Apple Pest Report: Saturday,
July 1,
2006
Vol. 14, No. 12
Scab
There has now been enough time for 2nd generation lesions from all of the scab ascospore infection periods to become visible. Scouting observations indicate that while scab can be found at low levels at some locations, scab suppression overall seems to have been effective.
The wetting period duration required for new fruit scab infections to take hold increases as fruit continue to develop. As the emergence of new foliage declines with the arrival of terminal bud set in the coming weeks, scab will not have tender young leaves to host a new generation of lesions.
If you find more than 5-18 scab-infected leaves per 100 shoots, then a full-dose captan applied with high water volume to insure good coverage with a follow-up application about a week later will prevent spread to more fruit and leaves. If you find 3-5 scab infected leaves per 100 shoots, then maintaining protection with captan at 14-day or two inches rain between applications should be adequate. If you find 1-2 scab infected leaves per 100 shoots, then fungicide protection against flyspeck should be sufficient for scab suppression.
Keep on eye out for scab high in the canopy of taller trees. When checking for scab, note that water sprout leaves seem more likely to be infected than horizontal shoot leaves. Include water sprouts in sampling, but only to the degree that they occur in random shoot selection.
Flyspeck and Sooty blotch
Normally, there is a break between the end of fungicide applications to protect against apple scab and before the beginning of protection against flyspeck. Where scab control has been successful, late June – early July is usually a time when the interval between spray trips can be extended until apple maggot emergence requires an insecticide application.
But this year the persistent cloudy, wet weather has accelerated progression of flyspeck infection risk. There is much left to be discovered about flyspeck, including answering the question of whether there is more than one fungus that causes the symptoms we call flyspeck. The following paragraphs reflect current understanding.
Flyspeck overwinters on the waxy cuticle of brambles, maple trees and many other host plants in the orchard border. But it does not overwinter to a significant degree within the orchard itself. Ascospores from overwintered colonies start releasing around the time of McIntosh Pink until about two weeks after Petal Fall. As with other biological phenomena, this process follows a bell-shaped curve, with most of the spores being released towards the middle of this period. After landing on susceptible host tissue, it takes about 270 wet hours for a new colony to become visible. When it becomes visible (and possibly even before), the colony begins producing asexual (i.e. cloned) conidial spores. After another 270 wet hours, those infections can produce another generation of conidia, with the cycle repeating until harvest.
The fungus originally identified as the cause of flyspeck shows optimum growth in lab tests at 60 – 75 degrees F, and requires very high humidity. Accumulation of “leaf wetness” hours in the field has been found to correlate well with observations of flyspeck symptoms.
There are several reasons why growers do not need to factor flyspeck ascospore infection potential into spray decisions. Scab fungicides applied at the same time are more than adequate to prevent flyspeck. In addition, because flyspeck overwinters primarily in the orchard border, the first round of infections primarily occurs on other border row plants. The susceptible tissue of concern in orchards is fruit surface. Flyspeck does not cause leaf infections. During the time of flyspeck ascospore releases there is little apple fruit surface to protect.
After infections on border row host plants have had time (= 270 wet hours) to produce a new generation of conidia, the amount of inoculum in proximity to the orchard greatly increases. By then, there are fruit in the orchard, and where scab control was successful, fungicide applications to prevent apple scab have ended.
Using Petal Fall as a single reference point for when flyspeck spore are released, we can count the accumulation of wet hours to estimate when the risk of flyspeck infection increases with the appearance of a new generation of conidia-producing colonies. This marks the beginning of the period when protection against flyspeck infection is needed for apples.
At the other end of the season, infections that occur close enough to harvest such that they will not have time for exposure to enough wet hours before harvest to develop visible symptoms do not require prevention. On average, it usually takes about 30 days for 270 wet hours to accumulate. Therefore, if fungicide protection is effective to within about 30 days of harvest date, then there should be little risk of flyspeck appearing before harvest in a normal year.
Summary of the above: To prevent flyspeck appearing on apples, the fruit surface should have effective fungicide residue from 270 wet hours after Petal Fall, until there is little chance of there being 270 or more wet hours before harvest.
That is the basic story; here are adjustments and their management implications.
The number of days it takes to accumulate 270 wet hours varies tremendously between orchards. Large trees with thick canopies, and trees in low spots that hold morning dew, accumulate wet hours much faster than small open trees on a windy hilltop. The wet hour accumulations posted on Orchard Radar run towards the high end, meaning that they reflect conditions in high risk trees, and are “conservative” (overstate the risk) for small open-pruned trees with good air drainage.
We do not have very good understanding of how long fungicide coverage will remain effective at preventing new flyspeck infections. After recent articles by, and discussion with, Drs. Dave Rosenberger (Cornell) and Dan Cooley (UMass), I am adjusting at the flyspeck management guidelines recommended for Maine growers.
The best fungicides for protecting against flyspeck are Topsin M and the strobilurins (Flint, Sovran, Pristine). Application of one of these materials with good coverage can be relied on for protection for up to 21 days or 2 inches of rain, whichever comes first.
Full-dose captan or ziram (i.e. Captan 50WP at 2 lbs. / 100 gals. dilute, or Ziram 76DF at 1.5 lbs./100 gals.), is reliable for up to 14 days or 2 inches rain, whichever comes first.
Captan or Ziram at half rate (i.e. 1 pound captan 50 WP per 100 gallons dilute, or Ziram 76DF at 3/4 lb./100 gal. dilute), is reliable for up to 10 days or 1.5 inches rain, whichever comes first.
Dr. Rosenberger’s earlier field trials indicated that postinfection application of a strobilurin or Topsin M fungicide could control flyspeck infections that were within the first 100 hours of the 270 needed to become visible colonies. This meant that the first fungicide application using one of those fungicides could be delayed until 270+100 = 370 wet hours after Petal Fall. This approach has been recommended without resulting in any known control failures for over five years. However, a more recent test suggested that a postinfection application made at only 67 hours after infections started may not be able to provide adequate control. As a result, I have revised the current guideline for Maine growers to count on no more than 50 hours postinfection activity.
Opinion has gone back and forth about whether postinfection application kills the flyspeck, or just causes it to stop development until fungicide protection wears off, allowing the flyspeck to resume development when fungicide residue and the "fungistatic" effect wears off. If flyspeck infections are only stopped but not killed by postinfection application, then the 270 hours of wetness before harvest would need to be reduced for infections that had already made some progress before being suspended by a postinfection fungicide application. The likelihood for the “stopped but not killed” hypothesis being true currently seems high enough that preventive coverage rather than postinfection application is preferred for flyspeck management. If postinfection control is used, then the number of wet hours in the delay should be deducted from the allowance for wet hours between the end of fungicide protection and harvest.
For apples going into storage, it is prudent to allow for the possibility that apples will be wet at harvest, and will stay wet until fruit temperature in storage becomes too cool for further flyspeck development. This reduces the allowance for wet hours between the end of fungicide protection and harvest to 220 hours, and to less than 220 hours if a postinfection fungicide delay occurred earlier in the summer.
At Highmoor Farm, 270 wet hours after Petal Fall accumulated by June 14. That is an earlier than normal date for the beginning of flyspeck infection risk. But of greater concern is that another round of 270 wet hours will be completed by around July 6. That is about a month earlier than normal. If summer rain frequency between July 6 and harvest is normal, this would mean that flyspeck will have an extra generation cycle during the 2006 growing season. We do not know enough to say for sure, but the wet fall in 2005 could also mean that the overwintered flyspeck inoculum level was higher than normal this year.
Put it all together: a wet fall in 2005; a wet spring and early summer in 2006; recent data suggesting that fungicides do not work quite as well against flyspeck as previously thought. This is a year to be cautions with flyspeck. Take care of flyspeck and you will not have problems with sooty blotch.
Coverage is important for summer disease control. Summer pruning can help a great deal in reducing flyspeck in big thick trees. Slow tractor speed and high water volume are recommended for fungicide applications. Consider adding a spreader to the tankmix.
Insects and Mites
In dry years, apple maggot emergence can be delayed by dry soil conditions. I do not think that applies to this year. The first apple maggot flies could appear in the coming week at Highmoor Farm, where unbaited red sticky ball traps were set on June 30.
Roundheaded apple tree borer females are beginning to lay eggs. The eggs hatch into larvae that tunnel into the trunk and kill small diameter apples trees where the borers have not been prevented or controlled. There is little need for concern where insecticide was applied for plum curculio or where carbaryl was used as a thinner. This pest does not even make it onto the radar screen for commercial orchardists, yet it is the number one cause of death of backyard trees.
Young trees that have not received insecticide for other reasons should be given trunk applications of insecticide every 2-3 weeks in July and August. Because it binds with the bark for prolonged protection, a single Lorsban trunk application is good for the remainder of the year.
Codling moth does not require separate control where insecticide applications were used for plum curculio. Organic growers who have had codling moth problems in the past should have B.t. protection in place to control first generation codling moth larvae.
European red mite levels remain low in blocks where oil was applied prebloom. Second generation mites will begin maturing to lay third generation eggs starting around July 5 at Highmoor Farm. By around July 14, nymphs from those eggs will be maturing to begin laying fourth generation eggs. Thus, July 5 to July 14 is a good time to scout mites. If control is needed, treating before a new batch of eggs is deposited increases the chances for success.
Leafhoppers have appeared in several locations. It is difficult to assign a treatment threshold for white apple leafhopper, but if you have had problems with picker nuisance in previous years, then control may be needed if leafhoppers are easily found now. Potato leafhopper feeding on the growing points of shoots can cause serious damage on young trees and should be controlled if there is more than 1 potato leafhopper nymph per leaf.
Other items
The Maine State Pomological Society Meeting and Summer Tour will be held July 18 at Highmoor Farm in Monmouth.
Directions:
From the North:
Take Exit 109b off I-95 in Augusta and travel west on Route 202 for
about 16 miles. Highmoor is on your left.
From the South: Take Route 202 from Lewiston for about 16 miles, Highmoor at the top of a hill on your right.
Sincerely,
Glen