Apple Pest Report: Friday, June 16, 2006
Vol. 14, No. 11
Scab
The end of primary scab does not mean the end of scab management
Early indications are that growers were able to achieve effective scab control in May. However the jury is still out. The final primary ascospore infection period has accumulated enough growth time to appear as 1st generation lesions on leaves only within the last couple of days. Confirmation that scab control was successful requires monitoring until all primary scab infection periods have had time to appear as 1st and then as 2nd generation lesions.
As of June 15 at Highmoor Farm, 100% of primary scab infection periods have had time to appear as 1st generation lesions. So the next few days of warm and sunny weather will be a great time to scout the orchard. About 80% of this years primary scab infection potential has had time to appear as 2nd generation lesions. It won't be until around Saturday, June 24 that potential lesions from the final ascospore infection period will have had time to appear as 2nd generation lesions.
Because the final infection period was a severe one, keeping an eye out for scab until the 100% 2nd generation lesion date has passed is especially important this year. The unusually large amount of rain in early June adds another level of uncertainty which only scouting can resolve.
The following article was written by Dr. Dave Rosenberger of Cornell University's Hudson Valley Lab back in 2003, but it all seems applicable today. It only applies to you if you are finding more than 5-18 scabby leaves per 100 shoot and fruit clusters.
"What is the best approach for keeping apple scab off of fruit in orchards with a moderate level of scab on terminal leaves? Unfortunately, there is no simple answer to this question. Below are four options to consider.
Option 1:
Make at least two applications of captan alone at the maximum label rate per acre. Applications at this time of year can be 10-14 days apart unless rainfall (>1.5 inches) removes captan residues before 10 days have elapsed. Captan is very effective for protecting fruit, especially when combined with high temperatures of 80-85F. However, if cool wet weather persists into July, then continued applications (more than two sprays) using high rates of captan may be essential. If weather becomes more normal (hotter and drier), then the risk of fruit infection will subside until September when scab might become active again.
Option 2:
Apply captan at maximum label rates as noted above, but
tank-mix the high rate of captan with an SI fungicide (Nova,
Rubigan, or Procure). The full rate of captan is needed because the
SIs will shut down scab on leaves but will do little to protect the
fruit. This option is considerably more expensive than Option 1.
It will prove more effective than Option 1 IF orchards do not
contain SI-resistant populations of scab and IF weather stays
cool and wet for another month. Option 2 also provides extended
control of mildew. However, if weather become hotter (days >
80-85F) and drier, then Option 1 will probably work just as well as
the more expensive Option 2 for controlling scab.
Option 3: Apply captan at 50% of maximum label rates in a
tank-mix with Flint or Sovran. Flint and Sovran will provide better
protection of fruit than the SI fungicides so a half-rate of captan
should be adequate. The fact that Sovran and Flint bind to
cuticular waxes should make them more resistant to wash-off than is
captan. However, Sovran and Flint will be less effective than the
SI fungicides (in the absence of resistance) for arresting
incubating but still invisible infections in leaves. Thus, the
trade-off here is potentially better control of fruit scab with
Sovran or Flint compared with greater reduction of total inoculum
when SIs are applied to leaves with incubating lesions.
Option 4:
Apply captan at 50% of maximum label rate in combination with a
full rate of dodine (Syllit). This is an extremely risky approach
because we cannot accurately predict which orchards have dodine
resistance. However, if you have not used ANY dodine, not even in
the first spray of the season, for at least 10 years, then this
approach might be very effective. Only one spray of dodine should
be applied and the follow-up spray should be the full rate of captan
because a single spray of dodine may be enough to re-select for
dodine-resistant strains that survive at low levels in most
orchards.
The specifics of the orchards involved should be considered
when choosing among the options noted above. There is significantly
more risk of getting scab on fruit of susceptible cultivars such as
McIntosh and Ginger Gold and than on more scab-resistant cultivars
such as Empire and Delicious. Therefore, captan alone might suffice
for the more resistant cultivars, whereas an SI-plus-captan might be
warranted for McIntosh blocks. Similarly, vigorous young trees that
are still actively growing have the potential for several more
cycles of leaf scab, whereas older orchards with a heavy crop will
soon set terminal buds. The SI-captan combination is more likely to
pay for itself in orchards with vigorous terminal growth because the
SIs can quickly reduce the amount of inoculum available for
infecting new leaves (unless, of course, the scab is SI-resistant)."
*********************************
One change since that article was written is increased concern and evidence for scab strains resistant to sterol inhibitor fungicides in New York and elsewhere. The status of SI resistance in Maine is not documented. My guess is that the probability of SI resistant scab is low for orchards where SIs have been used at full rates and/or in combination with a protectant fungicide, within the 96 hour post-infection time limit, and where SIs were not used more than twice in succession before rotating to another type of fungicide. But only testing can measure the degree of shift towards resistance for a local scab population.
Enough with the precautions, I don't have scab, so what's next?
Many orchards received renewed fungicide coverage on Monday-Wednesday June 12-14. Unless the forecast changes and more heavy rains arrive, those applications will provide solid protection against secondary scab until June 24-26. If you use less cautious assumptions about duration of effective residue, those applications can be considered as providing adequate protection against low level secondary scab until the end of June. By the time the protection ends, it will be late enough to check the orchard again to make sure that 2nd generation lesions from the year's final scab ascospore release on June 2 have not appeared.
If the orchard is scab-free at that point, then it is reasonable to rely on the June 12-14 applications beyond the estimated depletion date for effective scab protection. You may not need to renew fungicide until mid-July when apple maggot risk creates need for another spray trip to renew insecticide coverage.
If you are wondering why you need any protection at all if the orchard is scab-free, the answer is that there is probably always some scab out there, especially on larger trees. So if pays to maintain some degree of protection even if you are seeing scab lesions.
And of course, scab is not the only fungus pest in the orchard.
Flyspeck and Sooty blotch
We have less precise tools to estimate the timing and severity of flyspeck infection risk than we have for scab. But there is substantial confidence now in the idea that the risk of flyspeck infection in apple orchards does not become significant until flyspeck infections on border row host plants have had time to begin producing conidia (secondary flyspeck spores). That is estimated to occur when about 270 leaf wetness hours have accumulated since Petal Fall.
Because of the consistent rain and cloudy weather in early June, the estimated June 19 beginning of flyspeck infection risk at Highmoor Farm is earlier than normal.
The recommended fungicide intervals for protection against flyspeck are longer than the intervals recommended for protecting against primary scab. And the recommended interval depends on which fungicide is used. For the best flyspeck fungicides (Flint, Sovran, Pristine, mancozeb), the guideline is that a full dose, full orchard application is good for 21 days or 2.5 inches of rain, whichever comes first. For the next-best options, (Topsin M + half rate captan, captan alone at full dose, or full dose ziram), the guideline is 21 days or 2.0 inches of rain, whichever comes first.
Depending on the weather, a June 13 full dose captan application will likely remain effective at preventing flyspeck infection until about June 28. But fungicide protection against flyspeck does not necessarily need to be renewed as soon as the previous coverage wears off. Fungicides seem to be able to stop early flyspeck infections from ever becoming visible if applied within the first 100 leaf wetness hours after the infection started. So if coverage lasts until June 28, then renewed fungicide coverage could be delayed until as late as roughly July 12. Of course that estimate is completely dependent on the amount of rain and hours of leaf wetness accumulated.
The flyspeck respray intervals for Highmoor Farm are online at http://pronewengland.org/AllModels/MEmodel/RADARME-Monmouth.htm#FLYSPECK. They show fungicide depletion date estimates corrected for observed weather and the latest 10-day weather forecast.
If you control flyspeck, then you have most likely done enough to keep sooty blotch and low level secondary scab in check, as well as black rot and bitter rot.
Black rot and Bitter rot - the other summer fungal diseases
The other two summer diseases of some consequence in Maine orchards are black rot and bitter rot. Unless there is either unusually hot, humid weather or an unusually high amount of inoculum (such as dead trees left in the orchard).
Black rot fruit infections are usually found on older trees where a branch canker is dropping spores onto underlying fruit. While it manifests as leaf and fruit infections, black rot is probably more important for the branch cankers it can cause. Black rot infection is facilitated by winter damage. Removing dead and dying wood will help keep black rot and opportunistic wood rot fungi from taking advantage of a free food supply in the orchard.
Bitter rot is rarely a problem in Maine, presumably because it likes hotter temperatures and more prolonged high humidity than Maine weather usually provides. But it has occasionally been found to cause fruit infections in Maine orchards, so there is some degree of risk. It too invades dead wood, and sanitation to remove dead branches and trees is also key to managing this disease.
Fire blight
If there were any fire blight blossom blight infections young trees with late blooms still on the tree by June 1, then wilting and discolored tissue on the fruit clusters will become apparent by June 18. Shoot blight infections could have started on tender new growth on that day, but that risk would only have occurred in orchards with active fire blight cankers from last year.
I hope this is the last mention of fire blight for this year's newsletter, so I'll end with a few tidbits:
* Honeycrisp is much more susceptible to fire blight than McIntosh.
* Trees 3-6 years of age are at peak risk.
* Trees on M26 rootstock have elevated risk of tree death from a fire blight infection.
*** So if you have 3-6 year old Honeycrisp on M26 rootstock, then you should be on the lookout for fire blight blossom and shoot blight symptoms even in an apparently low risk year.
For an excellent review of how shoot blight spreads in orchards, see an article in the recent Scaffolds newsletter by Dr. Dave Rosenberger. It is at http://www.nysaes.cornell.edu/ent/scaffolds/2006/060612.html#disease
Insects and Mites
If insecticide was renewed after June 11, that application should last well beyond the end point for the plum curculio protection period. The date at Highmoor for the end of this period is June 18.
If insecticide was not renewed as a perimeter or full block spray after the June 7-11 rain, then the end of insecticide protection around June 11 left almost half of the plum curculio protection period uncovered. For blocks with even a moderate degree of plum curculio pressure that could lead to extensive damage.
Orchards without insecticide protection since wash-off on June 7-11 could also be susceptible to roundheaded apple borer attack. The risk is highest on young trees with less than 4 inches trunk diameter. Trees near unmanaged infested host trees in adjacent brush or woods are more likely to be attacked. Normally, insecticide coverage against plum curculio kills the adult female borers before they can lay eggs.
At Highmoor Farm and elsewhere in central Maine, European red mites (ERM) are in their annual trough between 1st generation adult die-off and 2nd generation egg hatch. That lull will end with the accelerated hatching of eggs starting around Saturday June 17. Egg hatch has apparently already begun in southern Maine.
ERM were over threshold at petal fall in Highmoor blocks that did not receive oil this spring. Observations suggest that the easy winter was kind to overwintering mites. I thought that the heavy rains in the past month would have knocked mites back down to negligible levels, but that may not be the case. The week of June 17-24 is a critical time to asses the mite population and to respond with treatment if needed.
If over threshold, a mite population becomes more difficult to control as more eggs are laid for the following generation. The current best guess is that by June 24 at Highmoor the earliest hatching 2nd generation mites will begin reaching maturity to begin laying 3rd generation eggs.
The mite treatment threshold from June 15 to July 15 is if more than 59% of middle-aged fruit cluster leaves have living, hatched mites present, or if there is more than an average of 2.5 mites per leaf. If you check 40 leaves and find 15 or fewer leaves with mites, then it is safe to assume the mites are below threshold. If you find 30 or more infested leaves out of 40, then treatment is recommended. I've updated the Apple Pest Scouting Pocket Card and put it online at http://pronewengland.org/INFO/PROpubs/AppleScoutCard-2006.pdf
Having the chart with you when you scout makes checking for mites quick and easy. Here are instructions:
Mite Monitoring:
Dr.
Jan Nyrop of Cornell University devised the Presence/Absence sequential sampling
scheme for efficient determination if pest mites on apple foliage require
suppression. I have adjusted the dates for use in Maine.
Procedure:
Start with a randomly selected tree and choose subsequent trees to get leaves
from throughout the block. Collect 4 leaves per tree, with leaves from each
quadrant (north, east, south, west) of the canopy. Choose only middle-aged leaves from the middle of fruit
clusters or shoots. Do not collect the youngest or oldest leaves as they can
give a biased count. Collect 40 leaves from 10 trees (a baggie is handy).
Count to confirm you have 40 leaves checking them. You will be
surprised at how many times you end up with some number other than 40.
Examine the top and bottom of each leaf with a magnifying lens. A pair of inexpensive 3.5X reading glasses sold at "dollar stores" provide adequate magnification. 3.5X is at the high end for reading glasses, most are 1.75 to 2.5, so make sure you pick a 3.5X pair.
Count as infested only those leaves that have one or more living, hatched European red mites or twospotted spider mites. Mite eggs do not count toward the threshold. If a mite is sitting still, you can nudge ith with your finger nail to see if it moves. If it doesn't, you can consider it dead.
Mite Threshold Chart for June 16 to July 15
| Number of leaves examined | <--- Number of leaves with mites present ---> | ||
| column 1 | column 2 | column 3 | |
| Far below threshold | Below threshold | Above threshold | |
| 40 | 8 | 15 | 30 |
| 60 | 15 | 28 | 42 |
| 80 | 23 | 40 | 54 |
| 100 | 36 | 58 | 59 |
Interpretation:
< column 1
–
If the number of infested leaves is equal or less than the number in column
1, the mites are far below threshold and resampling can wait for 11‑16 days
(depending on temperatures). Optimized resampling intervals for Highmoor
Farm are online at
http://pronewengland.org/AllModels/MEmodel/me-Monmouth-ERMScoutJune.htm
between columns 1 and 2 – If the number of infested leaves is greater than column 1, but equal or less than the value in column 2, the mites are below threshold now, but need to be resampled in 6‑10 days (depending on temperatures).
between columns 2 and 3 – If the number of infested leaves is greater than column 2 and less than column 3, more leaves must be examined to decide if the mites are above or below threshold. A decision is always reached if sample size reaches 100 leaves.
> column 3 – If the number of infested leaves is equal to or greater than the value in column 3, mites are above threshold for this time of year and prompt treatment is recommended.
White
apple leafhopper
(WAL) were
reported as abundant in one orchard this week. WAL may be
more of a pest to humans than to trees. A threshold of at least 3
per leaf is suggested to prevent tree stress from WAL feeding. But if you have had from picker nuisance caused by
numerous leafhopper adults in September, then a much lower
threshold of 25 WAL nymphs per 100 leaves may be more
appropriate.
WAL are not known to spread diseases.
Carbaryl (Sevin) used as a thinner provides control of this insect. Provado at low rates is also very effective. Actara, Assail, Avaunt, Calypso and Thiodan are other options, as well as pyrethroids. Organic options are azadirachtin (Aza-Direct, Neemix) and pyrethrin (Pyganic). As we are now more than two weeks past petal fall, Agri-Mek, which is yet another alternative, is not a good choice because leaves that have begun toughening up may not absorb enough Agri-Mek for prolonged activity.
EPA notice
Based on a reevaluation of the risks and benefits of phosmet (Imidan), the EPA has proposed the following changes to restricted entry intervals after Imidan applications.
|
Crop |
Current REI |
Proposed REI |
|
Apples |
3 days |
7 days |
|
Apricots |
3 days |
14 days |
|
Blueberries |
24 hours |
3 days |
|
Grapes |
14 days |
14 days |
|
peaches/nectarines |
3 days |
7 days |
|
Pears |
3 days |
7 days |
|
plums/prunes |
3 days |
7 days |
In addition, at the end of the background document explaining the reasoning behind the proposed longer REIs, a single line without any attached rationale really caught my eye. Here it is:
"- require label amendments to eliminate phosmet use in pick-your-own operations"
It may be that EPA has not fully considered the impacts of such a change on small scale retail growers. The REI decision gives 8 pages of explanation for the background toxicology and operational issues that were considered. There was no background explanation for the ban on use in Pick you own orchards.
The stakeholder input EPA received may not have adequately represent the interests of retail growers. 'Pick your own' marketing is a major component of the New England apple industry. A coordinated response by New England growers to the EPA proposal could result in a changed decision. The reason for the EPA announcement was to solicit such input. This looks to be a perfect opportunity to exercise that process. The deadline for public comments is August 8.
EPA also announced a proposal to cancel registration of azinphosmethyl (Guthion) on apples in 2010.
Sincerely,
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