Evaluating Post-Patent Fungicides By A.R. Chase

Over the past few years, we have seen post-patent fungicides enter the ornamental marketplace. They are sometimes less costly and usually have a very similar label to the brand-name fungicides we are familiar with. We have also started seeing products with similar active ingredients marketed as the same or superior (as is often the case with Aliette (Bayer Environmental Science) and other phosphonates).

The companies that sell post-patent fungicides do not routinely trial their products against the better-known trade-name fungicides. Each year at Chase Research Gardens, Inc. we compare fungicides with the same or very similar active ingredients in side-by-side trials. These comparisons can result in one of three conclusions: 1) the post-patent and brand-name products give the same level of control, 2) the trade-name product gives better control than the post-patent product and 3) the post-patent product gives better control than the brand-name product. For obvious reasons, the manufacturers do not want to gamble on the outcome, but I do. I present the following review of some of our trials for the enlightenment of growers.

Phosphonates And Aliette

We compared four phosphonates (phos acid alternatives) to Aliette 80WP as a weekly spray (three applications) on rosemary plants with powdery mildew infections. We used the products at the labeled rates (generally) or followed advice from the manufacturer. Disease severity was rated every week right before the fungicides were applied (see Figure 1, left).

The trial indicated that phosphonate 1 (24 oz. per 100 gal.), phosphonate 2 (64 oz. per 100 gal.) and phosphonate 4 (64 oz. per 100 gal.) gave a little better control than Aliette (16 oz. per 100 gal.) and phosphonate 3 (64 oz. per 100 gal.). In another powdery mildew trial, we saw similar results. Since the use rates were not identical, some analysis of efficacy versus cost must be made before deciding which product gives the best value.

In another trial on two phosphonates and Aliette, we compared drench and spray applications for prevention of Phytophthora aerial blight on annual vinca (see Figure 3, right). Top grade reflected disease severity (1 = dead to 5 = excellent, no disease). Both Aliette treatments provided excellent control of Phytophthora as did all application methods of the two phosphonates. In this case, the phosphonates were as effective as Aliette, but again the rate-cost comparison must be made to determine actual value.

The final comparison I present is a summary of safety of phosphonates compared to Aliette (see Figure 2, above). A few years ago, I heard claims that certain phosphonates were safer than Aliette since they did not contain aluminum. After testing them in more than 20 trials (Phytophthora, downy mildew and Pythium, especially), I believe the opposite is actually true.

Aliette was used at 16 oz. per 100 gal. while most of the other products were used at rates of 32-64 oz. per 100 gal. The roots were negatively affected by the high rates of phosphonate 2 and 3 while phosphonate 4 actually had the highest root quality. Be sure to compare all factors when considering a post-patent fungicide. The cost per unit of the product is not the only aspect to compare. The amount of product needed to achieve the same effect and safety considerations can make the product as expensive or not worth the savings.

Iprodione And Thiophanate Methyl

We have also been able to make some comparisons of different fungicides with the same active ingredient. In the first instance, we were primarily comparing the wettable powder formulation of iprodione known as Chipco 26109 (Bayer Environmental Science) to a newer, flowable formulation, Chipco 26GT (Bayer Environmental Science), but we had an additional flowable formulation to compare in the Alternaria leaf spot trial on impatiens (see Figure 4, above). We found, in general, the products performed to the same degree of safety and efficacy in the same trial for Alternaria leaf spot on impatiens, Botrytis blight on primrose and Rhizoctonia cutting rot on poinsettias or damping-off on annual vinca. Control in the two Rhizoctonia trials was not impressive for either formulation.

We finally compared high and low rates of several thiophanate methyl fungicides to 3336 (Cleary Chemical Company) for control of Thielaviopsis basicola, the cause of black root rot (BRR) on pansy. The exact rates of each thiophanate methyl are given in Figure 5, above. The data are given as the fresh weight of tops in grams and higher weights indicate control of the disease. In this case, even the 4-oz. rate of 3336 gave excellent control of BRR while the lower rates of TM1 and TM2 were not as effective. The higher rate chosen for each of the thiophanate methyl fungicides each gave excellent results.


The most important message I can send at this point is that if you choose post-patent products based solely on their cost per unit, you may be successful and you may not. In some cases, you will have effective and safe control while in others the brand-name fungicide will be more effective or safer and actually result in a better value. The question of post-patent fungicides versus brand-name fungicides is yet another example of having to know more than perhaps you would like. Unfortunately, there are few things in life that are simple.

A.R. Chase

A.R. Chase is plant pathologist at Chase Agricultural Consulting LLC and can be reached at archase@chaseresearch.net.

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