Basil

Ocimum basilicum

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Description

Basil, Ocimum basilicum, is a short lived annual or perennial plant in the family Lamiaceae grown for its leaves which are used as a herb. The basil plant grows from a thick taproot and has silky green opposite (paired) oval leaves which grow to be 3–11 cm (1.2–4.3 in) long and 1–6 cm (0.4–2.4 in), branching out from the central stem. The plant produces small white flowers which are clustered on a single spike at the top of the plant. Basil plants are often grown as annuals but may survive for several seasons with some care and can reach heights between 30 and 130 cm (11.8 and 51.2 in) depending on the variety. Basil may also be referred to as sweet basil, St. Joseph's wort, thai basil, lemon basil or holy basil depending on the variety and is native to India and other tropical regions of Asia.

Uses

Basil is commonly used as a fresh or dried herb in cooking and is popularly used in beverages in Southeast Asia. Essential oil can be extracted from the leaves and used in cosmetics, dental products and perfume.

Propagation


Basic requirements
Basil is a warm season crop which will grow optimally in areas where daytime temperatures are consistently above 21°C (70°F) and nighttime temperatures stay above 10°C (50°F). Basil is very sensitive to frost and will need protected if a late cold snap is forecast. The plant will grow best in a fertile, moist soil with a pH between 6 and 7. Basil requires around 6–8 hours of sun every day and benefits from some shade in the afternoon.

Sowing seeds
Basil seeds should be sown indoors 6–8 weeks before planting outside. Sow seeds in a sterile seed starting mix in seed trays or pots 0.2–1.0 cm (0.08–0.4 in) deep and water gently. Ensure the temperature remains between 15.5 and 27°C (60–80°F). Seeds should germinate in about 5 days at 21°C (70°F).

Transplanting
Basil seedlings can be transplanted to the garden when they are between 6 and 8 weeks old, about 2 weeks after the last frost date. Plants should be spaced approximately 30 cm (12 in) apart, allowing 45 cm (18 in) between rows. Pinching back the growing tip of the plants after transplanting will encourage the growth of new shoots.

Harvesting
Basil leaves can begin to be harvested any time after the plants have reached a height of 15–20 cm (6–8 in). Harvest leaves by pinching the leaves from the tips of the stems to encourage the more branching. Leaves should be pinched regularly to keep the plants productive and prevent them from going to seed.

Diseases

Aphids (Various spp.) Insect Various

Symptoms

Small soft bodied insects on underside of leaves and/or stems of plant; usually green or yellow in color, but may be pink, brown, red or black depending on species and host plant; if aphid infestation is heavy it may cause leaves to yellow and/or distorted, necrotic spots on leaves and/or stunted shoots; aphids secrete a sticky, sugary substance called honeydew which encourages the growth of sooty mold on the plants.

Comments

Distinguishing features include the presence of cornicles (tubular structures) which project backwards from the body of the aphid; will generally not move very quickly when disturbed.

Management

If aphid population is limited to just a few leaves or shoots then the infestation can be pruned out to provide control; check transplants for aphids before planting; use tolerant varieties if available; reflective mulches such as silver colored plastic can deter aphids from feeding on plants; sturdy plants can be sprayed with a strong jet of water to knock aphids from leaves; insecticides are generally only required to treat aphids if the infestation is very high - plants generally tolerate low and medium level infestation; insecticidal soaps or oils such as neem or canola oil are usually the best method of control; always check the labels of the products for specific usage guidelines prior to use.

Cercospora leaf spot Fungus Cercospora ocimicola

Symptoms

Circular to irregular dark spots on leaves with light centers

Comments

Management

Avoid overhead irrigation and splashing plants with water, instead water plants from the base and apply a layer of mulch around the plants to reduce water splash; remove and destroy any symptomatic leaves; minor infections can be controlled by spraying weekly with a fungicide containing potassium bicarbonate

Cutworms, loopers, owlet moths, and underwings Insect Spodoptera exigua
Trichoplusia ni

and others

Symptoms

The early stage larvae feeds on terminal clusters. Later stage larvae skeletonize the leaves. Also, they will cut the seedling stem near the base resulting in heavy loss.

Comments

The insect has a wide host range.

Management

Hand pick the larvae and kill them. Remove and destroy weeds and crop residue. Spray biocontrol agent (bacteria/virus) to kill insects. If infestation is severe, spray suitable insecticide.

Downy mildew Fungus Peronospora belbahrii

Symptoms

Yellowing leaves; discoloration often begins around middle vein and spreads outwards; gray fuzzy or downy growth on lower surface of the leaves; brown to black angular necrotic patches on the plant.

Comments

Yellowing leaves may be mistaken for nutrient deficiency; can be spread by contaminated seed.

Management

Grow tolerant varieties; apply protective fungicide; ensure good air circulation around greenhouse grown plants; use drip irrigation to avoid wetting foliage.

Flea beetle Insects Phylotreta spp.

Symptoms

Small holes or pits in leaves that give the foliage a characteristic “shothole” appearance; young plants and seedlings are particularly susceptible; plant growth may be reduced; if damage is severe the plant may be killed; the pest responsible for the damage is a small (1.5–3.0 mm) dark colored beetle which jumps when disturbed; the beetles are often shiny in appearance.

Comments

Younger plants are more susceptible to flea beetle damage than older ones; older plants can tolerate infestation; flea beetles may overwinter on nearby weed species, in plant debris or in the soil; insects may go through a second or third generation in one year.

Management

In areas where flea beetles are a problem, floating row covers may have to be used prior to the emergence of the beetles to provide a physical barrier to protect young plants; plant seeds early to allow establishment before the beetles become a problem - mature plants are less susceptible to damage; trap crops may provide a measure of control - cruciferous plants are best; application of a thick layer of mulch may help prevent beetles reaching surface; application on diamotecoeus earth or oils such as neem oil are effective control methods for organic growers; application of insecticides containing carbaryl, spinosad, bifenthrin and permethrin can provide adequate control of beetles for up to a week but will need reapplied.

Fusarium wilt Fungus Fusarium oxysporum

Symptoms

Yellow, wilting leaves; brown streaks on lower surface of leaves; stunted growth; death of plant.

Comments

Disease favors warm, wet conditions.

Management

Use only disease free seed; treat seeds with hot water to kill fungi prior to planting; if present in field, rotate crop every 2-3 years with crop other than basil or mint.

Grasshoppers Insect Various sp.

Symptoms

Both adult and nymphs feed on foliage, buds, and tender stems. The first stage nymphs feed on leaves by forming circular holes. As the insect develops it feeds on entire foliage. Grasshoppers are very eager feeders.

Comments

It feeds on several crops including weeds.

Management

Encourage birds in the field. If population is severe, spray suitable insecticide.

Gray mold Fungus Botrytis cinerea

Symptoms

Dense, brown to gray fuzzy growth on stems and leaves and fallen plant debris; leaves dying and dropping from plant; severe lesions on stem may cause plant death.

Comments

Promoted by high humidity and poor air circulation.

Management

No chemical treatment available; avoid working in field in rainy conditions; remove infested leaves and/or plants; avoid overhead irrigation.

Japanese beetle Insect Popillia japonica

Symptoms

Leaves skeletonized (only veins remaining); flowers and buds damaged; plant damage may be extensive; adult insect is a metallic green-bronze beetle with tufts of white hair protruding from under wing covers on each side of the body; adult beetles are approximately 13 mm in length; larvae are cream-white grubs which develop in the soil.

Comments

One beetle generation every 1-2 years; pheromone traps may actually attract more beetles to home gardens and should generally be avoided; beetle overwinters as larvae in soil; beetle has an extensive range of over 300 host plants.

Management

If beetles were a problem in the previous year, use floating row covers to protect plants or spray kaolin clay; adult beetles can be hand picked from plants and destroyed by placing in soapy water; parasitic nematodes can be applied to soil to reduce the number of overwintering grubs; insecticidal soaps or neem oil can help reduce beetle populations.

Leafminers Insects Liriomyza spp.

Symptoms

Thin, white, winding trails on leaves; heavy mining can result in white blotches on leaves and leaves dropping from the plant prematurely; early infestation can cause fruit yield to be reduced; adult leafminer is a small black and yellow fly which lays its eggs in the leaf; larave hatch and feed on leaf interior.

Comments

Mature larvae drop from leaves into soil to pupate; entire lifecycle can take as little as 2 weeks in warm weather; insect may go through 7 to 10 generations per year.

Management

Check transplants for signs of leafminer damage prior to planting; remove plants from soil immediately after harvest; only use insecticides when leafminer damage has been identified as unnecessary spraying will also reduce populations of their natural enemies.

Leaf spot Bacteria Pseudomonas cichorii
Colletotrichum spp.

Symptoms

Angular or irregular brown or black water-soaked spots on leaves; streaks on stems.

Comments

High humidity and overhead watering promotes spread of disease.

Management

No treatment when present; use disease free seed and/or transplants; use wide field spacing to promote air circulation around plants; remove diseased leaves from plant and soil surface immediately.

Root knot nematode Nematode Meloidogyne spp.

Symptoms

Galls on roots which can be up to 3.3 cm (1 in) in diameter but are usually smaller; reduction in plant vigor; yellowing plants which wilt in hot weather.

Comments

Galls can appear as quickly as a month prior to planting; nematodes prefer sandy soils and damage in areas of field or garden with this type of soil is most likely.

Management

Plant resistant varieties if nematodes are known to be present in the soil ;check roots of plants mid-season or sooner if symptoms indicate nematodes; solarizing soil can reduce nematode populations in the soil and levels of inoculum of many other pathogens.

Root rot Fungus Rhizoctonia solani
Pythium spp.

Symptoms

Failure of seeds to germinate; germinated seedlings collapsing; brown, shriveled area at base of stem; roots brown and water-soaked.

Comments

Promoted by high humidity and poor air circulation.

Management

Plant seeds in sterile soil; plant basil in well-draining soils.

Slugs & snails (Gray garden slug, Spotted garden slug, Brown garden snail, European garden snail , etc.) Mollusc Decoratus reticulatum
Limax maximus
Helix aspersa
Cornu aspersum

Symptoms

Irregularly shaped holes in leaves and stems; flowers and fruit may also be damaged if present; if infestation is severe, leaves may be shredded; slime trails present on rocks, walkways, soil and plant foliage; several slug and snail species are common garden pests; slugs are dark gray to black in color and can range in size from 2.5 to 10 cm (1-4 in); garden snails are generally smaller and possess a rounded or spiral shell.

Comments

Slugs and snails prefer moist, shaded habitats and will shelter in weeds or organic trash; adults may deposit eggs in the soil throughout the season; damage to plants can be extensive.

Management

Practice good garden sanitation by removing garden trash, weeds and plant debris to promote good air circulation and reduce moist habitat for slugs and snails; handpick slugs at night to decrease population; spread wood ashes or eggshells around plants; attract molluscs by leaving out organic matter such as lettuce or grapefruit skins, destroy any found feeding on lure; sink shallow dishes filled with beer into the soil to attract and drown the molluscs; chemical controls include ferrous phosphate for organic gardens and metaldehyde (e.g. Buggeta) and carbaryl (e.g Sevin bait) for non-organic growers.

References

CABI Crop Protection Compendium. (2008). Ocimum basilicum datasheet. Available at: http://www.cabi.org/cpc/datasheet/36858. [Accessed 06 November 14]. Paid subscription required.

Meyers, M. (2003). Basil: An Herb Society of America Guide. The Herb Society of America, Ohio, USA. Available at: http://www.herbsociety.org/factsheets.... [Accessed 06 November 14]. Free to access.

MacKenzie, J. (Ed) (2007). Growing basil. University of Minnesota Extension. Available at: http://www.extension.umn.edu/garden/y.... [Accessed 06 November 14]. Free to access.

Tran, T. (2011). Basil Diseases. Cornell University Department of Plant Pathology and Plant-Microbe Biology. Available at: http://plantclinic.cornell.edu/factsh.... [Accessed 06 November 14]. Free to access.