How to Control Apple Scab:

Relearning What Your Father Knew




Reprinted from Southern Tier Produce News; March 2005

David A. Rosenberger, Professor of Plant Pathology, Cornell University




For the past 40 years, apple growers have benefited from development of new fungicides that allowed consistent control of apple scab along with increased flexibility in spray timing.  When ferbam was introduced in the early 1950s, apple growers marveled at its effectiveness compared to sulfur.  The introduction of captan, maneb, and mancozeb fungicides later in the 1950s provided more options for controlling apple scab and other apple diseases.


Then dodine, sold as Cyprex in the early 1960s, provided apple growers with the first fungicide that had both post-infection and anti-sporulant activities.  As a result, dodine proved very effective for slowing or arresting development of secondary scab in orchards where early-season control failures would otherwise have resulted in significant crop loss.  The 1970s brought the introductions of benomyl (Benlate) and integrated pest management (IPM).  By the time the DMI fungicides (Rubigan, Nova, Procure) were introduced in the late 1980s, IPM was an accepted part of fruit-grower jargon and everyone was looking for methods to reduce pesticide use.


The DMI fungicides were tailor-made for IPM programs because they provided a scab-control safety net that enabled growers to take risks that would have been inconceivable 20 years earlier.  By tank mixing DMI fungicides with a protectant fungicide (captan, mancozeb, or Polyram), apple growers were able to capitalize on the benefits of both fungicide chemistries.   The protectants in the mix provided excellent residual activity against scab on both leaves and fruit.  The DMI fungicides provided up to 96 hours of post-infection activity, excellent pre-symptom activity when applied after the 96-hour post-infection window, and anti-sporulant activity that suppressed production of secondary inoculum on leaves with visible lesions.


With DMI fungicides, scab sprays at green tip became optional, 10-day spray intervals were the norm, and alternate row spraying on a 10-day interval was common during the pre-bloom period.  Timing of scab sprays could be adjusted by a few days so as to coincide with insecticide sprays that had to be applied at pink or petal fall.  Even when pre-bloom fungicide programs were less than 100% effective, back-to-back applications of a DMI fungicide at petal fall and first cover would correct the problem.





Holes in the safety net!




Research in NY has shown that apple scab with resistance to DMI fungicides is now common in many orchards.  Unfortunately, DMI resistance often becomes evident only in the wake of disastrous control failures.  As a result of control failures, orchards with DMI-resistant scab often have exceptionally high levels of overwintering inoculum.  The situation is further complicated by the fact that scab in most orchards is also resistant to Topsin M and is sometimes resistant to dodine as well.



   Suddenly, most apple growers with less than 50 years of

experience are entering unfamiliar territory: how does one manage apple scab in high-inoculum orchards when no fungicides are available to arrest scab development after leaves become infected?  For the first time since the introduction of dodine, apple growers must consider the possibility that just a slight error in pre-bloom scab control can result in season-long scab problems and a high incidence of scabby fruit.  Pre-bloom scab control becomes analogous to a high-wire act with no safety net! 





Attitude Adjustments:




The scab control guidelines that follow might sound familiar to fathers and grandfathers of current-day apple growers.  For younger apple growers, a quick attitude adjustment is essential for minimizing potential losses to apple scab.  Following are a list of common misconceptions about dealing with DMI-resistant apple scab:



Misconception #1: DMI resistance is of no concern to me because the DMIs are still working in my orchards.  Wrong!  Even if the DMIs have always worked well in your orchards, you never know when they may stop working.  Why wait for an expensive control failure before shifting to more conservative strategies?  Furthermore, by adopting more conservative spray strategies immediately, you may be able to conserve the post-infection activity of DMI fungicides for another decade in your orchards, thereby preserving the only remaining tool that can be used to arrest development of secondary scab in those unusual years when your conservative program is less than perfect.



Misconception #2: We'll ask Cooperative Extension or our fungicide supplier to run a quick test and tell us which fungicides are still working in our orchards. Wrong! There are currently no quick tests for resistance to dodine or to DMI fungicides.  Wolfram Koller at Cornell and Vincent Philion in Quebec are working to develop reliable tests, but it may be several years before a testing methodology is perfected.  Even after a test becomes available, testing for fungicide resistance will probably cost several hundred dollars per sample and will require collection of scabby leaves or active apple scab lesions from an unsprayed sample tree. Availability of qualified service providers for this kind of work could also limit the usefulness of fungicide resistance testing.



Misconception #3: We can substitute Flint or Sovran for DMIs when DMI resistance appears.  Wrong!  Sovran and Flint are great fungicides when they are used as protectants.  However, they are less effective than the DMI fungicides because they have less post-infection activity (only 48 to 72 hours instead of the 96 hours for DMIs) and they have no pre-symptom activity to slow development of incubating scab lesions.  As a result, Sovran and Flint have almost always been a disappointment when they have been used to stop epidemics in orchards where primary scab is already well established.



Misconception #4: The green tip spray can't be that important because most ascospores aren't released until after tight cluster.  Wrong!  It is true that relatively few ascospores are usually released at green tip.  However, infections initiated between green tip and tight cluster pose greater risks to the apple crop than primary infections that are initiated after tight cluster.  The early infections begin producing conidia just when fruit and