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Fireblight

A common disease of crabapples and other species. Learn about the cause and the multiple control methods.

Fireblight is a common disease of many Rose-family species in North Dakota.  The list of hosts includes apples, crabapples, hawthorn, mountain-ash, cotoneaster, pear and others.  Fireblight is caused by a bacteria that enters the tree via succulent tissue or through fresh wounds.  The recent storms that have been rolling through our region have resulted in perfect conditions for this disease to flourish.

The most common symptoms (see below) are dead branch tips with leaves still attached, curved over at the tip into a ‘shepherd’s crook’.  There is usually a sharp dividing line between living and dead tissue, and in some cases a sunken canker will be found here.  The bacteria can be found even further into the living tissue which is very important when treating the problem.

fireblight, crabapple, Valley City

A heavy infection of fireblight on a crabapple in Valley City, ND.

Dormant-season pruning is the best option for treating fireblight.  Locate the dividing line between living and dead tissue, and go back at least 8-12” into the healthy wood before pruning at a branch connection.  If there are just a few dead branch tips, they can be pruned out during the growing season but it is critical to sterilize the pruning tool between cuts to minimize the potential for spreading the bacteria to new wounds.  Pine Sol® or a 20% bleach solution can be used as sterilizing agents.  Both are corrosive to metal, so rinse and oil pruning tools when finished.  Chemical treatments in the spring can help prevent the disease from establishing, and copper-based fungicides as well as streptomycin are labeled for this purpose.  Follow all label directions and treatment recommendations.

fireblight, canker, margin, crabapple, Valley City

Canker margin on fireblight; living tissue is on the left and dead tissue is on the right.  Pruning should be at least 8-12" into living wood and should occur at a branch connection.

One additional way to prevent fireblight is plant resistant varieties of the host species.  Fireblight ratings of several edible apple varities can be found at: https://www.ag.ndsu.edu/pubs/plantsci/hortcrop/h327.pdf.  More information on the basics of tree pruning is available at:

https://www.ag.ndsu.edu/publications/landing-pages/gardens-lawns-trees/pruning-trees-and-shrubs-h1036

-Joe Zeleznik

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Preparing trees for winter checklist

A great summary of the steps needed to minimize winter injury to trees and decrease disease pressure, ensuring a great start to next year's growing season.

The following article was written by Aaron Bergdahl, Forest Health Specialist with the NDSU/North Dakota Forest Service.  It first appeared in the NDSU Extension Crop and Pest Report, September 12, 2013.

White ash - NDSU 2013

With the coming of fall it is important to remember to prepare your trees for the tough North Dakota winter. The following checklist serves as a reminder of the most important considerations for fall tree care and proper tree winterization.

  • Water trees weekly in late fall until freeze-up. Two gallons per inch of stem diameter is recommended, if the soil is dry at a depth of six inches. This will help minimize winter injury.
  • Rake up and remove/destroy fallen leaves. This is the best thing you can do to reduce the amount of fungal leaf disease next year because many fungal leaf diseases overwinter on leaf litter.
  • Prune dead wood to decrease overwintering sites for tree diseases and insect pests.
  • Wait until the tree is dormant (at least November) to prune living branches, always using proper technique.
  • Wrap the lower main stem of trees that have not developed thick bark to protect them from sunscald and rodent feeding damage.
  • Wrap burlap around smaller, high-value (landscape) conifers or set up a burlap sun/wind shield to help minimize the chances of winter burn.
  • Throughout winter, use caution when applying deicing products near trees and shrubs. Salts and other chemicals contained in some products may cause harm.

By completing this checklist you will be doing your part to maximize the chances that your trees will make it through the winter in good health and will be ready for a productive growing season.

For more information, please contact: Aaron Bergdahl, Forest Health Specialist, NDSU/North Dakota Forest Service. aaron.d.bergdahl@ndsu.edu (701) 231-5138

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Iron chlorosis - brief overview

This articles touches on iron chlorosis in trees, including its causes and potential cures. Everything works some of the time, nothing works all the time.

(The following appeared in the ND Forest Service’s ‘Prairie Forester’ publication in 2009.)

-Joe Zeleznik 

Life is tough on trees in North Dakota. They face bitter cold winters, spring floods and summer droughts. One thing they don’t face is nutrient deficiency … usually. Iron chlorosis is the exception. It affects trees throughout the state and it is sometimes fatal.

Autumn Blaze maple, iron chlorosis, close-up of leaf, NDSU campus

The main symptom of iron chlorosis is yellow leaves with green veins. Iron is used by the trees to make chlorophyll, the green pigment that traps sunlight for photosynthesis. There is no predicting if or when a tree will be affected by iron chlorosis; a tree can be healthy for many years, then suddenly become chlorotic. Iron chlorosis can be the first step in a decline spiral that eventually kills the tree.

Some species are more susceptible than others to reduced iron. Silver maple trees are especially vulnerable. Silver maple is one of the parents of the Freeman maples such as Autumn Blaze® or Sienna Glen®; these hybrids are slightly susceptible to iron chlorosis, but not as much as silver maple. River birch and red oak are less sensitive than silver maple, but more sensitive than most other tree species.

Most of North Dakota’s soils have enough iron in them to support healthy plant growth. But sometimes the iron is in a form that’s not available to plants. The culprit is usually high soil pH. The soil is not acidic enough to keep iron in the available form. Low soil oxygen can also cause iron to be unavailable. Trees are more susceptible to iron chlorosis if they are growing in flooded or compacted soils. Even low temperatures can reduce iron availability.

Can chlorotic trees be “fixed” or “saved”? As with most things, the answer is “it depends.” In the case of iron chlorosis, prevention is more effective than after-the-fact treatment.

Determining the cause of the low iron availability is critical to developing a treatment approach. Obviously, we can’t change the weather – low-temperature induced chlorosis can’t be prevented. However, flooding and compaction – and therefore low soil oxygen – can be mitigated by improving drainage or aerating the soil.

Autumn Blaze maple, NDSU, iron chlorosis

For chemical treatment products, there are a lot of manufacturers’ claims. What does the science say? It’s not conclusive. Everything works some of the time, but nothing works all of the time.

If the cause of the chlorosis is high soil pH, then there are two options – lower the pH or add iron in an available form. Soil pH can be lowered by adding elemental sulfur or by adding acid-forming fertilizers. Some experts recommend combining these approaches by applying iron sulfate to the soil. Treatments must be repeated every two-to-three years. Iron can also be added as a chelate. Chelated iron comes in many formulations, so ask your local garden center what formulations are on hand.

Iron can also be added directly to the tree, either as a foliar spray or as a trunk injection into the vascular system. Iron chelate sprayed onto the leaves offers a quick fix to the problem, but the effects are often short-lived. Trunk injections have shown mixed results. Sometimes, injections are effective and trees regain their health and vigor. However, if the damage is too severe, then trees will not recover.

Iron chlorosis can be found in trees throughout the state. But with proper management, it can be prevented, and sometimes, cured.

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A leaf of a different color

Most leaves are green during the growing season. Leaves that are off-color are often showing signs of some type of stress. However, that's not always the case, as seen in these photos.

Leaves normally change color in the fall – we all know this.  If leaves begin to change color in the summer, that’s usually not a good sign.  Insects, diseases, nutrient deficiencies, environmental stresses – or some combination of these – can all cause leaves to turn yellow, brown or other colors during the growing season.

However, some trees and shrubs are supposed to have two-toned leaves during the growing season.  On other trees, you may see individual leaves of one color, and other (individual) leaves of a different color.  In some cases, that’s okay!  See the pics below.

-Joe Zeleznik

09.AUG.13

‘Schubert’ chokecherry – ‘Schubert’ chokecherry (a.k.a. ‘Canada Red’ chokecherry) is valued for its purple foliage during the growing season and bright red fall color.  The bright green leaves at the tip of this branch are a new flush of growth appearing in mid-season, and they will turn purple in one to two weeks.  This variety of chokecherry is native to North Dakota and is extremely hardy.  Suckering from the roots may cause problems.

Newly formed Schubert chokecherry leaves, showing green color, before changing to a purple tone.

Poplars –This hybrid poplar produces leaves that start off with a purple hue which then change to green – just the opposite of the Schubert chokecherry.  There are many poplar trees that are either native to North Dakota or adapted to the environment here.  Most poplars grow quickly but die young – especially the hybrids.  This can be handy when establishing a windbreak, as the poplars can provide immediate protection while the more-permanent trees establish slowly.

Newly formed leaves of a hybrid poplar show a purple hue, changing later to green.

Ivory Halo dogwood – This species has variegated leaves.  That is, the leaves are naturally two-toned.  There are many other trees and shrubs that are variegated, but finding ones that are tough enough to handle the North Dakota environment can be difficult.  Ivory Halo is hardy to Zone 3, and it does best in protected areas with good soil moisture. 

Two-toned, variegated leaves of Ivory Halo dogwood.

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Ash anthracnose recently observed.

Ash anthracnose is a recurring problem. This year (2014), it was a little later than normal, but has finally arrived.

Some ash trees have recently lost their leaves because of a problem called ash anthracnose.  We see this in spring, especially following wet weather, as we’ve experienced lately.  Additional symptoms can include black blotches on leaf margins, causing leaf distortion, and small purple-to-brown spots in the middle of leaves. The leaf symptoms may not necessarily be visible on fallen leaves, since the infection that triggered leaf drop is likely on a petiole or other inconspicuous location.ash anthracnose.  Notice the dead leaflet margins, and the curved growth of the leaflet(s).

Treatment with fungicides is usually not warranted. Fungicides are only effective as a preventative treatment, usually as leaves are expanding.  Treating trees now can prevent mid-season infections, but infection is more common in the wet spring, rather than the drier summer.  For most large trees, fungicide applications aren’t very practical.  However, there are cultural practices you can implement now, such as a light fertilization, to help reduce recurring stress on ash. “Light” fertilization would be 1-3 pounds of nitrogen per 1,000 square feet of soil surface around the tree.  If you already fertilize your lawn, then there should be more than enough to meet the tree’s nutritional needs.

The fungus that causes ash anthracnose overwinters in the upper parts of trees on seeds, on twig cankers, and on any other plant part that remains attached to twigs, so raking and destroying fallen leaves and twigs may only help reduce inoculum rather than completely eliminate it. As a result, ash anthracnose is a recurring problem on ash as long as we have wet, cool weather in the spring. Disease severity varies from one year to the next, and among individual trees.

-Joe Zeleznik

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“Winter Injury” on conifers

Discolored needles on evergreens, noticed in late winter or early spring. Causes are many, true cures are hard to come by.

Every year, starting in late February or early March, I begin to get calls and emails about evergreen trees that aren’t very green.  Some of the needles may be turning brown, red or even a shade of purple.  Generally, we categorize the problem as “winter injury”.  That’s somewhat vague, as the potential causes are many.

The classic example of winter injury is seen in the picture below.  All of the needles that are located above the snow are exposed to harsh winter conditions such as fierce winds and brutal cold.  Additionally, bright sunlight can reflect off the snow and create more problems. 

winter injury, conifer, evergreen, disease

The symptoms of winter injury in spruce trees are often somewhat different (below).  Needles turn different shades of purple or red or brown.  Usually, there is no specific pattern to the damage – the discoloration isn’t above the snowpack, or just on one side of the tree or even worse on one side.  It isn’t necessarily newer needles, or older needles, or needles on the tops of the twigs that are affected.  That makes diagnosis a little difficult – environmental problems usually show some type of pattern.  Nevertheless, if symptoms like these develop over the course of the winter, after January, then it’s almost certain that the problem can be categorized as winter injury. 

 spruce, conifer, evergreen, winter injury, stress, disease

While rough winter conditions can kill needles, the buds that contain next year’s growth often survive.  The trees in the first picture above were damaged in the 2009-2010 winter.  The shoots had new growth in 2010 and the trees recovered well.  Winter injury stresses trees and minimizing stress over the following growing season is important in helping trees recover.  Watch out for pests and treat them as needed.  Spruce trees that have been damaged by spider mites are more susceptible to winter injury than those that haven’t been attacked.  Minimizing drought stress is also important.  Make sure the tree has enough water to meet its needs, but don’t overwater.  A rule-of-thumb for watering is once every 10 days, if there has been no rain.  In the fall, watering up until the ground freezes will help keep the tree hydrated going into winter.  This will help to minimize winter injury, though it won’t completely prevent it.  Helping the tree to build new leaf tissue can also support its recovery.  Adding a small amount of nitrogen fertilizer in the spring – 1 to 3 lbs of nitrogen per 1000 square feet of soil surface – can help the tree to recover. 

Winter injury is common and reveals itself as dead needles during the spring.  Minimizing stress during the growing season can help trees recover from winter injury, and in some cases, can even lower the probability of winter injury from happening in the first place.

Joe Zeleznik

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