Peter Konjoian
Talk to greenhouse growers across the country and chances are you'll hear that algae control is one of their more frustrating challenges. This problem has steadily become worse in recent years and growers are recognizing significant financial losses attributed to algae and the production problems they cause. It is frustrating because while algae are living organisms, eliminating them does not fall under the normal protocols of pest control. Algae are not insects and cannot be controlled by insecticides. Nor are they a fungus that can be controlled by fungicides.

Another source of grower frustration is that, while algae are not insects or fungi, their presence in the greenhouse encourages development of both of these groups of pests. Most growers can recite the logical progression of how algae development under the bench or on growing media provides a breeding area for fungus gnats, which then provides a breeding area for fungal diseases. Controlling algae can effectively interrupt the cycle.

To further add to growers' frustration, the products available to treat algae have not been effective enough to offer acceptable control. However, new research has identified a product that is capable of stopping algae: chlorine dioxide.

Using chlorine dioxide

You may recall several years ago that the Hart Senate Office Building in Washington, D.C., was contaminated with anthrax spores. The material used to decontaminate the building was chlorine dioxide, a compound that for 50 years has been known to be one of the most effective biocides.

Due to a recent technological breakthrough in the generation of chlorine dioxide, small and extremely pure quantities can now be generated economically. Chlorine dioxide gas is quite soluble in water and stable enough to generate stock solutions of 500 parts per million and higher in quantities of water as small as 1 quart to more than 50 gallons. Once the solution is produced, it can be injected into irrigation lines to eliminate algae and it can be used to sanitize various greenhouse surfaces, tools and containers.

The chlorine dioxide product Selectrocide comes as a dry packet that is placed in water. Within hours the solution is ready to use. The recommendation for irrigation system treatment is periodic shock treatments at concentrations of about 50 ppm accompanied by continuous injection at a concentration of less than 1 ppm.

While the current focus of my chlorine dioxide research is for greenhouse algae control, other beneficial uses are anticipated as the knowledge base expands. Chlorine dioxide may also become an effective part of an integrated management strategy for the control of common organisms responsible for plant diseases. Preliminary data support this hypothesis and research is continuing.

Algae are everywhere

Algae establish residence on a wide range of greenhouse surfaces, including sand, gravel and concrete floors, sidewalls of various construction materials, places where glazing materials contact structural components and evaporative cooling pads. When algae slime layers become established on floors and walkways, they create slippery surfaces that can be hazardous.

Growing media also encourage algae growth. Peat-based growing mixes provide the right combination of moisture, temperature, light and nutrients to provide another area of residence. Propagation environments, in particular, are challenging and too many growers have seen plug trays covered with algae.

The most overlooked component of a greenhouse that encourages algae growth is the irrigation system.

New PVC lines can be installed and, within a month, a visible layer of algae can become established if the line is dedicated to constant feed. Where are the algae coming from? Is it in the municipal water supply? If the water source is not municipal, are the algae coming from a well, a pond or other water source?

Two summers ago I was working in a 5-foot-deep ditch cutting into 2-inch PVC water and fertilizer mains to service a new greenhouse. I was surprised to find the same layer of green algae was lining the buried fertilizer pipe that was in the above-ground lines. What perplexed me was how the algae were thriving 5 feet underground without sunlight. I had assumed that algae needed sunlight to survive. What I learned from that experience and this research project may help explain why so many growers are frustrated trying to control algae.

Futile actions to stop algae

Many growers have told me that they've been advised to avoid using schedule-40 PVC in their greenhouses for fertilizer lines because this thinner, white pipe allows sunlight to penetrate, which encourages algae development. These growers have spent two to three times more money to install schedule-80 PVC pipe instead, under the assumption that the thicker, gray pipe blocks sunlight and prevents algae from growing inside the lines. Unfortunately, anyone who has installed this thicker, darker pipe has poured money down the drain. If algae can grow 5 feet underground, they're not going to have a problem growing in a pipe that is 1/8 inch thicker.

While I haven't switched from schedule-40 to schedule-80 pipe, my ignorance of this subject has led me to perform a silly practice of my own. I have buried schedule-40 irrigation lines running along the ground an inch beneath the sand floors in my greenhouses to block the sunlight and prevent algae growth. I now know I don't need to waste my time doing this anymore.

What is biofilm?

Two years ago I was approached by representatives of Selective Micro Technologies about conducting research on chlorine dioxide as an algaecide for the horticulture industry. The team of scientists with whom I work includes a physical chemist, chemical engineer, mechanical engineer and a microbiologist. It was the microbiologist who explained to me how algae were growing in underground greenhouse lines.

The explanation focuses on a new word: biofilm. Biofilm is a complex of living organisms and both organic and inorganic matter that establishes a presence on the inner surfaces of irrigation lines. The organisms live comfortably in this environment, and when fertilizers rich in nutrients are injected into the lines, suddenly life goes from being comfortable to downright luxurious.

Algae and biofilm are able to form a symbiotic relationship -- what one needs, the other provides. It's a relationship that keeps feeding itself, which is a major point in understanding why algae control is so difficult. Biofilm is able to provide algae with enough nutrients to substitute for their need of light to create such nutrients. This allows algae to flourish in irrigation lines, even 5 feet underground where it's dark.

Biofilm causes major problems

Biofilm presents a challenge to many industries besides horticulture. Its presence requires attention in beverage production, brewing, dentistry, food and meat processing, medical laboratory analysis and water purification. It is not always its association with algae that causes problems. In the beverage and brewery industries, biofilm negatively impacts taste through an association with yeasts. In the high-purity water industry, anything that's not supposed to be in the water needs to be removed.

A reason growers are so frustrated with algae control efforts is that existing products, while effective in killing algae, are not effective in removing biofilm from the water lines. Many growers experience varying degrees of initial control followed by recontamination over time, resulting in perpetuation of the problem.

Another piece of my algae control problem has been resolved. The microbiologist I'm working with has documented through water sampling and analysis that algae are indeed coming into my greenhouse in my town water. Cynically speaking, I'm paying the local water department to deliver algae to my greenhouse.

For more: Selective Micro Technologies LLC, 66 Cherry Hill Drive, Suite 23, Beverly, MA 01915; (978) 927-6610; fax (978) 927-6088; www.selectivemicro.com.

Peter Konjoian is president, Konjoian's Floriculture Education Services Inc., 48 Brundrett Ave., Andover, MA 01810; (978) 683-0692; peterkfes@aol.com. Technical advice and assistance with the research for this article was contributed by Landon Merrill, a microbiologist at Selective Micro Technologies LLC, 66 Cherry Hill Drive, Suite 230, Beverly, MA 01915; (978) 927-6610; fax (978) 927-6088; www.selectivemicro.com.

. © 2003 Branch-Smith Publishing