There was a time when selection of a fungicide for brown rot control was based solely on efficacy and cost of the product. However, the brown rot pathogen, Monilinia fructicola, has become resistant to DMI fungicides and to some extent QoI fungicides in many stone fruit growing regions in the eastern United States. This occurrence has underscored the need for a greater degree of integration of different fungicide chemistries as a resistance management strategy.
In developing fungicide programs for brown rot, one must recognize that the pathogen produces inoculum (spores) that is easily transported by wind or blowing rain. Although each cultivar may be planted in its own block, or row in the case of many farm markets, this inoculum will readily move between blocks or rows. Thus, the pathogen can be exposed to many successive preharvest fungicide sprays, even though each block or row is receiving only a few sprays.
For example, if 10 cultivars are going to be harvested in succession and each receives two-to-three preharvest sprays, then strains of the pathogen could be exposed to as many as 20 to 30 consecutive fungicide applications that season alone. Think of the New Jersey peach harvest season from July through September as one large epidemic.
Available fungicides. Three new fungicides (chemistry) were added this year to the 2013 New Jersey Commercial Tree Fruit Production Guide: Fontelis (SDHI), Merivon (QoI + SDHI), and Inspire Super (DMI + AP). These materials join the current arsenal of DMI, QoI, and SDHI fungicides that are available as either single or dual active ingredient products. The question is “How do we integrate all of these materials into programs that provide both excellent disease control and minimal risk of resistance development?”
In terms of brown rot control, there exists three main chemistries to integrate: DMI, QoI, and SDHI. The overall strategy is to rotate usage of these three chemistries throughout the entire harvest season so that each chemistry is approximately one-third of the total number of preharvest sprays. Possible programs consist of two types: single-active programs and combination dual and single active programs.
Single active programs. The simplest single active preharvest program consists of alternating products of the three types of chemistries. For example, the first preharvest application could be a QoI (Abound, Gem), the second application a DMI (Indar, Orbit, Orius, etc…), and the third application an SDHI (Fontelis). This same program would be repeated throughout the season for each successive cultivar harvest so that each fungicide chemistry was applied one-third of the total number of sprays.
As an example, a Gem / Indar / Fontelis program was applied at 17, 8, and 1-day preharvest, respectively, in our research test block during the 2012 growing season. This program provided 96% control of brown rot under heavy disease pressure (70% fruit rotted on non-treated controls). Given the choice of two QoIs and a variety of DMIs, many other programs are possible. Note that the final spray in the above program was applied prior to the first picking so as to provide protection during any infection events occurring between pickings.
Combination single and dual active programs. Many newer fungicides are dual-active products; that is, they have two active ingredients, each of a different chemistry. Currently, three types of dual active fungicides are available for brown rot control: DMI + QoI (Quadris Top); DMI + AP (Inspire Super); and QoI + SDHI (Merivon, Pristine).
To fulfill the three-chemistry strategy, one would be tempted to simply alternate the dual active material with a fungicide have a different single-active chemistry. For example, Pristine (QoI + SDHI) could be alternated with a DMI, such as Orbit (PropiMax, Bumper), for all preharvest sprays throughout the season. At first this program seems reasonable, and may work very well, but the DMI is being deployed alone 50% of the time (half the sprays).
Another perhaps better approach is to apply two applications of the dual active compound to every one application of the single active compound. Using the example above, the program would be Pristine / Orbit / Pristine for the three preharvest sprays. This reduces the single-active DMI to one-third of the sprays, the same exposure level as in the single-active programs. In this program the dual-active does have twice as much exposure, but this is acceptable given the two active ingredients are mixed. However, for this program to provide resistance management, both active ingredients in the dual-active must be active against M. fructicola.
Combination dual active programs. Examination of the available fungicides reveals that two combination dual active programs are possible: Inspire Super (DMI + AP) alternating with either Merivon or Pristine (QoI + SDHI). At first, these programs look interesting since they contain four chemistries. However, as stated in the previous section, both active ingredients in a dual active compound should be active against brown rot.
In the case of Inspire Super, the majority of control probably emanates from the DMI component, difenoconazole, not from the AP component, cyprodinil. The fungicide Vangard, which contains cyprodinil as its active ingredient, does provide good blossom blight control but is not considered to be as active against the fruit rot phase; note Vangard is only labeled for blossom blight control in the eastern U.S. Consequently, with respect to developing brown rot programs, Inspire Super should probably be viewed simply as a DMI fungicide rather than as a dual-active product.
Rates. If the label for an at-risk fungicide (single or dual active ingredients) lists a rate range, then application of a middle-rate is suggested as the starting point. This rate can be adjusted accordingly as one gains experience with the product and program, or to compensate for favorable conditions, inoculum levels, etc… Unless results of field trials indicate otherwise, use the low end of the rate range with caution.
Protectant fungicides. Several protectant fungicides (PHI), namely sulfur (0 d), ziram (14 d), captan (0 d), and thiram (7 d) have low enough PHIs to allow one or more sprays during the preharvest period. However, these materials are generally not recommended for brown rot control during this period due to their low degree of efficacy.
For example, Captan 80WDG was applied during the 2012 preharvest period at 5 lb/A. Applications were made at 17, 8, and 1 day preharvest to the peach cultivar ‘Suncrest’. Under heavy disease pressure (70% rot on controls), captan at this maximum label rate only provided 59% control of brown rot.
Program selection. A variety of integrated programs can be created using the above “rules” which are primarily designed to reduce selection for resistance. These programs will not necessarily be equal in their efficacy to control brown rot, nor is this necessary. Determining which program to apply is dependent on the needs of the orchard (e.g. inoculum level), the environment (weather conditions), and the cost of the program.
By examining fungicide efficacy tables, each program can be given a rating for control of brown rot. In most cases, a combination of fungicides have good to excellent ratings should suffice. The overall cost of each program can also be determined from current prices. One then selects the least expensive program that provides the degree of control needed. Finally, as always, ready labels thoroughly prior to fungicide usage.