First phase almost over

     The rain on Sunday, June 1 caused an important primary scab infection period in Maine orchards.  This was the last major infection period of the year.  But prudence suggest that even Sanford area orchards treat the next infection period as requiring full protection.  As of Sunday afternoon, the next rain is forecast to arrive Thursday night, June 6.

     A commonly used guideline is to consider the first rain after ascospore maturity is expected to have reached 95% as the estimated date of the final significant primary scab infection period.  But there is still considerable margin of error in that estimate.  In 1 year out of 20 the true maturity on that date could be less than 80%, with 20% of the season's ascospores yet to mature.  Because I can't afford to be that wrong even one year out of 20, I use more conservative guideline.  The date I use to say primary scab season is over is when there is a warm soaking rain after there is at least a 95% chance that at least 95% of the ascospores have matured. 

     Daytime rain on Friday June 7 would surpass that criterion for the Sanford area, and be just a day or two short in the Monmouth and Morrill areas.  Applying the criterion strictly, the last significant primary scab infection period in Monmouth and Morrill will be the first warm (= over 50 F) soaking (= at least 0.1 inch during daylight hours) rain on or after Sunday June 8.

     Sanford is at 100% McIntosh petal fall.  Monmouth should be at 95% McIntosh petal fall on Monday, with Morrill a day or two behind.  With the heavy rain just before petal fall and scab not quite over, every orchard of scab-susceptible trees needs renewed protection this week.

     If the previous application was made on or before Monday May 26, then you should consider using a post-infection fungicide as soon as possible to stop any infections that might have started on unprotected tissue.  Tuesday morning, June 3 looks like the first opportunity for quiet enough wind to make that application.

     A full dose captan, mancozeb, or Polyram application made on or after Tuesday morning May 27 should have given adequate protection through the Sunday June 1 rain.  But that protection was largely depleted by Sunday's rain.  Because the next rain still poses some scab infection risk, even an orchard sprayed on Saturday May 31 should have protection renewed prior to the next infection period forecast to occur on Thursday - Friday, June 5 - 6.

Next phase just starting

     Unsprayed trees in a high scab block in the Monmouth area showed 17 infected leaves on 100 shoots checked on Friday, May 30.  Now in a commercial orchard that would be enough to call for back to back fungicide applications for lesion suppression.  But in an unsprayed block of highly susceptible trees, it is very low for this time of year.  Only infection periods that occurred before May 16 had time to develop visible lesions by May 30.  This year, those infection periods did not amount to much infection pressure.  The big action started on May 21.  Those infections will have had time to develop into visible lesions starting on Tuesday and Wednesday of this week.

     That makes this week an important time to check apple trees to see if scab control has been effective.  If it has, great, but keep checking as first generation lesions from later infections will have completed their development time and if present, will appear over the next two to three weeks.

     If scab control has not been effective, all is not lost if you become aware of the situation early and react before infections have a chance to spread to fruit.  First generation infections are primarily on foliage because fruit tissue is not exposed at the time those infections occurred.  While not the preferred situation, it is possible to prevent fruit scab even after foliar infections have become established.

 Captan Caution

      The following article is by Dr. David Rosenberger of the Cornell University, Hudson Valley Lab, reprinted from the May 27, 2003 edition of the Scaffolds newsletter. 

"Fruit growers should be very cautious about using captan during the next 7—10 days because weather conditions over much of the state have left apples, peaches, plums, and cherries unusually susceptible to captan injury. Captan is an effective, broad-spectrum fungicide that is labeled for many fruit crops. However, when absorbed into plant tissue, captan causes phytotoxicity that appears as leaf spotting, shot-holing, and leaf yellowing. When combined with other products that enhance uptake into leaves, captan applied at this time of year can cause complete defoliation of peach and nectarine trees. To be safe, growers should avoid applying captan until trees have had several days of sunny, dry weather.

The risk of captan injury is greatest when the annual spring growth flush of fruit trees coincides with an extended period of cloudy, cool, damp weather. The growth flush on fruit trees begins when terminal shoots start growing during or shortly after bloom. The cuticle (the waxy layer on the leaf and fruit surfaces) develops in response to heat and water stress. During cloudy and damp weather, there is little danger from heat or water loss and trees therefore produce only a thin cuticle to protect the newly formed leaves and enlarging fruitlets.

The same waxy cuticle that serves to prevent water loss also prevents captan from entering and injuring living cells beneath the plant cuticle. Some varieties of plums and cherries almost always develop a leaf spot or shot-hole after captan is applied because, even under the best conditions, some captan enters and kills leaf cells of these varieties. For most other fruit crops, captan causes little or no injury except during unusual seasons when weather conditions inhibit cuticle development.

Even when plant tissue has only a thin cuticle, captan by itself will rarely cause phytotoxicity (except to those plum and cherry varieties that are especially susceptible to captan injury). Problems often arise, however, when captan is mixed with other agrichemical products. Spray adjuvants that enhance the transport of captan through the plant cuticle can greatly increase the phytotoxicity of captan, especially when the plant cuticle is thin at the time spray is applied. Adjuvants that enhance uptake of captan include spray oils, some spreader-stickers, and other petroleum-based carriers commonly found in products that are formulated as liquids or emulsifiable concentrates.

Where apple scab symptoms are appearing in orchards, the best option for stopping further spread of apple scab is to apply a combination of an SI fungicide along with the maximum label rate of captan. To avoid phytotoxicity problems, however, growers may need to use an SI-plus-mancozeb combination for the next week to avoid the potential phytotoxicity that could result if captan is applied at this time. This is especially true if Sevin XLR Plus will be applied for thinning or if spray oil will be applied with a miticide during the next week. Those who opt to apply captan despite the risks are advised not to use spray adjuvants that might enhance trans-cuticular movement of captan."

 Calyx end rot

Consider Topsin M for a petal fall fungicide if there is block history of calyx end rot.  This disease tends to increase with cool wet weather around bloom and petal fall.  Captan also controls one of the two fungi implicated as causes of calyx end rot.