Crop Production Guidelines
By: Randall T. Hamasaki, Hector R. Valenzuela, Dick M.Tsuda, and Janice Y. Uchida
University of Hawaii, Cooperative Extension Service, Associate
County Extension Agent, Associate Extension Vegetable Specialist,
Research Associate, and Assistant Professor of Plant Pathology,
Basil (Ocimum basilicum) is a popular herb grown for the fresh market or for its dried aromatic leaves which are used as a spice or flavoring. This herb, a member of the mint family, is a native of Central Asia and North West India. It is a perennial, adapted to warm growing conditions, but is typically grown as an annual. It is commonly used in tomato dishes, pesto, and as a flavoring in soups, salads, minced beef, and sausages. North America annually imports about 2,000 tons of basil. Basil is produced commercially in Egypt, France, Hungary, Israel, Mexico, Indonesia, and the United States. In Hawaii, basil is grown on about 55 acres with an estimated annual farmgate value of $1.2 million. Many types of basil are available varying in size, leaf shape, flavor, and in leaf color ranging from green to purple. Product quality is determined by appearance (color and absence of decay or insect damage), flavor, and moisture content for the fresh market, and volatile oil content and total insoluble ash content for the processing market. Basil markets include local hotels and restaurants, local wholesalers and retailers, mainland produce wholesalers, and processors of pesto and other products using basil. Basil for export from Hawaii is mainly grown during the winter months.
CULTURE AND MANAGEMENT
Basil seed should be obtained from a reputable source to assure that the seed is true to type. The soil should be worked well to allow for proper germination. Basil is direct seeded to a depth of 1/4 inch or transplanted into the field when about 6 in tall. When direct seeding, over-seed expecting an 80-90% germination rate. Optimum temperature for germination is 70oF (20oC), but the seeds will germinate well between 65-85oF (15-30oC) in about 7 days. Basil can be grown year-round in Hawaii in well-drained soils at elevations below 700 ft (250 m), and from May to October at higher elevations.
Rows are spaced 24 to 36 in apart, and plants are spaced 6 to 24 in in the row depending on culture. Basil can also be planted in beds with a spacing of 24 to 36 inches between bed centers. Three rows are planted per bed with a spacing of 12 inches between rows. In large scale operations, basil may be planted at a high density for once over harvesting. In smaller operations, basil is planted at a lower density and harvested over a long growing season.
Mulches and Windbreaks
Basil requires adequate soil moisture throughout the growing season to maintain quality and yields. The field may be mulched with organic material between the rows to help control weeds and conserve moisture. A mulched field will also reduce the amount of soil splashing onto the leaves which will simplify the washing of foliage at harvest time and help reduce the incidence of certain diseases. High winds can damage foliage and cause rapid moisture loss in the field. The use of windbreaks may be beneficial under high wind conditions.
Fertilizer practices may vary depending on the previous crop and on the current fertility of the soil. Use soil tests to determine the levels of available plant nutrients in the soil. Over-fertilization will diminish the quality of sweet basil at harvest and may increase the soluble salt content of the soil to undesirable levels. Plants overfertilized with nitrogen often show dark discolorations in the cooling rooms after harvest. Pre-plant fertilization rates of 120-120-120 lbs per acre of N-P205-K2O are recommended for soils deficient in these nutrients. Basil does best when soil pH is between 6.0-7.5. Nitrogen may also be side-dressed at the rate of 20-30 lbs per acre soon after the first harvest. Experiments in Indiana showed that ammonium sources of nitrogen may decrease petiole and stem yields and leaf content of essential oils when compared to nitrate-N sources. Growers who plan to market basil labeled as "organic" should check with Federal and/or State guidelines for organically grown produce.
Basil is a specialty crop with only a few pesticides registered for use on the crop. Therefore, growers should develop and follow a pest management program to minimize the incidence of pests and diseases in their field and follow strict measures during and after harvesting to ensure that live insect and mites do not infest shipments. Field plantings of basil generally have a healthy population of beneficial arthropods such as parasitic wasps, spiders, and other general predators which can help keep pest populations to moderate levels. Learn to recognize both pests and beneficials and regularly monitor their populations. The early detection and management of pest problems can often prevent major problems. Growers producing basil for export should pay particular attention to insects that are quarantine pests or are likely to remain on harvested basil, and remove them before export.
Growers should also be careful that their product is not contaminated with pesticides that are not registered for use on basil. Properly labeled shipments and record keeping help to protect the industry as a whole and to identify the source of contaminated products.
Insect, Mite, and Mollusk Pests
At least 30 known insects and mites are found on basil grown in Hawaii. Although only a small portion of these cause significant crop damage, growers shipping to the continental U.S. have to comply with strict quarantine regulations which essentially require arthropod-free shipments. Therefore, growers should also monitor insects which may not be economically important in the growing of the crop, but which could pose quarantine problems later. Identification of the pests attacking your crop is simplified by classifying them based on the type of symptoms typically observed in the field.
Chewing Type Damage
Chinese rose beetle, Adoretus sinicus Burmeister. Chinese rose beetles cause heavy damage to basil when population levels are high. Adult Chinese rose beetles are brown, about 1/2 inch long, and are nocturnal. The chewing type damage may appear as leaves with holes or a lace-like appearance when the areas between leaf veins are eaten. Chinese rose beetles leave small, dark colored particles of frass which are excreted on the plant while feeding. Chinese rose beetles feed on a wide variety of ornamental and food crops in Hawaii. The larvae feed on decaying plant material and do not damage basil.
Slugs, various species. Slugs are nocturnal mollusks which thrive during wet weather but can also be troublesome during dry weather in irrigated fields and greenhouses. Their feeding damage appears as entirely eaten leaves or foliage with holes or missing parts, girdling or scraping of the bark, or the entire destruction of seedlings. Slugs leave a shiny slime trail as they move about. They hide during the day under plants, rocks, mulch, pottery, fallen leaves, or in loose soil.
Celery leafminer, Liriomyza trifolii (Burgess). The leafminers infesting basil are yellowish larvae (maggots) of tiny flies. Leafminer larvae feed inside the leaf tissue causing tunnels or mines within the leaf. Leafminer populations have usually been relatively light on basil and have not resulted in significant in-field crop losses. However, because leafminer larvae live and feed inside of the leaf tissue, they cannot be dislodged by shaking or rinsing the harvested basil. For basil grown for export, the leaves containing live larvae must be plucked off before packing. The adult fly is mostly black with some yellow and is about 1/16 inch long. Adult flies makes tiny punctures on leaf upper sides while feeding and laying eggs. Leafminer populations in Hawaii are usually kept below damaging levels by a number of parasites. Beneficial parasite field populations may be significantly reduced when broad-spectrum type insecticides are applied. Because biological control does not provide total leafminer control, grower/shippers must carefully inspect the harvested basil to avoid noncompliance with quarantine regulations.
Beet armyworm, Spodoptera exigua (Hubner). The destructive stage of the beet armyworm is the larva or caterpillar which causes a chewing type of damage on basil. The younger leaves are most commonly infested with more than one of the small, recently hatched caterpillars. The 1 1/4 inch long caterpillars are light green with pink to red stripes on their sides. The caterpillars develop into moths which are nocturnal.
Pinkwinged grasshopper, Atractomorpha sinensis Bolivar. Basil may be attacked by several grasshopper species. The pink winged grasshopper commonly feeds on basil and other herbs. Both nymphal and adult stages cause a typical chewing type of damage to the leaves. Grasshoppers are more of a pest in home gardens than in commercial plantings where they are of minor importance.
Sucking Type Damage
Brazilian leafhopper, Protalebrella brasiliensis (Baker). Brazilian leafhoppers commonly infest herbs such as basil, perilla (shiso), and mint. The adults are black and yellow, whereas nymphs are light green. Both stages are capable of hopping or jumping when disturbed but only the adult, with its fully developed wings, are capable of flying. Brazilian leafhoppers are sucking type insects which can cause leaf scarring, sometimes confused with the mines of leafminers. Several other leafhopper species attack basil. These insects generally cause no significant in-field damage even with high populations levels. However, leafhoppers have resulted in noncompliance with quarantine regulations, especially with the eggs and small nymphs which are not easily detected.
Banded greenhouse thrips, Hercinothrips femoralis (O.M. Reuter). Several thrips species infest and cause significant problems on basil grown for export. Thrips are small, slender insects which in the adult stage generally have wings with hair-like fringes. The banded greenhouse thrips is very common on basil and can usually be found on the lower leaves of established plants. Their feeding leaves whitish scars or a silvered appearance on the leaves. The fecal spotting left by thrips is also noticeable. Live thrips in harvested basil can be easily overlooked because of their small size and cryptic habits. Because it is difficult to identify immature thrips to the species level, the presence of live thrips in basil shipments has been of great concern to the local fresh herb export industry.
Silverleaf (sweetpotato) whitefly, Bemisia tabaci
(Gennadius) and greenhouse whitefly, Trialeurodes vaporariorum (Westwood). Both whitefly species infest basil. The nymphs and adults feed on plant sap and produce a sticky substance called honeydew which can serve as a substrate for sooty mold. Whitefly immature stages, which tightly attach to the leaf undersides, have caused problems with basil grown for export.
Other Pests. Other pests which cause sucking type damage to basil foliage include:
Southern green stink bug, Nezara viridula (Linnaeus)
Broad mite, Polyphagotarsonemus latus (Banks)
Red and black flat mite, Brevipalpus phoenicis (Geijskes)
Diseases of Basil
Information on basil disease etiology, epidemiology, or control is very limited. Although basil is plagued by many diseases, no fungicides or bactericides are currently registered for this crop in Hawaii. Growers, therefore, must rely on early disease recognition and use of cultural practices such as the use of windbreaks and rain shelters to prevent and manage diseases. By recognizing the first symptoms of disease, growers can remove diseased plants and continuously monitor fields for signs of pathogen recurrence or spread. For disease control: 1) use clean seed; 2) grow the crop in disease-free soil; 3) maintain disease-free fields; 4) rotate basil with non-susceptible crops such as oats, buckwheat, and sunhemp; 5) remove diseased leaves or plants to reduce field inoculum levels; 6) control moisture since high humidity or free water on plants strongly favors disease; 7) increase the spacing between plants to improve air movement and reduce leaf wetness periods; and 8) increase the organic matter in the soil which will enhance microbial activity and favor pathogen reduction. Laboratory analysis is recommended to identify the causal agent(s) when diseases are first encountered.
Various organisms cause leaf spots on basil in Hawaii. Some of these are new causal agents, and research on identification, pathogenicity, and spread of these organisms has been initiated. These include the following organisms:
Colletotrichum sp. This fungus causes dark leaf spots, defoliation, tip dieback, stem lesions, and sometimes loss of entire plants. At times, dead tissue within leaf spots fall away, producing holes in the leaves. Like most leaf spotting fungi in the tropics, it is especially troublesome during wet and humid weather. Fungal spores are produced on diseased plant parts, splash to healthy leaves or plants, and initiate new spots.
Cornyespora cassiicola. This fungal pathogen produces numerous, small gray to black specks or leaf spots on basil. Small spots coalesce and produce larger diseased areas. Older leaves are lost following heavy spotting. Stem lesions are light brown to brown.
Bacterial pathogens. Bacteria cause water-soaked, dark, angular, or irregular leaf rots, and brown to black, wet stem rot. When dry, diseased areas are brittle and easily cracked. Bacterial diseases are extremely severe during wet weather and also cause stem rots. Quality and total production will be greatly reduced. Postharvest losses are also very high.
Aphelenchoides sp. (foliar nematode). These microscopic round worms cause angular leaf rots during wet weather. Spots are large, water soaked, and black. Aphelenchoides sp. swim in the thin film of water covering the plant surface in high humidity or rain. This nematode penetrates the leaves through stomates, then feeds and multiplies in the internal leaf tissue.
Wilt, Die-back or Decline
Fusarium sp., causes a major production-limiting disease in commercial farms and in home gardens. Early signs of this fungal disease include slow growth and yellowing of the young shoots. Advanced symptoms include wilting, die-back, and discolored stems. Infected plants may show a darkened discoloration of the stem beneath the bark. Longitudinal slices in the stem will reveal this symptom. Under moist conditions, a light white to pink, cottony, growth can be observed along the infected stem areas. Fusarium can persist in the soil for many years, thus, growers must prevent contamination of their farms or new fields. The spread of Fusarium can be reduced by quick removal of all diseased plants and leaves. Do not bring infected plants or soil onto the farm from contaminated areas. Avoid planting in wet, poorly drained areas.
Other soilborne pathogens such as Rhizoctonia solani,
Pythium spp. and Phytophtora sp. have been associated with diseased plants. Further research is needed to establish their roles in root disease.
Decline Caused by Nematodes
Basil is very susceptible to root-knot nematodes, Meloidogyne spp. These microscopic round worms damage the roots and impede the plant's ability to take up water and nutrients from the soil. As a result, affected plants may show symptoms of nutrient deficiency, wilting, and yield decline. Galling and root rot occur on plants that are heavily infected. Growers should check with agricultural chemical representatives, County Extension Agents, or Plant Disease Specialists for chemicals that are registered for soil treatments for nematode control. The addition of organic matter such as chicken manure (1 lb per hill) may improve soil conditions and reduce the effect of nematodes and other pathogens. Other cultural practices for disease control include rotation with non-susceptible plants and soil solarization. Soil solarization techniques employ a clear plastic sheet placed over tilled soil for six or more weeks. The plastic cover retains heat in the soil and reduces nematode numbers. Soil solarization is effective only if sufficient sunlight is received to accumulate heat beneath the plastic cover.
Tomato Spotted Wilt Virus
Tomato spotted wilt virus is spread by several species of tiny insects called thrips. This disease is not a major problem with basil but is currently a severe problem in many other crops grown in Hawaii such as tomato, pepper, lettuce, and chrysanthemum. Infected plants may have dark colored, ring-like spots on the leaves, especially on younger ones.
Postharvest Problems from Infected Fields
Unhealthy fields produce leaves of poor to marginal quality. Leaves harvested from fields with disease problems are rapidly rotted by bacterial and fungal pathogens following packing. A minor field problem involving low levels of bacteria can result in major postharvest losses. Avoid harvesting from fields with known bacterial pathogens present. Select clean leaves from the tips only and avoid branches near the base of the plant.
High density plantings and physical or mechanical weed control are the most common practices used to prevent weed problems in basil. Weed control is enhanced with proper field preparation prior to planting. Both plastic and organic mulches may also be utilized to minimize weeds in the field. Check with your County Extension Agent, Department of Agriculture, or chemical sales representative for herbicides which may be registered for use on basil.
HARVEST AND POSTHARVEST PRACTICES
Hawaii relies on the high quality of its horticultural products to maintain competitiveness in the local and export market. High quality, including rich flavor and attractive dark green or purple foliage, is achieved by following proper cultural and postharvest handling practices. Proper field culture is essential to obtain a quality end-product. Growers should determine the product characteristics desired by particular market outlets. When growing basil for export maintain a line of communication with Department of Agriculture personnel to determine acceptable and unacceptable cultural and postharvest management practices which may have an effect on the quality of your product.
Basil is ready for hand harvest about 30-35 days after planting. In Hawaii, 4-6 inch shoot tips with two to four sets of true leaves are picked. However, because variations exist depending on the intended use of the product, check with your buyer for desirable product characteristics (stem length, etc.). Basil fields are picked daily when plants are rapidly growing, or every 3-5 days. Heavier picking (tips > 6 in) will delay the timing between harvests. Regular harvesting prevents flowering and seed setting and encourages vegetative growth. Basil will grow about one inch per day under the proper environmental conditions. During the picking operation try to hold the tips from to stem, to minimize contact with the fragile foliage tissues.
Handling and Packing
Fresh basil is very tender and is easily damaged by rough handling, desiccation, and chilling. To ensure and maintain product quality, minimize bruising when harvesting and packing. Harvested basil is usually dipped in cool water to reduce the "field-heat" and to help dislodge soil particles and some of the insects pests that are not strongly adhered to the plant. It is important to drain off the free water prior to packing the product. Basil may be packed in bulk or packed in twelve bunches to the pound in polypropylene bags placed in paper cartons, sometimes lined with moist paper. For the local market, basil is packed in 1-2 oz plastic bags.
Storage and Transportation
The shelf-life of basil is relatively short as compared to other herbs such as rosemary, oregano and thyme, and 30% losses during shipment are not uncommon. Basil is susceptible to chilling injury and should not be stored below 40oF (5oC) for extended periods. Basil that has been damaged by cold (chilling injury) turn black and are rendered unsuitable for sale. Store and ship fresh basil at 45 to 55oF (5-13oC) and 95% relative humidity. Diseases affecting basil in the field will likely reduce the shelf-life of the harvested product. Basil for export must be carefully inspected before packing to help ensure that it is free from live insects which will result in noncompliance with Federal and State quarantine regulations.
Other Postharvest Tips for Basil Include:
_ Harvest early in the morning when temperatures are lower.
_ If packaged in bags to reduce moisture loss, maintain at a constant temperature to prevent condensation.
_ Packages may be perforated for ventilation or may be of a polymer which is partially permeable to water vapor.
_ Do not store or ship with fruits or vegetables that release ethylene.
Cantwell, M. 1992. Handling of fresh culinary herbs. Small Farm News. Univ. California Small Farm Center, Davis. July/August.
Dostal, D. L. 1990. Postharvest Storage of Sweet Basil. M.S. Thesis. Michigan State University.
Flavin, D.J. 1987. Tropical herb production and marketing. pp. 77-79. In: J.E. Simon and L. Grant (eds.) Second Nat. Herb Growing & Marketing Conf. Proc. Indianapolis. 19-22 July. Purdue Agr. Expt. Stat. bull. 530.
Hamasaki, R. T. and D. M. Tsuda. 1993. Survey of Arthropod Pests on Commercial Herbs Grown In Hawaii. Unpublished.
Hawaii Agricultural Statistics Service. 1992. Hawaii Herbs: Fresh Production, Farm Price, and Farm Value.
Joyce, D. and M. Reid. 1986. Postharvest handling of fresh culinary herbs. The Herb, Spice, and Medicinal Plant Digest 4(2): 1-7.
Uchida, J. Y. 1991. Disease of Herbs In Hawaii. Presentation at the Third Annual Hawaii Herb Association Conference.
Notice: Reference to a firm or trade does not imply endorsement over firms or products not mentioned.