Sunscald

Sunscald, as the name suggests, is damage to a plants leaves, stems or trunk due to the sun.  It is also referred to as sun scald, sun bleaching, and leaf whitening.

Sunscald on tomato leaves

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Leaves & Seedlings

The lightened areas are sun scald, not glare or reflection.

The lightened areas on the leaves (above) were bleached by the sun and are not glare or a reflection.

Prevention

Increase sun exposure in stages. 

  1. At first keep them under a porch roof or shade cloth (ie. no direct sunlight). 
  2. After a few days, expose them to full sun for short periods (1 hr to start with)
  3. Increase the length of full sun exposure until they are handling 3 or more hours without any problems.

Its usually not a good idea to plant them out in the garden until this process is complete.

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Diagnosis

The lightened color of the leaves is distinguished from other diseases by its airbrushed appearance.  Edges of lightened areas fade into the normal green areas.  Surfaces that were shaded due to the curves of the leaf are not as bleached.  Lightened areas range from white to creamy white to yellowed. 

Causes

Sudden Light Changes: Leaves that are used to part shade conditions will sun scald if suddenly exposed to full sun.  Seedlings that are started indoors are very susceptible to getting bleached leaves when they are moved outside for the first time. 

Factors that make it worse:

  • Water droplets on leaves act as a lens, focusing the light and increasing the probability of sun damage.
  • Drought:  Plants are less able to cope with bright sun if they don't have enough water.
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Treatment

  1. Move seedlings back into a part shade environment.
  2. Remove badly bleached leaves.
  3. Water if they need it.
  4. Reintroduce to full sun in a couple days to a week.

Usually the plant will recover just fine.  Unlike pathogens, sunscald is just a temporary effect.  Remove the cause and an otherwise healthy plant will snap right back.  If all its leaves were bleached, however, the plant usually dies.

Trunks of Trees

Cherry tree with sunscald
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Prevention

White Latex Paint:  It looks strange but works really well and lasts for several years.  Also defeats insect camouflage so birds have no trouble picking your tree clean.

White paint on a apple tree prevents sunscald.

Loose opaque tube protectors:  The tube blocks the sun from shining directly on the trunk.  The larger the tube the better.  A big cold air space between the tube and the tree will help ensure the trunk doesn't warm up.

Tree wrap:  Only use a light or white colored wrap.  A dark wrap will heat up just like the tree would, and with no air space, that heat will dissipate into the tree.  Remove it in the spring as they become a home for bugs.

Diagnosis

There are a number of things that can cause fissures and damage to bark.  The biggest clue that it is sunscald is that the damage is only on the South-Southwest side of the treeThe tree on the left has mild damage on the sunny side (left) The bark is pealing off on this side but is completely intact on the shady side (right).  Severe sunscald can cause live tissue death, resulting in complete loss of bark and/or cracking.

Causes

During winter, the tissues of the tree are below freezing, but can be thawed when exposed to bright sunlight.  At night the cells freeze again.  This repeated cycle of freezing and thawing damages cells.  If the temperature change is dramatic enough to thaw the living tissue below the bark then the damage is more serious as these are the cells that grow new bark and carry water to the tree.  Trees with thin bark such as cherry and birch are more susceptible to sun scald.

Treatment

  1. Carefully remove or trim off any bark "flags",  that is, pieces of bark that have pulled away from the tree.  These provide cracks for pests to hide.
  2. Seal any areas that have lost bark with a tree wound spray or several layers of latex paint.
  3. Keep an eye out for fresh insect damage such as newly drilled holes, weeping sap or the bugs themselves.   Insects will often be attracted to the damaged side of the tree and attack it.  If this happens you'll need to kill the insects.
  4. Employ one of the sunscald prevention methods (listed on the left) in the fall.


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