The high cost of human health care, especially prescription
medications, has led many of us to that very question. Can I use the generic,
less-expensive product or must I purchase the name brand? Although we do not
have true generics available for our plant health care needs, we are faced with
essentially the same question: What about using a generic?
As with most simple questions, the answer is complex. Over
the past couple of years, Chase Research Gardens has sought an answer to this
question for ornamental disease control through a series of trials with
side-by-side comparisons of fungicides with similar active ingredients. As one
might expect, these tests are not popular with all fungicide manufacturers, so
many of these trials are my own fault with no one to share the blame. Some
companies will be gratified by the results we found while others will not. I do
apologize for leaving products out of these tests, but they were my own idea and
thus subject to my whims. The chemical class, active ingredient and rates of
the products tested are summarized in Table 1, page 32.
Coppers and Mancozebs
Perhaps the oldest fixed copper fungicide is copper sulfate
pentahydrate, first used in the 1800s. 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
marketplace. Examples of products with this active ingredient are Champ and
Kocide. Most recently we have Camelot (copper salts of fatty and rosin acids).
In the fall of 2000, we collected some 1-gallon azaleas with
powdery mildew and tried to eradicate the problem with a couple of fungicide
sprays. The products were copper and mancozeb (alone or in combination
[Junction]). It is clear from this test (see Figure 1, top right) that all
products worked reasonably well (to about the same degree) to reduce the leaf
area affected by powdery mildew.
Dithiocarbamate products such as mancozebs have been used to
control many foliar diseases on ornamentals for years. The azalea powdery
mildew test showed similar control with Junction (cupric hydroxide and
mancozeb), Dithane NT Rainshield and Protect T/O (both mancozeb). We also compared
some mancozeb fungicides (as well as sterol inhibitors) for control of
Alternaria leaf spot on Pittosporum. These liners had severe disease when we
potted them up and started the fungicide therapy program. In this trial Protect
T/O, Dithane NT Rainshield, Zyban (mancozeb and thiophanate methyl) and RH-0611
(myclobutanil and mancozeb) all gave excellent disease control (See Figure 2,
above). In addition, the sterol inhibitors (triazoles) Terraguard, Systhane and
Banner Maxx were also excellent.
The severe outbreak of poinsettia scab in the late summer of
2000 led to the test reported here. We trialed quite a few different
fungicides, and several containing mancozeb were among those tested. All
mancozebs tested (Stature is a combination of dimethomorph and mancozeb) gave
excellent preventative control of poinsettia scab (see Figure 3, above right).
The final group of fungicides we were interested in
comparing were those containing chlorothalonil — a long-time standard for
leaf spot control on ornamentals. Our first test was on impatiens with
Alternaria leaf spot. The plants were healthy when we started so this trial was
preventative in nature. Products included were two formulations each of Daconil
and PathGuard. The results we obtained showed that each of the chlorothalonil
fungicides provided very good to excellent prevention of Alternaria leaf spot
on this bedding plant (see Figure 4, below).
Some of the same products were included in a trial last
spring on daylily leaf streak. In this case, we also included two formulations
of Concorde as well as two combination products containing thiophanate methyl
and chlorothalonil (ConSyst and Spectro). While disease prevention was only
moderate, it certainly was equivalent with all of the fungicides included (see
Figure 5, below).
Which to Use?
For the few products and diseases tested under these
controlled settings, I would be hard-pressed to find any differences between
fungicides with the same or similar active ingredients. I have overall found
the fungicides within any one of these groups (copper, mancozeb, chlorothalonil
or sterol inhibitor) to control many diseases to a similar degree.
Keep in mind that the diseases I have reported here are
relatively easy to control. In cases where disease pressure is exceptionally
severe or the disease itself difficult to control, even slight differences can
be critically important. For example, Phyton 27 has given slightly better
control of bacterial disease than other copper products tested. Further, one cannot
control Fusarium wilt on Cyclamen with any sterol inhibitor — only
Terraguard has proven effective in our trials. Finally, we have found
significant Á differences between the strobilurin fungicides despite the
similarities of their active ingredients.
So what’s the answer to the question I posed at the
beginning of this article? Sometimes you can go with the least-costly
fungicide, while other times you must use a specific fungicide. No simple
answers in this life.
Editor’s Note: The use of specific trade names in this
article does not constitute endorsement of these products in preference to
others containing the same active ingredients. The use of trade names is solely
for the purpose of providing specific information and does not signify that
they are approved to the exclusion of others. Mention of a product does not
constitute a guarantee or warranty of the product by the author or the
When disease breaks out, what is the most important characteristic of a chemical: cost or efficacy? New data from Chase Research Gardens helps you combine the two