HORTICULTURE Fire Blight Management in Southwest Michigan
Adapted from E-154 Michigan Fruit Management Guide
With the probable future loss of streptomycin for fire blight control due to the development of resistance in the pathogen, non-chemical aspects of control take on increasing importance and must be considered as a top priority.
Sanitation.
The
first step in managing fire blight is to remove the overwintering cankers from dormant
trees. The bacteria overwinters in cankers. Cankers should be cut out during dormant
pruning in the Winter and Spring to reduce the amount of bacteria in the Spring. Cut the
branch 8 to 12 inches below the canker margin. When dormant pruning carefully examine the
tree and remove all evidence of fire blight cankers.
Trees that show symptoms of fire blight for several years in row should be removed and burned. This continuous infection is an indication that the fire blight bacteria is systemic in the tree and it will continue express symptoms each year and serve as an infection source inoculating the orchard every year. Continuous pruning of the same infected trees year after year is not sustainable, since the trees would probably become unproductive due to the infection and pruning, and only serve as a source of inoculum for the rest of the orchard.
Pruning out strikes during years when fireblight is not a major problem allows growers to reduce the small amounts of bacteria that will be present in their orchards in the years that the bloom conditions are right for the spread of the disease. "I tell my growers to scout for and prune out strikes when the weather in dry and don't go out until the dew has dried." Pruning out infected shoots to limit the spread of shoot blight is recommended on young or small trees, particularly those on M.9 or M.26 rootstocks. Removing infected shoots resulting from the extension of dormant canker is recommended on large trees. If infection to spurs and shoots is severe, however, it is of doubtful benefit to remove blight from large trees. To be effective in limited spread, prune out strikes as soon as they appear.Orchard Risk Assessment. At the beginning of each growing season, make a risk assessment of each orchard, categorizing its risk of infection by fire blight as low, medium, or high. This judgment should be based on such factors as the previous occurrence of fire blight in the orchard, the susceptibility of the varieties grown and their rootstocks, the age and vigor of the trees, and the number of overwintering cankers. The orchard risk assessment rating is used in combination with a "daily risk assessment," described below, to determine whether to apply antibiotic sprays.
In making orchard risk assessments, consider the relative susceptibility of the varieties to fire blight. Table listing the susceptibility of many new and common apple varieties is provided for your convenience. One table is a large table derived from variety trial at SWMREC in 1991. This table has a lot of differences between closely related strains showing more the variability of the disease as well as general variety susceptibility. A second more concise table was created after the 2000 fireblight epidemic. Trees propagated on M.26 rootstocks tend to be more susceptible than others, and the rootstock itself is very susceptible. Young, vigorously growing trees are generally more susceptible than mature ones. Most commercial pear varieties are susceptible.
| Chart for Assessing Orchard Risk | |||
| Daily risk rating |
Orchard risk rating |
||
| Low | Moderate | High | |
| Low | None | None | Marginal |
| Moderate | None | Marginal | Worthwhile |
| High | Marginal | Worthwhile | Critical |
Copper
is often applied at green tip to reduce inoculum produced by any remaining cankers and to
prevent bacteria from colonizing the emerging bud. This will not give season long control
and use of copper later in the season will cause severe russeting of the fruit. Russeting
decreases the value of the fruit for the fresh market and if severe, the fruit are only
acceptable for processing.
Two bactericides, Agrimycin (streptomycin) and Mycoshield (oxytetracycline), are effective against the blossom phase of fire blight. Mycoshield can only be used on apples in 2003 if a special Section 18 emergency registration is granted by EPA.
Streptomycin has been the standard for control of fire blight on apples since it was introduced in the 1950s. It is a more effective product than Mycoshield where the fire blight bacteria are not resistant. Streptomycin is used during the bloom period to control blossom blight and in summer up to 50 days of harvest to control infections associated with storms. Timing of bloom sprays is often based on infection prediction models. When these models are used, streptomycin applications can be reduced while retaining excellent disease control.
Since being discovered in one orchard in Van Buren County in 1990, streptomycin-resistant strains of the fire blight bacterium (Erwinia amylovora) have been detected throughout Van Buren County and, beginning in 2000, in some areas of Berrien county adjacent to Van Buren County. Resistant strains also have been detected in Kent County and in Newaygo County.
Mycoshield is useful as a replacement for streptomycin in areas where streptomycin resistance has been a problem. Mycoshield can also be used in a combination spray with streptomycin as a part of an anti-resistance management program. Apply it as a protectant during bloom for blossom blight control. The product has no effect on fire blight unless applied to open flowers under weather conditions that are likely to lead to blight infection. It does not provide economic control of blight if applied even a few hours after infection has occurred, and it does not prevent infection of shoots or fruit during the summer.
Timing Antibiotic Sprays.
Time antibiotic sprays carefully. The bacteria thrives in the stigma of open blossoms during bloom. Under warm conditions bacterial numbers quickly rise to incredibly high numbers. Rain washes the bacteria down to the base of the flower where the bacteria enter the plant begin growing in the plant. Antibiotics are timed to kill the bacteria before infection takes place.
With Agrimycin and Mycoshield, timing is everything. Timing of bloom sprays is often based on infection prediction models such as MARYBLYT and CougarBlight. MSU has posted predictive models for Michigan's automated weather stations system at the MSU Integrated Crop Management website allowing you to run predictive models of diseases including fireblight. This site is open to all. You need to create an account so that you can go back and the website will remember you. This information can be used to estimate the infection risk if an automated station is located close to your orchard. Disease Models and Michigan Weather data are also available from MIFruit.com.
In the absence of information from a predictive model, reasonable control of fire blight should be achieved if the antibiotics are applied using the "daily risk system." These daily assessments are determined after the onset of bloom. On days with rain and days following a rain, note the maximum temperature and the amount of rainfall, then refer to the following chart to determine the daily risk rating.
 |
Chart for Assessing Daily Risks | ||
Maximum temperature |
Rainfall |
||
| Less than | More than | ||
| 0.1 inch | 0.1 inch | ||
Daily risk rating |
|||
| Under 65° F | None | Low | |
| 65° F to 69° F | Low | Moderate | |
| 70° F to 80° | Moderate | High | |
| Over 80° F | High | High | |
For example, on a day with a maximum temperature of 75° F and more than
0.1 inch of rain, the daily risk rating would be high. On a day following a rain with the
same maximum temperature but no rainfall, the rating would be moderate.
When determining daily risk ratings, use the "Orchard Risk Chart" described earlier to determine whether it is worthwhile to apply an antibiotic spray to control fire blight. For example, if the daily risk rating is moderate and the orchard risk rating is low, a spray is not needed. If both the daily and the overall orchard risk ratings are high, on the other hand, applying a spray would be critically important.
When using the "daily risk system," remember that a single day with rain occurring early in the bloom period is usually inconsequential; in low-risk orchards, a spray is probably not needed. During the early bloom stage after a spray has been applied, a second application is usually not needed until a significant number of new blooms have opened. After full bloom, an application is effective for about four days. If rainfall of 0.5 inch or more occurs, however, it will end the application’s effective period.
