Managing Apple Scab in High Inoculum Orchards

Posted: March 26, 2012

In apple orchards where scab was poorly controlled last fall, growers will need to compensate this spring for what we might call the five curses of high-inoculum, which are outlined in this article.


Dr. Dave Rosenberger and Dr. Kerik Cox, Cornell University Plant Pathologists

Modified from Scaffolds Fruit Journal, Cornell University



1. Expect more ascospores: Using data from a study by Gadoury and MacHardy (1986), New Hampshire orchards that had less than 1% leaf scab in autumn produced an estimated 888,000 ascospores/A as compared to 6.1 billion spores/A for an orchard with 20% leaf scab. These data suggest that orchards with 20% leaf scab may produce nearly 7,000 times more ascospores than orchards that had less than 1% leaf scab. Thus, orchards with a lot of scabby leaves in fall are indeed "high-inoculum" orchards.


2. Expect that there were more ascospores at green-tip: This is a logical corollary to the previous item. However, it is noted separately because the spores that are discharged early in the season pose the greatest risk for generating economic losses in commercial orchards. If ascospores initiate infections at green tip, then the first generation of conidia will become available about the time that trees are in bloom, and that is a period when fruit and leaves are at maximum susceptibility. Also, fungicide protection sometimes lapses toward the end of bloom if a fungicide spray is delayed with the objective of combining the fungicide with petal fall insecticides. Thus, having more ascospores at green tip escalates the risk of getting green-tip infections that will produce conidia before petal fall, which in turn ratchets up the risk of fruit scab.


3. Conidia may overwinter in buds: Work by Holb et al. (2005) in the Netherlands showed that when scab incidence in autumn exceeded 40% of terminal leaves, then small numbers of viable conidia would often survive through winter inside bud scales. Although the numbers of conidia surviving in buds under the worst-case scenarios reported by Holb are dwarfed by the numbers of ascospores that would be produced in those orchards, the conidia in buds are perfectly positioned to cause infections as buds begin to grow in spring. Thus, conidia in buds can be expected to have much greater infection efficiency than ascospores since the majority of ascospores released at green tip will never find tissue where they can cause infections. Incidentally, viable spores have been found inside buds on at least several occasions in New York, so it seems probable that the results reported by Holb from studies in the Netherlands are also applicable to high-inoculum orchards in northeastern United States.


4. Expect more infections from marginal infection periods: In low-inoculum orchards, relatively small numbers of ascospores are released during any given wetting period, and only a few of those released will be deposited on host tissue and complete the infection process in the minimum time listed for infections in the revised Mill's table. As the duration of wetting increases, more and more spores can be deposited on host tissues, so the severity of infection periods increases with time at any given temperature. In high-inoculum orchards, the total spore contingent is much higher (perhaps 7000 times higher as pointed out in #1 above), so many more spores will succeed in completing the infection process during short or "marginal" infection periods.


5. Fungicides will seem less effective: For example, if one assumes that 2% of the total season's ascospores could be released at green tip, that only 1% of those released will succeed in causing infections in unsprayed orchards, and that a green-tip fungicide spray will be 99.9% effective (which may be optimistic), then one might expect only 0.18 scab infections/A for orchards that had less than 1% leaf scab last year whereas orchards with 20% leaf scab last year might see 1,218 infections per acre. The only options for changing the odds are to either improve fungicide efficacy via higher rates, shorter intervals, and better spray coverage, or to implement inoculum reduction practices in the high-inoculum orchards.


Failure to control early infections vastly increases the risks of economic losses. Two steps you should have already taken are:


1.  To have applied one or more inoculum-reduction strategies to reduce the potential ascospore load. Four proven options for reducing ascosporic inoculum include (A) treating orchards in either late fall or early spring by applying 40 lb/A of urea dissolved in water and sprayed over the orchard floor (Sutton et al., 2000); (B) flail chopping leaf litter to speed leaf degradation (Sutton et al., 2000); (C) applying dolomitic lime to the orchard floor at the rate of 2.25 tons/A (Spotts et al., 1997); or (D) raking or vacuuming the leaf litter and removing it from the orchard. More details on methods for urea treatment or flail mowing can be found in a Scaffolds article published in 2009 (Rosenberger, 2009).


 2.  To have started fungicide applications at silver tip or green tip. Having a fungicide in place before the first infection period after bud break is absolutely essential, especially in orchards where the DMI fungicides are no longer effective.


A third important step is to use higher rates of fungicides or fungicide combinations. Even in low inoculum orchards, we know that higher rates of fungicide are needed beginning just before tight cluster because 3 lb/A of mancozeb used alone is not adequate to control scab during the period of peak ascospore discharge between tight cluster and petal fall. A combination of mancozeb at 3 lb/A plus Syllit at 1.5 pt/A was a good option for green-tip and half-inch green sprays, and this combination may still be a good option at tight cluster in orchards where growers are fairly certain that dodine resistance is not a problem. (The new Syllit label no longer contains the restriction against using apple pomace from Syllit-treated trees for cattle feed.)  However, in orchards where the status of dodine-resistance is uncertain, using Syllit at tight cluster or thereafter poses more risks because the 3 lb/A of mancozeb that is combined with Syllit may not provide adequate control of dodine-resistant scab during the peak of inoculum release between tight cluster and petal fall.  Thus, combinations of mancozeb 3 lb/A plus Captan 80W at 3 lb/A might be preferable for applications at tight cluster and pink where there is concern that Syllit, strobilurins, and DMI fungicides are no longer working. 


Where strobilurin fungicides are still effective, mancozeb at 3 lb/A plus either Luna Sensation or Fontelis would be excellent options for high inoculum orchards.  Luna Sensation and Fontelis will provide excellent control of powdery mildew whereas the combination of mancozeb and captan will not.  These new products are best combined with mancozeb rather than captan because both Luna and Fontelis will provide only marginal control of rust diseases.  Having mancozeb in the tank mix ensures better control of rust.


Literature cited:

Gadoury D.M., and MacHardy, W.E. 1986. Forecasting ascospore dose of Venturia inaequalis in commercial apple orchards. Phytopathology 76:112-118.

Holb, I.J., Heinje, B., Jeger, M.J. 2005.The widespread occurrence of overwintered conidial inoculum of Venturia inaequalis on shoots and buds in organic and integrated apple orchards across the Netherlands. European J. of Plant Pathology 111:157-168.