2008
Fire Blight Alert

Tuesday, June 17 update:

The reports of fire blight are pouring in, and it seems that 2008 will be a "fire blight year," with more blight on apple and pear than for the past several seasons. Some of the blight on pears has developed slowly due to cold weather, and will become full blown strikes over the next few weeks. True "shoot tip" infections are being seen, which is very uncommon in the Pacific Northwest. (see the shoot tip infection section further down on this page.)

The most serious infections that are currently visible likely came from one of two infection events:

1. There was a short warm period in early May that caught early district apples with numerous secondary blossoms. In the rest of the state, those blocks with a 2007 occurence of blight in their neighborhood were likely under "high" infection risk on May 6-7-8. This infection period also caught pears in the later districts late in the primary bloom/petal fall period. Dew formation seems to have been the wetting event that triggered this early May infection. As in early district apples, risk of infection in the late blooming pear districts was especially high if there had been any blight in the local area last year. Bosc and Concorde pears seem to be most commonly infected. Both of these cultivars are especially prone to damage, once infected.

2. A "Extreme Risk" infection event occured on May 20, as a period of very nice warm (but not hot) weather was followed by widespread rain. Many pear orchards were wetted on that date, from Chelan north, the Wenatchee River Valley, and much of the Yakima Valley. There were "side-blossoms" on the fruit clusters in many pear orchards at the time. Also, some apple orchards in the area north of Spokane were in full bloom during the warm, then wet week, and growers report a significant primary bloom infection.

The full extent of damage across the state from this May 20 infection event is unknown, as the infections are difficult to see from a distance. Few late bloom were out on apples, except in the later regions of the state on May 20. The most vulnerable pear cultivar during the May 20 warm/wet period is likely to have been Bartlett, which had numerous "side-blossoms" on fruit clusters in the northern half of the state. The side-blossom infections on bartlett were beginning to appear as of early June, and will develop into full-blown strikes during June and early July. Ask your hand thinning crew to watch for the blight. As blight is currently active in many areas this season, "rat-tail" bloom could be infected more than during normal low blight years. If you have blight in your neighborhood, monitor the number of fresh rat-tail bloom on the pears (especially bartletts), and spray if the blossoms are wetted during a period of warm weather. Once the dry, hot summer (90F+) weather starts, rat-tail infections seem to become much less common.

SHOOT TIP INFECTIONS:

Most of the shoot tip blight in this region comes when blight bacteria move inside the tree from blighted flower clusters to the highly susceptible growing shoot tip. The fire blight then occurs on the tip, and symptoms move down the tree. True shoot blight occurs when blight bacteria contaminated growing tips are wetted and injured at the same time, often by windy rain storms. (Sounds like Spring, 2008) This infection of shoots is relatively independant of temperature, unlike the situation with flower infection. Shoot tip infection seems much more common if blight is already active in the neighborhood. Once the tree sets terminal. buds, it is no longer susceptible to new shoot tip infections. So, anything you can do to encourage the tree to slow or stop growing will help. Though shoot tip infection has been a significant problem in the central and eastern USA for 200 years, it is a poorly understood process.

Despite great effort, testing all the common blossom protective sprays, no researcher has found a treatment that seems to directly reduce shoot tip infection. Some good research has shown a very positive effect if the tree growth hormone spray that reduces shoot growth has been applied around apple blossom time. This is a response caused by the slower growth of the shoot tip, which reduces its' susceptibility to blight, and is not a direct effect on the blight bacteria. As the growth rduction effect requires application around bloom time to be effective, the shoot growth reduction hormone spray is not likely to reduce shoot tip infections if applied at this time of year.

CURRENT OUTLOOK:

Bartlett are just now developing a crop of "rat-tail" blossoms, which are vulnerable to infection duing this next several days, and which should be removed or sprayed after wetting, especially if active blight is in the orchard or neighborhood.

As usual, people are applying a truck-load of sprays to reduce damage to already-blighted orchards. Nobody anywhere has found a product that reduces damage in a valid test. The proof that these products help is from limited observations, comparing the amount of cutting between orchard blocks with different varieties and ages of pear or apple. I don't blame anyone for trying post-infection sprays, but sprays applied after you see symptoms are not going to help. When we find a real post-infection treatment, one that will drill down through the bark and kill this bacterial disease in place, we will be very pleased to tell you.

The statement below was posted on May 21:

Under the conditions we have experienced May 20 - 21, fire blight infection has been common in past years. The orchards at greatest risk are those that have a recent history of blight in the neighborhood, young and newly planted blocks with flowers open, and highly sensitive culivars with late bloom. However, all apple and pear orchards with blossoms have some potential for blight under current conditions.

Will every unprotected apple and pear orchard become infected? No, that never happens. Will some orchards become infected? Judging from years past, yes. How many? It depends on the current carry-over of the Erwinia bacteria that is out on blossoms now.

.(see fire blight basics explained below.)

THE WSU DAS FIRE BLIGHT MODEL:

The WSU Decision Aid System has the fire blight model for all AgWeatherNet sites, and is updated very frequently. It is in its' second year of automated form, but is far better than the way I have had to present it in the past. Do not rely on this automated system as your only risk assessment method. However, the heat risk levels for each current day are very valid, and the current date is most important. The forecasted temperatures are often lower than actually occur, so forecasted risk may be lower than it actually is. Keep checking daily during dangerous periods.

Here are some suggestions for using this new DAS interface:

Orchard History Setting:
Be certain to set this to match your areas' recent blight situation. If you or your immediate neighbors had almost no blight last year, take the default. If you or your near-by neighbors had blight last year, you must set the model accordingly. With higher starting numbers, the bacteria can grow to dangerous levels with lower relative daily temperatures.

Temperatures and degree hours:
The forecasted high temperatures come from a relatively conservative source. They often forecast lower than other commercial sites, especially for warm sites in Central Washington orchard areas. Look at predictions for your nearby and other regional sites, and look at the forecasted temperatures used in the DAS automated forecast, and look for nearby AgweatherNet sites with current and forecast temperatures that are similar to those at your orchard.

Wetting:
An Important note about remote monitoring of wetting events: Generally, 2 to 3 hours of wetting on a leaf wetness sensor, or rain at more than a trace (0.01 inch or more) is sufficient to trigger infection if contaminated flowers are present and degree hours are in the high or extreme range. Rain gages and wetness sensors are often the weakest aspect of weather monitoring systems. They don't always report wetting vs no wetting accurately, so use them, but do not make your final control decision based on dryness indicated by a leaf wetness sensor.

Remember that the wetting sensors on the AgWeatherNet equipment are on non-irrigated land outside of the orchards, not in a frost pocket or draw, or any other situation that is prone to dew or wetting by sprinklers. The fact that wetting is not indicated by AWN does not mean that it has not happened in your orchard as dew, or as mist from irrigation, or a local rain shower.

A number indicating wetness on the table means "Yes, wetting was detected at the AWN site."

A "dew" indicator has been added this year, a factor that can be critical in an infection. Two to three hours of dew is sufficient for infecton if flowers are infested with high numbers of bacteria following warm weather.

If there is no indication of wetting on the table, that means "the AgWeatherNet sensors detected no wetting, however, maybe wetting occurred somewhere else on your orchard. Only you should be the judge of that."

Please use this new system. (See the link to this free system on the "Current Models" page on this web site. or Click here. )

For blight infection to occur, you need four factors to occur, and they must occur in order.

1. First, you need flowers open on a potential host. This is usually obvious, except during the secondary bloom period, which is the most dangerous time. We now have this, and will have flowers in some blocks into early June. Some cultivars scatter bloom off and on all spring and early summer, which makes them more likely to be blighted.

2. Secondly, the blight bacteria must contaminate the flowers. This you can not see. This is possible anywhere, and any year, but is even more likely when your orchard neighborhood had blight last year.

3. Third, and most critical to the understanding of the infection process, you need warm weather. During primary bloom and petal fall, you need ABNORMALLY warm weather. After bloom, the normal temperatures are often warm enough. The sort of daily high temperatures we must be wary of in most orchards start in the mid to high 70's F, and are especially dangerous in the 80's F. These sorts of warm days can occur during primary bloom, and should alert you to

the possibility of blight infection when they occur, especially when it is warm for two or more days in a row. The graphic above indicates that hot, summer-like weather (95F+F) lasting for three or more days, followed by blossom wetting, did not lead to blight infection in 2006.
4. And finally, after you have those first three factors occur, you must have blossom wetting. Rain, heavy dew, irrigation. The wetting moves the bacteria from the stigma tips down into the nectaries, which then may lead to infection of the fruitlet and the tree.

There is an automated version of the Cougarblight fire blight infection risk model on the WSU Decision Aid System. (See the link to this free system on the "Current Models" page on this web site. Click here. )This model uses the AgWeatherNet data to run a fire blight model for all monitored sites, and is updated hourly. Set the situation relative to blight around your orchard last year, watch the degree hour totals and forecasts, and watch the rain, wetness, and dew point monitors on the upper left part of the page. When the degree hour total is near or over the threshold, flowers are present and wetness is indicated, blight is possible.

A few random observations about fire blight:

TREATING TREES THAT HAVE FIRE BLIGHT: There are no products that lessen the damage to trees after infection, though there are some that claim to, so your efforts are best spent preventing infection, and/or removing strikes as soon as possible after you see them. There are no studies done under controlled conditions that show any benefit from any spray applied post infection, though many products have been sold for this purpose over the years. There is usually at least one product being sold for this purpose on any given year, but they come and go. When a well-tested, registered product is actually found to have the ability to "stop" bight or reduce damage to the infected tree, I'll let you know as soon as possible.

Hail: Hail does not cause fire blight. A block that is free of fire blight does not have the bacterial contamination necessary for infection. Most of the time, blight DOES NOT follow hail.

However, if a hailed-on block has fire blight present at the time of the wounding and wetting caused by the hail, the bacteria that are on the tree gain access to the shoot tip and fruit tissues that are highly susceptible to infection, and blight follows within one to three weeks. This is one reason we recommend removing blight whenever you see it.

There are no good treatments that prevent this infection, but it is worth a try. As soon as possible after the hail, a treatment of oxytetracycline (Mycoshield, FlameOut) might reduce the number of strikes. It is probably best to have this treatment on within a few hours, but it might be helpful within 24 hours.

It is not at all likely that any other product would have an effect on reducing the degree of infection, including copper products, biologicals, or any other of the so-called post infection treatments.

Newly planted apples:

Apple trees planted four or five weeks prior to the mid-May infection period may have blossom buds when planted, so they may bloom far later than the usual date for the area. A first year apple will almost always die from this early infection. You will also have a blight problem in that affected block the next year, and perhaps for several seasons after. It is very simple to remove these blossom clusters, and it is less expensive to do it by hand than to spray the block. Complete removal of blossoms leads to complete control of the current fire blight infection. You should carefully assess the blossom situation in your blocks prior to deciding not to do any blight prevention.

Blossom wetting:"Wetting of blossoms" does not include spray wetting, as that does not wet the blossom long enough to induce infection.

Aerial Application: Most blight control products work directly on the blight bacteria as it grows on the stigma tip or, after an infection event, within the nectaries. If you do not get very good spray solution coverage of the interior of the flower, your terramycin antibiotic will not work. It is not likely that many potential flower infections are stopped by aerial application of oxytetracycline. The lable allows aerial application without any scientific validation of efficacy by controlled testing.