Your Mother Was Right: Wash Your Hands (and Everything Else)
Whenever I talk about sanitation during a disease management program, I can almost hear the eyes rolling. Although participants are always too polite to actually roll their eyes (at least while I'm looking), I suspect that they aren't too impressed with my basic message: "Wash your hands. Oh, and keep everything else clean while you're at it." After all these years, you would think that plant pathologists could be more innovative. Shouldn't new fungicides and resistant plants offer some relief?
It's true that several new active ingredients have been discovered and registered for use against many of the troublesome plant pathogens. Some of these fungicides are considered "reduced risk" by the Environmental Protection Agency (EPA), a label that assures the user that the product poses a reduced risk of harm to people and the environment. However, you must match the right product with an effective fungicide. Even when you find a fungicide that works well, you shouldn't overuse it. Disease-causing pathogens may not respond to a fungicide that's used over and over.
Genetic resistance is always the best way to combat disease. Breeders and growers alike note which verbena cultivars fare the best against powdery mildew. Troubled by downy mildew on snapdragon? Some cultivars have been shown to be less susceptible to disease than others. But the world of ornamentals is ever changing. Unlike a tomato variety that may stay in production for decades (e.g., 'Beefeater'), the ornamental industry is fueled by all that is new. A vibrant color or novel growth habit may make a new cultivar a market favorite even when disease resistance is lacking.
Where to Begin
So, we're back to washing hands and everything else. Where do you start? Start with the likely entry points for pathogens, such as doorways and vents. Pathogens that have spores or "fungal seeds," which move with air currents, can readily enter greenhouses through vents. This means that dumpsters containing discarded plants and plant parts should be kept closed, placed away from production areas and out of range of fans bringing air into the greenhouse. Botrytis gray mold readily produces spores as a fuzzy mat on plant parts lying in a dumpster or other trash container. The moment that the container's lid is picked up, the air currents are ready to bring the Botrytis spores into action, making them available to infect nearby susceptible plants should conditions be favorable (i.e., moist and wet).
Pathogens making their home in the soil — so as to invade plant roots — tend to spread via soil particles and dust that can cling to shoes, benches, pots, equipment, etc. Plants that arrive in a soil- or peat-based medium, such as plugs or prefinished material, are also prime candidates to carry pests such as Pythium, Phytophthora, Rhizoctonia, Fusarium and Thielaviopsis.
Using footbaths filled with disinfectant at greenhouse entrances to reduce the threat of soil particles (and accompanying pathogens) dropping off shoes is usually met with mixed reactions. How effective can this really be? Part of the answer depends on whether there is a commitment to keep the footbaths clean and refreshed with newly mixed disinfectant each day — and whether greenhouse workers are mandated to use them. The ability of footbaths to keep out major disease problems may be unproven, but we all know that walking around them to get into a greenhouse can't help either. For all other visitors, such as sales representatives, consultants, university extension agents and plant pathologists, I suggest disposable plastic booties that slip over shoes that can be tossed after the visit.
I've always liked the idea of using a disposable "lab coat" while working in the greenhouse. This coat would be put on when entering the greenhouse and be removed before leaving. The thought behind this strategy is that any would-be pathogens picked up by the coat while the worker moves among the plants would be contained to that greenhouse. This approach prevents a disease problem from spreading from one greenhouse to another.
What About Hands?
This part's easy. You'll need soap, water and the ABCs. Liquid soap from a pump is less likely to become contaminated than a bar of soap. However, a bar of soap is better than no soap. Wash your hands for the length of time that it takes you to sing or hum the ABCs (the long version, no shortcuts). Be thorough and scrub all surfaces: top, bottom, around nails and between fingers. What about hand sanitizers? Don't use them in place of washing your hands.
I think we can take a page from the medical community: They've linked good hand washing with soap and water to a decrease in patients suffering from pathogens commonly picked up while in the hospital, such as staph infections.
Eliminate Greenhouse Insects
Controlling weeds that may establish themselves on the greenhouse floor reduces the insects that can transmit viruses to ornamentals. Aphids, whiteflies, shoreflies and fungus gnats are each capable of carrying specific pathogens that can cause a lot of damage to plants of all ages.
Check Irrigation Water
Take a look at your irrigation water. If you're using a holding pond, reservoir tank, ditch or horizontal well as your source of irrigation water, then it is possible to introduce a water mold such as Pythium or Phytophthora to your crop. Persistent root rot caused by either of these pathogens may prompt you to look at your water as a source of contamination.
Sanitizing plant trays, benches and other plant containers is not an easy task. First, do not reuse any container that has held a diseased plant. When you toss the plant, throw out the container, too. Remember, any soil particles or pieces of roots left clinging to the surfaces of the pot will likely contain the very pathogen that you're trying to get rid of.
If you're serious about cleaning up pots, trays and benches, take a two-step approach. First, clean the surface with a power-washer or a good old-fashioned scrubbing with a clean brush and lots of water and soapy suds. Once the surface looks clean to the eye, follow up with a soak in a commercial disinfectant. Make sure the solution is fresh and the concentration is accurate and according to the label.
Giving trays only a quick dip in the disinfectant — or stacking trays and then placing them in the disinfectant — will not optimize your sanitation efforts. Think of it as more of a soak than a dip, and make sure the disinfectant has contact with all surfaces of the containers being sanitized.
If you insist on reusing trays or pots that held diseased plants, then I would suggest keeping these containers separate from the others and using an especially rigorous sanitation program that includes repeating the soak in a disinfectant. The pathogen that causes Thielaviopsis black rot is especially tenacious and will readily carry over from crop to crop unless an aggressive cleaning program is used.
Final Sanitation Tidbits
Sanitation is a state of mind. No hoses that have been on the floor should be on the plant bench. Pots and flats that have been cleaned should be encased in shrink-wrap and kept out of the production area. Remember, dust particles that contain plant pathogens may be picked up on air currents and deposited back onto the pots and flats that you've just sanitized. Shoes and feet should never be placed or rested on a plant bench because any pathogens that are on the floor will be moved up into the growing environment.
You may not be able to control the market or the weather, but you can control whether your operation is clean, keeping your disease pests to a minimum. Try it, and reap the benefits. Your mother would be pleased.