Brown Raised Spot Disease in Orchids

Causes

• Unfortunately no plant is immune to edema; however, with changes to environmental conditions, an orchid’s susceptibility can be limited. Edema occurs most often when orchids are overwatered during periods of warm days and cool nights. The combination of warm soil and cool, moist atmosphere results in the roots absorbing water at a faster rate than is lost through transpiration. If the conditions persist, plant cells will swell until they rupture, resulting in one or more raised lesions.

Symptoms

• Edema is generally characterized by the appearance of brown, swollen, round spots that may turn corky over time. Symptoms are most frequently observed on the underside of plant leaves. Each blister is between 1 and 2 mm in diameter; however spots will often combine to form a line or the appearance of a large infected area.

Possible Alternative Diagnosis

• Edema is sometimes misidentified as infestation by scale insects. Scale is a very serious, sucking pest that attaches to the surface of plant leaves. The waxy covering of the scale insect appears as a raised spot on leaves. To determine whether edema or scale is the problem, try rubbing the spot off the plant. If the blister easily rubs off, the problem is likely scale. While the plant tissue under edema remains green, its blisters cannot be rubbed off.

Prevention

• Cut back on watering and spraying orchids, particularly during periods of warm days and cool nights. Avoid watering, or water strictly in the morning, on overcast, humid days. Periodically examine the growing media to ensure that there is no excess water present. Allow orchids to dry out before rewatering. Provide an increased amount of circulation space in between orchids to improve air flow. If possible, increase the intensity of light to plants.