Fire Blight: Integrated Management for 2000 and beyond
By Dr David Manktelow, HortResearch
There are no silver bullets for fire blight control and the only long-term assurance for avoiding a recurrence of the1988-89 and 1998-99 epidemics is to implement an integrated management plan that focuses on disease avoidance. The following checklist covers steps in fire blight control that need to be addressed every season.
ISSUE #1) Sources of inoculum
 | Identify cankers on pears/apples on your orchard. |
| Identify cankers on pears/apples on near-by orchards. |
| Identify Alternative hosts. |
| Cut out and burn infected wood and cankers. |
| Apply late-dormant copper sprays to suspect blocks. |
| Remove (or copper spray) alternative hosts. |
ISSUE #2) Identify at-risk blocks
| Consider cultivar susceptibility. |
| Identify block proximity to inoculum sources. |
| Consider streptomycin resistance status of property. |
ISSUE #3) Monitor in-season infection risks
| Open blooms are most at risk. |
| Bloom infection risks can be identified using weather data (high temperatures with wetting events) – use predictive models. |
| Shoot infection risks are more difficult to predict - reduce risks by reducing lush growth, removing inoculum sources and avoiding leaf curling midge and other leaf damage. |
| Stressed trees (e.g. following ANA thinning sprays) may be more at risk. |
ISSUE #4) Understand preventative control options
| Streptomycin and low rate copper sprays have only a short period of efficacy (ca. 3 days). |
| Application of these chemicals up to 48 hours before, or 24 hours after, an infection period should provide best protection against bloom infections. |
| Streptomycin use must be minimised to avoid resistance problems. |
ISSUE #5) Identify and manage fire blight outbreaks
| Symptoms usually appear ca. 7-21 days after bloom infection – the MaryBlyte model gives a reasonable prediction of when symptoms will be visible. |
| Frequent scouting and strike removal (ca. twice per week) may be required if an outbreak occurs. |
| It is TOO LATE to apply streptomycin when symptoms are present. While this may protect newly opened blooms, it will promote resistance development. |
Fire blight infection risk prediction systems
Fire blight bloom infection risks are strongly influenced temperature and a number of temperature-based infection risk models have been developed overseas. Virtually all models will indicate a risk of bloom infection under conditions when average daily temperatures begin to exceed ca. 15o C. It is easy to monitor average daily temperatures using a max-min thermometer placed out of direct sunlight. Infection risk models simply provide a prediction of what could be happening in the real world. Because of variations in inoculum load and weather conditions between blocks it is essential that any disease predictions are reality checked against actual conditions. Every orchard should have a max-min thermometer.The fire blight infection risk warnings produced by Crop Health Services and HortResearch in the 1999 spring were based on predictions from the North American MaryBlyt and Cougarblight models. The Cougarblight model uses a four day running houly heat accumulation to predict infection risks. The MaryBlyt model uses a running hourly heat accumulation, combined with high average temperatures on the potential day of infection, to identify infection risks. Both models recognise that wetting by rainfall or dew is also required before infection can occur. An important point of difference between MaryBlyt and Cougarblight is that Cougarblight includes potential inoculum levels as a factor. Four different potential inoculum levels are recognised (based on potential presence of cankers on or near the orchard). The inoculum level selected has a major bearing on the infection risk predictions from Cougarblight. This mirrors the real world – the more potential inoculum, the greater the infection risk.
Forecast and monitored weather information were used to generate fire blight bloom infection risk warnings over the 1999 spring. MaryBlyt and Cougarblight model predictions were used to rank infection risk for up to 48 hours ahead. Risk assessments were based on the weather data monitored from three Hawkes Bay weather stations and short term MetService weather forecasts. The graph summarises the warnings given versus the times last season when infection was believed to have occurred. Infection was considered unlikely given a moderate risk prediction unless blocks were exposed to high inoculum and/or were warmer than monitored weather station sites. Infection was anticipated following high risk predictions if blooms and inoculum were present in the block. It was never envisaged that a warning service would run every season. Both of the models used to produce the 1999 disease warnings can be accessed directly by growers or consultants using the HortPlus MetWatch software1.
Fire blight chemical control options
Pre-season sprays
Copper "clean up" sprays applied post-harvest, over winter and/or pre-budbreak have been used for many years. However, copper accumulates in the soil and excessive use needs to be avoided. It is hard to justify copper use in an IFP programme if the spraying target cannot be identified. In the case of fire blight, only the late-dormant copper sprays can be expected to have any significant impact on disease risks. These sprays will help to kill bacterial inoculum produced from cankers. Copper has no eradicant effect on established fire blight disease, so late autumn and winter sprays cannot be expected to provide any fire blight control.
In-season sprays
Blooms that have been open for less than four days are highly susceptible to infection. If blooms are present and average daily temperatures begin to exceed 15o C you need to consider whether a streptomycin or low-rate copper application may be required. Ideally, try to avoid the use of either chemical (streptomycin because of resistance risks and copper because of risks of fruit damage). The effective life of either of these chemicals is only in the order of three days, so it is important to time any applications to closely match when infection periods occur. As a rule of thumb application up to 48 hours before, or 24 hours after, an infection period can be expected to give best protection against bloom infections of fire blight.
There is still some debate over what forms and rates of copper are most effective for fire blight control and what sort of fruit damage can be expected from their use (e.g. see the grower comments in the "case studies" summary). However, the following points can be made;
| Cupric hydroxide (usually as Kocide) is probably the least phytotoxic form of copper for in-season use. As a rule of thumb, the safer the copper formulation, the shorter its persistence. |
| Hawkes Bay organic apple and pear growers have found that they can achieve acceptable black spot control using low rates of cupric hydroxide mixed with sulphur. Fruit russet is highly likely to occur if cupric hydroxide rates are in excess of ca. 50 g per 100 L of dilute spray mix. Most organic growers use cupric hydroxide at around 35 g per 100 L and have achieved excellent fruit finish at this rate. |
| Organic growers have provided anecdotal evidence that low rates of copper (ca. 25-50 g per 100 L cupric hydroxide) will protect trees against fire blight infection. |
| Dr Chin Gouk conducted laboratory tests in the summer of 1999 to see how well copper residues washed from field sprayed apple leaves suppressed growth of the fire blight pathogen. She found that rates between 25 and 50 g/100 L of cupric hydroxide did suppress pathogen development – but that as copper rate was reduced, the duration of pathogen exposure required to achieve control had to be increased. |
| Bloom infections only occur on recently opened blooms, copper and streptomycin are not systemic, so blooms that open after a spray has been applied will not be adequately protected from infection. Successful fire blight management may require a succession of closely spaced sprays over bloom to prevent infection when weather conditions favour disease. |
| It is better to use both streptomycin and low-rate copper sprays over bloom, rather than rely on one chemical alone. |
Streptomycin resistance status
There is some uncertainty still about the levels and spread of streptomycin resistance in Hawkes Bay and what this implies for the use of this chemical for fire blight control next season. No orchards on which resistance has been detected have returned 100% resistant isolates in subsequent samples. There may be some potential to maintain the effective use of streptomycin on such orchards – however there is also a real possibility that streptomycin use on blocks with a high proportion of resistant bacteria will actually increase damage to fire blight (because potential antagonistic bacteria will be killed off). At risk blocks need to use both copper and streptomycin in their fire blight spray programmes.
This article made available for publication on this website with the permission of the author.
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