Disinfecting the Greenhouse

First Steps to a Clean Greenhouse
Attention to greenhouse sanitation and disinfecting are the first steps, that growers can take in the Fall to prepare their greenhouses for the Spring growing season. Pest problems, whether they be diseases or insects are easier to prevent than to cure. Begin with a clean greenhouse. Walkways should be free of soil, organic matter and weeds. Benches, preferably made of wire, should be disinfected and pots, flats and trays must be new or disinfected. Hose ends should always be kept off the floor and growing media should be kept in a clean area and covered. No plant material should be held in the media mixing area. Do not accumulate contaminated pots or media near the growing area. These practices will help prevent diseases and pests from becoming a problem this spring. Next, plan to disinfect your growing and plant handling areas.

Benefits to Disinfecting the Greenhouse
Over the course of growing a crop, infectious microbes and algae accumulate. Disinfecting your growing and plant handling areas of invisible but infectious microbes or pathogens can be done through the proper use of chemicals labelled for that purpose. Disinfecting should be done routinely, however timing does not always permit this extra effort. The greatest opportunity for this procedure occurs in between crop cycles when greenhouses are totally empty. For many growers it is now, prior to the spring growing season.
Most diseases are controlled, to some degree, by the use of disinfectants. For example, Botrytis grows on plant debris. Spores from this fungus become air-borne. Disinfectants will kill these spores. Dust particles from fallen growing medium or pots can contain bacteria, Rhizoctonia, or Pythium. Disinfectants will also control this potential problem. In addition to plant pathogens, disinfectants are useful for managing algae.

Managing Algae
Algae growth on walks, water pipes, equipment, and greenhouse coverings, on or under benches and in pots is an ongoing problem for most growers. Algae form an impermeable layer on the media surface that prevents wetting of the media and can clog irrigation and misting lines, and emitters. It is also a food source for insect pests like shore flies, and can be a liability risk for workers and customers using slippery walkways. To manage algae effectively requires some basic management practices in addition to chemical disinfectants. Algae need light, moisture and nutrients to survive. At first glance, these things may seem impossible to eliminate in the greenhouse. However, with some changes, the algae life cycle can be interrupted by eliminating any one of the three for a period of time.
Algae need sunlight for only a few hours each day. If sunlight can be minimized, then the growth period for algae also will be shortened. To exclude light avoid using clear or translucent hoses, covers, tanks or water distribution pipes. Use black tubing in irrigation lines and cover fertilizer tanks.
Train employees on proper watering practices. Over watering of crops with constant moisture frequently leads to algae build-up on the surface of the growing media. Avoid over watering crops, especially early in the crop cycle, to allow the upper surface of the media to dry out between watering.
Surface water from lakes and shallow wells may be high in nutrients that will contribute to algae growth. Use water from deep wells or municipal supplies whenever possible.
Avoid excessive fertilizer runoff and puddling water on floors, benches, and greenhouse surfaces to discourage algae growth. The greenhouse floor should be level and drain properly to prevent the pooling of water. A physical weed mat barrier helps to prevent both weed and algae growth. Avoid using stone on top of the weed mat. This will trap soil and moisture, creating an ideal environment for weeds, diseases, insects and algae.

Greenhouse Benches
If possible, use benches made of wire that can be easily disinfected. Wood benches can be a source for root rot diseases and insect infestations. Algae tend to grow on the surface of the wood creating an ideal environment for fungus gnats and shore flies, and plant pathogens can grow within the wood. Plants rooting through into the wood will develop root rot if conditions for pathogen activity are present. Disinfect benches between crop cycles with one of the products listed below. Keep in mind that the following disinfectants are not protectants. They may eradicate certain pathogens, but will have little residual activity.

Disinfectants for Greenhouses
There are several different types of disinfectants that are currently used in the greenhouse for plant pathogen and algae control. They are chlorine bleach, quaternary ammonium compounds such as Green-shield, Physan 20 and Triathlon, and hydrogen peroxide such as Dewa T-50. Alcohol, although not used as a general disinfectant is mentioned here because it is used by growers to disinfect propagation tools. All these products have different properties. If possible, disinfectants should be used on a routine basis both as part if a pre-crop clean-up program and during the cropping cycle.

Chlorine bleach
Chlorine is an effective sanitizer and has been used for many years by growers. Note that the half-life (time required for 50 percent reduction in strength) of a chlorine solution is only two hours. After two hours, only one half as much chlorine is present as was present at first. After four hours, only one fourth is there, and so on...To ensure the effectiveness of chlorine solutions, you should prepare them fresh just before each use. The concentration normally used is one part of household bleach (5.25 percent sodium hypo chlorite) to nine parts of water, giving a final strength of 0.5 percent. Chlorine is corrosive. Repeated use of chlorine solutions may be harmful to plastics or metals. Objects to be sanitized with chlorine require 30 minutes of soaking and then should be rinsed with water. Bleach should be used in a well-ventilated area. It should also be noted that bleach is phytotoxic to some plants, such as poinsettias.

Quaternary ammonium chloride salts
Q-salt products are used regularly by growers in Massachusetts. Q-salts currently available for greenhouse use include Greenshield, Physan 20 and Triathlon. Q-salts are quite stable and work well when used according to label instructions. Q-salts are labelled for fungal, bacterial and viral plant pathogens, and algae. They can be applied to floors, walls, benches, tools, pots and flats as disinfectants. Physan 20 is also labelled for use on seeds, cut flowers and plants.
Carefully read and follow label instructions. Recommendations may vary according to the intended use of the product. For example, it is recommended that objects to be sanitized should be soaked for 10 minutes, whereas walkways may require an hour or more. Instructions recommend that surfaces be air dried after treatment except for cutting tools. For disinfecting cutting tools, the label states to soak for 10 minutes before use, then use the wet tool on plants.Q-salts are not protectants. They may eradicate certain pathogens, but will have little residual activity.
Contact with any type of organic matter will inactivate them. Therefore, pre-clean objects to remove dislodgeable organic matter prior to application. Because it is difficult to tell when they become inactive, prepare fresh solutions frequently (twice a day if in constant use). The products tend to foam a bit when they are active. When foaming stops, it is a sign they are no longer effective. No rinsing with water is needed.
Hydrogen Dioxide Dewa T-50 kills bacteria, fungus, algae and their spores immediately on contact. It is labelled as a disinfectant for use on greenhouse surfaces, equipment, benches, pots, trays and tools, and is also labelled for use on plants. Recommendations state that all surfaces should be wetted thoroughly before treatment. Several precautions are noted on the label. Dewa T-50 has strong oxidizing action and should not be mixed with any other pesticides or fertilizers. When applied directly to plants, phytotoxicity may be of concern, for some crops, especially if applied above labelled rates or if plants are under stress. Dewa T-50 may also be applied through an irrigation system. Dewa T-50 concentrate is non-corrosive, but always careful read and follow label precautions.

Alcohol (70 percent) is a very effective sanitizer that acts almost immediately upon contact. It is not practical as a soaking material because of its flammability. However, it can be used as a dip or swipe treatment on knives or cutting tools. No rinsing with water is needed.
If possible, disinfectants should be used on a routine basis both as part of a pre-crop clean up program and during the cropping cycle.
This information is supplied with the understanding that no discrimination is intended and no endorsement implied. Due to constantly changing regulations, we assume no liability for suggestions. If any information in this article is inconsistent with the label, follow the label.