Apple Pest
Report: Monday, April 23, 2007
Vol. 15 No. 1
Greetings,
Welcome to another season for the Apple Pest Report. I’ll try to be brief in future issues, but there’s some catching up to do in the first couple of issues. McIntosh trees were reaching Green Tip at Highmoor Farm on Monday, April 23. Rogers Macs in Sanford reached Quarter Inch Green. With high temperatures in the 60’s forecast for Tuesday and Thursday, buds development will progress quickly this week before temperatures cool with rain forecast for Friday night – Saturday morning. The forecast suggests that leaves will stay wet long enough on Saturday to generate the year’s first apple scab infection period.
Scab
There have been three very interesting articles on apple
disease management in the Cornell Scaffolds Newsletter.
1. “Organic Disease Control for Apples” by Dr. David Rosenberger discusses the copper and sulfur phytotoxicity issues, sulfur’s limited retention and yield suppression effects, and other guidelines for using these organically certifiable compounds for control of apple scab and summer fruit rots. The article recommends that organic growers have liquid lime sulfur on hand as an eradicant in case scab infections begin to appear on leaves. The article also recommends alternating applications of 1% liquid lime sulfur and low rate copper fungicide from mid-July into September to suppress summer fruit rot pathogens. The full article is online at http://www.nysaes.cornell.edu/ent/scaffolds/2007/070319.html#disease
2. “Apple Scab and Post-infection Fungicides (part 1)” Dr. Wolfram Koeller discusses the lack of scab resistance to protective fungicides, the need for post-infection (i.e. ‘kickback’) fungicides when scab protection is inadequate due to heavy rains and inability to reapply protection, and the uncertainty around ascospore levels at Green Tip. The full article is online at http://www.nysaes.cornell.edu/ent/scaffolds/2007/070402.html#disease
3. “Early Season Copper Sprays” by Dr. David Rosenberger discusses the use of copper to suppress build up of fire blight bacteria oozing from their overwintering sites in trunk cankers on apple trees, copper for various stone fruit diseases, a potential advantage for finely ground “safe” coppers that may provide longer residual bacterial suppression, and a caution against the use of Aliette or any of the phosphite fungicides or plant nutrients (ProPhyt, Phostrol, Nutriphyte) for at least several weeks after a copper spray. The full article is online at http://www.nysaes.cornell.edu/ent/scaffolds/2007/070409.html#disease
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When to begin scab protection
This is a question that apple growers face each spring. In New York, heightened concern about strains of apple scab that are resistant to the post-infection fungicides has led to greater emphasis on an early start for a protective strategy starting at Green Tip. Unfortunately, that concern now applies to New England orchards also. The following excerpt is from a recent article by Dr. Dan Cooley in the UMass Healthy Fruit Newsletter:
“Of 14 orchards tested in New England over the past two years, 12 have scab that is resistant to the SI fungicides. The other two have scab that is partially resistant. No orchards could rely on SIs to be as effective as they were several years ago. In past years, if a few scab spores made it to apple leaves and started an infection, an application of Nova, Rubigan or Procure stopped it. Even if the first application went on over a week after the start of the infection, two consecutive SI applications stopped it. Not any more.
In most New England orchards, if SI fungicides are used, and they are mixed with a protectant fungicide (Captan, Dithane, etc.) then it is the protectant that’s doing the heavy work. There’s little or no contribution from the SI. At the same time, scab was resistant or moving towards resistance to dodine (Syllit) in all 14 orchards. No option for burning out scab there.
The strobilurines (Flint, Sovran) and the anilinopyrimidines (Vangard, Scala) were also tested. A shift towards resistance to strobilurines had started in four orchards (about 30%), and the rest still had sensitive scab. Surprisingly, a resistance shift to the AP fungicides had started in 7 (50%) of the orchards. And while the strobilurines and APs can suppress scab infections after they start, they do not have the long-term post-infection effectiveness that the SIs had.
So, resistance to fungicides means that if a few scab infections get started early, they can explode in a wet season. So, it may be time to forget about the SI’s in our orchards, and develop strategies that won’t require eradication of early-season mistakes.”
The tested orchards were not in Maine, and fungicide resistance is an orchard-by-orchard phenomenon depending on previous fungicide use patterns in each particular location.
So what does this mean for Maine apple growers this spring? Does it require using a protectant program beginning at Green Tip?
Well, let’s start with things that do seem clear. It is more important than ever to make use of cultural methods to reduce scab inoculum. Fewer ascospores arising from last year’s leaves means less chance of scab infection, less reliance on fungicide to prevent infections, and fewer opportunities for resistance to develop. Shredding leaves with a flail mower is best; chopping with a rotary mower will also help. Applying urea to expedite leaf breakdown also helps. Either or both can be done in the fall if leaf drop occurs before snow cover, and can be done now in early spring before scab infection potential is expressed. Good pruning to keep canopies open to sun and wind will hasten drying and suppress growth of all the fungal diseases.
Another thing that seems clear is that emphasis should be placed on a protective program. The post-infection fungicides are good to have to bail out of situations where scab protection fails, but their use should be minimized to preserve their post-infection efficacy for as long as possible. The rule of thumb for a significant shift to sterol inhibitor fungicide (Nova, Procure, Rubigan, and starting this year, Indar) resistance in the local scab population is a total of about 60 SI applications over the years. It seems to be a one way street, so even applications made when SI fungicides were first available count towards that threshold number. All of the SIs count toward the total, so switching between Nova and Rubigan doesn’t buy you any more time. In similar fashion, the less frequently you rely on strobilurin fungicides (Flint, Sovran, Pristine) the longer they will remain effective in your orchard.
Copper is an effective protectant fungicide that provides extra utility in suppressing growth of fire blight bacteria inoculum. Fire blight has been a nagging problem in a few orchards each of the past few years even without extremely high risk conditions during bloom to initiate blossom blight infections. Add to that its value as a nutrient, and its compatibility with oil, and copper looks like a good choice for a first fungicide application before Quarter inch green, or at the latest, Half inch green, where there is concern about fire blight. That includes orchards with any recent history of fire blight, or with trees planted on M26 or M9 rootstock, or with highly susceptible cultivars like Fuji, Gala, GingerGold, Honeycrisp, and PaulaRed; have good reason to start with an early copper application. While not as susceptible as the other cultivars listed, their habit of throwing late and secondary blossoms may make Cortland and Golden Delicious worth considering for the “fire blight concern” list. But due to phytotoxicity concern, copper should not be applied past Half-inch Green.
It is also clear that if scab control was inadequate last year, then you need to start scab protection before the first scab infection period that occurs after green tissue is exposed, i.e. after Green Tip. If there was noticeable scab on the apples last fall, you can bet that there is enough inoculum around to justify beginning protection as early as possible.
It is less clear whether orchards where scab control was excellent last year need to begin protection before the first possible infection period. Relative to the peak infection periods between Pink and Petal fall, scab pressure is much lower in the early infection periods before tight cluster, and especially those before Half Inch Green. So for low inoculum orchards, and where there is reason to believe there are still one or more effective post-infection options (i.e. SI, strobilurin, or Syllit) available if scab lesions do appear, the move to protective scab program may not necessarily require protection for the very earliest infection periods before Half-Inch Green. The only way to accurately check for low inoculum status is to have done a scab index the previous September and found fewer than 5 scabby leaves on 100 shoots. If you don’t have that information, relying on a tractor-seat estimate is risky.
By the time you get through the list of conditions arguing against skipping the first apple scab infection period(s) before beginning protection, chances are your orchard has been disqualified for one reason or another, and the recommendation is to begin protection at Green Tip. No matter how you cut it, waiting until scab pressure really takes off at Tight Cluster no longer seems to be a prudent option even for the cleanest of orchards. That approach puts too much reliance on post-infection fungicides whose use should be conserved.
Insects and Mites
It's not too early to apply oil to smother overwintered European red mite eggs. Use a 2% rate (= 2 gallons of oil for every 100 gallons water) for oil applications up to half inch green, then decrease the amount of oil to 1.5% for applications up to tight cluster, and use only 1% from tight cluster to early pink. Even a single oil application can prevent ERM problems for the rest of the season where pressure is light. Making two oil applications increases the odds for preventing the need for a summer miticide rescue treatment. But that can be problematic if you are using captan as your fungicide. Captan and oil should not be applied within 7-10 days of each other. A 7-day separation should be adequate unless there is an absence of rain in the interim. There are early season protectant fungicide options that are compatible with oil (e.g. Dithane, Manzate, Penncozeb, Manex, Polyram, Scala, Vangard).
For pears, one oil application at 2 gallons oil per 100 gallons water diluter, or two sprays at a 1 gallon oil rate will help repel female pear psylla from laying eggs. This also makes later insecticide applied against psylla later more effective as more of the population will be in the vulnerable nymph stage at one time.
The value of red trunk traps for leafminer detection is in the eye of the beholder. I find them slightly useful as an early indicator of where leafminer populations are high. But for whatever reason, (windy, cool or rainy weather during bloom being the prime suspect), high leafminer populations before bloom do not consistently leaf to enough leafmines after bloom to worry about. I find waiting to check for mines about a week after Petal Fall to be more efficient use of scouting effort and spray material. The downside to waiting is that if you wait too long, then control will becomes less effective starting at about 2 weeks after Petal Fall. Also, the list of available materials with excellent control rating against sap-feeding mines after Petal Fall is limited to Agri-Mek, Proclaim, Assail, Calypso, Provado, SpinTor, and Lannate. However, other materials applied at or shortly after Petal Fall such as Actara, Avaunt, or one of the pyrethroids (Danitol, Proaxis, Warrior) or also likely to provide adequate control.
White traps for Tarnished plant bug are more useful. Usually control is not needed, but in some years with warm dry springs, there can be enough TPB to justify adding a Prebloom insecticide. Without traps, it is hard to detect that situation.
New England Apple Pest Management Guide
The good news is that there will be an update New England Apple Pest Management Guide this year. The bad news is that printing has been delayed. The guide will be sold through Highmoor. Delivery is expected in about 2 weeks. I will let you know when we have reliable information on availability.
Scouting Co-op and Orchard Radar
Thanks to the Maine State Pomological Society, the apple scouting service is a go for 2007. The scout will visit each participating orchard once every two weeks. In some cases where it fits the driving schedule, the scout will visit every week. The scout provides independent observations of scab, mites and other pests in orchards of 1 acre or more from late May – early June until mid-August.
If you would like your orchard to be included in the scout co-op, please send an email to me at gkoehler@umext.maine.edu.
The person previously lined up for the job had a better offer for an internship in his field of study. If you know of a person at least 18 years old with a Maine driver’s license, a clean driving record, and who lives within commuting distance of Highmoor Farm, encourage them to pick up an application at Highmoor Farm, or to give me a call at 207-581-3882.
New publication
The Tree Fruit Field Guide to Insect, Mite, and Disease Pests and Natural Enemies of Eastern North America is an excellent reference tool for any apply grower. In addition to color photos of pests and damage, it has diagnostic keys to help identify problems and information on natural enemies. Dr. Renae Moran has a dwindling supply of copies for $22 at Highmoor Farm.
Other Stuff – Two recent pesticide queries
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Now that Indar is registered for use on apples, any guidance?
Other than that it’s great for brown rot on stone fruit, I haven’t paid much attention to Indar. Now that it has a supplemental label for use on apples, what I’ve learned from reliable sources is that it is better against fruit scab than the other sterol inhibitor fungicides, and is probably less likely to push the scab SI-resistance curve along, or at least may have a somewhat higher “magic number” than 60 uses.
The supplemental apple label simply gives a per acre rate of 2.67 ounces, leaving the user to assume that the stated dose is for the usual (and largely mythical these days) 400-gallon big tree orchard, which is usually the case when a single rate per acre is given. On a per 100 gallon dilute spray basis, the rate would be 0.67 to 1.0 ounces of Indar 76WSP per 100 gallons dilute, depending on whether you assume the acre referred to on the label is 400 gallon or 250 gallon tree row volume sized trees. (On peaches, the use rate for Indar is 0.8 ounces per 100 gallons diluter spray mix). Given that it is a sterol inhibitor fungicide, I would error on the side of caution and use it at 0.8 to 1.0 ounces of Indar 76 WSP per 100 gallons dilute spray mix. At that rate it is no more expensive per dose than Nova at its mid to high dose range.
Like the other SIs, it is also effective against powdery mildew and cedar apple rust. Unlike the other SIs, Indar also includes flyspeck and sooty blotch on the supplemental apple label. It is also much more effective than Nova against brown rot blossom blight and fruit rot. (Rubigan and Procure have little or no efficacy against brown rot).
As an SI, the best use for Indar on apples seems to be as a material held in reserve for when post-infection scab control is needed. But where you need to spray peaches or other stone fruit for brown rot control and want the convenience of one tank mix, Indar has the right efficacy spectrum for both. In that case, it should be mixed with a scab protectant fungicide to reduce the induction of resistant scab strains.
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Potassium bicarbonate as a fungicide
An organic apple grower was interested in potassium carbonate as a low-toxicity fungicide suitable for organic certification. Here is my reply about potassium bicarbonate’s potential as an alternative fungicide for apples.
A fungicide has to meet several conditions in order to be useful for organic apple scab control:
1. Be registered in Maine
2. Be registered for use on apples
(Having a particular disease on the label is not required, but the crop must appear on the label).
3. If you are interested in organic certification, the material must appear on one of the approved materials lists. OMRI and the Washington State lists are the two publicly available lists.
4. The material should be effective for the use intended.
There are four potassium bicarbonate products registered in Maine for 2007. Three of these products (“Ecomate Armicarb 0”, “Kaligreen”, “Milstop”) are labeled for use on apples, and are also allowable for organic certification. The fourth one, Greencure, does not appear on either the OMRI or Washington State lists of approved materials for organic certification, and I cannot find a label to see if it has apples on the label. There is another potassium bicarbonate fungicide on the OMRI list called “Bi-Carb Old Fashioned Fungicide”, but it does not appear to be registered in Maine.
While not required, it is of some value to see the disease you are trying to control listed on the label. While EPA does not require efficacy data to list a certain disease on the label, by doing so the manufacturer is making a promise that the product is effective against that disease. If you use it and it does not work, then you have a basis for making a claim that the product is faulty and at least getting your money back. If the product does not list the disease you are trying to control, there is reason to believe that the product is not effective against that disease.
“Kaligreen” only lists powdery mildew as the target disease. “Ecomate Armicarb 0” lists flyspeck, Botrytis, and powdery mildew, but not apple scab. Milstop has the most extensive lists of diseases controlled, including apple scab, flyspeck, powdery mildew, Penicillium, Phytophthora, and Botrytis. This probably reflects differences in how aggressively the different manufacturers are willing to use in their marketing instead of real differences between the products. I say that because all three products have similar concentration of the same active ingredient: 82-85% potassium bicarbonate. Whether the list of diseases on the Kaligreen label is overly conservative, or the longer list of diseases controlled on the Milstop label is overly aggressive is an unknown.
In a brief search, I was able to find information from only a few studies on the efficacy of potassium bicarbonate for apple disease control.
Methionine-Riboflavin and Potassium Bicarbonate-Polymer Sprays Control Apple Flyspeck and Sooty Blotch, 2001
http://www.plantmanagementnetwork.org/pub/php/research/apple/
Efficacy of Armicarb (potassium bicarbonate) against scab and sooty blotch on apples, 2006
http://orgprints.org/8075/01/LT_HJS_Armicarb_7.4.06.pdf
Field efficacy
of new compounds to replace copper for scab control in organic apple
production, 2007
http://orgprints.org/9449/01/heijne-etal-2007-scab-control_apple-production.pdf
First results of the use of potassium bicarbonate against scab in South Tyrol, 2006
http://orgprints.org/8824/01/ecofruit_12th_17.pdf (abstract in English)
The authors of those papers express optimism about the potential for potassium bicarbonate as an apple fungicide, and statistically significant suppression of scab and flyspeck with performance similar to sulfur is indicated. But the level of control is was lower than what most commercial apple growers would be willing to accept. It appears that developing potassium bicarbonate for use as an apple fungicide is a work in progress. Price, lenticel enlargement, fruit russetting, and increased bitter pit were mentioned as potential impediments.
Dr. Dan Cooley at UMass included potassium bicarbonate in a spray 2006 trial to evaluate alternative controls for fly speck and sooty blotch. Those results parallel what was shown in the studies linked above. (Many other materials were tested in that trial, only three treatments not shown here for clarity)
|
Material and amount per 100 gallons |
% of apples with Flyspeck |
% of apples with Sooty blotch |
|
No spray |
40% a |
41% a |
|
Armicarb 100 (potassium bicarbonate) (5 lbs.) |
22% ab |
27% ab |
|
Captan 50W (1lb.) + Topsin M (6 oz.) |
6% b |
7% b |
Dr. Cooley also shared data from a 2006 trial conducted in Illinois where Kaligreen performed as well as Captan + Topsin M in providing 0% flyspeck or sooty blotch, compared to 65% flyspeck and 71% sooty blotch incidence in the untreated check.
None of the three organic apple disease articles listed below mention potassium bicarbonate as a tool for apple scab or flyspeck/sooty blotch control. Perhaps that will change if future work on potassium bicarbonate shows good efficacy. Or it may turn out that potassium bicarbonate is similar to hydrogen peroxide (which the reviews below also do not mention) as an apple scab control tool that might be produce interesting data in a field trial, but which becomes less appealing when you look at the number of applications, expense, efficacy, and reliability compared to existing conventional and organic alternatives. In other words, the jury is still out.
Organic disease control for apples. David Rosenberger, Cornell Univ. 2007.
http://www.nysaes.cornell.edu/ent/scaffolds/2007/070319.html#disease
Organic apple disease management guide. Mike Ellis Ohio State Univ. 1997.
http://www.caf.wvu.edu/kearneysville/organic-apple.html
Organic Apple Production Guide for Nova Scotia. Gordon Braun, editor. Revised 2004.
http://www.organicagcentre.ca/DOCs/OACCappleGuide.pdf
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