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ATTRA Information Brief, by Christine Rugen and Janet Bachman. Revised: 5-25-90, GKA.
The genus Vaccinium contains several species of economic importance, but the highbush blueberry is the most widely cultivated. The highbush blueberry does, however, have geographical limitations. In the far North, lowbush blueberries (and a few "dwarf" highbush cultivars) are better climatically adapted than highbush types. In the deep South, rabbiteye blueberries are the adapted type. The focus of this information brief is the highbush blueberry. If you have a question concerning the species of blueberry best adapted to your area, call your county Cooperative Extension Service office.
On most sites, blueberries are relatively free of disease and insect pests, but weeds are an ever present problem. Field preparation at least one year in advance of setting blueberry plants is strongly recommended, since herbicides cannot be used in organic production. For details of pre- and post-plant weed control see ATTRA's "Overview of Organic Fruit Production."
Fertility Consideration.. The "Overview" also contains basic information on organic fertilization of fruit plants. However, blueberries are unique among the commonly grown fruit plants in their fertility requirements. Blueberries require an acid soil and prefer nitrogen in the form of ammonium. James Shoemaker discusses blueberry nitrogen requirements in his book, Small Fruit Culture (1): "One reason for failure of blueberries on neutral to basic soils may be a lack of ammonium nitrogen. Soil conditions there are usually favorable for Vitrifying organisms, resulting in a high concentration of nitrates. Ammonium nitrogen is short-lived under such conditions. But in the acid soils that are naturally favorable for blueberries, denitrifying organisms predominate, and nitrates may be quickly converted to the ammonium form." It seems that providing a low soil pH is the best way to insure a source of ammonium nitrogen for the blueberry plants.
A look at soil biochemistry can clarify this reasoning. Nitrifying organisms are those which convert ammonium to nitrates in the soil. Denitrifying organisms complete the reverse process by changing nitrates into ammonium. Both nitrifying and denitrifying organisms are simultaneously present in the soil therefore, the conversion process can go either way. The primary environmental factors that influence which nitrogen source will predominate include soil pH, moisture, and soil oxygen content. Denitrification, which results in ammonium formation, is greatly enhanced by an acidic soil condition. Because of this, it is important to focus on creating an acidic soil environment.
If blood meal or fish fertilizer is applied to a neutral soil, the nitrogen in the proteins will be converted first into ammonium and then into nitrate. To slow this process of conversion to nitrates, the soil pH must be lowered to promote the growth of denitrifying organisms. Sulfur compounds will reduce soil pH, creating an acid environment. Several sulfur products can be applied either through a drip system or as a top dressing. Most protein sources of nitrogen are also acidifying.
Because of the rather stringent water requirements of blueberries (due to a lack of root hairs they require frequent watering--the equivalent of 1" rainfall per week), many growers find it convenient to apply liquid or soluble fertilizers through the irrigation.
Fish fertilizer is a form of soluble protein that may be used to provide blueberries with nitrogen. Seagro Corporation (2) and J&G Agrow-Tek (3) produce wettable powder formulations which are water soluble. Paul Lanphere, Seagro's representative, thinks this type of product can be used in conjunction with a drip irrigation system. He stresses two constraints: there must be adequate agitation to keep the powder in solution; and there must be at least 20 pounds of pressure in the system. The product can also be applied as a foliar feed, providing an alternative method of supplying nitrogen to the plants. The wettable powder formulation makes the product more economical than an emulsion because of the reduced freight costs and added application possibilities.
Another potential liquid nitrogen source is blood. The technique of "spray drying" produces a water soluble form of blood meal. The product can be used through a drip irrigation system. The conventional process, ring drying, produces a product that is not soluble and must be applied as a top dressing. American Meat Protein Inc. (4), Inland Molasses Company (5), and CA Spray Dry Company (6) are sources for bulk orders of soluble blood. For smaller orders, try J&G Agrow-Tek (3).
Seaweed concentrates are not recommended as a source of major nutrients such as nitrogen. However, they may be used to supply some micronutrients.
Pests. The blueberry maggot, Rhagoletis mendax (Curran), can be a key blueberry pest in many production regions. It is very closely related to the apple maggot, and both are native to the United States. Alternate hosts for the blueberry maggot include wild blueberries, barberries, and huckleberries. In locations where large populations of wild hosts exist in areas surrounding the crop, the blueberry maggot is likely to be a problem.
This pest overwinters in the pupal stage, buried in the soil. The adult flies, which are approximately 6 mm in length, emerge over a period of a month or two in the summer. Eggs are laid in the ripe berries and the maggots eat the pulp of the fruits, causing many fruits to drop, spoiling the sale of others, and creating difficulties in postharvest care.
The text, Useful and Destructive Insects (7), mentions that rotenone applied at 0.5 pounds per acre at 7 to 10 day intervals after the berries turn blue, has given control by killing the adult flies. Rotenone, a botanical insecticide, can be used by organic growers. However, this product is toxic to beneficial organisms, swine and humans. This material, as well as other pesticides, should only by used if monitoring efforts have determined that an economically damaging population exists. Monitoring can also facilitate the proper timing of pesticide applications. Yellow sticky cards can be used to determine when the flies first begin to emerge.
Dr. Erwin Elsner (7), researcher with the Rutgers University Blueberry/Cranberry Research Station, has been developing an integrated pest management (IPM) program for blueberries and cranberries in New Jersey. He mentions that at this point he is not aware of any method for protecting the fruit from blueberry maggot attack; therefore some type of chemical application must usually be applied to prevent economic damage. The large population of wild plants in his area serves as a reservoir for these pests, which move into domestic plantings.
Dr. Elsner mentions that clean cultivation is a cultural technique which may reduce blueberry maggot populations. This is because the fly pupae seem to survive better in moist areas that are covered with grass and other weeds.
Another pest that may cause serious problems in blueberries is the red-banded leafroller. The adult of this species is a moth, which can be monitored with pheromone traps. The naturally occurring bacteria, Bacillus thuringiensis, is effective against the larval stage of this pest. Several products are available including Dipel (TM), Thuricide (TM), and Javelin (TM).
Two other pests, the sharpnose leafhopper and aphids, may cause problems on blueberries. These insects are primarily damaging because they transmit diseases, which local wild populations of blueberries and their relatives may harbor.
Leafhoppers transmit a microorganism that causes stunt disease. In areas where stunt disease is a known problem, leafhopper control is suggested. The botanical pesticide sabadilla, as well as insecticidal soap and Diatomaceous earth, are reported to be effective against leafhoppers. It is also important to prevent the spread of stunt disease by purchasing only certified, disease-free plants.
Aphid problems can be minimized by avoiding over-fertilization of the crop. For chemical control of aphids, organic growers can use insecticidal soap.
Yellow sticky cards can be employed to determine when blueberry aphids are beginning to fly in and infest the crop. However, monitoring efforts can be complicated since many aphid species (most of which do not infest blueberries) are usually present in the field. These species will also be attracted to the sticky cards, making the counting of blueberry aphids difficult. Another problem is that the first few generations of blueberry aphids are often not winged, and the sticky cards will be an ineffective monitoring tool for them.
Birds are often serious pests of the fruit. Various methods of control have been tried--such as "scare-eye" balloons, Mylar reflective tape, sonic devices, etc.--with various levels of success. The problem with most repellents or scare tactics is that the birds usually become habituated to the stimulus, and it no longer serves to repel the birds. Sometimes growers are able to overcome this problem by changing the stimulus often enough; e.g., switching from balloons to Mylar tape or moving the balloons from one site to another. Properly applied bird netting has provided consistent and predictable control, but it can be expensive to purchase and set up. For local information on bird control, contact your county Cooperative Extension Service office.
Diseases. Diseases that may occur in blueberries include, mummy berry, Botrytis, stem blight, phythophthora root rot, a mycoplasma-caused disorder, and several virus diseases. For proper disease identification, contact your county Cooperative Extension Service office.
Clean cultivation can reduce the incidence of mummy berry disease. This practice destroys the fallen mummified fruit, which harbors the innoculum for the next season's infection. Wettable sulfur sprays have also been effective in reducing mummy berry infection. In New Jersey, researchers used three sprays roughly one week apart with the first spray timed for leaf emergence in the spring (8).
Prompt picking of mature fruit, and good air circulation throughout the crop canopy can help minimize Botrytis problems. Sulfur sprays for mummy berry should also help to suppress Botrytis.
Stem blight is a fungal disease more common in the mid-South (North Carolina, Tennessee, Missouri, and Arkansas) than in cooler climates. It can be a serious problem in wet years and if old infected wood is not removed during the pruning process. Sanitation (removing infected wood) is currently the only control. Researchers in Missouri and elsewhere are looking for fungicidal controls. There is also some apparent variation in varietal susceptibility, but as yet no recommendations exist for resistant varieties (9).
Phytophthora root rot is another fungal disease somewhat more common in the South than in the North. The causal organism is favored by poorly drained and heavy soils. Planting on raised beds, incorporating plenty of organic matter at planting, and being careful to not spread the infection to clean sites via farm equipment are all helpful. The best control, however, is to be certain to purchase clean planting stock from a state certified nursery program.
Blueberry stunt is caused by a mycoplasma transmitted by the sharpnose leafhopper from wild blueberry plants. Control is limited to rogueing out infected plants and controlling the leafhopper (see the insect section). In some areas, it might be feasible to eliminate the alternate hosts, the wild blueberry plants.
Several virus-borne diseases can be troublesome on blueberries. These include blueberry mosaic, shoestring, red ringspot, and necrotic ringepot. Control is limited to purchasing clean stock and rogueing out infected plants.
Enclosed are several articles on blueberry culture. The importance of proper soil preparation, organic matter, good drainage, and the special moisture and pH requirements of blueberry plants are discussed.
The Secrets of Organic Blueberry Culture For The Home Gardener, a publication prepared by Gordon Watkins (president of the Ozark Organic Growers Association), discusses appropriate fertility management, insect, weed, and disease control for organic production of highbush blueberries.
For specific cultural information relating to your area, please contact your local Cooperative Extension Service office.
(1) Shoemaker, James S. Small Fruit Culture. AVI Publishing Company. 1978.
(2) Paul Lamphere, Seagro Corporation, 3601 10th S.E Wenatchee, WA 98801, (509) 884-1600. East
(3) American Meat Protein, 2515 Elwood Drive, Ames, IA 50010, (515) 292-1021
(5) Bob Bahn, Inland Molasses Inc., 5 Jones Street, Dubuque, IA 52001, (319) 588-4641
(6) CA Spray Dry Co., PO Box 5035, Stockton, CA 95205-0035, 209/948-0209
(7) Metcalf, C. L. and W. P. Flint, Destructive and Useful Insects, McGraw-Hill Book Co., 1962, p. 798.
(8) Dr. Erwin Wiener, Rutgers Blueberry/Cranberry Research Center, Chatsworth, NJ 08019, (609) 726-1590.
(9) Carter, R. E. and J. F. Moore, Jr., "The Plight of Blueberry Blight," Missouri Farm, Nov./Dec. 1989, p. 39.
Copyright © 1990. ATTRA.
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