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 flowersand cut foliage, tropical foliage plants and woody crops. Other diseases suchas Fusarium leaf spot on dracaenas and fairy ring leaf spot on dianthus affecta narrow range of ornamentals but remain serious concerns for producers ofthose crops. Fungal leaf spots rarely kill a crop, but on rooted cuttings, suchas pittosporum, Alternaria can result in massive losses. It is also possible toincur huge losses by planting plugs contaminated with foliar diseases. Unlessenvironmental conditions are bad for the disease, it will continue while theproduct finishes.
Plant pathologists continue to stress use of culturalcontrol strategies that minimize exposure to overhead irrigation and rainfallwhen possible; employ pathogen-free seeds, cuttings and plugs; and useresistant cultivars. Many of these methods are impractical when the productionarea is the great outdoors. In addition, despite nearly constant warningsregarding use of pathogen-free propagative materials, seeds are still commonlycontaminated with Alternaria and other pathogens, and plugs or rooted cuttingsinfected with a variety of leaf spot diseases are easily obtained.
Fungicides will remain, for the foreseeable future, the mostcommon and often the only way to manage some fungal leaf spots for manyornamentals. The difficulty faced in diagnosing even common leaf spots usuallymeans that growers will choose a broad-spectrum fungicide to cover all thebases. There are many available fungicides with relatively safe, broad-spectrumcharacteristics that allow for undiagnosed control of many leaf spot diseases.A further difficulty is that, due to the increasingly large number ofornamentals grown, many specific diseases cannot be found on a fungicide label,even when a diagnosis is made. Researchers across the United States recognizethese problems and struggle each year to fill the information gap for bothfungicide users and manufacturers. I summarized the most current information oncontrolling some fungal leaf spots a couple of years ago and decided it wastime to do another update.
The most commonly used fungicides have been available to ourindustry for the past 15-30 years in one form or another. Examples of theseinclude chlorothalonil, copper, mancozeb, iprodione, thiophanate methyl andvarious combinations of these active ingredients. More recent fungicideintroductions often include active ingredients such as fludioxinil, propiconazole,myclobutanil, azoxystrobin and trifloxystrobin. The newer fungicides representmany classes of chemistry, most quite different from previously availableproducts. This gives the grower a better tool for resistance management, sincerotations between chemical classes are believed to be our best tool in reducingthe potential for fungicide resistance. Certainly, we have a very large numberof different chemical classes to choose from for fungal leaf spot control. Theresponsible producer must practice rotation between classes and/or the use ofproducts with more than a single active ingredient.
Over the past 18 months, we have performed a number of leafspot tests. These include four studies of Alternaria leaf spot on impatiens, twoof Alternaria leaf spot on dusty miller, one on fairy ring leaf spot(Curvularia or Heterosporium) on dianthus, two on Fusarium leaf spot ondracaenas and one on Anthracnose on Cordyline. The tables and charts withinthis article show the results of these trials as well as a summary of previouswork.
During these tests, we also found that some of the growthregulators affect leaf spot development (See Figure 1, top left). While thismay not be as effective as a fungicide spray, the grower should be aware thatsome reduction in disease severity is occurring. Fludioxinil (Medallion) gaveexcellent control but must not be used on impatiens since it will kill them ifit gets into the root system. The label does not allow use on impatiens, butthe product should be excellent for Alternaria leaf spot on many otherornamentals. Iprodione (Chipco and Sextant) and chlorothalonil (Daconil,Concorde and PathGuard) each gave excellent control in many of the Alternariatrials. Mancozebs (Manzate and Stature) gave very good control, while Phyton 27gave good control. Additions of Capsil or Camelot did not appear to improveactivity of the fungicides in these trials. Curvularia (Heterosporium) causesfairy ring leaf spot on dianthus. The fungus is related to Alternaria, andindeed, the products that work best for Alternaria leaf spot are also veryeffective 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 ofthis 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 (suchas Dithane and Protect). The keys here are prevention of infection, or at lastearly detection, and application of the most effective product in your arsenal.
Finally, we performed a couple of trials on Fusarium leafspot on two dracaenas about 18 months ago. We were most interested in extendingthe interval of treatment with the strobilurin product, Heritage. The trialsdemonstrated clearly that using 4 oz per 100 gallons applied ç every 21days was as effective as 1 oz every 7 days (See Figure 4, above). Diseasepressure, ease of application and direct fungicide costs should be consideredwhen deciding how much and how often to apply products for leaf spot control.Remember that the most cost-effective application is preventative, which shouldbe used in combination with cultural disease controls such as the use ofpathogen-free plugs or cuttings and minimizing overhead irrigation.
As always, these products must be tested on your crops forsafety, and you must follow the labels. If the product is labeled for greenhouseuse only or for use outside of a greenhouse only, you must follow itsdirections. The advent of many "new" fungal spots on ourornamentals over the past few years makes choice of the most effective productdifficult. When this occurs, new products and familiar, older products shouldbe considered as potential candidates. Remember that the only way to choosecontrol strategies wisely and reduce costs is to obtain an accurate diagnosisfrom a plant pathology laboratory. Otherwise, you are spraying in the dark andcan cause more damage than good.