Apple Pest Report: Friday, May 12, 2006
Vol. 14 No. 5
Scab
Well I guess I don't have to spend any more time wringing my hands about conditions being too dry for scab ascospores to mature in a timely manner. I may be wringing water out of my socks instead. The overwintered leaves where scab resides and everything else are plenty wet now, and according to the forecast are going to stay plenty wet for the most of the next 10 days.
For growers using captan, mancozeb, Polyram or other surface fungicides, regardless of whether your most recent fungicide application was on Friday May 5 or Tuesday May 10, the situation is similar. The coverage is washed off as of Friday May 12, and any scab ascospores that mature and release after Friday will land on unprotected foliage until you can renew protection. Fortunately, cool temperatures on Friday-Sunday will reduce the percentage of ascospores maturing over those days. The forecast calls for a warm-up starting on Monday which will increase the infection pressure if the rain continues as expected.
If the latest fungicide application was with a surface fungicide before May 4, there is additional concern that trees were not protected through the high infection conditions on Tuesday May 9.
If the previous fungicide application was with a combination of a contact fungicide and a sterol inhibitor type (Nova, Procure, Rubigan), the situation isn't any different because the SI, while rain resistant after it moves into the leaves, is only good for a few days forward protection anyway.
The situation is a little better if your previous application was on or after May 5 and used a strobilurin fungicide (Flint, Pristine, Sovran). Once those materials move into the leaves they are much less affected by rainfall than the surface fungicides. However, they might be less resistant than the SIs because the strobilurins are held in the waxy cuticle layer and don't really move into the leaf tissue.
At best, a strobilurin applied on Monday or Tuesday, May 9 or 10, would provide protection until Tuesday or Wednesday, May 15-16. Normally strobilurins are considered good for six days forward protection, but with the cool temperatures slowing growth, they may have reached forward for seven days. On the other hand, the May 15-16 estimate is probably overly optimistic in locations where there has been 3+ inches rain since the application. So even where a strobilurin was applied as late as May 9 or 10, protection may be depleted by the end of Sunday May 14 or earlier.
In other words, we are moving into the period of peak scab ascospore maturation, peak foliar susceptibility, and there is no protection on the trees. That sounds dire, but we've seen this situation before (such as last year). With proper use of the fungicides available it is possible to come through this with good scab control nonetheless.
The fungicide depletion tables at
http://pronewengland.org/AllModels/MEmodel/me-Monmouth-ScabSprayContact.htm
and
http://pronewengland.org/AllModels/MEmodel/me-Monmouth-ScabSprayStrobilurin.htm
may be of some use for interpreting your specific situation if your
location has had similar recent rain events as Highmoor Farm.
All of this is a preamble to this week's attempt at words of
apple pest wisdom, so here they are: "As soon as you are able,
reapply fungicide and if possible, use a strobilurin for that
application."
If you can make a strobilurin
application sometime on late Sunday or Monday, (which currently looks
like the next possible spray window) then the postinfection activity
should be able to reach back and take care of most of the infection risk
that started on Friday-Sunday, May 12-14. As temperatures rise and
foliage growth resumes, the infection risk on Tuesday May 16 beyond will
likely be considerably higher, creating need for strong forward
protection also.
Actually, a sterol inhibitor might provide slightly better post infection activity. But with a lot more rain coming in the forecast, the sterol inhibitor's lack of forward protection is a liability. Even when combined with a protectant (and an SI should always be used in combination with a protectant) the SI-protectant combination will leave you unprotected again within 3-4 days after the application if the rains arrive as forecast. The Highmoor Farm forecast shows rain every day between May 12 and May 22. Nobody really knows how much or whether its going to rain that far ahead, but it's fair warning of what could happen.
The anilinopyrimidine fungicides (Vangard, Scala) have 48 hour postinfection activity (compared to the 48-72 hours attributed to strobilurins, and the 72-96 hours for sterol inhibitors). But Vangard and Scala are not recommended for use beyond tight cluster because they are not as effective as temperatures warm and are less effective on fruit scab than foliar scab.
I guess the ultimate fungicide cocktail in this situation would be a combination of sterol inhibitor AND a strobilurin but that has two disadvantages - it's a bad idea for resistance management to use them simultaneously, and the cost would be very high.
If the rainy forecast comes true, then after you get that next fungicide application on, be on the lookout to reapply again. In situations where the postinfection activity of a fungicide is put to the test because of long delay in the application after infections started, or because of high infection conditions, then a second backup application serves to insure good performance of the first postinfection application.
Fire blight?
Don't worry, be happy. Even if you are in southern Maine where apple trees reached bloom before development slowed, it's been too cool for fire blight bacteria to build up or for bees to spread them.
Insects
and Mites
European apple sawfly traps are set at Highmoor Farm. But
there has probably been little activity given the rainy cool weather.
Rain is "nature's miticide" so the chances of mite outbreaks are being
reduced as the rain comes down. At Highmoor, prebloom tarnished
plant bug and leafminer populations were low to begin with and if we get
a week of rainy days that will render even less of a threat.
Happy
Mother's Day!
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