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 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 reduces populations exposed to selection.

Fungicide Resistance Issues 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-specturm fungicide, like captan, metiram (Polyram), mancozeb, Ziram, thiram, sulfur, or ferbam. These highly effective materials should not be used once the major threat of scab infection is over. 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 bloom through petal fall. For brown rot preharvest sprays, using a premix (eg. Merivon) is recommended over using a single fungicide (eg. Gem).

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.

The type of fungicide used affects 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.

Resistant apple scab fungi and brown rot are not "super" fungi

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.

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").

Authors

More by Christy Hemler