Tree Fruit Disease Toolbox - Fungicide Resistance Management

Resistance has sometimes resulted in pest-management-program failures. Below are presented tactics to help delay resistance to fungicides.
Tree Fruit Disease Toolbox - Fungicide Resistance Management - Articles
Tree Fruit Disease Toolbox - Fungicide Resistance Management

Understanding Types of Fungicides

Pesticides used for managing fungi-caused fruit diseases are either fungicidal (they kill fungi) or fungistatic (they inhibit fungal growth). Fungicides can be separated into two categories: protectants and systemics.

Protectant fungicides protect the plant against infection at the site of application. Their characteristics are as follows:

  • They provide protection against infection.
  • They do not penetrate into the plant.
  • They require uniform distribution over the plant surface.
  • They require repeated application to renew deposit.
  • They have a multisite mode of action against fungi.
  • Fungi are not likely to become resistant to protectant fungicides. Some common protectant fungicides are Bravo, captan, copper, Dithane, Manzate, Polyram, sulfur, and Ziram.

Systemic fungicides prevent disease from developing on parts of the plant away from the site of application. Their characteristics are as follows:

  • They penetrate into the plant.
  • They move within the plant.
  • They control disease by protectant and/or curative action.
  • They often have a very specific mode of action against fungi. Some systemic fungicides are Elite, Flint, Indar, Rally, Merivon, Orbit, Pristine, Procure, Rubigan, and Sovran.

Cultural Control and Fungicide Use Patterns

Due to environmental conditions, disease is inevitable in the Mid-Atlantic growing region and use of chemical controls is a necessity; however, following cultural practices that favor decreasing disease pressure will help decrease the opportunity for resistance. Using resistant varieties, minimizing tree stress, and maintaining proper soil fertility reduces disease incidence since pathogens do not reproduce well on trees that are less susceptible to disease. As a result, the chance of resistance decreases. Avoid selecting sites with high disease pressure since this increases the chance of selecting for resistant fungi. Using dormant copper sprays and removing inoculum sources such as leaves (using urea or a flail mower), mummified fruit, and dead twigs/branches reduces the initial pathogen population. When using fungicides, use only when needed since this avoids unnecessary selection for resistant populations. It is important to be sure sprayers are appropriately calibrated and covering trees effectively. Achieving good spray coverage, tank-mixing with protectants, and alternating fungicides with different modes of action (FRAC group) reduces populations exposed to selection.

Fungicide Resistance Issues and Mitigation Strategies for Specific Diseases

Apple scab and brown rot

Fungicides in FRAC Groups 3, 7, 9, and 11 are highly effective against scab infection on apples and brown rot on stone fruit. However, apple scab and brown rot fungi can become resistant to these fungicides, especially if any of them are continually applied alone. Growers using one of these fungicides to control apple scab or brown rot must be certain to not only alternate it with an unrelated fungicide but also use it in combination with a broad-spectrum fungicide, like captan, metiram (Polyram), mancozeb, Ziram, thiram, sulfur, or ferbam. Another strategy to prevent resistance is to alternate the use of these materials throughout the season. The less any one of them is used in an orchard during a given season, the lower the chances that resistance will develop. At the present time, we know fungi causing apple scab and brown rot have shown high tolerance to fungicides in FRAC Group 11. As a result, growers are cautioned when using fungicides in this class, especially when it is not included as a premix. These fungicides should be avoided during peak primary apple scab spore dispersal, which is from late pink through petal fall.

Mitigating fungicide resistance for apple scab

Using cultural controls, such as removing inoculum sources (fallen leaves), is important for decreasing disease incidence; however, during seasons where the disease pressure is high (frequent rains, warm temperatures), fungicide applications will be important. It is critical to monitor disease conditions since this will play a crucial role in deciding which fungicides to use and when. In addition, if the alternate row middle (ARM) method is being used, it is very important to not stretch intervals, especially during frequent warm and rainy conditions. Sometimes this may mean shrinking intervals to five days, especially if disease conditions are favorable. There have been incidences where apple scab “broke through” as a result of stretching ARM intervals too long during very wet periods.

From green tip through tight cluster

Scab spores will begin to be dispersed from overwintering leaves starting at green tip; however, the spore numbers will be low, gradually increasing over time. If conditions are dry, focus on managing powdery mildew by using products such as Indar, Rally, Topguard/Rhyme, or sulfur tank-mixed with a broad-spectrum fungicide (EBDC, ferbam, metiram, ziram). Dry weather plus low scab spore numbers equals low disease pressure. Although some strong powdery mildew products are not as effective against scab, a broad-spectrum fungicide will keep the disease in check. If disease conditions are favorable for scab (warm and wet), then consider using other fungicides from FRAC Groups 3 or 9, such as Indar, Inspire Super, Procure/Trionic, Scala, or Vangard, during this period. Be sure to rotate FRAC Groups. Growers are highly encouraged not to use the FRAC Group 7 fungicides during this time period; these fungicides are best saved for peak apple scab pressure, which is from pink through petal fall.

From pink through petal fall

Scab spores will start to peak (the maximum number of available spores dispersing from the overwintering leaves) beginning late pink and will remain high through approximately late petal fall. In our experience with monitoring scab spore dispersal from overwintering leaves, available scab spores remain high (more than 10,000) for approximately two weeks (from pink through petal fall). During this time, it is best to use FRAC Group 7 (SDHI) fungicides, such as Aprovia, Fontelis, Luna Sensation, Luna Tranquility, Merivon, Pristine, or Sercadis, and tank-mix with a broadspectrum fungicide. Limit FRAC Group 7 fungicides to two applications during this period of high disease pressure. A maximum of four complete applications are allowed per year for FRAC Group 7 fungicides. Save two FRAC Group 7 fungicide sprays (if possible) for the end of the season when Luna Sensation, Merivon, or Pristine should be applied in order to mitigate late season and storage fruit rots.

From post petal fall through second cover

Although the number of overwintering scab spores drastically decreases after petal fall, spores are still available and can wreak havoc, especially if conditions favorable for disease are present. During this time, use products from FRAC Group 3 and 9, such as Inspire Super, Indar, Rally, Procure/Trionic, Scala, or Vangard, plus a broad-spectrum fungicide. One recommendation is to use an EBDC through first or second cover and then switch to captan for the later summer cover sprays. Use products that may have a long PHI (such as Scala) earlier rather than later. These products could also be used in rotation with the FRAC Group 7 fungicides that are used from pink through petal fall.

Mitigating fungicide resistance for brown rot

Many factors influence brown rot development. During dormancy, removal of brown rot blossom blight cankers and fruit mummies will decrease the number of available spores during the season. Green fruit are not susceptible to infection by the brown rot pathogen. However, immature fruit that are not properly pollinated or become injured can become infected and begin to rot. Remove any infected green fruit and drop them to the ground. Near harvest, as fruit are maturing, drop any rotting fruit to the ground to prevent fruit from becoming mummies, thereby reducing overwintering inoculum for next year.

Bloom through cover sprays

The relative efficacies of current fungicides available are listed in Table 4-14. Depending on disease conditions during bloom, one or two sprays will be needed for protection from blossom blight caused by the brown rot pathogen. Research from Rutgers has shown that captan cover sprays will adequately drop the number of available spores that could cause disease when harvest nears.

Preharvest sprays for brown rot

If frequent rains continue throughout the summer and harvest season, then a three-spray preharvest program is highly recommended. The recommended timing for this program is 18 days, nine days, and one day preharvest, with a final captan cover spray at 28 days preharvest. Note that the final preharvest spray can be applied immediately before the first picking, or alternatively between the first and second picking; the idea is to provide protection throughout the handling process. Of course, the fungicide used at this time must have a zero- or one-day PHI and appropriate REI. For resistance management reasons, a minimum of two different chemistries should be applied to each cultivar block (alternated). However, use of three different chemistries is strongly recommended given that some of these chemistries are rated as high risk for development of resistance. An excellent three-spray program that utilizes all three chemistries is Gem (FRAC Group 11), Indar (FRAC Group 3), and Fontelis (FRAC Group 7). For those fungicides composed of two active ingredients, simply alternate with the third chemistry. For example, apply a fungicide with FRAC Groups 7 and 11, FRAC Group 3, and FRAC Groups 7 and 11. As you progress through the harvest season spraying different cultivar blocks, simply continue with the rotation.

Powdery mildew

Frequent applications of fungicide may be required for mildew control. Fungicides in FRAC Groups 3 and 7 are effective for controlling powdery mildew. There are presently no documented cases of apple powdery mildew resistance to these materials.

Cedar apple rust

Only a brief part of the life cycle of the cedar apple rust fungus is spent on apple trees. Infection of apple leaves or fruit occurs between the pink and first cover spray periods. The cedar apple rust fungus survives 19 months or longer on red cedar. The contact between the fungus and the fungicide applied to apples is relatively short, reducing the potential for resistance to develop. If a resistant cedar apple rust fungus does develop, it must also survive on red cedar. Therefore, resistance of the cedar apple rust fungus to any fungicide is not likely.

Summer diseases on apple

Although resistance has not been reported for fruit rots or sooty blotch and flyspeck, it is important to be proactive by rotating fungicides and tank mixing with a broad spectrum chemical when controlling these diseases.

Frequently asked questions about fungicide resistance

Does the type of fungicide used affect the potential for a fungus to develop fungicide resistance?

Broad-spectrum fungicides like copper, captan, and sulfur act by interfering with several of the fungus's vital life functions. These fungicides have multiple modes of action, which allows little chance for resistance since the fungus must undergo multiple changes to counteract the fungicide.

Systemic fungicides like Inspire Super, Vangard, Scala, Flint, Sovran, Merivon, Pristine, Luna Sensation, Luna Tranquility, Fontelis, Rubigan, and Rally are highly effective against many tree fruit diseases. They are single-target-site fungicides interfering with one vital life function, so one change is needed for the fungus to become resistant. Thus, the potential for resistance to these fungicides is much greater than to broad-spectrum fungicides.

How do fungi develop resistance to a fungicide in an orchard?

As previously discussed, resistance is more likely to develop against fungicides that have a single mode of action, especially if they are used alone for a long time. In the orchard, resistant fungi may occur naturally in very small numbers even before the fungicide is first used. When a fungicide is applied, it reduces the number of susceptible apple scab and brown rot fungi. The few scab and brown rot fungi that are resistant to the fungicide are able to increase in number. As the fungicide is repeatedly used, the number of resistant fungi increases. The fungicide becomes less effective as the fungus becomes more tolerant to it.

Are resistant apple scab fungi and brown rot "super" fungi?

No, apple scab and brown rot fungi that are resistant to certain fungicides are still susceptible to others that have a different toxic action against the fungi. Using fungicide mixtures will delay the buildup of resistant scab and brown rot fungi. Mixtures are most effective when used before resistance becomes a problem. Alternating chemicals that have different modes of action/FRAC code is another strategy to prevent resistance from developing.

Can sensitivity can return to certain fungicide classes?

Fungicides are not created equal when discussing persistence of tolerance of the fungus to a particular class of fungicides. For instance, sensitivity can return to fungicides in the FRAC Group 3 if these fungicides are not used for a period of time. This is due to resistance being a fitness cost to the fungus, i.e. the fungus will not maintain resistance since it will prevent the fungus from surviving over time. In contrast, sensitivity will not return to fungi that are resistant to the fungicides in FRAC Group 11 since the nature of the resistance is based on the mutation of one gene that is stable and not linked to fitness.

The Future of Tree Fruit Disease Control

Growers can prevent resistance by practicing good cultural control methods, using fungicide mixtures, tank-mixing with a broad-spectrum protectant, and alternating chemicals by FRAC Group code group ("spraying by the numbers").


Apple and pear diseases Peach, cherry, other stone fruit diseases Tree fruit disease management

More by Kari A. Peter, Ph.D.