Edema or Oedema is a common problem with many types of seedlings, but it’s not widely recognized and is frequently mistaken for insect larvae or insect damage. Edema is an issue caused by overwatering, it happens in young plants when their roots absorb water faster than their leaves can release water through transpiration (the “breathing” and fluid movement within a plant’s vascular system). The water collects in the mesophyll cells (inner layers of the leaves), and soon the pressure begins to push its way to the surface in the form of blister-like pockets. To the human eye, these “blisters” appear to be bumpy crystal-like formations, similar in appearance to salt or sugar crystals. The blisters can happen anywhere on the plant, but most often they are found on the underside of the leaves and can be accompanied by purplish leaf surfaces.

If the excess water problem is not recognized and corrected, the blisters will rupture and scab over into fine, brown, dry pockmarks that look very similar to mite damage. Because the blisters look similar to eggs or larvae and the scab looks similar to mite damage, this problem can run on causing frustration and drastic use of unnecessary insecticides.

Once the watering cycle is cut back to allow for proper transpiration rates, new growth will appear and be unaffected. As the plant grows larger, the older affected leaves may be removed. In younger seedlings, if this problem is not recognized and corrected, the seedling will die due to the suffocation of dermal cells.