Apple Pest Report: Tuesday, May 16, 2006
Vol. 14 No. 6
Scab
With the
torrential rains in southern Maine during this week of peak scab
infection risk, apple growers may be wondering about fungicide activity
in these conditions. Dr. Dan Cooley of the University of
Massachusetts wrote an interesting analysis of the current situation.
It is shown below with a minor edit where noted:
"Thoughts on apple scab from the shores of Lake Hitchcock. The Connecticut Valley in Massachusetts is the bottom of a 10,000 year old glacial lake called Lake Hitchcock. In a few more days, it may be refilled. It’s safe to say that we’re not getting much of a tan on the old beach.
The honest answer concerning to how to handle scab in this sort of weather is, there is a lot we don’t know. The obvious answer is spray when you can. But with what? Is it worth going out in the rain? Let’s try to get a best guess by looking at the things we do know. First, scab biology. The best news at hand is that primary scab season is approaching an end. Right now, at CSO about 90% of the spores are gone, and by next week, the last of the ascospores will have matured, and likely been released.
And here’s another small piece of good news. Primary infections during this type of rain are probably rare. Most spores that are mature and ready to go are released in the first couple of hours or less of a rain. After that, spores continue to mature slowly, at least during cool weather like this, and as they do, they probably get released. And then they probably get pounded into the ground by the rain. So, the amount of primary inoculum landing on leaves after the initial flush is very low.
On the other hand, conidia don’t stop growing, and they’re designed to get knocked around in the rain. So, if there were some early infections that started to produce spores last week, they are still there, still producing spores, and those conidia are moving around into nearby trees. This long-term rain makes any early mistakes much more costly.
Next, let’s look at fungicides. Captan and mancozeb, if they get a chance to dry on leaves, can stick well. In a study done years ago in NH, no matter what the rain, effective residues of captan were still on the leaf after 7 days. That may still hold true. We can hope that it does. However, it’s a safe bet that after 6 or 12 inches the levels of either of these fungicides on leaves has gotten very low. And after 7 days from the application, protection is probably gone.
But what about the strobilurines, Flint and Sovran, and the SI’s? They get absorbed into leaves. Does that mean that they’re less likely to get washed off in a monsoon? Well, remember that the SI’s never were very good protectants, with about 3 or 4 days activity maximum. So 5 days into a rain, they are moot. The strobilurines are better protectants, but we just don’t know whether they will last through heavy, long-term rain any better than captan or mancozeb. So the safe bet is to not depend on it.
All this means is something you already know: at the next opportunity, spray. Spray something. Probably the best choices are either an SI, as long as there is no indication of resistance in the orchard, or the strobilurines. These materials are included to take care of those mistakes that may have been made early in the year and to try to prevent any new infections that may have occurred during the rain from growing.
If Rubigan, Nova
or Procure are used, they should be mixed with at least 3 lb/A mancozeb
or 3 lb /A captan 50W (or the equivalent). Given the long-term
forecast, it might be a better idea to increase those rates to from 4.5
lb to 6 lb /A. The protectant ability of the SI’s being what it is, the
only protection after 4 days will be from the captan or mancozeb. Also
remember that it is necessary to put on a second SI combination about 7
days after the first.
(Ed. note - see comments following this article).
Flint or Sovran are effective in a single spray. However, it may be advantageous to plan a pair, as the pressure for scab has been intense.
Alternatively, a 6 lb. /A rate of captan 50W or mancozeb alone will provide protection, and about 18 hrs. kickback. The problem is that there may have been long, wet hours after the last spay well washed away but spores were landing on leaves, and growing.
One less conventional way to avoid this is to try spraying in the rain. Some fungicide will land and stick on leaves, and protect for a while. The problem is we really don’t know how much sticks, and how long it will be effective. It’s a lot of uncertainty for a major effort.
Another fungicide which may be useful is Syllit (dodine). Usually applications are limited to early season, but where still effective, Syllit has good (up to 36 hours) kickback and suppresses spore development. Use at the 0.75 lb/ 100 gal. rate. Check the label for cautions against tank mixes and russet on Goldens.
So until next week, this is Dan from Cold Spring Orchard Research and Education Center..."
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A couple of other points of emphasis, some of which came up in communications with Dr. Cooley and Dr. Dave Rosenberger over the last two days:
Strobilurin fungicides have somewhat shorter range of postinfection activity of 48-72 than the SIs at 72-96. In orchards with resistance of course, SI post infection activity is not reliable. SI resistance has been documented in NY and MI, but I am not aware of any documented cases in New England yet.
Beyond this postinfection range, the SI fungicides have presymptom activity. The rationale for a back-up application 7 days after the first when a SI application is made for postinfection control is not entirely clear. It used to be stated as a dose-effect, i.e. the second dose was thought to be needed to kill scab that may only have been temporarily inactivated by the first dose.
But the benefit of a second SI application may have more to do with less than perfect coverage and lack of redistribution of the first application. Thus, the back up application may contribute to better results because by hitting the spots missed by the first application, and controlling any previously missed infections at those locations because the SI fungicides have "presymptom activity". Presymptom activity is the ability of the fungicide when applied too late to stop the infection completely, to greatly reduce the number of secondary spores (conidia) produced by the scab lesion.
The need for a back-up strobilurin application in similar situation is not clear. But lacking the rationale for SIs with their presymptom activity, the answer for strobilurins is probably no. But you should not expect the same degree of postinfection control from a single strobilurin application as you would from a pair of SI applications.
While the strobilurins have less extensive postinfection/presymptom control, they have a major advantage over the SIs in that they provide solid forward protection for 6-7 days, whereas the SI fungicides only provide 3-4 days forward protection.
And getting beyond all the fancy new fungicide technology and speculation about ranges of activity, and rationale for material selection and timing, there is evidence that the older protectant fungicides (captan, mancozeb, or a combination of the two with each at a reduced rate - 3 lb. per acre for 300 gallon per acre trees) applied at a 7 day interval with good coverage, can provide good protection even in high rain situations.
Glen
Glen Koehler
Pest Management Office
491 College Avenue
Orono, ME 04473
Voice: 207-581-3882
Email:
gkoehler@umext.maine.edu
Web:
PRONewEngland.org
Fax: 207-581-3881
What we call the secret of happiness is no more a secret than our willingness to choose life. - Leo Buscaglia