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December 2007


I came across this info in an OSU newsletter.


12 Steps to Manage Bacterial Canker of Sweet Cherry

Dr. Robert A. Spotts

OSU Mid-Columbia Agricultural Research and Extension Center

Hood River, OR


Pseudomonas syringae, which causes bacterial canker, is a major bacterial pathogen of young sweet cherry trees. Often, 10 to 20% of the trees in new orchards are killed by P. syringae within five years of planting. Control must integrate several techniques including the following:


• Do not interplant new trees with old trees, which are major sources of P. syringae.


• Keep irrigation water off the part of the trees above ground as much as possible for the first 2 or 3 years after planting. Consider withholding water in late summer so trees will “harden off” and not be as susceptible to low

temperature injury in early winter.


• Avoid all types of injury – mechanical, insect, frost. Paint all trunks white with latex paint to prevent winter

injury. Adding copper to the paint is probably of little benefit.


• Some studies show less bacterial canker when pruning is delayed until spring, even as late as after flowering in

May. Less disease also occurs when summer pruning is used. Prune only during dry weather if possible.


• Remove and destroy branches and trees killed by P. syringae from the orchard.


• Mazzard F12-1 is one of the most resistant rootstocks. Resistance of new rootstocks is unknown at this time,

but trees on Mazzard may have an advantage over trees on size-controlling rootstocks. Sweet cherry scion

cultivars generally are susceptible.


• Locate the orchard in an area less likely to be affected by frost and slow drying conditions.


• Provide optimal soil conditions for growth of cherries, including attention to pH and nutrition. Application of

excess nitrogen, especially late in the growing season, will promote late season growth that is susceptible to low

temperature injury in early winter, followed by bacterial infection.


• Control weeds. They often support large populations of P. syringae, especially grasses. Clover and vetch

ground covers support lower populations. Consider clean cultivation of row middles for the first 3 years.


• Fixed copper products or Bordeaux 12-12-100, applied in October and January may help, but strains of

P. syringae resistant to copper are widespread in the Mid-Columbia area.


• Test for and control plant pathogenic nematodes before planting if needed. High populations of ring nematode

have been associated with more bacterial canker.


• In the Parkdale area, plant trees in May rather than April.



I suspect the above, is a good program to follow if you're having problems with gumosis in new peach plantings.


Every year I get calls from growers asking, “How cold hardy do you suppose the peach buds are?” Below is a reprint from four years ago.

At what point is it safe to prune peaches in the fall? That’s a question that seems to come up each year. What makes it an issue? Peach buds (all fruit buds) progress through a series of steps each winter that cause them to be more or less sensitive to cold. When a pruning cut is made, there is an invigoration of the tissues around the wound to heal the area. This hormone release seems to deharden the tree. The greater the percentage of wood removed the greater the stimulation to the tree. Big cuts on small trees are the most dangerous. Weak trees are more susceptible than healthy ones (young or old). I’ve seen literature that states the effect of pruning lasts anywhere from 5 to 10 days. One thing is certain, the sensitivity decreases with each subsequent day. The goal then is to not start pruning until the buds are sufficiently hardy to withstand this dehardening process RELATIVE TO EXPECTED TEMPERATURES! The greatest danger occurs when temperatures drop quickly, especially if the previous days have been mild. Keep you eye on the long range forecast!


Once hardening develops the difference between a few dead buds and a lost crop can be a matter of a few degrees of temperature change. Proebsting and Mills (1966) determined the percentage of survival of ‘Elberta’ peach flower buds and expressed the deviation from the temperature required to kill 50% of the buds (T50 ). Only 4C above the T50, 90% of the flower buds survived and only 3C lower temperature killed all the buds.


The winter acclimation process is presumed to develop in three stages:

  • Stage 1 is initiated by shortening daylength. In peach and cherry this stage is usually reached in late August or early September and is usually limited to a hardiness level of approximately 21F.

  • During stage 2, freezing temperatures trigger metabolic processes that result in a lower temperature hardiness level. This occurs by early-November (soon after defoliation) and brings a hardiness level of – 4 to – 9F.

  • Stage 3 is induced by continual freezing (below 27F) within the tissues that draw water out of the cytoplasm resulting in increased osmotic concentration and continued hardening. How’s that for big words! In other words….as it stays dang cold, the water is sucked from the bud making it more resistant to really low temps. This hardiness level can reach – 17F for T50 of peach and –29F for cherry. This hardiness is lost up to – 4F when thawing occurs, provided temps remain below 39F. This level that thawing induces on a non-frozen bud is known as the minimum hardiness level. This level will remain somewhat constant until rest is completed and temperatures rise. It typically starts near 5F in October, decreases to near – 5F in December and January, then increases slowly as the bud completes rest and starts to develop on mild days.

Flower buds are slow to gain hardness. Cherry and peach buds harden at a rate of 3 - 4 F per day for peaches and 5F per day for cherries if the temperature is below 28 – 30 F (Proebsting & Mills). They can lose it at a rate of 1 to 2 degrees per hour if the temperature rises above freezing. Peach flower buds respond to temperatures above 28F to deharden to their minimum hardiness level.

- Peaches in Central WA develop the ability to deep super-cool (cooling below the freezing point of a liquid without solidifying) and withstand temps below 10F in early October. At this time the buds are more hardy than the woody tissue on which they grow (Probsting). As spring development occurs the minimum hardiness level rises slowly, rarely going above 5 F before budswell. The T50 usually remains below 10F until shortly before the buds show pink. As spring progresses the buds rehydrate which raises their hardiness to 28F at bloom time. As bud development progresses, the ability to reharden with freezing is lost.

- Apricot, cherry, sour cherry, peach, and plum showed no ability to supercool in September or October; all buds super-cooled in November and December. Plum lost it’s ability to super-cool in January. Peach, apricot, and cherry lost their ability sometime between February and March.

- Sweet cherries develop the capacity for deep super-cooling 1 – 2 weeks earlier that peaches (Andrews & Proebsting).

- Most of the acclimation of fruit trees is determined by the genetic make-up of the tree. Cultural practices can affect it only to a small degree (no pun intended). Here’s a few thoughts though. Any practice that extends growth into the fall will decrease hardiness. Acclimation is a metabolic process that requires photosynthesis. Maintaining the foliage in good condition not only though the season but into the fall, will produce the maximum photosynthate possible increasing cold resistance.

  • Use the above info as a “guideline”. When it comes to killing temperatures and your orchard, it’s never an “exact” science!

Last year I pulled peach bud samples through the winter and sent them to CSU. They put them in a freeze chamber to determine at what temperature death occurred. This chart shows the percent live buds remaining after exposure to sub-freezing temps.



















The message of Christmas is that the visible material world is bound to the invisible spiritual world.


May the spirit of Christmas bring you peace,


The gladness of Christmas give you hope,


The warmth of Christmas grant you love.


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