First Steps to a Clean Greenhouse
If you have had re-occurring problems with diseases such as Pythium root
rot or insects such as fungus gnats, perhaps your greenhouse and potting
areas need a good cleaning. Over the course of growing a crop, infectious
microbes accumulate and algae flourish on moist surfaces harboring fungus
gnats and shore flies.

Attention to greenhouse sanitation and disinfecting are steps that growers
can take now to prepare their greenhouses for the growing season. Some
growers wait until the week before opening a greenhouse before cleaning
debris from the previous growing season. It is better to clean as early as
possible to eliminate sites for pests to reduce their populations prior to
the growing season. Pests are much easier to prevent than to cure.

Begin by thoroughly cleaning the floor of soil, organic matter and weeds.
Install physical weed mat barriers if floors are bare dirt or gravel and
repair existing ones. Weed barriers not only prevent weeds, but also make
it easier to manage algae. Avoid using stone on top of the weed mat that
will trap soil and moisture, creating an ideal environment for weeds,
diseases, insects and algae.

Benches, preferably made of wire, should be disinfected and pots, flats and
trays should be new or disinfected. Bench tops and work tables should be
made of a non-porous surface such as a laminate that can be easily
disinfected. Avoid using bare wood for these tasks. Hose ends should always
be kept off the floor and growing media kept in a clean area and covered.
Avoid holding plant material and accumulating contaminated pots, media or
debris in the media mixing area. Next, disinfect the growing and plant
handling areas, and irrigation system.

Benefits to Disinfecting the Greenhouse

Many pathogens can be managed to some degree, by the use of disinfectants.
For example, dust particles from fallen growing medium or pots can contain
bacteria or fungi such as Rhizoctonia or Pythium. Disinfectants will help
control these pathogens. In addition to plant pathogens, some disinfectants
are also labeled for managing algae which is a breeding ground for fungus
gnats and shore flies.

Although disinfecting should be done routinely, timing does not always
permit this extra effort. Take the opportunity to thoroughly clean
greenhouses between crop cycles when greenhouses are totally empty.

Managing Algae

Algae are a diverse grouping of plants that occur in a wide range of
environments. Algae growth on walks, water pipes, equipment, greenhouse
coverings, on or under benches and in pots is an ongoing problem for
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 a food source for insect pests like shore flies, and causes
slippery walkways that can be a liability risk for workers and customers.
Recent studies have shown that algae are brought into the greenhouse
through water supplies and from peat in the growing media. Once in a warm,
moist environment with fertilizer, the algae flourish.

Proper water management and fertilizing can help to slow algae growth.
Avoid over-watering slow-growing plants and especially crops early in the
production cycle. Allow the surface of the media to dry out between

Avoid excessive fertilizer runoff and puddling water on floors, benches,
and greenhouse surfaces. The greenhouse floor should be level and drain
properly to prevent the pooling of water prior to installing a physical
weed mat barrier.

Irrigation Systems
Non-recirculating irrigation systems can be cleaned using a new product,
chlorine dioxide (Selectrocide). It is labeled for disinfecting irrigation
lines and greenhouse surfaces. To thoroughly clean irrigation systems, the
company recommends using two consecutive overnight treatments, and then
flushing the system with clear water, making sure to discard the dislodged
debris. Selectrocide can also be used as a continuous ultra-low dose

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 containers into the wood will develop root rot
if conditions are favorable for pathogen activity. Disinfect benches
between crop cycles with one of the labeled 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 quaternary
ammonium compounds (Green-Shield, Physan 20, and Triathlon), hydrogen
dioxide (ZeroTol, Oxidate), chlorine dioxide (Selectrocide) and chlorine
bleach. All these products have different properties. 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.

Quaternary ammonium chloride salts (Green-Shield, Physan 20 and Triathlon).
Q-salt products, commonly used by growers are quite stable and work well
when used according to label instructions. Q-salts are labeled 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 labeled 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, the Green-Shield label recommends
that objects to be sanitized should be soaked for 10 minutes, and walkways
for an hour or more. Instructions recommend that surfaces be air-dried
after treatment except for cutting tools. The label recommends soaking
cutting tools for 10 minutes before use, then using the wet tool on plants.
One way to do this is by having two cutting tools, one pair to use while
the other is soaking.

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 dislodge 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

Hydrogen Dioxide (ZeroTol, OxiDate) Hydrogen dioxide kills bacteria,
fungus, algae and their spores immediately on contact. It is labeled as a
disinfectant for use on greenhouse surfaces, equipment, benches, pots,
trays and tools, and for use on plants. Label recommendations state that
all surfaces should be wetted thoroughly before treatment. Several
precautions are noted. Hydrogen dioxide has strong oxidizing action and
should not be mixed with any other pesticides or fertilizers. When applied
directly to plants, phytotoxicity may occur for some crops, especially if
applied above labeled rates or if plants are under stress. Hydrogen dioxide
can be applied through an irrigation system.

Sodium Carbonate Peroxyhydrate (GreenClean Granular Algaecide, TerraCyte)
Both algaecides are granular and activated with water. Upon activation,
sodium carbonate peroxhydrate breaks down into sodium carbonate and
hydrogen peroxide. GreenClean is labeled for managing algae in any non-food
water or surfaces. TerraCyte in addition to being an algaecide is labeled
to control moss, liverworts, slime, molds and their spores and is labeled
for use on plants. Non-target plants suffer contact burn if undiluted
granules are accidentally spilled on them.

Chlorine Dioxide (Selectrocide) Chlorine dioxide is a new disinfectant in
the horticulture industry for controlling algae, bacteria, viruses, fungi
and other microbial pests on greenhouse surfaces and in greenhouse
irrigation systems. Currently it is labeled to be used to clean out
irrigation lines with a periodic treatment at a moderate dose or to keep
lines from becoming re-contaminated by treating irrigation water flowing
through the system with a continuous ultra-low dose. Research continues to
be conducted on this product to expand its use in the industry.

Chlorine bleach. There are more stable products than bleach to use for
disinfecting greenhouse surfaces. However, when used properly, chlorine is
an effective disinfectant and has been used for many years by growers. A
solution of chlorine bleach and water is short-lived and 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, it
should be prepared fresh just before each use. The concentration normally
used is one part of household bleach (5.25 percent sodium hypochlorite) 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. Some would say that rinsing is not
necessary. Bleach should be used in a well-ventilated area. It should also
be noted that bleach is phytotoxic to some plants, such as poinsettias.

Disinfectants should be used on a routine basis both as part of a pre-crop clean up program and during the cropping cycle.

Dr. Teri Hamlin
North Region Agriculture Education
Georgia Department of Education
204C Four Towers University of Georgia
Athens, Ga 30602
706-552-4461 / 706-540-0032
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