Disease Management for Vegetables and Herbs in Greenhouses Using Low Input Sustainable Methods
Vegetable Disease Information Note 2 (VDIN-002)
Charles W Averre, Extension Plant Pathologist
Jean B. Ristaino, Research Plant Pathologist
Jonathan G. Shultheis, Extension Horticulturist

There is increased interest among greenhouse vegetable and herb growers in producing crops with little or no input ("organic") of pesticides. Some growers occasionally want to use fungicides and nematicides in minimal amounts during emergency situations to reduce a disease. This information note provides information on how to manage a greenhouse vegetable or herb crop to avoid harsh ecological disruption of the environment and to avoid or minimize losses due to plant diseases.

Because plant population densities in greenhouses are usually very high and closely confined by the greenhouse walls, some virus diseases, foliar blights, leaf spots, stem and fruit rots, root rots and other diseases can become severe very quickly. The importance of using sound crop management practices and integrated pest management (IPM) practices must be emphasized.

Most of the steps described below are compatible with "organic culture". In "organic culture" we assume that common sanitizers such as soap, water, and bleach can be used. Occasionally, even under the best management systems, a fungicide to reduce gray mold (Botrytis) will be necessary if plant density is exceptionally high and temperatures are mild.

Before the greenhouse is constructed, give much consideration to the location and design of the greenhouse structure to ensure:

    Good soil drainage. This is especially important for ground culture.
    Freedom from shade caused by buildings, trees, and greenhouse overhead trusses and equipment.
    Easy access from office/living quarters.
    Good air and water quality.
    Good air exchange with large exhaust fans with cooling pads on the opposite side.
    Continuous introduction of fresh air with a small fan for humidity control.
    Continuous air movement across the foliage. This is done with a large overhead "polytube" in the center of each greenhouse or greenhouse bay when the greenhouse is not being ventilated. The holes in the tube should be positioned at about 40 degrees from the ground on both sides to direct a stream of air into the plant canopy. The system usually works best when rows are oriented perpendicular to the polytube.
    Uniform heating with a heater that does not release flue gases into the house.

Use strict sanitation procedures for germinating seed and growing transplants. This should be done in a specially designated small, well screened, separate greenhouse. No other plants, including weeds, should be permitted in the plant propagation house.

    All seed should be obtained from reliable sources. Tomato and possibly other seed should be sanitized just prior to seeding. Dip tomato seed 40 minutes in a solution of 1 part commercial bleach (5.25% sodium hypochlorite) and 4 parts water. Dry the seed by placing in a cloth bag and running in a clothes dryer without heat.
    Seedlings originating from different sources must be kept separate from each other in the greenhouse to avoid cross contamination.
    Hands and tools must be washed with 5% commercial bleach or soap and water before handling a new group of transplants.
    Avoid bringing nonsterile soil into the greenhouse on shoes, tools, and supplies.

Prior to setting transplants, the growing medium (soil, soil-less mix, or hydroponic solution) should be checked for nutrients, then monitored weekly for hydroponic culture or every two weeks for soil and soil-less mix culture until the crop has been harvested. Plant and soil samples should be taken according to directions provided by the laboratory and sent to the Agronomic Division, North Carolina Department of Agriculture, Raleigh, NC 27607 (located on Blue Ridge Rd., 919/733-2656) for plant nutrient and pH analyses. This is of prime importance when growing in hydroponic systems.

Maintain the relative humidity in the house as low as possible by:

    Covering the soil surface with a white plastic sheet.
    Not wetting foliage during irrigation.
    Avoiding standing water.
    Introducing fresh air continuously into the greenhouse when the exhaust fan is off.

Diagnose all problems promptly! These include diseases, insect, nutritional, and growth problems. See your county agent for assistance and for details on how to collect and send specimens to the Plant Disease and Insect Clinic at North Carolina State University.

Promptly remove all diseased leaves, fruits, or entire plants to avoid spread of disease. Discard debris in a paper bag in a compost pile for use in outdoor culti-vation. Do not reintroduce this compost into the greenhouse.

As soon as the crop is harvested sanitize the greenhouse by:

    Removing all plant material and roots from the greenhouse and discard as previously stated.
    Hosing down with water all surfaces of the greenhouse structure, tools, and equipment.
    Tilling the soil, if plants were grown in the soil, and removing all remaining plant parts.
    Solarizing house by moistening the interior of the greenhouse, and closing it for two weeks during a hot, sunny period in the summer. The inside temperature should reach at least 145 F each day and the humidity should remain high.

These procedures are recommended for all greenhouse vegetable and herb growers to minimize the risk of introducing plant pathogens, reduce disease severity if pathogens are present, and to lessen dependency on labeled fungicides and nematicides. Pesticides, including biological control agents that may be commercially available in the future, must be used exactly as stated on the current label; very few pesticides are labeled for food crops grown in greenhouses.

Move Directly To:

Plant Disease and Insect Clinic Homepage
North Carolina Insect Notes
North Carolina Agricultural Chemicals Manual
CALS Home Page
NCCES Home Page
NCARS Home Page

For assistance with a specific problem, contact your local
North Carolina Cooperative Extension Service

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Published by
North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Reformatted Dec. 2000 by Plant Disease and Insect Clinic