Severe Apple Scab Affecting this Year's Crop in Pennsylvania

Posted: August 1, 2011

Over the last month, I have seen levels of apple scab that I never imagined possible in conventionally sprayed commercial orchards. As I write this, I have just returned from seeing two commercial orchards with complete control failure. In one of the blocks where I obtained samples, an entire crop of 'Fuji' is so heavily damaged that it may not be accepted for juice.
Severe apple scab on Fuji (Left) and Rome (Right) apples at a commercial orchard near Biglerville, Pennsylvania.

Severe apple scab on Fuji (Left) and Rome (Right) apples at a commercial orchard near Biglerville, Pennsylvania.

This is a 30-acre block that was subjected to a textbook spray program typical for fresh-market apples in the eastern U.S. And yet, this was not the worst diseased of this grower's blocks -- a 'Rome' block nearby is heavily defoliated, with the few remaining leaves covered with 'sheet-scab', and the few fruit still on the trees being heavily cracked. What is most troubling to me is that this is not a one orchard situation; I had just driven past another orchard that I saw last week where the situation is equally bad if not worse, and I have been seeing this level of damage on two to four orchards each week over the last month. It is a very humbling experience for a plant pathologist.

A few things about the orchards with control failure. These are certainly not poor fruit growers; they are among some of the best orchardists in Pennsylvania with many years of experience. Orchards with problems range from small-sized operations to some of the largest in the state and nearly all have crop management consultants on contract.

Cultivars affected range from those for processing to those grown primarily for fresh market. In my preliminary review of their spray programs, most of the operations had what I would characterize as good to very good disease management programs. There is of course variation among orchards and even within blocks of the same farm but overall, affected farms generally have very severe disease given the spray program they implemented.

Take the case of one of the large growers in Adams County. There are nine separate farms which are under the same production manager who assured me that a very similar program was used across the nine farms, of which two have a severe problem. Given the diligence I observed in their record-keeping, I can say that in this particular case the manager did what could be done to keep the crop clean so there must be something unique about these two farms.

Why such high levels of disease? This may sound scary but it is my considered opinion that if the level of devastation from scab reoccurs in the coming years, it would threaten the Pennsylvania apple industry. There are two reasons for making this almost alarmist statement:

  • Growers cannot sustain this level of loses repeatedly, a situation that is exacerbated by the fact that, in spite of obvious crop loss, they still have to spray the trees to minimize premature defoliation and ensure reproductive bud set for next year's crop
  • As I illustrate below, whatever cause of the control failure I can think of, there are no easy solutions to recommend.

It is therefore imperative that we try to understand what happened in order to come up with the appropriate responses. First let us deal with potential causes. I have often said in meetings that whenever you observe severe disease in a conventionally managed orchard there are three main reasons that can explain control failure:

  • either the disease control program used was ineffective for one of several reasons
  • the products used were not effective (i.e., the pathogen had become resistant)
  • a combination of these two reasons (i.e., a poor job with the spray program made worse by resistance to the products used)

A disease management program may be ineffective for many reasons including extremely favorable weather for disease development, wide spray intervals, spray coverage (a product of many other factors such as tractor speed, sprayer calibration, wind, canopy architecture, etc), and inappropriately timed applications.

The weather was particularly suitable for apple scab this year. In the primary scab season, we averaged almost twice the number of predicted severe infection periods that we observe in a typical year. For example, May alone had more severe infection periods this year than the total number observed in six out of the nine years for which data are available. The frequent rainfall events were a double trouble; growers often could not get into the orchards to spray, and whatever they applied often got washed off, especially the protectant component of the fungicide program intended to 'protect' the site-specific fungicides from selection for resistance in the apple scab fungus.

Number of days with severe apple scab infection periods predicted at PSU-FREC in Biglerville, Pennsylvania
Month Year
2003 2004 2005 2006 2007 2008 2009 2010 2011
*Predictions counted up to Jun 15 each year, i.e., end of primary scab.
April 4 6 3 4 4 4 9 7 17
May 9 15 4 6 5 8 12 7 24
June* 7* 11* 15* 2* 11* 3 * 9 * 8* 9*
Seasonal total 20 32 22 12 20 15 30 22 50

Resistance of Venturia inaequalis, the apple scab fungus, to major classes of the fungicides labeled for disease control is an equally important factor to consider. In 2010, my laboratory confirmed presence of resistance to sterol demythylation inhibitor fungicide (DMI) in many orchards in Pennsylvania (Pfeufer et al., 2010; Ngugi and Pfeufer, 2011).

About 75% of the orchards tested had resistance to at least one of the three different DMI fungicides that we evaluated, and we also confirmed high levels of cross-resistance to these fungicides. Unfortunately, this information could not have come at a worse time for our industry. The DMI fungicides would have been the best tool in a year like this with frequent rainfall events, multiple contiguous severe infection periods, and the associated difficulty in maintaining correct spray intervals.

Early on I recommended the use of the highest legal rates of the DMI fungicides in orchards with moderate levels of resistance. This recommendation was prudent especially because the label for one of the popular DMI fungicides was amended to allow the doubling of rate from 5 to 10 oz per acre. The nature of V. inaequalis sensitivity to DMI fungicides is such that it is possible to get good control of resistant populations by increasing the fungicide rate, until economical and horticultural concerns (e.g., effects on fruit size) become limiting. Anecdotal evidence suggests that growers that opted for the high rates have a cleaner crop than those who switched to strobilurin-based programs but this needs to be confirmed.

One consequence of the resistance to DMI fungicides was that some growers relied heavily on QoI fungicides (the strobilurins) as the 'big guns' for primary scab control. Without any real data, I can speculate based on what we know about the properties of these fungicides that they would not have been an ideal substitute to DMIs in a year like this. More worrying, however, is the high likelihood that we may have resistance to this class of fungicides in Pennsylvania orchards. Resistance to QoI in commercial orchards is likely given the levels of disease that I am seeing and the fact that resistance has recently been documented in Indiana and Michigan (Kimberly et al., 2011).

We are currently examining isolates from orchards with control failure to determine if resistance to strobilurins was part of the problem. The concern I alluded to earlier is that if resistance to strobilurins is confirmed, we shall almost simultaneously, have lost two of the most important site-specific fungicides labeled for apple scab control in Pennsylvania. This would therefore be a worse situation than the loss of benomyl (Benlate) turned out to be.

I am concerned not only because we do not have very good alternatives to these chemistries, but because some of the prospective replacements are mixture-products involving one of the two chemistries. Even in the event that we have a new, highly effective site-specific fungicide, it should be obvious by now that we are dealing with a very formidable and shifty enemy and that at best, such a fungicide will only buy us a few years. I am persuaded that we are dealing with what in medicine is termed as 'multi-drug resistance'....or call it 'super bug' if you may...problem with the apple scab fungus, hence my statement on there being no easy solution.

What are the implications of these observations for the Pennsylvania apple industry? It is important to consider a few scenarios. First, this was an unusually bad year but not all growers face the same level of apple scab severity even though I have seen enough control failures out there to be alarmed. If resistance to fungicides turns out to be the main cause of the control failure, then I am certain growers with a clean crop are not very far off from getting resistant V. inaequalis populations if we continue to rely on site-specific fungicides. Of course there are those who would argue that we can produce apples using protectant fungicide programs. I will not be the one to contest this view given that I have an acre of clean organic 'Gala' apples that was protected with regular complete sprays of lime sulfur and sulfur. What I will question is whether our growers are set up to effectively support protectant programs especially in a year like 2011.

Regardless of one's views on alternate-row-middle spray programs, the fact is that they have become fully accepted by growers in Pennsylvania. As a consequence, few of the large growers especially have the labor, sprayer number and configuration and the wells necessary to revert to complete spray programs without undertaking major investments. And I will not even venture into a discussion on the label limitations on some of the protectant fungicides. There are also concerns about the consequences of increased use of some of the protectant products on beneficial insects such as predatory mites.

How do we respond to this severe apple scab epidemic? In the short term I propose/suggest that: We need to get an estimate of the losses faced by the growers. In collaboration with Dr. Tara Baugher, we intend to survey growers to get an estimate of how wide-spread the problem is. Among other things, we hope to anonymously collect information on your spray programs, the level of scab and estimated crop loss. These data are important in order to highlight this problem and to support our requests for research funding and if necessary a petition for emergency registration of effective fungicides.

One of the studies that we need to urgently undertake is an assessment of whether we have resistance to strobilurins. The assessment I am doing at this stage is a very limited survey, which is not funded having been initiated only after we began to observe control failure last month. It is therefore not a substitute to a comprehensive survey such as we just completed for the DMI fungicides and which I think is overdue for the strobilurins.

Later in the fall or winter, I propose we (growers, researchers and consultants) get together and have a discussion on apple scab. We are going to have to think very carefully and prepare ourselves for radical changes in the way we manage apple scab if we are to have a sustainable apple industry. It is accurate to say that effective fungicides are not being replaced at a rate that can match that with which they are being lost to resistance; this requires a paradigm shift on our part. In the meantime, we are following-up on the status of registration for two new products that we expect to be available next year. I hope we don't have to resort to emergency registration but all options are on the table.

For orchards with a severe problem, I intend to propose a thorough clean-up program on an individual by individual basis. Such a clean-up is essential even if new fungicides become available next year. Indeed, I would argue it is required before you introduce a new fungicide in order to buy the new product more time. There scientific evidence that populations of the apple scab fungus with resistance to one mode of action progress faster towards resistance to an unrelated chemistry than isolates that are not resistant. Please watch this space in the near future for further details.


  • Kimberly S. Chapman, George W. Sundin, and Janna L. Beckerman (2011). Identification of Resistance to Multiple Fungicides in Field Populations of Venturia inaequalis Plant Disease. 95: 921-926.
  • Kimberley E. Lesniak, Tyre J. Proffer, Janna L. Beckerman, and George W. Sundin (2011). Occurrence of QoI Resistance and Detection of the G143A Mutation in Michigan Populations of Venturia inaequalis. Plant Disease. 95: 927-934.
  • Ngugi, HK, and Pfeufer E.E. (2011). Resistance to DMI (SI) fungicides in populations of Venturia inaequalis, the apple scab pathogen, from Pennsylvania apple orchards. Pennsylvania Fruit News Vol. 91 (1):22-27.
  • Pfeufer EE, Travis JW, & Ngugi HK (2010). Resistance to DMI fungicides in Venturia inaequalis from Pennsylvania. Phytopathology 100:S100.

Dr. Henry K. Ngugi, Penn State Fruit Research and Extension Center Plant Pathologist