What determines the longevity of a scab fungicide application?

       Protectant scab fungicides like captan, Syllit, and EBDCs (Polyram, mancozeb) work by killing scab spores before they can penetrate into susceptible apple tissue.  Expanding leaves are the most important site for scab infection.  But apple leaf tissue is only fully susceptible to scab infection for a few days after emergence.  The residue of a protectant scab fungicide application loses its protective ability primarily because of tissue growth that “outruns” the ability of the fungicide to redistribute and cover new tissue in subsequent rains.

      Full dose captan at 7 day intervals can prevent scab against even very high inoculum levels and heavy rains.  While the other protectant fungicides may not quite equal captan’s protectant abilities, the 7–day rule has been successfully used with them also.  The 7-day rule isn’t really based on the number of days, but on the typical amount of new tissue expansion that occurs in 7 days.  Therefore, if there is an unusually warm string of days, and rapid expansion of susceptible tissue, then the 7 day interval may need to be shortened.

       Polyram is a good choice for protectant fungicide when the spray will include oil.  In addition, the zinc content in Polyram is enough that orchards where Polyram is used are less likely to have foliar zinc deficiency. 

Powdery mildew was unusually common last year throughout the Northeast.  This was attributed to the sequence of several winters without low enough temperatures to kill powdery mildew infected buds where the fungus overwinters.  With multiple periods of temperature below -11 F this past winter, the threat of powdery mildew should subside this year.  But if powdery mildew has been a repeated problem in your orchard in recent years, Pink would be the time to use a scab fungicide that also is effective against powdery mildew.  In addition to the sterol inhibitor fungicides (Nova, Rubigan, Procure) and the strobilurins (Flint, Sovran), Topsin M offers some protection against powdery mildew.

 INSECT and MITE PESTS

       Only two Leafminer moths were found on 8 traps in the “Lower portion” blocks at Highmoor on May 7.  Four traps in a third block up on the hill had leafminers over threshold at an average of 14 per trap.  But in the past, even prebloom leafminer trap counts much higher than this did not always correctly predict subsequent above-threshold number of mines.  In addition, treatment against larvae after Petal fall is more effective than spraying to kill the adult moths before bloom.  Therefore the recommendation is to use the traps as an early warning signal, but to base a treatment decision on the number of mines per 40+ leaves shortly after Petal fall.

Tarnished plant bug trap captures at Highmoor Farm on Wednesday, May 7 (when McIntosh buds were about two days before Tight cluster) averaged less than 1 per trap.  The threshold at Tight Cluster for a retail apple crop is 5 per trap.  The wholesale crop threshold at that time is 3 per trap.

 European red mite egg hatch will start soon (estimated dates May 12 in Sanford, May 16 in Monmouth).  Prebloom oil to smother European red mite eggs is most effective when applied at Tight cluster and early Pink, just before the eggs hatch.  While less effective after egg hatch begins, it is still worth doing if oil was not applied previously.  Fruit buds become more sensitive as they develop, so no more than a 1% oil solution should be used when the buds move beyond Tight cluster and approach full Pink stage.

             If oil is applied more than once, each application should be made at a concentration effective on the day of application and without regard to an earlier, or planned follow-up, application.  The “split oil” idea has been discarded, and “double oil” is a more accurate term.  The phytotoxicity of oil with captan when they are applied less than a week apart constrains either the fungicide choice or oil timing.

            Tight cluster to Pink is a good time to check for the various species of caterpillars that like to feed on apple buds.  If you find more than 5 out of 100 buds being used as caterpillar food, then a prebloom insecticide for caterpillar control might be worthwhile.  The species you are most likely to find are yellowish obliquebanded leafroller and greenish green fruitworms.  Green pug moth larvae are more likely to be found in low-spray trees.  As they mature, these larvae develop a dark brown stripe along their back.

          Pink is the time to hang traps for European apple sawflies (EAS).  The traps are useful to indicate where insecticide is needed right at Petal fall because EAS activity was high.  Otherwise, insecticide for plum curculio can be delayed until just before the first warm humid night after Petal fall.  EAS is a major pest in organic orchards.  Organic growers planning on using Surround might consider starting before bloom as Surround residue on the leaves during bloom may reduce EAS egglaying.