Utah Pests News Winter 2008-09

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Abiotic Diseases of Spruce and Other Conifers

 
  Fig. 2. Salts accumulate at the tips of the needles and progress toward the base. Soil analysis from this site showed high levels of salt.
   
 
  Fig. 1. Abiotic symptoms occur in uniform patterns. Symptoms of varying causes can be similar, so it’s important to give a detailed history of the site.
   
The Utah Plant Pest Diagnostic Lab received several spruce samples this fall with abiotic diseases.  Abiotic (meaning “non-living”) diseases can be caused by water stress, nutrient deficiency, salt accumulation, or chemical damage.  Abiotic symptoms are typically uniform, where all needles on a branch (Fig. 1) or all branches on a certain part of the tree are affected, whereas symptoms caused by pathogens are random.  The three most common problems on spruces are over or under watering, salt stress, and chemical damage.

Water Stress

Water stress is caused by either a shortage or an excess of water.  Spruce trees are shallow-rooted, and native to the mid-mountain range of the Rockies where optimal water occurs.  Mimicking the native environmental watering regime takes practice, and when inadequate, can result in disease.  In general, a water deficit develops when plants lose more water from transpiration than they are absorbing from the soil.  In dry soil, roots cannot absorb as much water as has been lost and stress symptoms begin to appear.  If water in the tissue decreases enough, the tree will significantly deteriorate and die.  Drought-stressed trees may show symptoms such as drooping or wilting, yellowing, browning at the tips (scorch), curling, or a combination of these symptoms.  In conifers, the oldest needles will show symptoms first and may drop prematurely.  Dieback of twigs and limbs will occur as symptoms progress over the years.  Drought conditions will often appear throughout a group of trees because the soil conditions will be similar.  Severe drought can also predispose spruce to infection by opportunistic pathogens and insects.

Too much water can also cause problems in spruce, which are more sensitive to saturated soils than deciduous trees.  Excess water stifles the roots through a reduction in available oxygen.  Roots deprived of oxygen are unable to absorb water and nutrients, and plant metabolic processes stop, growth ceases, and trees begin to decline or die.  Excess water can also predispose trees to root rot caused by Phytophthora and Pythium, which need periodic flooding for germination and dispersal.

Salt Stress

Saline soils are common in the West because an arid climate, inadequate rainfall, and poor drainage cause naturally occurring salts to concentrate.  Spruces have a low to moderate tolerance to salts.  When growing in saline soils, roots are unable to absorb water, resulting in symptoms similar to scorch.  Nutrient availability is also diminished because sodium ions replace calcium and other nutrients on soil particles. In highly salty soils, roots eventually start to absorb salt ions where they accumulate in needles and twigs at toxic levels (Fig. 2).  Needle damage starts with browning at the tips and progresses inward toward the base.  Consecutive years of salt injury results in dieback.

Although not as commonly used in the West as in the eastern U.S., de-icing salts applied to roads during winter can also cause toxic effects in spruces.  When moisture is present, salt ions from road spray are able to enter and accumulate in needles.  Symptoms include brown needle tips, premature needle drop, and twig death.

To combat salt stress, add large amounts of water to the root zone to leach salts further down into the soil profile.  Do not use irrigation water that is already saline, such as from a water softener.  Reducing soil compaction will aid in water absorption.

Chemical Damage

 
  Fig. 3. The abnormal growth of the new needles in this sample is indicative of a symptom seen with chemical damage.
 
  Fig. 4. Chemical damage is the suspect cause of the symptoms in this spruce, causing chlorophyll disruption.
Common causes of herbicide injury include using more than the recommended amounts, applying them unevenly or unintentionally to the root zone of trees, or drift to non-target trees.  Herbicide damage is greater on plants exposed or treated during hot weather.  Drought and heat stress can enhance the toxic effects.

Symptoms of herbicide damage will vary depending on the type of herbicide used (Figs. 3 and 4).  Hormone-type (phenoxy) herbicides will be translocated to growing points and cause abnormal growth in new needles or leaves.  Symptoms can develop several days or several weeks after exposure, or they may appear in the spring after exposure in the fall.  Conifers are relatively tolerant of phenoxy herbicides, but the shoots may curl and the needles may drop in response to damaging doses.  If affected, spruce trees will outgrow the symptoms in one or two years. 

Herbicides applied to turf can often injure trees in the area when taken up by the roots.  Non-selective herbicides (such as glyphosate, paraquat, or picloram) can persist in the soil for 45 days to a year or more, causing residual effects on later plantings.  These herbicides tend to stop growth and cause chlorosis in new and old leaves or needles.  Trees injured by these herbicides are less likely to recover than trees injured by hormone-type herbicides.

Contact herbicides (such as paraquat) can cause a localized necrosis similar to spots caused by some fungi.  The Plant Pest Diagnostic Lab does not have the ability to test for chemical toxicity.  Any further testing should go through Clark Burgess at Utah Department of Agriculture and Food.



-Erin Frank, Plant Disease Diagnostician