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Disease Management Strategies Following the Challenging 2011 Season

Posted: March 7, 2012

This article provides a short review of the early-season fungus diseases of apples, a quick review of fungicide properties, a short discussion of how epidemics arise, some ideas about how to maintain fungicide efficacy and delay resistance, and finally, some disease management examples, including the integration some of the new SDHI fungicides into an effective program for 2012.

In most locations, two or three fungal diseases comprise the early season disease complex that must be managed with an integrated approach. Apple scab is usually the earliest disease, with ascospores often trapped in March and first infections occurring at or near the green tip stage of bud development. These early infections, if allowed to occur, often drive a disease epidemic that may lead to significant crop loss. The apple scab fungus overwinters in leaves on the ground, and has ascospores that are discharged by early spring rains, which are usually mature by the green tip stage and mostly over by the petal fall to first cover stage. Early infection may cause fruit and leaf drop, and severe defoliation can lead to poor return bloom. The length of wetting required for spores to germinate and infect at a given temperature is well known and is routinely determined using tables that depict temperature and wetting duration (the revised Mills table, for example, but there are others). Optimum conditions for infection are six to nine hours at 65° F; infection drops off sharply at above 80° F, but can occur if wetting exceeds 35 to 48 hours at 33 to 36° F. A wide variety of fungicides are available for disease management; however their specific properties and the erosion of their effectiveness by genetic changes in the pathogen population need to be considered so that good choices are made.

Powdery mildew is our dry weather disease. It can become a recurrent problem, with the potential for fruit infection at the pink stage of bud development. The primary inoculum comes from mildew in overwintered lateral buds that break dormancy as blossoms near the tight cluster stage. Conidia from the primary infections produce secondary infections during bloom and later. It is important to protect new tissues and developing buds from infection since mildew has chronic effects on yield. Generally, good mildew management should be focused on the tight cluster through second cover period. Fungicides that are effective against mildew include the demethylation inhibitor fungicides (the DMI’s or SI’s, e.g. Rally, Procure, Topguard, Inspire Super), although decreased efficacy of some of these fungicides has been noted in the Winchester area by Dr. Yoder; the strobilurins (QoI fungicides) such as Flint and Sovran), and the SDHI’s are effective during the period that they are included in the schedule. Sulfur is generally inexpensive and has suppressive effects on mildew, although a tighter spray interval is needed to achieve good mildew control. Topsin-M (applied for SB/FS and summer rots) is effective later in the season to prevent infection of developing buds. The EBDC's, ziram, captan, Vangard, and Scala are ineffective for mildew.

Rust diseases, mainly cedar apple rust and quince rust, are good examples of diseases where local pressure can be very high. Rust can affect tree vigor by repeated defoliation and fruit losses can be high if environmental conditions are favorable at the pink through bloom stage. Both rusts have Eastern Red Cedar as their alternate host. For cedar apple rust, inoculum levels are related to the numbers of new galls that were initiated two years earlier by the aeciospore stage of the fungus. Quince rust inoculum is more regular from year to year because the galls are perennial. Rusts are generally controlled with the DMI’s, EBDC’s, and ziram. Captan, sulfur, and Vangard or Scala are not effective against rusts; and QoI’s and SDHI’s may be weak under high disease pressure.

Disease Management Principles

In order to understand how fungal populations can develop to damaging levels, let’s consider a few principles of plant pathology that can be used to illustrate how we can get into trouble. First, consider that no fungicide is 100% effective, and that there are always escapes and resistant individuals in natural populations. The data from Koller et al. (1996) from Nova Scotia are useful to illustrate the shift in the proportion of a non-exposed wild-type population of the scab fungus toward resistance following the use of DMI fungicides. We see from these data that fungal pathogens evolve against selection pressure, thus mutation and genetic shifts are part of the agricultural ecosystems that we try to manage. Of course, problems emerge when resistant scab populations become too large to control with the mancozeb component in a DMI mixture when applied on a 10-day interval.

Disease progress is enhanced by the establishment of primary infections (in our specific discussion here, these are the infections that arise from overwintering ascospores); therefore the interruption of the disease cycle by the prevention of primary infections often forms the basis for the management of many plant diseases. This is why plant pathologists like to show disease cycles. Disease progress is delayed by reducing the amount of inoculum that is available to initiate primary infections; and fungicides delay epidemic progress, but they are effective only when they are present and need to be re-applied after they are removed by rainfall or when new non-protected plant tissues arise.

There are three epidemiological principles that function to give rise to a disease management strategy. The three principles are: 1) reducing initial inoculum (and initial disease, y0) can reduce overall disease during the course of the epidemic; 2) slowing the rate (k) of the epidemic; and 3) shortening the time (Δt) of exposure of the crop to the pathogen. The control of disease would be maximized by a combination of the three principles. In apple disease management, the main way that we maximize our “k effect“ – that is, slowing the rate of the epidemic - is by the timely application of effective fungicides.

Looking at our 2011 scab weather summary from Biglerville, we see how difficult it was last year to maximize our k effect because of fungicide removal due to frequent and intense precipitation. For example, PSU-FREC reported 2.0, 4.0, and 0.25 inches of precipitation in April, May, and June (to June 15) of 2010, compared to and 11.35, 4.32, and 1.42 in April, May, and June (to June 15) of 2011. These rain events were associated with a total of 22 infection periods in 2010 and 55 infections periods in 2011. This is remarkable!

Another factor that works against our ability to slow epidemic progress is the development by the pathogen of resistance to some of our important fungicide groups. The erosion of fungicide effectiveness has been a continuous process for single-site type fungicides, such as the DMI’s. This class of fungicides gave rise to The “Golden Age” of early-season disease control, according to Dr. Dave Rosenberger from Cornell University. The foundation of integrated disease management during “The Golden Age” was based on the DMI class and some of their extraordinary properties, including, their 96-hour post-infection activity, their after infection pre-symptom activity, and their post-symptom activity after conidia formation. These properties allowed for delayed sprays in orchards with no or light scab, or until after the first infection period; extended intervals between sprays; and lower rates of protectant fungicides in mixtures with the DMI’s.

Managing resistance has been part of disease management since the emergence of resistance to Benlate in the 1970’s. Site-specific mode of action materials are more likely to have pathogens develop resistance than multi-site materials. Paradoxically, some of our disease management practices that might favor resistance development have included low protectant chemical rates, occasional sprayer calibration issues, tree-row-volume applications, and exposing high populations of the pathogen to a site-specific fungicide with post-infection/post-symptom applications.

Multi-site vs. Site-specific Fungicides

A group with the acronym FRAC (Fungicide Resistance Action Committee) assigns class numbers to our fungicide groups and issues learned opinions about fungicide resistance management. We can break down all the fungicides into two large groups, the multi-site materials and the single-site materials. The multi-site materials act in more than one location in the pathogen and include captan, the EBDC’s, ziram, sulfur, and fixed copper. The site-specific materials are subdivided into several classes, including (for classes used on apples), the DMI’s (group 3), the QoI’s (group 11), AP’s (group 9), and the SDHI’s (group 7). The last group contains some of the recently labeled materials that we’ll talk about later.

Each of the two major groups has its own pros and cons. For the multi-site materials, the pro’s include: they provide a barrier to infection, have no resistance problems, and are relatively inexpensive. The con’s include: they have little, if any, kickback; no systemic movement so frequent re-application is sometimes necessary; they can be washed off; and they are used at pound per acre rates. For apples, the main multi-site materials are captan and the EBDC’s. Each of these has its own pro’s and con’s; EBDC use is generally limited to 3 lb. per application (although we’ll look at the 6 lb. rate a little later) causing us to mix the materials into something Dr. Rosenberger calls “captozeb”; EBDC’s control rust; and EBDC’s are cheaper. Captan may be more active against scab but also is more likely to wash off in heavy rain events. With these items in mind then, Dr. Rosenberger has concluded that where an EBDC is not needed for rust or cost containment, full rates of captan may provide better scab control than “captozeb”; use higher rates of captan in the mix when spraying ahead of predicted heavy rains; and where rust pressure is heavy, use “captozeb” (in our area (eastern W. Va.) where rust pressure is often very heavy, I choose to supplement the EBDC with ziram, hence the occasional use of “captozizeb”).

The site-specific fungicide classes that are available for use on apples include the QoIs (quinone outside inhibitors) or strobilurins, i.e., Flint, Sovran, and Pristine. These are powerful protectants that control scab, rust, mildew, SBFS, and black and white rots; however, they were initially positioned as DMI replacements and they failed in that role. Current labels limit their use to 4 applications per year.  Resistance, resulting in complete loss of activity against scab (as opposed to the “shift” we see with DMI’s) is negating their usefulness in Michigan, and is beginning to limit their usefulness in our region (with reports of complete resistance to them in VA, MD, PA, and NY). Because the resistance is complete, they fail completely when used in orchards with scab that is resistant to this class.

The next and largest class of site-specific fungicides for apples includes the DMI’s. Choices from this group include: Topguard (flutriafol), Rally (myclobutanil), Procure (triflumazole), Vintage (fenarimol), Tebuzol (tebuconazole, also a component of Adament), Indar (fenbuconazole), and Inspire (difenoconazole, which when combined with Vangard, an AP class fungicide, becomes Inspire Super). Not all are the same with respect to managing our complex of diseases, with some showing more effectiveness against mildew and others showing greater effectiveness against scab. Some of the problems with the DMI fungicides are resistance is becoming wide-spread, the level of resistance present in our scab populations often results in control failures, and it is difficult to predict where resistance occurs and how quickly it will emerge. While some of the “second generation” DMI’s, such as Indar and Inspire, show greater effectiveness relative to older compounds, Dr. Kerik Cox has found that in DMI-resistant orchards, Inspire and Indar did not control scab as well on McIntosh, which is highly susceptible to scab, but performed better on Empire, which is less susceptible. From this, a general recommendation would be to be cautious with Inspire Super and Indar for scab control in orchards where DMIs have previously failed.

The newest class of site-specific materials includes the SDHI’s (succinate dehydrogenase inhibitors). While this class has been around for several decades, the discovery of highly active molecules has moved them into the market place for disease management on specialty crops. Products that are becoming available now, or will be available soon include: Endura (boscalid), a BASF product and when combined with pyraclostrobin (QoI) makes Pristine; Fontelis (penthiopyrad), a DuPont product that will not be pre-mixed with another material, but which should be combined with something of your choice; and two products in the Luna series from Bayer, Luna Sensation (fluopyram) in a pre-mix with Flint (a QoI), and Luna Tranquility (fluopyram) in a pre-mix with Scala (pyrimethanil, an AP class fungicide). Another SDHI that we may see later this year or next year is Merivon (fluxapyroxad), a product from BASF that will be combined with pyraclostrobin (a QoI). In general, these products have good activity against scab and mildew, they penetrate leaves and are resistant to wash-off; they may be weak on rusts, are prone to resistance, so must be used in mixes with EBDC’s or captan. It is unfortunate that there is resistance already present to some of the pre-mix partners for some of these materials. They are not comparable to the DMI’s for post-infection, pre-symptom, and post-symptom activity. Furthermore, several of the SDHI components have not been widely tested alone, so their individual efficacies are not widely known.

There are a couple of other fungicide choice options that should be mentioned. The AP’s (the anilinopyrimidines class of fungicides), Scala and Vangard, provide 48 to 72 hours of post-infection activity against scab; and they may be useful for early-season missed scab infection periods.  They do not redistribute very well to new leaves and provide relatively poor fruit protection. Interestingly, we have observed a resistance shift to the AP class, even in scab populations where they have not been used, and in association with a shift in resistance to the DMI class. For this reason, they are most effectively used pre-bloom in combinations with captan or mancozeb. Another potential option for some orchards is Syllit (dodine). This older fungicide has historically provided excellent scab control but lost favor when resistance appeared following intensive usage. We have found that it can still be very effective against scab in most orchards when used in the first two sprays of the season. It has excellent retention and redistribution properties and provides a high level of fruit protection. Although the resistance problems create uncertainties, our data show that the persistence of the resistance may be less wide-spread than expected. However, because of the uncertainty, Syllit should always be combined with mancozeb or captan, and only used in the first one or two spray applications.

Another potential option for some orchards is Syllit (dodine). This older fungicide has historically provided excellent scab control but lost favor when resistance appeared following intensive usage. We have found that it can still be very effective against scab in most orchards when used in the first two sprays of the season. It has excellent retention and redistribution properties and provides a high level of fruit protection. Although the resistance problems create uncertainties, our data show that the persistence of the resistance may be less wide-spread than expected. However, because of the uncertainty, Syllit should always be combined with mancozeb or captan, and only used in the first one or two spray applications.

Some Potential Management Options

In this final section, I will illustrate some potential management plans for situations where overwintering levels of scab are high and/or where resistance to one or more classes of fungicides may be present, and some ideas for orchards where past scab management has been relatively successful. Also, I will show an option for disease management without the use, or only limited use, of site-specific fungicides.

Option 1. Potential disease management plan in orchards where scab was severe in 2011 and potential overwintering scab populations are high. At any point beginning at pink, if rust is a concern substitute mancozeb for captan (except when mancozeb is already mixed with captan, then supplement that mixture with ziram).
  Timing Material Choice Rationale/alternative
  Prebloom Urea 40 lb/A Or leaf shredding for inoculum reduction
1 Green tip Cu or mancozeb Cu for fire blight

2

Half-inch green

Syllit + captan

Or Syllit + mancozeb
Or captan + mancozeb if resistant to Syllit
Or Scala + mancozeb

3

Tight cluster

Syllit + captan

4

Pink

QoI + captan

If QoI resistance, consider Fontelis + mancozeb

5

Bloom

QoI + captan

6

Petal fall

DMI + captan

DMI at PF & 1C may be needed for rust; Rally or Topguard recommended for mildew; Inspire Super or Indar better against scab; caution with McIntosh and other highly susceptible cultivars

7

First cover

DMI + captan

 

Option 2. Potential disease management plan in orchards where scab was moderate or noticeable in 2011 and potential overwintering scab populations are moderate. At any point beginning at pink, if rust is a concern substitute mancozeb for captan (except when mancozeb is already mixed with captan, then supplement that mixture with ziram).
  Timing Material Choice Rationale/alternative

 

Prebloom

 

 

1

Green tip

Cu or mancozeb

Cu for fire blight

2

Half-inch green

mancozeb + captan

3 lb EBDC + 2 lb captan 80W
Or Scala + mancozeb or Scala + captan

3

Tight cluster

mancozeb + captan

4

Pink

QoI + captan

If QoI resistance, consider Fontelis + mancozeb

5

Bloom

QoI + captan

6

Petal fall

DMI + captan

DMI at PF & 1C may be needed for rust; Rally or Topguard recommended for mildew; Inspire Super or Indar better against scab; caution with McIntosh and other highly susceptible cultivars

7

First cover

DMI + captan

 

Option 3. Potential disease management plan in orchards where scab was light in 2011 and potential overwintering scab populations are very low and potential ascospore dose has been determined. At any point beginning at pink, if rust is a concern substitute mancozeb for captan (except when mancozeb is already mixed with captan, then supplement that mixture with ziram).
  Timing Material Choice Rationale/alternative

 

Prebloom

 

 

1

Green tip

Cu or mancozeb

Cu for fire blight

2

Half-inch green

 

Or Syllit + mancozeb
Or Scala (or Vangard) + mancozeb if resistant to Syllit

Consider Rally or Topguard here for mildew

3

Tight cluster

Syllit + captan

4

Pink

QoI + captan

If QoI resistance, consider Fontelis + mancozeb

5

Bloom

QoI + captan

6

Petal fall

DMI + captan

DMI at PF & 1C may be needed for rust; Rally or Topguard recommended for mildew; Inspire Super or Indar better against scab; caution with McIntosh and other highly susceptible cultivars

7

First cover

DMI + captan

 

Option 4. Potential disease management plan in orchards where multiple fungicide resistance is present or suspected and the preference is to use minimally, or not use, site-specific fungicides.
  Timing Material Choice Rationale/alternative

 

Prebloom

Urea 40 lb/A

Or leaf shredding for inoculum reduction only is used when scab was noticeable last year

1

Green tip

Cu or captan

Cu for fire blight

2

Half-inch green

Mancozeb 6 lb/A

Sulfur for mildew begins
Add Scala if back action is needed

3

Tight cluster

Mancozeb 6 lb/A

4

Pink

Mancozeb 6 lb/A

For re-applications following wash-off events, apply captan; do not go past bloom with mancozeb if 6 lb/A used at any time; good time to introduce SDHI if desired

5

Bloom

Mancozeb 6 lb/A

6

Petal fall

Captan 80W 5 lb/A

Include, as needed, ziram for rust and sulfur for mildew; if ziram is added, captan can be reduced

7

First cover

Captan 80W 5 lb/A

In conclusion, the fungicide schedule should be planned to maintain the long-term use of our most effective fungicides, using mixtures and alternations of our multi-site and site-specific fungicides. In addition to managing scab, the period from tight cluster to second cover should be evaluated for the need to manage mildew and rust. Try to use mixtures of multi-site and site-specific materials whenever possible, as these will reduce the likelihood of resistance to the site-specific materials, reduce the potential for crop loss in a year when resistance appears, and slow the rate of an epidemic in a year when resistance appears. It would be prudent to plan for your first choice and second choice of season-long fungicide options, in case resistance appears or market fluctuations affect price or availability of products. There are many valid options, in addition to those shown above, for scab and other early-season disease control programs, with no single option that will meet everyone’s needs.

Acknowledgements:

Appreciation is expressed to Dr. Dave Rosenberger, Cornell University; Dr. Keith Yoder, Virginia Tech; and Dr. Noemi Halbrendt and Brian Lehman, Penn State, for slides, data, photos, discussions, and ideas that went into this presentation.  This is a summary of a presentation given at the Penn State Extension President’s Day Fruit Growers Educational Meeting on February 20, 2012, Biglerville, PA.

Prepared by Dr. Alan R. Biggs, Professor of Plant Pathology and Extension Specialist, West Virginia University Tree Fruit Research and Education Center