Stem Rot on Basil

Damping-off Diseases

• Although the stem of your basil is affected, the culprit could be a fungal disease called pythium root rot, which sometimes appears on above-ground parts of plants. This disease can strike mature basil plants, but it is more common in young ones. Along with other fungal pathogens, it constitutes a disorder known as "damping-off," in which seedlings die soon after being transplanted. According to Horticultural Development Company, symptoms of pythium root rot include poor emergence of seedlings, stunted growth, yellowing, wilting and eventual death. Browned or blackened spots may appear on the stem, with affected areas becoming narrowed and constricted. Phytophthera, a similar pathogen, also causes stunted growth and yellowing; cutting open the stem reveals brown discoloration inside. Yet another fungal disease, Rhizoctonia solani, causes reddish-brown discoloration and dry, cracked lower stems. A characteristic fine gray webbing may appear on leaves and soil.

Fusarium Wilt

• Fusarium wilt, caused by a fungus called Fusarium oxysporum, attacks the water-conducting tissue, or xylem, of the stem. Plants will often appear unaffected and grow well until they are between 6 and 12 inches tall, then become rapidly stunted and brown, particularly on the tips of branches. Wilting and eventual death follows. As with basil plants affected by pythium root root and phytophthera, fusarium-infected specimens should be pulled out and discarded in a closed container.

Gray Mold

• Gray mold, or Botrytis cinerea, infections begin on cuts on stems after leaves have been harvested. The infection then moves down the stems, causing leaves to drop off; clusters of dead leaves at the base of the plant are a hallmark of the disease. If the mold reaches the main stem, the disease can be fatal. Clemson Cooperative Extension reports that you may be able to salvage plants by promptly cutting out infected parts, as well as gathering and discarding fallen leaves.

Prevention

• In order to thrive and gain resistance to soil-borne fungal disease, basil requires at least six hours of full sun a day, along with properly-drained soil with a pH between 6.0 and 6.5. Make sure you ensure sufficient air circulation by providing enough room between your basil plants; BasilGardening.com advises planting them 12 to 15 inches apart. You can cut the odds of disease by planting resistant cultivars; Clemson Cooperative Extension recommends the Genovese varieties. Because spores can be spread in moisture, you should avoid overhead watering and harvesting when leaves are wet. Planting seedlings in modular trays can help reduce the spread of infection. Finally, never plant basil in areas where stem rot has been a problem before; spores from fungal-borne diseases can live in soil for several years.