Is it really mold?

Is it really mold?

For many companies, discovering black discoloration on your wood packaging products can be troublesome. At first glance it might look like mold fungi, which are a great cause of concern regarding human health, but it might be something else entirely. There are types of fungi that grow on lumber called blue stain but they are not linked to human health concerns.

Interestingly, there are also other types of naturally occurring defects in lumber that might look like mold but in fact are not biological. Before you “jump the gun” and ask your supplier to replace all your wood pallets with fresh ones, keep in mind there are many types of naturally occurring, non-biological defects that may look scary, but are not caused by microorganisms (fungi, bacteria, etc).

Understanding the difference could save you time, money, and a great deal of worry.


Iron Stain

Iron stain is a common discoloration found on wood. Oak, redwood, cypress, and cedar are particularly prone to iron stain because these woods contain large amounts of tannin-like extractives. The discoloration is caused by a reaction between extractives naturally occurring in wood and iron-containing materials, such as metal fasteners and sawblades. Staining is spread by water movement, so the affected area is less for woods that have low moisture content, although staining can occur if water is reintroduced to the surface of the wood. The staining is cosmetic and does not affect the strength properties in wood.


brown stain or zebra stain

In western hemlock, a type of discoloration occurs only after the wood is dried in a kiln. Whereas the unaffected areas appear light yellow, affected areas appear dark brown, making for noticeable differences in surface color variation. Below the surface of susceptible pieces, sometimes the brown stain will appear black after the wood is dried. This is known as zebra stain. Zebra stains happen when iron or manganese darkens the browning and makes it turn black.


sun exposure

If your wood product is left outside and exposed to the sun, over time it will darken (like a sun tan) and may make the wood appear dirty or damaged. The impact of sun exposure causes a chemical change in the tannins of the wood that, over time, react to the sun’s exposure. If this happens to your wood packaging product, or other lumber product, it’s said to be “weathered.”



Blue stain is not mold; it is a common cause for the discoloration of lumber. Certain darkcolored microscopic fungi cause a bluish or grayish discoloration in the sapwood of the tree. Not all blue stains are blue. Other stain shades can be blue, bluish black, gray, brown, red, yellow, orange, or purple. Blue stain has no effect on the performance and strength of lumber. It can be used for exactly the same purpose as non-stained lumber. A simple test can help you determine whether or not a piece of lumber is blue-stained or growing mold. Lightly rub the affected surface of the wood. Mold grows on the surface and can be brushed off or smeared, whereas blue stain penetrates deep into the wood and cannot be removed. Blue stain poses no health risk, and blue-stained lumber is safe to handle.


Follow these steps to reduce the chances of fungi from impacting your wood packaging inventory.

1. Keep it dry. Blue stain thrives in wood that has a moisture content greater than 19%. Keeping it dry and in low-humidity conditions will prevent it from growing. If your inventory is stored outside, tarps or paper wrap are useful, but make sure there are holes that allow for ventilation.

2. Keep it ventilated. Storing wood pallets in an unventilated space creates ideal conditions for new blue stain growth, especially in warmer weather. If you must store your wood products inside, providing sufficient air ventilation will reduce the likelihood of blue stain.

3. Keep it clear. Remember, surface blue stain transmits via insects. Ensure the area surrounding your wood products is clear of vegetation or debris that might harbor insects
or pests that transmit blue stain.

4. Keep it off the ground. A 6 to 8-inch elevation will ensure that the bottom layer will stay dry from puddles of rain that might form. This will keep your products dry.


Red alder, oaks, beech, maples, and other hardwood species are commonly susceptible to enzymatic discoloration. This is the reaction of enzymes or polyphenolic compounds in living cells. This produces a grayish or brownish tone in sapwood.


Typically seen in the forms of dark lines or streaks in oak, green or brown patches in sugar maple, or purple to black areas in yellow poplar; mineral discoloration sometimes develops in standing or fallen trees in mineral rich soils. Preventing discolorations caused from iron stain and weathering are quite manageable. If you store wood products outdoors, keep them covered yet ventilated to prevent weathering. Also, keep your ferrous metals from having direct contact with lumber to prevent black ink stains. Other types of black stains and discolorations, like zebra stains, enzymatic discolorations or mineral discolorations, are naturally occurring and challenging to control.

For more information go to