Edema

Symptoms:  Excess water is absorbed by the roots quicker than it is lost by the leaves, causing swelling of plant cells and producing a blister-like lesion. Occurs when plants watered during warm days and the nights turn cool or during periods of cool weather when water quantity and/or frequency is not reduced.

Treatment: No treatment.

Prevention:  Water early in the morning when nighttime temperatures drop below 65F. Reduce watering in the fall when plant growth slows.

Read More: Edema caused by overwatering when nights are cool, by Robert Cating, IFAS Extension of the University of Florida.

Cold Damage from Cold Weather and/or Cold Water

Symptoms of Chilling: Slowed growth, or limited growth flush. This may be difficult to detect without non-chilled plants for comparison or a thorough knowledge of the orchid’s normal growth rate. Other symptoms include:
-   Surface lesions, pitting, large, sunken areas and discoloration
-   Water-soaking in tissues, usually followed by wilting and browning
-   Internal discoloration (browning)
-   Accelerated rate of natural death
-   Increased susceptibility to attack by fungi and bacteria

Symptoms of Freezing:  Obvious symptoms may not be present until after the plant has been stressed by warm temperatures. Freeze symptoms include:
-   Desiccation or burning of foliage
-   Water-soaking in tissues, usually followed by wilting and browning
-   Water-soaked areas that progress to necrotic spots on leaves
-   Death of sections of the plant or the entire plant

Treatment:  Remove the dead tissue to prevent secondary bacterial infection. As a precaution, create a slurry with cupric hydroxide (Kocide or Champion) combined in equal parts with mancozeb (Manzate or Dithane M45) and apply to damaged tissue with a toothbrush.

Prevention:  Know the temperature requirements of your orchids and watch the projected nighttime lows during winter.

Read More:  Cold Damage by Robert Cating, University of Florida, Treating Cold Damage in Orchids and Leaf Reddening after Exposure to Cold by Dr. Martin Motes, Cold Damage and Mesophyll Cell Collapse by Susan Jones, American Orchid Society.

Sunburn

Symptoms:  Sudden appearance of black spots on leaves when the leaves become overheated from too much light caused by a sudden increase in light during the change in season or moving plants outdoors in the spring. The burn often will occur on the highest point of the leaf where it is exposed to the most sun. Burn fades to thin tannish leaf scar over time.

Treatment:  Sunburn is irreversible and leaf damage could be an invitation to secondary infection.

Prevention:  Move plants slowly into brighter light, moving them into slightly greater light over a 2 to 3 week period so they gradually become acclimated to the higher light conditions.

Fertilizer Burn on Flowers

Symptoms:  If you get water soluble fertilizer on your flowers while you are watering, you have the potential for burning the flowers, particularly if you use a full strength rather than dilute fertilizer. The fertilizer salts will burn the flower leaving a water soaked spot. Fertilizing blooming plants may also shorten the blooming time.

Treatment:  After you’re done fertilizing, come back with a plain water spray on the flowers to wash the fertilizer off the flowers to prevent their spoiling.

Prevention:  It is safer to use a more dilute fertilizer more frequently than full strength fertilizer. Try using one quarter to one eighth of the labeled strength weekly (when you water), unless you determine your plant needs a higher fertilizer concentration.

Salt Toxicity    photos and text courtesy of the American Orchid Society

Salt Buildup:  Salts present in your water supply and added by fertilizers accumulate over time. Salt buildup looks like whitish to brownish crusts on the medium and around the pot, or on the surface of the mounting substrate on mounted orchids, and can be a sign of over-fertilizing. If allowed to remain, those salts will negatively impact the health of your plant. Excess fertilizer salts burn and kill orchids

Symptoms:  Lack of root growth may indicate an unhealthy concentration of mineral salts in the medium, on up to full fertilizer burn. If this is suspected, decant the plant and check its roots. Dead root tips, brown roots or salt crust on the potting medium surface are signs of trouble. In later stages, brown leaf tips, leaves and eventually pseudobulbs may appear, indicating burned roots. If allowed to continue, fertilizer burn will eventually kill the plant.

Treatment:  Flush the pot monthly by watering with copious amounts of water to solubilize salts and then watering again an hour later to flush accumulated salts from the pot.

Prevention:  It is preferable to use a dilute fertilizer, say one quarter to one eighth of the labeled strength.