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
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.
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.
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
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.
With more than 20 chemicals labeled for use on leaf spot diseases, deciding which product to use can be difficult. New research from Chase Research Gardens will make that decision easier.