Fungicides for Leaf Spot Diseases of Ornamentals

Fungal leaf spots are the most common and obvious diseases present during ornamental crop production. In most cases, they are easily noticed, and the temptation to guess specific causes is great. While certain characteristics may often be present with each type of leaf spot disease, there are no hard and fast rules about diagnosing by the “seat of your pants.” Most serious diagnosticians recognize the need to culture the pathogen before making a recommendation for the best treatment.

Pathogens such as Alternaria and Colletotrichum (anthracnose) affect most ornamentals, including bedding plants, cut flowers and cut foliage, tropical foliage plants and woody crops. Other diseases such as Fusarium leaf spot on dracaenas and fairy ring leaf spot on dianthus affect a narrow range of ornamentals but remain serious concerns for producers of those crops. Fungal leaf spots rarely kill a crop, but on rooted cuttings, such as pittosporum, Alternaria can result in massive losses. It is also possible to incur huge losses by planting plugs contaminated with foliar diseases. Unless environmental conditions are bad for the disease, it will continue while the product finishes.

Plant pathologists continue to stress use of cultural control strategies that minimize exposure to overhead irrigation and rainfall when possible; employ pathogen-free seeds, cuttings and plugs; and use resistant cultivars. Many of these methods are impractical when the production area is the great outdoors. In addition, despite nearly constant warnings regarding use of pathogen-free propagative materials, seeds are still commonly contaminated with Alternaria and other pathogens, and plugs or rooted cuttings infected with a variety of leaf spot diseases are easily obtained.

Fungicides will remain, for the foreseeable future, the most common and often the only way to manage some fungal leaf spots for many ornamentals. The difficulty faced in diagnosing even common leaf spots usually means that growers will choose a broad-spectrum fungicide to cover all the bases. There are many available fungicides with relatively safe, broad-spectrum characteristics that allow for undiagnosed control of many leaf spot diseases. A further difficulty is that, due to the increasingly large number of ornamentals grown, many specific diseases cannot be found on a fungicide label, even when a diagnosis is made. Researchers across the United States recognize these problems and struggle each year to fill the information gap for both fungicide users and manufacturers. I summarized the most current information on controlling some fungal leaf spots a couple of years ago and decided it was time to do another update.

Available Chemicals

The most commonly used fungicides have been available to our industry for the past 15-30 years in one form or another. Examples of these include chlorothalonil, copper, mancozeb, iprodione, thiophanate methyl and various combinations of these active ingredients. More recent fungicide introductions often include active ingredients such as fludioxinil, propiconazole, myclobutanil, azoxystrobin and trifloxystrobin. The newer fungicides represent many classes of chemistry, most quite different from previously available products. This gives the grower a better tool for resistance management, since rotations between chemical classes are believed to be our best tool in reducing the potential for fungicide resistance. Certainly, we have a very large number of different chemical classes to choose from for fungal leaf spot control. The responsible producer must practice rotation between classes and/or the use of products with more than a single active ingredient.

Fungicide Efficacy

Over the past 18 months, we have performed a number of leaf spot tests. These include four studies of Alternaria leaf spot on impatiens, two of Alternaria leaf spot on dusty miller, one on fairy ring leaf spot (Curvularia or Heterosporium) on dianthus, two on Fusarium leaf spot on dracaenas and one on Anthracnose on Cordyline. The tables and charts within this article show the results of these trials as well as a summary of previous work.

During these tests, we also found that some of the growth regulators affect leaf spot development (See Figure 1, top left). While this may not be as effective as a fungicide spray, the grower should be aware that some reduction in disease severity is occurring. Fludioxinil (Medallion) gave excellent control but must not be used on impatiens since it will kill them if it gets into the root system. The label does not allow use on impatiens, but the product should be excellent for Alternaria leaf spot on many other ornamentals. Iprodione (Chipco and Sextant) and chlorothalonil (Daconil, Concorde and PathGuard) each gave excellent control in many of the Alternaria trials. Mancozebs (Manzate and Stature) gave very good control, while Phyton 27 gave good control. Additions of Capsil or Camelot did not appear to improve activity of the fungicides in these trials. Curvularia (Heterosporium) causes fairy ring leaf spot on dianthus. The fungus is related to Alternaria, and indeed, the products that work best for Alternaria leaf spot are also very effective for fairy ring leaf spot (See Figure 2, left).

Anthracnose diseases are a little more difficult to control. These trials were performed on azalea, Cordyline, euonymus and lupine (much of this work was performed by University researchers) (See Figure 3, top right). The best products overall appear to be fludioxinil (Medallion), strobilurins (such as Cygnus and Heritage), triazoles (such as Systhane) and mancozebs (such as Dithane and Protect). The keys here are prevention of infection, or at last early detection, and application of the most effective product in your arsenal.

Finally, we performed a couple of trials on Fusarium leaf spot on two dracaenas about 18 months ago. We were most interested in extending the interval of treatment with the strobilurin product, Heritage. The trials demonstrated clearly that using 4 oz per 100 gallons applied Á every 21 days was as effective as 1 oz every 7 days (See Figure 4, above). Disease pressure, ease of application and direct fungicide costs should be considered when deciding how much and how often to apply products for leaf spot control. Remember that the most cost-effective application is preventative, which should be used in combination with cultural disease controls such as the use of pathogen-free plugs or cuttings and minimizing overhead irrigation.

As always, these products must be tested on your crops for safety, and you must follow the labels. If the product is labeled for greenhouse use only or for use outside of a greenhouse only, you must follow its directions. The advent of many “new” fungal spots on our ornamentals over the past few years makes choice of the most effective product difficult. When this occurs, new products and familiar, older products should be considered as potential candidates. Remember that the only way to choose control strategies wisely and reduce costs is to obtain an accurate diagnosis from a plant pathology laboratory. Otherwise, you are spraying in the dark and can cause more damage than good.