Assessing the Risk of Disease in Greenhouses

You can use this article as a guide to assess the risk of disease in your growing facility.  Although you believe you know all the details of your facility, it is suggested you talk to employees to be certain you know exactly how they do various tasks. For example, in a greenhouse where trowels or dibble boards are used, where does the employee put them when not in use? If placed on the floor, they may become contaminated with pathogen-infested soil. If they are hung on the wall or always left on a pathogen-free potting mix, they are not likely to become contaminated. Even these small details are important.

I. Potting Mix

A. Grower-prepared

Components

• What components are used to make the potting mix? Sand, soil, and peat are likely to carry pathogens, while vermiculite, hydrogels, and perlite are, by virtue of their manufacturing, free of pathogens and insects.
• Fertilizer used to amend the potting mix, if stored too near herbicides, could absorb herbicides.
• Potting mix-incorporated pesticides, if not incorporated in the proper amount and manner, may be toxic. These should be incorporated after steaming, fumigation, or other treatments and prior to potting.

Treatment to Eliminate Pathogens and Other Pests

• If components are used that may carry pathogens, is the mix treated to eliminate those pathogens? Heat can eliminate all pathogens if applied properly. Chemical fumigation generally kills most but not all organisms. All chemicals leave some residue, to which certain plants are very sensitive.
• Storage of the potting mix: Is the potting mix stored in a location and under conditions where pathogen-contaminated soil is not likely to get into the mix?

B. Commercial, Soilless Mix

• Treatment: Since some brands appear to harbor pathogens, the mix used for stock plants should be steamed or fumigated.
• Storage: Is the potting mix stored in a location and under conditions where pathogen-contaminated soil is not likely to get into the mix? Are bags or plastic-covered bales kept intact without holes or rips?

C. Moving the Mix from Storage to the Potting Area

• Is all the equipment used to move potting mix used only for that purpose? Are all the tools and equipment, including the buckets of a front-end loader, cleaned and disinfested (sanitized) prior to moving the mix if also used for other purposes?

II. Potting Area

• Are potting benches and floors cleaned and sanitized periodically?

III. Pots and Flats

• Are cell packs, pots, flats, and other containers new? Or, if used, are they thoroughly washed to remove all lumps of soil and then sanitized?

Storage of Pots and Flats

• Are pots and flats stored in the original boxes (closed) or in a covered area so that pathogen-contaminated soil is not likely to get onto them?

IV. Planting

• Do workers wear clean clothes that are not contaminated with soil or plant sap that may harbor pathogens? They should not have been handling plants elsewhere in the facility prior to planting.
• Is any machinery used in planting cleaned and sanitized periodically? Are dibble boards or other tools used to make holes in the potting mix cleaned and sanitized periodically and kept off the floor?

V. Benches, Floors, or Surfaces on Which Pots Are Placed

• Are benches raised at least 24" above the floor? Are solid-topped benches thoroughly cleaned and then sanitized prior to plant placement? Are expanded metal bench tops, wooden slats, or lath bench tops thoroughly cleaned and then sanitized between crops?
• Are porous cement floors or other paved surfaces on which pots are placed thoroughly cleaned and sanitized between crops?

VI. Plant Material

• List all the plants that are grown.
• Do the pathogens and pests of some of the crops readily infect or infest the next crop to be grown in that space or nearby crops?

A. Seedling Production

• If starting plants from seed, were the seeds fungicide, hot water, or chlorine treated by the seller to eliminate pathogens? If seedling plugs are purchased, is the supplier reliable? Have there been disease problems on plants from this supplier in previous years, soon after receiving plants?

B. Vegetatively Propagated Material

Stock Plants

• Are new stock plants established every 12 months or more often?
• Is the stock plant material culture indexed and virus-indexed?
• Are stock plants always under a strict insect and disease management schedule and monitored regularly?

Propagation

• Are cuttings removed by breaking out the branch by hand? Or, if knives are used, are they cleaned and sanitized when moving from one stock plant to another?
• Are the cut ends of cuttings dusted with the hormone and not dipped in a common container? If a common container becomes contaminated with a pathogen, all cuttings subsequently dipped may be inoculated with the pathogen.
• Is the cutting bed divided, or are flats, rooting cubes, pellets, or similar products separated so that if it is later discovered that a few cuttings carried a pathogen, not all the cuttings will have been exposed? Do not place the entire crop at risk by rooting everything in a common bed.

VII. Spacing of Plants

• Are stock plants in rows parallel to airflow, with space between rows?
• Are seedlings and plants to be grown with space to allow adequate air circulation? Good air circulation that fosters a reduction in relative humidity inhibits gray mold (Botrytis) activity.

VIII. Handling of Plants during Production

• Are stock plants handled only by one or two of the most trusted employees early in the day, before they handle any other plants?
• While disbudding, harvesting cuttings, cleaning the potting soil surface, and removing the fading lower leaves and spent flowers are important tasks, they can spread pathogens. Do each of these tasks separately. Or assign one person to each task so that the rest of the plant is not touched.
• Do workers always handle plants known to be healthy first and work toward areas where insects or diseases are known to occur so that they minimize the risk of spreading pathogens on their hands and clothing?
• Do workers use snuff, chewing tobacco, or other tobacco products in a location away from the plants and then wash their hands thoroughly before going back to work? Do they store the tobacco products in a location where their work clothes and tools will not become contaminated? Tobacco products can contain tobacco mosaic virus, which is easily spread on hands and tools to healthy plants.

IX. Greenhouse Structure

• Is the stock plant greenhouse completely separate from all other parts of the operation?
• Have you designed the greenhouse to minimize condensation from dripping onto plants below?  
• Are heat and venting systems set up and used to maintain low humidity and good air circulation?
• Are shoe dip stations located at each entryway? Are workers told that it is important to always pass through the shoe dip in order to minimize the spread of pathogens into the greenhouse? Is the dipping solution replaced daily?
• Are hand-washing stations readily accessible and close to where work is being done?
• Are the greenhouse vents, cooling pads, and other open areas screened with a mesh size that will prevent the entry of aphids, whiteflies, and larger insects? Is air movement within the greenhouse designed to keep insects out?

X. Watering Practices

• Is well water or municipal water used? Or is water from a surface water supply used? Surface water may carry Pythium, Phytophthora, and Erwinia.
• Is overhead irrigation, which readily splashes pathogens from plant to plant, employed? Or, is capillary mat, drip, ebb and flow, or trickle irrigation watering, which tends not to spread pathogens and keeps the plant surfaces dry, used?
• Are hose ends hung and not left on the ground? Are hose ends cleaned and sanitized periodically, and especially immediately if they fall on the ground?
• Is watering done early in the day so that relative humidity decreases before nightfall?

XI. Cultural Practices

• Do fertilization practices avoid overfertilization, nutrient deficiencies, and nutrient toxicities?
• Are plant growth regulators, if used, applied in the manner defined by the product label only to plants listed on the product label?

XII. Worker Training and Practices

• Are workers trained to recognize symptoms of disease and insect activity?
• Do workers understand the most common places where pathogens and insects are harbored?
• Are new workers trained to recognize what is normal and what is abnormal for each crop?

XIII. Miscellaneous Greenhouse Practices

• Is debris disposed of promptly in appropriate covered containers outside the greenhouse, away from intake vents, until removed from the area?
• Are weeds, including escaped crop plants, removed from under benches?
• Are weeds immediately outside greenhouse vents cut or removed on a regular basis? These may harbor pathogens, mites, and insects.

XIV. Chemical Treatments

• Is there a specific reason for using each chemical being applied? What is the rationale for the timing of their use?
• Are records kept on each lot of plants treated so that it is known what specifically has been used on particular plants, when it was applied, and what environmental conditions prevailed during and soon after application?

XV. Integrated Pest Management (IPM) Methods

• Does someone have the responsibility of regularly scouting the greenhouse? Is that person trained and updated frequently? Is the person given the necessary amount of time needed to thoroughly scout each week? What is the established scouting schedule? Are records kept on findings?
• Are soil or tissue tests run regularly?
• Where does the scout go for backup on diagnoses when it is needed?

XVI. Problems Encountered by the Grower

• Are records kept on the occurrence of root rots, stem rots, cutting rots, damping-off, leaf spots, flower blights, general yellowing, or edema, including when they occurred, crops affected, and how the problem was resolved? Do particular problems recur each growing season? What is the likely source of these problems?

Prepared by Gary W. Moorman, Professor Emeritus of Plant Pathology