Ornamental Plants Annual Reports and Research Reviews 2000

Special Circular 177-01

New Weapons in Ornamental Disease Control

Hannah Mathers

Introduction

Disease control in ornamentals can be very difficult. Four reasons for the difficulty of control are listed here. First, many ornamentals are vegetatively propagated. Vegetatively propagated materials are generally more prone to disease infestations than sexually propagated materials. Second, vegetatively propagated crops have greater uniformity. The severity of a disease outbreak is greatest when the uniformity of the host plant is increased. Third, at several production stages, ornamentals may be grown at close spacing, and the closer the plants are, the greater the potential for disease to spread. Fourth, effectiveness of a pesticide increases when the inoculum potential decreases. In some situations, production practices have caused a buildup of inoculum. An example is the increase in frequency of root rot pathogens, Phytophthora and Pythium, particularly with the use of recirculated water.

Fortunately, over the past 10 years, a large number of new products have been researched and introduced into the ornamental market to make disease control easier. Most of the newer products represent classes of chemistry quite different from those previously available. Therefore, they provide the grower with new tools for resistance management. Rotations between chemical classes are believed to be our best tool in reducing the potential for fungicide resistance. This article will discuss fungicide resistance and a unique new Botrytis fungicide (class: Hydroxyanilide) and fungicides with activities based on pathogenesis-related proteins, activated oxygen compounds, and RootShield. This article will also cover some general disease-control principles, a list of registered ornamental fungicides, and three new fungicides with strobilurin chemistry. One strobilurin, Compass, will be introduced in the fungicide resistance section of this article but discussed in additional detail later.

Fungicide Resistance

A case of fungicide resistance in ornamental production is Botrytis resistance. Botrytis resistance is present and increasing in ornamental production. Years have passed without a new mode of action fungicide to control Botrytis. A new product called Compass, labeled by Novartis, offers a new mode of action to control fungicide-resistant strains of Botrytis. The active ingredient in Compass is trifloxystrobin, which is a strobilurin. Compass has a new type of activity; it is a mesostematic fungicide. Table 1 compares the characteristics of a mesostematic fungicide with the more common activity types of systemic and protectant or contact fungicides. A mesostematic fungicide has a high affinity with the plant surface and is absorbed by the waxy layer. Compass is a broad-spectrum fungicide providing excellent control of Botrytis and is considered a reduced risk fungicide by EPA. Compass is also effective against Pythium, powdery mildew fungi, and was shown to have some curative properties in terms of apple scab control.

Another new Botrytis fungicide is Decree. Decree is a reduced-risk fungicide and is safe on non-target beneficial organisms. It can therefore be used in Integrated Pest Management (IPM) programs. It is registered as Elevate in the stone fruit and small fruit market. The active ingredient is fenahexamid, which is a hydroxyanilide and is registered by Tomen Agro Incorporated and introduced by SePro Corporation. Decree is a unique class of fungicide with a novel mode of action for use in nurseries and greenhouses. Decree controls the spread and sporulation of Botrytis by interfering with fungal metabolism.

Two other new active ingredients with effectiveness against Botrytis are cyprodinil and fludioxonil. A product with cyprodinil as the active ingredient is Vangard, registered by Novartis. A product with fludioxonil as the active ingredient is Maxim. Maxim is a seed treatment for certain seed and seedling diseases, including Botrytis, and is registered by Novartis. Maxim has a new mode of action. It causes lysis of the cell wall. Maxim provides no control of oomycetes, but if mixed with Apron, it is effective against Pythium in sweet corn. Fludioxonil is also the active ingredient in Medallion. Medallion has activity on Botrytis and is also effective against several other foliar diseases and several stem, crown, and root diseases. Dr. Ann Chase has found that Medallion gave the best control of Cylindrocladium in azalea cuttings with one application as a drench.

Three other ways to combat Botrytis resistance are listed here. First, apply no more than two applications of a fungicide without rotating to another fungicide, preferably from another class. Second, never apply the fungicide at below label rate. Third, use mixtures of chemicals rather than applying fungicides in sequence.

Fungicides with Their Basis from Nature

Although many thousands of pathogens are present in nature, individual plant species are susceptible to only a very small number of these pathogens.

In the early part of the 20th century, evidence that plants could be protected against infection by prior infection of the plant with another pathogen began to accumulate. This phenomenon became known as induced or acquired resistance to disease (Hammerschmidt and Becker, 1997). One of the characteristics of acquired resistance is that it is effective against a broad spectrum of pathogens.

The first Systemic Activated Resistance (SAR) change detected in plants was enhanced peroxidase activity (Hammerschmidt and Becker, 1997). In addition to peroxidase, many plants also systemically accumulate a group of proteins collectively known as the Pathogenesis-Related (PR) proteins. These proteins have been classified into several major groups that have been given names PR1 through PR5.

A new product registered by Eden BioScience, Messenger, enhances one of these PR proteins called harpin. Messenger has a broad spectrum of control with activity against downy and powdery mildews, bacterial blights, white rust, black spot of rose, and grey mold. Trials with Messenger have revealed 10-20% increases in growth with the test plants. Actigard, registered by Novartis, is an inducer of host resistance, imitating the SAR response. Both Messenger and Actigard are known as "Activators." Messenger and Actigard have no direct activity against target pathogens. Actigard confers broad-spectrum control of both bacterial and fungal plant pathogens.

Like most "Activators" other than Messenger, Actigard has been shown to "cost" the plant in terms of growth reduction. Neither Messenger nor Actigard are currently registered in ornamentals, but these novel plant-health products will play a role with ornamentals in the future.

In addition to enhanced peroxidase activity, plants also may produce other "activated oxygen" species such as hydrogen peroxide, which have a role in Systemic Activated Resistance (SAR). Activated oxygen species have recently been given attention as an integral part of the active defense responses to pathogens (Hammerschmidt and Becker, 1997).

One role that activated oxygen species may play is the strengthening of cell walls (Ye et al., 1992). ZeroTol and OxiDate from BioSafe Systems have been sold more as general sanitation products but have activity as activated oxygen compounds. ZeroTol is labeled as a broad-spectrum algaecide/fungicide for use in nursery, greenhouse, retail, landscape, and water treatment. OxiDate is labeled as a broad-spectrum bactericide/fungicide in greenhouse vegetable, fruit, and nut production. Both products have new activated peroxygen chemistry. Although ZeroTol and OxiDate have their basis in nature, because they are manufactured, they are not considered natural fungicides. Both, however, are approved for use in organic production and have zero hour re-entry intervals (REI).

A fungicide that is considered from nature is RootShield. RootShield does not have a role in Systemic Activated Resistance (SAR). It is not an "Activator." RootShield is called a biological fungicide. It contains the fungus Trichoderma harzianum strain T-22. It protects roots from diseases caused by Pythium, Rhizoctonia, and Fusarium spp. and permits stronger, healthier root systems. It is manufactured by Bioworks, Inc., and can be applied as a drench or granular. It is safe around water sources, humans, birds, and animals. Dr. Ann Chase has also done trials with RootShield and Cylindrocladium with azalea cuttings and found good control was provided with one application as a drench. One caution: RootShield is a protectant, not a curative, and will not work against foliar pathogens such as Botrytis.

Disease Control Methods

Disease control can take many forms in nursery and greenhouse operations. Physical control may take the form of removing infected plant parts with hand pruners. A common problem with using hand pruners on infected materials is maintaining sterile conditions on the blades in order to avoid spread of diseases. Modified hand pruners, known as the KlipKleen Pruning System, have a vial attached that slowly drips a disinfecting solution over the blades to maintain sterile conditions.

Biological control is another form of disease control that is just beginning in nurseries.

Cultural control includes removing plant debris after pruning or leaf fall, spacing plants to improve air circulation, improving fertility and soil pH, providing appropriate drainage and irrigation to avoid water stress, and controlling weeds. Cultural controls may also include a general sanitation program, which may include chemical disinfectants such as Greenshield, Triathlon, ZeroTol, Bromine, or Chlorine.

Monitoring is another important part of any ornamental disease-control program. Monitoring is the regular inspection of plants to detect the presence of damaging insects, weeds, diseases, nematodes, or other adverse environmental conditions. Monitoring provides information to pinpoint the location of pests and to apply controls in the most effective and timely way. Monitoring also provides information on the presence and activity of beneficial organisms that may eliminate the need for other controls, and tells the manager how effective previous controls have been.

Monitoring is accomplished by visual inspections and a variety of trapping devices, including spore traps, and may be facilitated by recording environmental data, such as temperature, rainfall, and humidity. In nurseries, recording of cultural information, such as fertilizer applications, leachate analysis, and water-quality evaluations, is also important. One person should be assigned to maintaining a scouting and monitoring program. However, every staff person should be trained to be on the lookout for signs of pest activity and occurrence.

The core of any plant-protection program should always be prevention. Essentially, a healthy plant is more resistant to disease and can compete better against disease organisms. Fungicides are often used in a prevention program. Chemical controls with synthetic fungicides are the most common type of disease control. Fungicides should be viewed, however, only as tools in a disease-control program. They should never be the whole program.

Several fungicides that were registered for ornamentals as of January 1999 are listed in Table 2. Remember, the newest products are not always the best (Chase, 1999). When comparing older products to some more recently introduced on the market, Chase found excellent control was achieved by many of the older products (Chase, 1999). The purpose of this article is not to advocate the newer chemistries over the old products, but merely to make the readers aware of the array of alternatives for their disease-control programs.

Strobilurins

The original compound Strobilurin A. was isolated from a fungus called Strobiluris tenacellus, which was found on decaying wood in a European forest (Chase, 1999). There are three active ingredients in the Strobilurin fungicide type - kresoxim-methyl and, more recently, azoxystrobin and trifloxystrobin.

Kresoxim-methyl was the first Strobilurin to hit the ornamental market under the trade name of Cygnus (Chase, 1999). Cygnus was released by BASF Corp. Cygnus has translaminar movement, which means the chemical moves into and through the leaf, enabling distribution on the opposite leaf surface. This means that the chemical can reach leaves not directly sprayed, which is advantageous when trying to control many of the leaf diseases. Cygnus is used as a protectant against powdery mildew, downy mildew, rust, black spot, Botrytis gray mold, anthracnose, and several leaf spot diseases.

Heritage was the second Strobilurin in the ornamental market (Chase, 1999). The active ingredient is azoxystrobin. Azoxystrobins have translaminar and systemic properties. Heritage is registered by Zeneca and is the only systemic Strobilurin fungicide available. It has excellent residual activity, systemic action, and potency against oomycetes, ascomycetes, basidiomycetes, and deuteromycetes. Azoxystrobin is xylem translocated and moves upward and outward through the plant from the point of application, so good coverage with a surfactant is recommended. There is no control achieved below the point of application. When Heritage is applied as a drench, the chemical will move into the root system and throughout the plant. However, again, if sprayed onto leaves it will not move downward into the stem and roots.

Heritage is labeled for control of two conifer blights - Phomopsis and Sirococcus; many leaf blights and spots including anthracnose, Alternaria, and Septoria leaf spots and black spot of rose; powdery mildews; rusts; several flower blights including Botrytis blights; aerial and shoot Phytophthora blights, and three soil-borne pathogens - Rhizoctonia solani, Sclerotium rolfsii, and Fusarium spp. Heritage also offers excellent control of Cylindrocladium in azalea cuttings. Heritage inhibits spore production and germination.

Note some fungicides sometimes have an effect on reducing rooting if used on cutting materials as a drench. If reduced rooting occurs, use the fungicide as a spray rather than as a drench. Heritage presents no significant risk to beneficial insects, as well as earthworms and honeybees, making it an ideal choice for IPM programs. Heritage was also the first product registered by the EPA under its Reduced Risk and Safer Pesticide Program. It poses minimal risk to workers and has a four-hour re-entry interval (REI).

The third Strobilurin in the ornamental market is called Compass, labeled by Novartis. The active ingredient in Compass is trifloxystrobin. Flint is the trade name for the fungicide with the same active ingredient that delivers excellent control of important diseases of many fruit and vegetable crops including grapes, cucurbits, and pome fruits.

Compass has a new type of activity, as explained earlier in the article. It is a mesostematic fungicide. A mesostematic fungicide has a high affinity with the plant surface and is absorbed by the waxy layer. Compass is a broad-spectrum fungicide providing excellent control of Botrytis, Pythium, powdery mildew, rusts, downy mildew, and leaf spot diseases. Flint and Compass have protective and curative activity.

Summary

Disease control in ornamentals is still very difficult; however, the wealth of new products for disease control gives many interesting and valuable alternatives to ornamental producers. Doing your own trials with new products is the best way to determine which works best on which diseases and if these products are better than what you have used in the past.

Another important aspect of a disease control program is to know how to submit a good sample to outside consultants for confirmation of a disease. When samples are submitted to a plant diagnostic laboratory, you are looking to confirm or refute the presence of an infectious disease.

Infectious diseases are often referred to as biotic disorders, meaning the causal organism is alive. However, 60 percent or more of samples presented to most diagnostic laboratories are abiotic in origin. Abiotic means the cause of the problem is not a pathogen. Some common non-infectious or abiotic disorders are physiological disorders like graft incompatibility, chemical injuries, nutritional deficiencies or excesses, planting problems, environmental injuries, or salt damage.

Determining the difference between abiotic and biotic problems in the field is not always easy, but it is essential to developing a good disease-control program.

Literature Cited

Chase, A. R. 1999. Bringing in the new and bringing back the old in the new millenium. Greenhouse Product News. December 1999.

Hammerschmidt and Becker, 1997. Horticultural Reviews. Jules Janich, Ed. John Wiley and Sons, Inc., New York, N.Y. Vol. 21. 1997.