Tree Fruit Disease Toolbox - Fungicide Resistance Management

Understanding Types of Fungicides

Pesticides used for managing fungi-caused fruit diseases range from being fungicidal (they kill fungi) or to being fungistatic (they inhibit fungal growth), with the difference strongly dependent on the concentration of the fungicide. Fungicides can be separated into two categories: protectants and systemics.

Protectant fungicides protect the plant against infection at the site of application. These are commonly known as contact fungicides and are classified as FRAC Group M Fungicides, which:

• Provide protection against infection.
• Do not penetrate into the plant.
• Require uniform distribution over the plant surface.
• Require repeated applications to renew the deposits.
• Have a multisite mode of action against fungi, which are unlikely to become resistant.
• Include chlorothalonil, captan, copper, ferbam, mancozeb, sulfur, thiram, ziram.

Systemic fungicides prevent disease from developing on parts of the plant away from the site of application and are classified as FRAC Group fungicides with numbers, which:

• Penetrate into the plant.
• Move within the plant, either systemic locally (within the leaf, or translaminar, e.g. Aprovia, Indar, Luna Tranquility, Merivon, Topsin) or throughout the whole plant (e.g. Aliette, ProPhyt, Rampart, Ridomil Gold SL).
• Control disease by protectant and/or curative action.
• Often have a very specific mode of action against fungi.

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

Fungicides in FRAC Groups 3, 7, 9, and 11 are highly effective against scab infection on apples. However, apple scab 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 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, mancozeb, Ziram, 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 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.

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 not to 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" because of stretching ARM intervals too long during very wet periods.

Dormancy

Applications of copper to apple trees during late winter or early spring will help limit available apple scab spores and fire blight bacteria for the coming season. Use copper at the rate of 2 pounds of metallic copper per acre. To determine the amount of copper product to use, pay attention to the percent metallic copper equivalent (and amount of metallic copper per unit) listed on the label of the copper being used.

From Green Tip Through Tight Cluster

Mature 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 in FRAC Group 3 and tank-mixed with a broad-spectrum fungicide (captan, mancozeb, ferbam, or ziram) or sulfur alone. 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. Include a spreader-sticker in the tank mix to allow the broad-spectrum fungicide to be rainfast (if not using a rainfast mancozeb product). This will allow the fungicide to persist longer during rainy periods. If disease conditions are favorable for scab (warm and wet), then consider using other fungicides from FRAC Groups 3 or 9 during this period. Be sure to rotate FRAC Groups. Growers are highly encouraged not to use the FRAC Group 7 fungicides during this time; these fungicides are best saved for peak apple scab pressure, which is from late 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 when wet) beginning late pink and will remain high through approximately late petal fall. In our experience with monitoring mature scab spore dispersal from overwintering leaves, available mature scab spores remain high (more than 10,000 to 20,000) for approximately two weeks (from pink through petal fall). During cooler springs, this may extend to three weeks. During this period, growers are highly encouraged to use complete sprays instead of ARM, especially if frequent rain events favor extended wetness periods; this is most important during bloom. During this time, it is best to use FRAC Group 7 (SDHI) fungicides tank-mix with a broad-spectrum 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 summer season, including before harvest. In addition, tank-mix with a rainfast mancozeb  or add a spreader-sticker to allow the mancozeb or ziram to be rainfast. This will allow the broad-spectrum activity to persist longer during very rainy periods.

From Petal Fall Through Second Cover

Although the number of overwintering mature 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 plus a broad-spectrum fungicide. One recommendation is to use a mancozeb through first or second cover (whenever 77 day PHI occurs) and then switch to captan or ziram for the later summer cover sprays. Use products that may have a long PHI 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.

After Harvest

To reduce the available inoculating spores for next season, spray trees with urea as close to leaf drop as possible. Spores need the leaf tissue to survive the winter and urea assists in the microbial breakdown of the tissue: leaves with extra nitrogen stimulate the growth of these beneficial microbes. Using urea will reduce inoculum by 50 to 80 percent for the next season. Dissolve 40 pounds of feed-grade urea in 100 gallons of water (5 percent solution), spraying 100 gallons per orchard acre. Feed-grade urea is recommended due to the ease of dissolving it in water. If you choose to not use urea, be sure your nitrogen comes from an ammonium source. Good coverage of the leaves is desired for leaves to absorb the urea. If the leaves have already fallen off the tree, urea can also be sprayed on the fallen leaves on the orchard floor. Additional breakdown of the leaf tissue can be assisted by using a flail mower, which will chop up the leaves. Using urea and a flail mower can reduce spores for the next season by at least 90 percent. When there are no sources of spores on the orchard floor or within 100 feet, there is a very low risk of early infections from these diseases. Finally, late season urea application does not compromise cold hardiness and has shown to help with tree health for the next season.

Brown Rot

Fungicides in FRAC Groups 1, 3, 7, 9, and 11 are highly effective against brown rot on stone fruit. However, 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 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, thiram, sulfur or ziram.

Many factors influence brown rot development. During dormancy, the 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.

Dormancy

Applications of copper to stone fruit trees during late winter or early spring will help limit available fungal (brown rot) and bacterial pathogens (bacterial spot) for the coming season. Use copper at the rate of 2 pounds of metallic copper per acre. To determine the amount of copper product to use, pay attention to the percent metallic copper equivalent (and amount of metallic copper per unit) listed on the label of the copper being used.

Sprays for Bloom Through Cover

The relative efficacies of current fungicides available are listed in Table 4-14 in Part IV. Depending on disease conditions during bloom, one or two sprays will be needed for protection from blossom blight caused by the brown rot pathogen. For cover sprays, sulfur, and captan will keep brown rot spores in check. Research from Rutgers has shown that captan sprays during the final two cover sprays before the preharvest brown rot sprays are very important. The rate should be no less than 3.125 pounds per acre.

Preharvest Sprays 

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 a final captan cover spray at 28 days preharvest, then other products at 18 days, nine days, and one day 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, the use of three different chemistries is strongly recommended given that some of these chemistries are rated as high risk for the development of resistance. For example, an excellent three-spray program that utilizes all three chemistries is Flint Extra (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.

Fungicide Rates

In general, a good starting point is the middle of the rate range, and using somewhat higher rates, but not necessarily the maximum, as conditions become more disease favorable. The FRAC Group 3 fungicides are a special case in that using higher rates is also a resistance management tactic. In this regard, note that Indar has an EPA 24(c) special local needs registration in Pennsylvania and Maryland that allows application up to 12 fluid ounces per acre.

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 in Pennsylvania to date.

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.

Bitter Rot

Bitter rot is an important apple rot pathogen during warm wet weather, especially on highly susceptible cultivars such as Empire and Honeycrisp. It is caused by various species in the Colletotrichum acutatum and C. gloeosporioides species complexes. The most effective products for managing bitter rot are the broad-spectrum fungicides mancozeb and captan, and the single-site fungicides pyraclostrobin (found in Merivon and Pristine), tryfloxystrobin (Flint Extra and Luna Sensation), benzovindiflupyr (Aprovia), and if applied at the high rate, fluazinam (Omega). For post-harvest applications, fludioxonil (Scholar) is the most effective product. Under moderate disease pressure the broad-spectrum fungicides ziram and the single-site fungicide penthiopyrad (Fontelis) also provide good control, while kresoxim-methyl (Sovran), and thiophanate methyl (Topsin M) provide fair control. FRAC Group 3, Group 9, and Group 7 fungicides (except for benzovindiflupyr and penthiopyrad) provide little to no control of bitter rot. The Colletotrichum species that cause bitter rot have not become resistant to these fungicides; they have never been sensitive to these fungicides.

Resistance to FRAC Group 1 fungicides (thiophanate-methyl) has been found in several orchards in Pennsylvania. Because of this, thiophanate-methyl should not be applied alone to manage bitter rot and should be used with a broad-spectrum fungicide like captan. There have been a few rare cases of resistance to FRAC Group 11 fungicides (pyraclostrobin, trifloxystrobin, and kresoxim-methyl) in orchards in Illinois and Maryland, In Pennsylvania, resistance to FRAC Group 11 has been found in the species C. siemense, which has been found in very low incidence and is not the dominant species occurring in Pennsylvania orchards. For this reason, FRAC Group 11 fungicides, especially pyraclostrobin (found in Merivon and Pristine) and trifloxystrobin (Flint Extra and Luna Sensation), are still recommended for bitter rot control, as long as they are tank mixed with a broad spectrum fungicide and the label restrictions are followed with no more than four FRAC Group 11 fungicide applications per year and no more than two consecutive applications.

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 those in FRAC Groups 3, 7, 9, and 11 (and their pre-mixes) 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 within the population 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 can 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. It is also important to note there are some fungi, such as the Colletotrichum species that cause bitter rot, that are naturally resistant to certain classes of fungicides.

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 and tank mixing with broad-spectrum fungicides are strategies to prevent resistance from developing.

Can Sensitivity Return to Certain Fungicide Classes?

Fungicides are not created equal when discussing the 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.

If I Experience Fungicide Failure During the Season, Should I Automatically Believe It Is Due to Fungicide Resistance?

No. Many variables must be ruled out first before considering fungicide resistance is to blame. Ask yourself the following questions, which could impact the efficacy of fungicide applications:

• Were the spray intervals during the primary apple scab period too far: more than 7 days for complete sprays or more than 3.5 days for alternate row middle sprays?
• Was it raining during pink through petal fall and alternate row middle spraying used (specifically for controlling apple scab)?
• Were fungicides not reapplied after a very rainy period?
• Were fungicides not applied prior to a rain event from late June through harvest (specifically for controlling bitter rot)?
• Was the sprayer calibrated?
• Is the affected location in a low-lying area and not drying out completely?
• Has the pH of the spray water been tested: was the pH too high? (High pH can decrease the effectiveness of fungicides.)
• Was enough water used to achieve adequate coverage of the entire tree?
• Was it windy during the time of application such that the spray did not contact the trees?
• Other considerations: suddenly adjusting the fan speed or speed of the tractor?
• Has it been raining all of the time with little chance of applying fungicides appropriately?

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 ("spraying by the numbers").

Spraying by the Numbers: Fungicide Resistance Management Tables for Tree Fruit Diseases—These downloadable tables will help you to avoid resistance by "spraying by the numbers."