Bronzing

Bronzing your baby's shoes is one thing; bronzing in the garden indicates a problem.

What is bronzing?

Bronzing refers to how some leaves or fruit turn purplish or bronze-colored due to mineral imbalances, pest feeding, chemicals, environmental conditions, or disease. Bronzed leaves are often smaller, and damaged areas cannot perform photosynthesis. Bronzing damage may look similar to sunburn damage, except that sunburned leaves tend to turn gray rather than bronze. Bronzed fruit has a dry, rough texture.

Blueberry bronze leaf curl (OPENi) CC BY 3.0

Mineral imbalances
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Too much or too little of certain minerals can cause bronzing. Regularly adding organic material to your garden soil helps minimize mineral imbalances. But it’s still a good idea to know what to look for when scouting your foodscape:

• Chlorine deficiency – More likely in sandy soils, symptoms include bronzing, stunting, necrosis, chlorosis, and wilting.
  
• Copper deficiency – Copper deficiencies are rare. When they occur, they make trees look more like shrubs than trees. Bronzing, shoot, twig and needle dieback, and witches’ broom are common symptoms.
  
• Iron toxicity – Iron toxicity often appears as bronzing and reddish spots. These symptoms are from iron oxidizing the chlorophyll used in photosynthesis. In areas with heavy clay soil, insufficient iron is more likely.
  
• Manganese deficiency – Manganese is an immobile nutrient, so deficiencies are seen in younger leaves first. Manganese deficiencies look very similar to iron (Fe), magnesium (Mg), and nitrogen (N) deficiencies, with interveinal chlorosis and bronzing. Brown specks may also be visible.
  
• Sodium toxicity – Too much sodium can cause severe chlorosis, bronzing, and leaf drop. Stunting and other water stress symptoms are also common.
  
• Zinc deficiency – Rare in most areas, zinc deficiencies appear as twig dieback (necrosis), yellowing (chlorosis), and leaf bronzing, often caused by too much phosphorus in the soil. Zinc deficiencies are more common in container plants.

[Did you know that rice farmers rate their plant varieties using leaf bronzing scores (LBS)? They rank rice varieties according to their ability to tolerate excessive iron in the soil.]

Pest feeding

As pests feed, leaf and fruit bronzing may occur. Most of these pests are sap-suckers:

• Mites can cause leaf stippling and bronzing. Fruit can also take on a russet, bronzed appearance.
• Nematode feeding may cause bronzing and stunted growth of boxwood.
• Thrips may cause bronzing and downward leaf curling of lettuce, strawberries, blueberries, boxwood, and many other plants.
• Aphids, scale insects, and whiteflies may also cause bronzing.

Chemical damage

Damage can occur when herbicides, pesticides, and fungicides are incorrectly applied. Also, the wind can carry herbicides and other chemicals from neighboring gardens and yards that may cause leaf bronzing. Bronzing, necrosis, interveinal chlorosis, desiccation, and distorted growth may indicate chemical misuse or overspray.

Environmental conditions

Air pollution often causes high ozone (O3) levels in the atmosphere. Ozone, combined with high temperatures and bright sunlight, can cause purple-brown discoloration, or bronzing, on the upper surface of leaves. Bean plants are especially vulnerable to air pollution.

Disease

Many plant diseases include bronzing as a symptom. These include spotted tomato wilt (carried by thrips), phomopsis stem canker in sunflowers, alfalfa mosaic, cotton root rot, and blueberry bronze leaf curl.

Use bronzing as a clue when you walk through your garden. The brownish or purplish discoloration of bronzing is a clear sign that something is amiss.