Plant Health Problems
The disease problems of basil are similar to those of other herbs. See Herbs for a detailed discussion of these diseases.
Diseases caused by Fungi:
Fusarium wilt, Fusarium oxysporum f. sp. basilicum.
Plants develop a general or partial wilt. The affected tissue can also turn dark and water-soaked. The water-conducting tubes inside the stem will become discolored. The fungus which causes the disease is seedborne. However, the fungus can persist in the soil for long periods of time once it is established.
Control can be achieved by using clean seed and fresh potting soil. Seed can be disinfested by soaking in 10% household bleach (1 part bleach to 9 parts water) for 1 minute, thoroughly rinsing with tap water, and planting immediately. Outdoor plantings should be rotated out of basil. Affected plants should be rogued out and discarded. The cultivar "Nufar" has shown some resistance.
Damping-off, Rhizoctonia solani, Pythium spp.
Seedlings either rot and do not germinate or germinate and collapse. The disease frequently attacks the base of the seedling and leaves a shriveled dried-up stem at the base of the seedling.
Control can be achieved by using sterile potting media, clean pots, and fungicide-treated seed. If seeds in a seed tray become symptomatic, discard all of the seeds since asymptomatic seeds may be infected.
Root and crown rot, Rhizoctonia solani, Pythium spp., Fusarium spp.
Symptoms appear as wilting and a slow or rapid collapse of the plant. The roots can appear brown and water-soaked instead of white. A water-soaked lesion can often appear at the base of the stem.
Control can be achieved by using sterile potting media, clean pots, and fungicide-treated seed. Affected plants should be rogued out and discarded.
Botrytis blight, Botrytis cinerea.
Botrytis blight occurs on many different kinds of herbs. The disease appears when plants are grown under conditions of high humidity, such as in a terrarium. The disease appears as tan to brown spots on leaves that can be associated with a grayish mold colonizing the damaged area. Senescing flowers are particularly susceptible and frequently have papery spots on them.
Botrytis blight can be suppressed by avoiding overhead irrigation and watering in the morning to allow time for leaves and flowers to dry. Proper plant spacing can improve air drainage and rapid leaf drying. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are neem oil and thiophanate-methyl. Consult the label for dosage rates, safety precautions, and indoor use.
Diseases caused by Nematodes:
Foliar nematodes, Aphelenchoides spp.
Patches can appear on the leaf and appear light or dark brown. The disease is caused by microscopic worms called nematodes that tunnel in the leaf.
Affected leaves should be cut off and discarded. Avoid splashing water from leaf to leaf. The nematodes can live as saprophytes in soil or potting mix for a short time so replanting the plant into fresh soil mix at the time the leaves are picked off can increase the control.
Diseases caused by Physiological/Environmental Factors:
Cold water injury, cold water used for watering.
When basil is grown indoors, irregular or ring-like pale tan spots appear on the upper side of the leaves. These may vary greatly in size and severity since some cultivars are more sensitive than others. Injury occurs when water which is colder than the surface of the leaf is used for watering. Symptoms are often more pronounced during the winter months.
This problem can be avoided by watering with tepid or room-temperature water and by not wetting the foliage during watering.
Soluble salt injury, excessive fertilization and accumulation of high levels of soluble salts in potting medium.
When herbs are grown indoors, they can develop root injury from excessive levels of fertilizer in the potting medium. Symptoms are similar to those caused by root rot so analysis of the soil is required for correct diagnosis. An indication of high soluble salts is the presence of white crystalline deposits on the rim of the outer surface of the pot or on the surface of the potting medium.
Soluble salts can sometimes be leached from the potting mix by running a stream of water through the soil for approximately 20 minutes. However, when salt levels are excessive, the plant can be repotted into fresh sterile potting mix and a clean pot. It is also important to avoid excessive use of fertilizer during periods of slow growth.