The Black Root Rot Disease in an Orange Tree

The Fungus

• The black root rot fungus produces two types of soil-borne spores. Round-cornered rectangular endoconidia die after a few months. Barrellike chlamydospores survive for several years. The chlamydospores separate when mature, with each spore capable of germinating. Water, soil and fungus gnats or shore flies that feed on infected plants all transport the spores to healthy orange tree roots.

Favorable Soil Conditions

• Black root rot thrives in soil temperatures between 51 and 63 degrees Fahrenheit and soil moisture at 70 percent capacity. The fungus requires a moderately acidic to alkaline soil pH, above 5.6. It begins dying at a pH of 5.5; a 4.8 acidity reading destroys it. Because oranges and most citrus also perform best at pH levels between 6 and 8, soil-acidifying amendments may damage the trees as they are eradicating the root rot.

Symptoms

• Fibrous roots of oranges infected with black root rot develop dead, black lesions. They sometimes exceed 1/2 inch in diameter. This condition causes stunting in orange seedlings. Thielaviopsis-related collar rot results in stained, seeping bark on the mature orange trunks. As the infection advances, it may girdle the trunk and cut off the tree's nutrient and water flow. The tree's foliage and stems may wilt and die.

The Peat Connection

• A study by researchers at the University of Florida's Citrus Research and Education Center identified soil-less, peat-based growing medium as the source of black root rot infections in greenhouse-raised orange seedlings. Their findings, published in the December 1991 "Plant Disease," identified commercial peat-based medium and baled Canadian bog peat moss as sources of contamination. They also found that traces of the fungus remained in the air of a greenhouse two months after the root rot-infected plants were removed. The fungus survived in peat debris and as airborne spores. Their research suggested that, in greenhouse conditions, the disease could also spread to other peat-medium-raised plants.

Control

• Planting orange seedlings after soil temperatures have risen above 63 degrees discourages black root rot infestation. An effective chemical treatment is a soil drench of benzimidazole-based fungicide, applied according to the manufacturer's directions. Steaming peat-based growing medium for 30 minutes at 140 degrees makes it safe for container seedlings.