Fungus on Basil Plants

Fusarim Wilt

Fusarium oxysporum is the fungus that causes fusarium wilt in basil and hundreds of other plants. This fungal pathogen thrives in warm weather and in acidic, sandy soils. Three types of spores or races are detected so far. This soil-borne fungus can persist without a host plant for many years.

According to the report "Diseases of Basil and Their Management" (University di Torino, Italy), this fungus can show airborne behavior. For example, a greenhouse fan can circulate and disperse the airborne propagules or spores. Sprayer operations may accidentally spread the fungus that threatens the plants intended to be saved.

This fungus enters the plant's xylem or water-conducting vessels and then blocks and breaks down the xylem. Symptoms include yellowing, wilting and asymmetric growth. Leaf cells on one part of the leaf outgrow the bottom ones, and cause the leaf to curve downwards. Diseased plant stems show pale pink-orange layers. Necrosis or plant death follows. Young basil plants can die within seven days after first symptoms. Older plants may last longer.

The fungus Colletotrichum causes black spot. Symptoms include lesions with circular or irregular dark spots on leaves and stems. Older lesions show dryness with shredded tissue. Lesions enclose the stem and develop to plant death. Severe cases develop in very dense crops.

In greenhouses, after warm, humid nights followed by cool nights, the lower night temperatures and dew can enhance black spot growth. Steps to avoid black spot include eliminating contaminated basil seeds and applying fungicidal soil treatment where permitted.

Contaminated soil may lead to a soil-borne disease.empty planting pots image by askthegeek from Fotolia.com

Different soil-borne and seed-borne fungi cause damping off, which can destroy seeds or seedlings. Pythium ultimum and Phytophthora are fungal-like micro-organisms that thrive in abundant moisture in soil and growing medium and cause root rot. Sources include contaminated soil and water, unsterilized pots and trays, dirty tools, infected cuttings or plug plants and debris, according to the Horticultural Development Company.

Spores of both Pythium and Phytophthora pathogens can survive many years without a host. Germinated spores then disperse by water traveling in soil and circulating irrigation. Symptoms show poor seedling emergence and rotted, infected roots.

Pythium root rot occurs in cool, wet, poorly-drained soils. Adult shore flies and sciarid flies can transmit this fungus. Resting spores germinate and grow rapidly to invade plant roots. Stems show slimy and black lower portions. This fungus develops from root tips upward and results in seedling stem collapse, often in one direction.

Phytophthora root rot usually affect roots of mature plants, but can affect damping off that results in seedling death. This fungus invades root tips and results in a brown to black rot. This fungal pathogen can survive for years as a resting spore in the soil.

For basil plants set in module or plug trays, the risk of disease between seedlings reduces.