Algae invasion

Whenever I am asked to do an article, my first step is to check the literature to see what research has been done. That is where I started on this algae article but I could not find anything that could be called published research on algae control in horticultural production. I ran across some work I performed while I was a professor at the University of Florida some 20 plus years ago but that was almost all of it. Significant work has been conducted on liverwort, moss and many weeds but little to nothing has been done with algae on any surface or even in water treatment. There is much made of using disinfestants like chlorine, ozone and hydrogen peroxide, with claims of algae prevention or eradication but I failed to find the research to back-up the claims.

It is interesting to me that so little attention has been paid to one of the most common problems faced in greenhouse production. It is found in nearly every greenhouse, nursery and sometimes even landscapes where ornamentals (and other crops) are produced. Algae grows wherever we have carelessly supplied it with water and food. It can be found on pots, potting media surfaces, soil, benches, walkways and even all over plant leaves and stems. Algae are a nuisance, a worker health risk (slipping), a challenge to plant nutrition as they use water and fertilizer meant for plant growth, and an irrigation challenge because a dense algal mat can make water penetration impossible. They are also food for other pests including shore flies and fungus gnats.

Control methods
The first step in algae control is to understand the conditions that promote its growth. The high moisture, light and temperature so abundant throughout the greenhouse industry are ideal for algae growth. When this environment is coupled with excess nutrients, such as found in ornamental production as well as some landscapes, algae populations explode. Even when fertilizer is applied sparingly and directly to plants a certain amount is not used by the plant and provides food for the algae.

Methods used to control algae differ depending upon the surface they are growing on. Walkways, pots and benches can be treated with some chemicals which are very toxic to plants. Sodium hypochlorite (commercial bleach), hydrogen peroxide, copper, and quaternary ammonium compounds are available for algae control on nonliving surfaces. In addition, algae control in cooling pads is generally achieved with chemicals, especially the quaternary ammonium compounds. Algae control on plants has been tested using a variety of “fungicides” including the hydrogen peroxide, chlorine, dithiocarbamates and quaternary ammoniums. Results vary depending on sensitivity of the plant and type of algae present. It is always better to avoid the problem through careful management of the environment than to eradicate an infestation on the plant surface.

One of the most common ways algae spread in a greenhouse is through water. What makes a pest (like algae) suited for spreading in irrigation water? The most important factor is that algae makes propagative structures like spore that can be carried by the water. Algae, water-molds (like Pythium and Phytophthora) and bacteria all have a stage with flagella which allow them to actually direct their movement and swim in water films. Our need to contain and often reuse irrigation run-off makes an ideal way for algae to move throughout the greenhouse or nursery. Water treatment may become the favored method for algae control but it is only part of the solution.

Algae can be especially hard to control in recirculated water systems due to their “close” relationship with the plants we grow. The conditions that make the plants grow well usually are the same as those that the algae require for optimal growth. Applying fertilizer in the irrigation water is one of the things that favor algae explosions. Finally, many products or approaches that would kill the algae could damage our crop making the results unacceptable.

Product review
There are quite a number of products currently labeled for algae control. I reviewed some important characteristics of some of them.

There are many quaternary ammonium (QA) compounds (including Green-Shield, Physan 20 and Triathlon) that can be used on floors, walls, benches, tools, pots and flats as disinfectants. Pre-cleaning these surfaces is usually a good idea for maximum benefits since contact with any type of organic matter inactivates these QA compounds. They are usually recommended for use on plant material directly and do not have any residual once the solutions dries.

One notable exception is the fourth generation quaternary ammonium product (KleenGrow). It has higher organic tolerances and longer residual activity on surfaces. It can also be applied through drip irrigation systems or flood systems (0 REI). It is also labeled for use directly on plants surfaces (48 hours REI for this use only). The only time we saw damage from KleenGrow was on poinsettia when we applied it as a foliar spray seven weeks in a row.

Selectrocide is a chlorine dioxide-generating product that can be injected into irrigation lines to remove algae and the bioslime surrounding the algae. It is also used as a shock treatment to clean out irrigation lines but should not be used to treat water for immediate use to irrigate crops due to phytotoxicity. The label recommends a much lower rate continuously used to inhibit algae growth on clean surfaces.

Oxidate and ZeroTol are two hydrogen dioxide products that are labeled as disinfectants for use on greenhouse surfaces, equipment, benches, pots and trays. Oxidate has the added benefit of being an organic product.

Hydrogen dioxide is a strong oxidizing agent and should not be mixed with any other pesticides or fertilizers. When applied directly to plants, phytotoxicity can occur.

Sanidate is a combination of hydrogen peroxide and peroxyacetic acid. It is labeled as a disinfectant for use on greenhouse surfaces and equipment, pots, trays, and walkways and in greenhouse irrigation systems.

X3 (Xeroton-3) is a three way combination of hydrogen peroxide, peroxyacetic acid and octanoic acid. It may be used as an algaecide on greenhouse structures. In trials conducted over 3-4 years, we found good efficacy and safety on many foliar diseases of plants.

Our trials also showed that although X3 was effective against algae growing on the potting medium surface, ZeroTol usually performed better. X3 is also labeled for use in chemigation.

GreenClean Pro is sodium carbonate peroxhydrate and is labeled for algae control in greenhouses. The product recommends treatment at the first sign of algae. In cases where algae has become established, bubbling, bleaching and discoloration of algae is immediate. GreenClean Pro is activates when it contacts water breaking down into sodium carbonate and hydrogen peroxide. Avoid contact with any plant material. More products are labeled and some of them are further summarized in a good article by the University of Connecticut at www.hort.uconn.edu/ipm/greenhs/htms/algaemgmt.htm.

Prevention is best
Remember that all pests, diseases and other problems faced by plant producers are easier to prevent than to eradicate. Eradication takes higher rates or stronger chemicals that can damage crops at times. Try to be aware of your practices that promote algae problems from how you fertilizer and irrigate to the volume of each. Use the optimal rate and interval for plant growth and avid excess.

Ann R. Chase is a well-known plant pathologist and expert on diseases of ornamental plants throughout the United States. She is a widely published author, teacher and speaker.