Oh dear! It’s Oedema.

Oedema on the young leaves in this begonia basket.

Oedema, that physiological disorder that appears during periods of low light and high humidity.  There’s been quite a bit of it reported in Ontario greenhouses this spring, and unfortunately it’s related to the long rainy (or snowy!) spring we’ve been having.  If you’ve noticed salt-like crystals, odd tumour-like growths or water-soaked spots on either side of your plant leaves this disorder might be the culprit.

The disorder affects a wide variety of greenhouse ornamentals.  It’s usually noticed in spring crops like sweet potato vine (ipomea), geranium, begonia and/or petunia. Tropical plants and succulents are also susceptible as are veggie transplants like peppers and tomatoes. The damage can look similar to thrip or spider-mite feeding damage, but scouting will typically reveal low numbers of these pests (or you may have more than one problem!).

Figure 2. High humidity makes it hard for the plant to transpire, allowing water to collect in leaf tissues.
High humidity makes it hard for the plant to transpire, allowing water to collect in leaf tissues.

Causes: Humidity and Low Light Levels

If the relative humidity is high in the greenhouse, the plant has a hard time transpiring.  A well-irrigated plant in this environment will have roots that readily take up water, and leaves that cannot release it since the air around the plant is already full of water.  Humidity is related to temperature, so this can also happen when the media the plant is grown in is warm and wet, and the air surrounding the plant is cooler.

Both of these are a product of the type of weather we’ve been dealing with so far this spring. The water eventually pools in leaf tissue giving a swollen appearance until cells become either malformed (bumpy/bubbly or a crusty salt-like appearance) or the area around them collapses leaving a necrotic spot.  Oedema may be seen the top or bottom of affected leaves.  The impact of humidity on leaves is species specific, meaning that certain plants are more susceptible to this type of damage.

oedema eggplant
Oedema is present on the top (as necrotic spots) and bottom (as malformed “bumpy” areas of growth) of this eggplant leaf.

Treatment: Patience and…

  • Cut back on irrigation
  • Use air circulating fans and strategic venting to remove some of the humidity
  • Cross your fingers and hope for some sun

Lucky for us, the forecast shows warmer weather with some sun this weekend and into next week, so we should see some relief.  Remember that new growth is typically fine once the source of the problem has been corrected.  The new leaves will usually cover the damaged growth, but damaged older leaves can be removed if needed.

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