Assessing the Risk of Disease in Nurseries

The following is a guide to assessing the threat of disease in a nursery operation.
Assessing the Risk of Disease in Nurseries - Articles

Updated: August 8, 2017

Assessing the Risk of Disease in Nurseries

Although much of the information applies to container production ** notes those practices that also apply to field production. Various procedures and equipment used in a nursery can decrease or increase the chances of a disease outbreak. There is a blank form at the end of this guide for you to use in your particular operation. No detail is too minor to disregard. Although you believe you know all the details, it is suggested you talk to employees to be certain you know exactly how they do various tasks. For example in a containerized production nursery using bark media, where does the employee put tools used to handle it when not in use? If placed on the ground, they may become contaminated with pathogen-infested soil. If they are left on bark pile or in a clean bucket, they are much less likely to become contaminated. Even these small details are important.

I. Container Mix

A. Grower-prepared


What components are used to make the container mix? Sand and soil are likely to carry pathogens while bark, vermiculite, hydrogels, and perlite are generally free of pathogens and insects

**Fertilizer, if stored too near herbicides, could absorb herbicides.

Container 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 field soil or sand are used, is the mix treated to eliminate those pathogens? Heat can eliminate all pathogens and leave some beneficial organisms in the mix 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 container mix:

Is the mix stored in a location and under conditions where pathogen-contaminated soil is not likely to get into the mix? Is it on a cement or other impervious pad so that loaders do not scoop up untreated soil from beneath the pile? Is the mix kept in a shed or covered storage area so that field soil is not blown into the pile?

B. Commercial, soilless or bark mix


Since some brands appear to harbor pathogens, the mix used for stock plants should be steamed or fumigated.


Is the 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 bails kept intact without rips?

C. Moving the mix from storage to potting area:

Is all the equipment used to move potting mix used only for that purpose? Or, are the tools and equipment, including the buckets of a front-end loaders, cleaned and disinfested prior to moving the mix if also used for other purposes?

II. Potting Area

Are potting carts, benches, and floors cleaned and disinfested periodically?

III. Containers and Flats

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

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 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 disinfested periodically? Are trowels, shovels, or other tools used to make holes in the mix disinfested periodically and kept off the floor?

V. Placement of pots

Are benches raised at least 24" above the ground? Are solid-topped benches thoroughly cleaned and then disinfested prior to plant placement? Are expanded metal bench tops, wooden slats, or lath bench tops thoroughly cleaned and then disinfested between crops?

Are porous cement floors or other paved surfaces on which pots are placed thoroughly cleaned and disinfested between crops?

If containers are placed outdoors, are the raised off the ground on a 4-6" layer of stone or coarse gravel? Is the area graded so that containers do not sit in puddles of water?

VI. **Plant Material

List all the plants that are grown.

Do the pathogens and pests of some of the crops readily infect the next crop to be grown in that space or nearby crops?

**Stock Plants:

Are new stock plants established on a regular basis? Is the stock plant material culture indexed and virus indexed? Is the stock plant area completely separate from all other parts of the operation? Are stock plants always under a strict insect and disease management schedule and monitored regularly?


Are cutting knives and shears disinfested when moving from one stock plant to another?

Are cutting-ends 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.

**Spacing of plants:

Are stock plants in rows parallel to air flow with space between rows? Good air circulation that fosters a reduction in relative humidity inhibits the activity of most fungi and all bacteria.

VII. **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 are important tasks because they can spread pathogens.

**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 their spreading of pathogens on 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 that is easily spread on hand and tools to healthy plants.

Are hand washing stations readily accessible and close to where work is being done?

VIII. **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 drip, ebb and flow, or trickle irrigation watering, which tend not to spread pathogens and keeps the plant surfaces dry, used?

Are hose-ends hung and not left on the ground? Are hose-ends disinfested periodically and especially immediately if they fall on the ground?

**Is watering done early in the day so that leaf surfaces dry before night?

IX. **Cultural Practices

Do fertilization practices avoid over-fertilization, nutrient deficiencies, and nutirient toxicities?

**Are plant growth regulators, if used, applied in the manner defined by the product label only to plants listed on the product label?

X. **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?

XI. **Misc. Practices

Is debris disposed of promptly away from valuable plants and the production area?

**Are weeds, including escaped crop plants, removed from the area? Are weeds immediately outside greenhouse and lath house vents cut or removed on a regular basis? These may harbor pathogens, mites, and insects.

XII. **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?

XIII. **IPM methods

Does someone have the responsibility for regularly scouting the nursery? 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?

XIV. **Problems encountered by grower

Are records kept on the occurrence of root rots, stem rots, cutting rots, damping-off, leaf spots, flower blights, general yellowing, 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?

Nursery Assessment Worksheet

I. Container Mix

A. Grower-prepared

  • Components
  • Treatment To Eliminate Pathogens And Other Pests
  • Storage Of The Mix

B. Commercial, soilless mix or bark

  • Treatment
  • Storage

C. Moving the mix from storage to potting area:

II. Potting Area

III. Containers and Flats

  • Storage Of Pots And Flats

IV. Planting

V. Benches, floors, or surface on which pots are placed

VI. Plant Material: List all the plants that are grown

  • Stock Plants
  • Propagation
  • Spacing Of Plants

VII. Handling of plants during production

VIII. Watering Practices

IX. Cultural Practices

X. Worker Training and Practices

XI. Miscellaneous Greenhouse Practices

XII. Chemical Treatments

XIII. IPM methods

XIV. Problems encountered by grower