much nitrogen fertilizer, heavy pruning, and over-irrigation. Management relies on maintaining trees in the proper range of vigor, applying protective blossom sprays (bactericides and biologicals), and most importantly, promptly finding, removing, and destroying blight strikes.

Blossom applications of copper materials, the antibiotics streptomycin and terramycin, or rotations with both are necessary in pear-growing areas to reduce the spread of fire blight bacteria. The timing of the first application is critical. In California, average daily temperatures or degree-hours are used to schedule fire blight sprays. Resistance to streptomycin is well documented, and in the past years, there have been indications of decreasing sensitivity to terramycin in the upper Sacramento Valley. Resistance to terramycin was reported several years ago in one Sacramento Delta orchard and in the Marysville and Yuba City production areas. Applications should be made in the context of a total IPM program. For detailed information on these methods, see Integrated Pest Management for Apples and Pears 2nd ed., UC ANR Publication 3340.

Antagonistic microorganism formulations are commercially available to prevent colonization of the blossoms by Erwinia amylovora during bloom. Certain products also suppress frost formation and fruit russeting, thus having multiple uses. They are most effective when used in conjunction with antibiotic treatments such as streptomycin, but cannot be tank mixed with terramycin, and are incompatible with copper and certain fungicides, particularly mancozeb (Manzate).

One active overwintering canker can cause infection of surrounding trees, and a few such cankers per acre can render a well-timed preventive spring and summer spray program ineffective. Remove and destroy cankers and diseased limbs by cutting at least 8 to 12 inches below the visible injury. This helps to stop disease movement in the tree and reduces the source for new infections. Be sure to sterilize pruning shears and saws with a 1:10 dilution of chlorine bleach or Lysol in water whenever they come into direct contact with diseased tissues and periodically throughout pruning.

Organically acceptable methods include cultural and biological controls along with sprays of terramycin, streptomycin, some copper products, and Bordeaux.

Several mean temperature and degree-hour models are available to assist in predicting infection periods and the need for control. All are based on the minimum and maximum temperature thresholds above and below which bacterial growth and subsequent infection ceases. These models are utilized in most commercial California pear districts to time antibiotic and copper treatments.

The UC model recommends the first spray at bloom when mean temperatures reach 62°F in March, 60°F in April, and 58°F in May. This model recommends treating every 3 to 5 days until the end of rattail bloom regardless of changes in weather that would inhibit bacterial growth and infection.

The newer degree-hour models are based on assessing actual conditions for bacterial growth and infection and also indicate when treatment is unnecessary. One such model is the degree-hour model, which takes into account early bloom and periods of continuous cool weather, allowing adjustments in treatment timings. Use of the degree-hour method requires a recording system that provides a continuous temperature reading in or near your orchard. One degree-hour equals 1 degree above 65°F for 1 hour. For example, a temperature of 70°F for 2 hours equals 10 degree-hours (70°F is 5 degrees above 65°F and for 2 hours, 5 X 2=10).

Accrue degree-hours each hour of the day unless 3 consecutive days with maximum temperatures below 66°F occur. In this case, the number of degree-hours adding up over time is reset to zero until temperatures again exceed 65°F and degree-hours accrue again. The degree-hour total is not reduced by continuous cool temperatures if the total has surpassed 400 degree-hours and has coincided with precipitation or simultaneous warm, humid infection periods of at least 57°F and 90% relative humidity. If the orchard is being irrigated, the humidity threshold is reduced to 80% relative humidity as measured outside the orchard. If possible, start the season with a full soil water profile so irrigation during bloom can be avoided.

In the Sacramento Valley, treat within 24 hours preceding rain if 1 to 150 degree-hours have accumulated. In the North Coast region, treat within 24 hours preceding rain when more than 150 degree-hours have accumulated. Treatments for both areas are recommended every 3 to 4 days when accumulation exceeds 150 degree-hours (Sacramento Valley) or 250 degree-hours (Lake County). Alternate-day treatments are recommended in the Sacramento Valley whenever more than 500 degree-hours occur in conjunction with major bloom periods.

Two other models developed to predict fire blight infection periods are the Maryblyt model, used primarily in the mid-Atlantic states, and the Cougarblite model from the Pacific Northwest. These models have not yet been adequately validated under California conditions. For more information on these models, see the UC IPM Disease Model Database.

Rain or hail may require immediate respray of the orchard if temperatures conducive to fire blight development exist. If conditions conducive to fire blight development have occurred and frost conditions develop that are severe enough to cause the pear skin to rupture, re-treat immediately. Varying degrees of bacterial resistance to streptomycin exist in California. As noted above, several cases of resistance to terramycin were reported, and less sensitive strains of E. amylovora have been associated with reduced performance under commercial conditions in the Sacramento Delta and upper Sacramento Valley.

More information on pear bactericides

• Efficacy and timing of pear bactericides