Disease Control: Bringing in the New, Bringing Back the Old in the New Millennium
Disease control on ornamental plants remains a critical problem for most producers. Leaf spots, stem rots and root rots result in significant and serious losses whether the crops are plugs, potted flowers or landscape plants. Over the past few years quite a number of new bactericides and fungicides have been investigated for control and safety in the ornamentals market. Part 1 of this article, which appeared in last month’s GPN, focused on the newest of these products – those based on strobilurin chemistries.
This month I will discuss an old standby, copper, which has been the active ingredient in some of the oldest fungicides still on the market. Copper also is the active ingredient in most of the bactericides currently on the market. It remains a valuable component of ornamental disease control.
Perhaps the oldest fixed copper fungicide is copper sulfate pentahydrate, first used in the 1800’s. Today, Phyton 27 is the best-known example of this type of copper product. In 1889, basic copper was the first factory-made fungicide. Nearly 70 years later, cupric hydroxide entered the market place. Examples of products with this active ingredient are Champ and Kocide. Although numerous products based on the toxicity of copper are used today, only a few are legally marketed for disease control in ornamentals.
How copper works
Copper products act as preventatives for bacterial and fungal diseases; copper also controls algae. Products containing copper sulfate pentahydrate, copper (cupric) hydroxide, and basic copper sulfate have been used in the ornamental trade for years. These products are thought to act best on bacterial diseases, but they also have wide ranging efficacy for fungal diseases.
The action of copper occurs at multiple sites to prevent spore germination. Copper products present a relatively low risk of resistance development for fungal pathogens, but the level of risk is high for bacterial pathogens. The consequence of multiple applications to a single crop for bacterial diseases is frequently resistance of that bacterial population to copper (and subsequent loss of control).
Some products are formulated with other active ingredients (i.e.,. mancozeb) as a means of managing resistance development in fungal populations. With so many available choices in today’s fungicide market, growers are strongly encouraged to routinely alternate between copper and other groups. This is problematic in bacterial disease control; although development of resistance is a serious problem, few legal and/or effective alternatives to copper are available.
Some of the available copper products include Camelot, Kocide 2000 TNO and Junction (Griffin L. L. C.); Champ (Agtrol International); and Phyton 27 (Source Technology Biologicals, Inc.).
Camelot is a cupric hydroxide product that is labeled on 25 ornamentals and has a 12-hour REI (re-entry interval following application). Kocide 2000 TNO is labeled on 107 ornamentals and has a 24-hour REI. Junction is a combination of cupric hydroxide and mancozeb that is labeled on 59 ornamental crops and has a 24-hour REI.
The final cupric hydroxide product is Champ. The product has a 48-hour REI (most recent label I can find) and is broadly labeled on all ornamental crops. There is a label precaution to test Champ on your crops prior to wide use on those not listed.
Phyton 27 is a copper sulfate pentahydrate product with 111 ornamentals listed on its label and a 24-hour REI. It is especially useful in fighting Xanthomonas blight on geranium – perhaps the greenhouse ornamental industry’s most notorious bacterial disease.
Grower experience, as well as research, has shown that copper is active in controlling a large number of ornamental diseases, including those caused by fungal and bacterial plant pathogen (Table 1). Examining the labels of five of the most common copper products reveals the breadth of activity of these fungicide-bactericides. They are generally effective against foliar diseases as well as a few soil-borne pathogens (drench applications). Over the past few years, tests have been conducted to compare copper products with some of the newer chemicals such as the sterol inhibitors and strobilurins. Some of these tests are summarized in the tables on pages 27 and 28.
Copper control on foliar diseases
Alternaria and Colletotrichum. Leaf spot diseases caused by fungi present continuing problems for many ornamental plant producers. Regardless of crop, many are affected by such common fungal pathogens as Alternaria and Colletotrichum (anthracnose - also Glomerella), which can attack a variety of crops. Others are affected by pathogens such as Didymellina (iris leaf spot), which although they affect a smaller range of crops, cause very serious losses.
These pathogens represent only a small subset of the many that can and do cause leaf spots on ornamentals. Thus, one of the biggest challenges is identifying which fungus is causing the leaf spot you are looking at.
The use of a broad-spectrum product such as copper for control of leaf spots diminishes the need for a specific diagnosis. In our trials over the past three years we have observed some control to good control of leaf spot diseases by copper fungicides. The degree of control varies with disease severity and the fungus involved (Table 2). In addition to Alternaria and Colletotrichum (anthracnose), coppers have been effective against Entomosporium and Didymellina (Table 3).
Botrytis blight. One of the most common diseases of ornamental crops is Botrytis blight or gray mold. This disease occurs on nearly all ornamentals. The fungus, Botrytis cinerea, is a pathogen that attacks flowers, leaves, stems, bulbs and seedlings (damping-off diseases). But Botrytis also has the capacity to grow as a saprophyte on dead and dying plant tissue. Because Botrytis has the ability to aggressively attack damaged tissues, applying marginally effective fungicides at rates conducive of phytotoxicity can actually increase the severity of Botrytis blight.
Phyton 27 has been somewhat effective for Botrytis control, although phytotoxicity on some crops under some conditions is a significant concern (Tables 2 and 3).
Other copper fungicides that can give a high level of Botrytis control include Camelot, Kocide TNO and Junction (Figure 1).
Mildews. Downy mildews have gained a strong foothold in the horticultural industry throughout the U.S. and Canada over the past few years. Currently, they are causing serious losses in many floricultural crops including rose, snapdragon (cut and bedding), pansy, viola, alyssum, statice, salvia, and rosemary.
Despite the sound-alike name, powdery mildews have little in common with downy mildews, which attack different plants under very different conditions. Some of the fungicides used to control powdery mildew are not the same as those used to control downy mildew; thus, an accurate diagnosis is critical prior to starting any control program. Phyton 27 has been very effective for downy mildew on snapdragon and alyssum (Tables 2 and 3). All fungicides tested to date must be used as preventives; once ornamentals are infected, these fungicides cannot stop a downy mildew disease.
Powdery mildews also cause serious damage to many floral and ornamental crops grown in greenhouses and outdoors. The diseases on different plants have similar symptoms – small white, powdery or dusty colonies (1/4 to 1/2 inch) that form in circular patches on upper or lower sides of leaves, stems, petioles and even flower petals.
Many fungicides are effective in controlling powdery mildew on ornamentals. Phyton 27 and Camelot have been very effective in powdery mildew control on gerbera daisy, miniature rose and New Guinea impatiens (Figure 2).
Rusts. Rust diseases occur on some of our most important ornamentals including rose, aster, fuchsia, geranium and snapdragon. Copper products have shown variable activity against rust. Phyton 27 is very good for snapdragon rust. In a separate trial on snapdragon rust, Camelot, Junction and Kocide 2000 TNO failed to provide significant control. It is possible that the activity of the copper on copper pentahydrate is superior to that of cupric hydroxide for rust control on this plant.
Rhizoctonia, Cylindrocladium and Thielaviopsis. Rhizoctonia spp. are among the most diverse of plant pathogens, causing root, stem and foliar diseases on many of our most important herbaceous and woody ornamentals. The pathogen is soil-borne, which means it lives in the soil or potting media, and causes both pre- and post-emergence damping off of many seed-propagated crops such as vinca, impatiens, stock and snapdragon.
Apparently, few of the copper fungicides have been tested extensively for control of Rhizoctonia diseases on ornamentals. The few reports I found indicate a low level of control for these diseases.
Two other soil-borne fungi linked to root rot are Cylindrocladium and Thielaviopsis. These pathogens are very difficult to control even with the best products. Soil drenches with Phyton 27 have proven somewhat effective for control of Cylindrocladium root and petiole rot on spathiphyllum when disease pressure is not extreme. Similar tests for black root rot (Thielaviopsis) control on pansy have not shown significant control with soil drenches of Phyton 27.
Phytophthora and Pythium. Phytophthora parasitica causes serious losses to many plants including vinca, petunia, snapdragon and pansy. Aerial blight may not occur at all if temperatures are significantly lower than 90° F, if humidity levels are low, or if rainfall or overhead irrigation is minimal. Preventative sprays or drenches can be very effective during production, but are of little value on landscape plantings infected with this pathogen. Preventative sprays with Phyton 27 compare favorably to those of Aliette in controlling this disease.
Some of the most common and costly diseases of ornamental plants are caused by Pythium spp. These diseases can affect cuttings, seedlings, and all stages of crops until finishing. Pythium spp. primarily attack plant roots, but can cause stem rots, cuttings rot and foliar blight under the right conditions.
The same products used for Phytophthora should be highly effective for Pythium. These products must be applied as soil drenches to deliver the product to the site of damage – the roots. The results with copper fungicides for control of Pythium root rot have been variable, ranging from no control to good control (Tables 2 and 3).
In general, the degree of root rot control achieved with soil drenches of copper fungicides is not sufficient to warrant this use under most conditions. However, control of stem diseases or aerial blights, which are caused by soil-borne fungi, is significant. Thus, control of Rhizoctonia stem rot or aerial blight, or Phytophthora aerial blight can be excellent with preventative sprays of some copper fungicides.
I have tried to summarize some of the information available on copper products and their uses in ornamental disease control. I do not intend this to be an exhaustive review, but I do want to indicate some potential uses for copper products. Apologies are extended to manufacturers of those products not included.
Please remember that considerations such as cost, safety, ease of application, availability and spectrum of activity come into play when choosing a fungicide for a particular purpose. Although it can be tempting to assume that the newest products are the best for the job, do not forget that some of the standards that have been around for years remain excellent choices.