Orchid Pests and Diseases - Environmental Problems

Edema

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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: Physiological Disorders of Orchids: Oedema (Robert Cating et al, IPA)
Edema Blisters on Orchids (Sue Bottom)

Calcium Deficiency

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Symptoms: Calcium deficiency has a similar appearance to a bacterial or fungal rot, but is really the same problem as blossom end rot in tomatoes. It affects cattleyas, especially the Guarianthe, primarily during periods of rapid growth. Calcium, a lesser macronutrient, is used to build cell walls. Deficiencies usually occur in spring and summer during periods of active growth. New leaves may turn black at the tips. The affected area has an advancing yellow band. Bud growth may be inhibited or buds may develop improperly. A calcium deficiency may also cause death of root tips. (extracted from Orchid Species Culture by Margaret L. and Charles O. Baker)
Treatment:  Supply extra calcium when the plant is in active growth. Use a good cal mag fertilizer, add calcium nitrate at the rate of 1/4 to 1/2 teaspoon per gallon to your water (alternating with Epsom salts) between fertilizations, add 1 tablespoon of dolomitic lime per gallon or top dress with powdered dolomite as a source of calcium and magnesium during the hot months when calcium requirements are high. You can also make the calcium present in well water more available to the plant by using an acidity generating fertilizer like 20-10-20.
Prevention:  Supply extra calcium via a good cal mag fertilizer, calcium nitrate or dolomitic lime.
Read More: More Black Tips (Rockhampton Orchid Society)
Calcium and Magnesium (Sue Bottom, SAOS)
Calcium Deficiency in Cattleyas (Sue Bottom, SAOS)
Blackened Tips in New Cattleya Leaves (Sue Bottom, SAOS)

Magnesium Deficiency

Symptoms: Magnesium deficiency can result in cupped leaves, a reduction in growth and marginal or interveinal chlorosis (yellow along leaf edges or between veins) and usually is exhibited in the middle or older leaves (extracted from Orchid Species Culture by Margaret L. and Charles O. Baker)
Plants grown with a magnesium deficiency can exhibit chlorotic mottling after exposure to extremes of temperature. When growth begins in the spring, the older leaves on plants grown without sufficient magnesium will yellow and drop as the plant translocates magnesium to the new growth. Dr. Martin Motes suggests an increase in anthocyanin (red/purple coloration) in leaves, particularly after exposure to cooler temperatures, can occur as a result of magnesium deficiency
Treatment: Supply extra magnesium when the plant is in active growth. Use a good cal mag fertilizer, add Epsom salts at the rate of 1/8 to 1/4 tsp weekly or 1/2 teaspoon per gallon to your water each month, add 1 tablespoon of dolomitic lime per gallon or top dress with powdered dolomite.
Prevention:  Supply extra magnesium via a good cal mag fertilizer, Epsom salts and/or dolomitic lime.
Read More: Leaf Reddening from Magnesium Deficiency after Exposure to Cold (Dr. Martin Motes)
Calcium and Magnesium (Sue Bottom, SAOS),

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: Treating Cold Damage in Orchids and Leaf Reddening after Exposure to Cold (Dr. Martin Motes)
Cold Damage and Mesophyll Cell Collapse (Susan Jones, American Orchid Society)

Sunburn

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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.

Bud Blast

Symptoms: Bud blast can take many forms. Buds may wither and fall from an otherwise healthy plant, buds can fail to open into flowers or the buds may rot in the sheath.
Causes: If you've just brought an orchid home, the most likely reason for bud blast is the damage to the sensitive bud from its being moved or radical changes in the amount of light and water the plant is used to.
  It can also be caused by water: too dry and the moisture can be drawn from the sensitive buds or too much water can cause root rot and the plant can't sustain the emerging flower.
  Radical temperature changes can also cause it: drafts from an air conditioner or heater can cause bud drop and condensation from day night temperature changes can cause buds in the sheath to rot.
  Chemicals from fumes and ethylene from combustion engines can cause the buds to age and distort flowers. Insects like aphids and thrips can feed on the buds and cause them to drop or be deformed after opening.
  Other reasons include drying out from too low of humidity or being too close to grow lights and chemical damage from fertilizers. Sometimes bud blast occurs and there is no obvious reason for it.
Prevention:  The best prevention is good cultural practices maintaining the proper moisture and temperature for your plants to thrive. If a sheath starts to yellow or darken, carefully peel it back to prevent condensation from rotting the buds. If there is evidence of aphids or thrips, spray the plants with a chemical that won't damage the flower (such as a dilute Orthene).
Read More: Bud Blast (Susan Jones, AOS)
Bud Blast and Flower Blighting (Sue Bottom, SAOS)

Orchids, Weeds and Ferns Don't Mix

Symptoms: Once ferns start growing in an orchid pot, the tough fibrous rhizomes and roots will quickly fill the orchid pot and suffocate the orchid roots. The orchid will go into a gradual decline as the rotting roots can no longer support the top growth.
Treatment: Remove ferns and weeds as soon as you see them starting to emerge in the pot. If you wait too long, you'll have to remove the plant from the pot, clean all potting material and fern growth from around what's left of the orchid roots and repot in new sterile mix.
Prevention:  Don't grow ferns in the vicinity of your orchid plants and keep your growing area weed free. The spores easily go airborne and always seem to find a home in orchid potting mix
Read More: Ferns, Weeds and Orchids Don't Mix (Sue Bottom)