May 7th Apple Talk Conference Call Summary

Apple Talk Conference Call Summary

Tuesday May 7th, 8:00 – 9:00

Presenters: John Aue, Threshold IPM; George Sundin, Michigan State University (MSU).

Moderator: Peter Werts, IPM Institute of North America; questions or comments,

Tree phenology

Tree phenology from northern Illinois to the Twin Cities, including southern half of Wisconsin, varies from pink on early varieties, e.g., Zestar, in southeast Wisconsin and northern Illinois to1/4″ green in the Twin Cities. Most growers have had one or more rain events since green tip, plus snow in early May from Lacrosse to Hastings Minnesota.  Weather stations have not shown infection periods during these wetting events.

 Insect monitoring and management

Growers should be scouting for mite-egg hatches, rosy apple aphid (RAA) and overwintering obliquebanded leafroller (OBLR).  The OBLR will be emerging from hibernation and will first appear as ¼ inch long larva feeding on blossoms and shoots.   Traps for redbanded leafroller (RBLR) and spotted tentiform leafminer (STLM) should also be hung.

Internal Lepidoptera monitoring and management

The Trece codling moth long-life (CM L2) lure is still the recommended choice for codling moth (CM) monitoring.  Growers using pheromone mating disruption should also be using the CMDA lure which attracts both male and female codling moth.  CM traps should be hung next week for most growers across the region.

See recent Scaffolds article on mating disruption,

Growers who did not have a crop last year should be monitoring for Oriental fruitmoth (OFM) and lesser appleworm (LAW).  OFM and LAW have three generations in our region and damage is very similar to CM, yet flight is off set enough if we had a populations and are not monitoring, it could appear as if we had early CM damage.

Q & A with George Sundin, MSU Plant Pathologist

Apple scab and powdery mildew

What’s your opinion of Venturia inaequalis development this spring compared to development of tree phenology?

Growers are recommended to apply protectant fungicides before infections at green tip.  The scab fungus has timed itself to closely follow the crop phenology.  If an infection occurs at green tip, the conidia that develop will release spores during bloom.  Once lesions are in the leaves, it is easier for new spores to find their target in the trees.  This year spore monitoring in Michigan did identify that some spores were released prior to McIntosh green tip.

If apple scab in MI is largely strobilurin resistant, how are you recommending using the class 7 (SDHI) fungicides? Do you see any utility in the combination products such as Merivon (pyraclostrobin + fluxapyroxad) or Luna Sensation (trifloxystrobin + fluopyram) or Luna Tranquility (pyrimethanil +fluopyram)?  Did your observations of strobilurin failures in MI agree with a qualitative resistance mechanism?

Post-infection applications of SDHI, strobilurin or sterol inhibitor (DMI) fungicides, should be avoided.  If DMI resistance is suspected, what is the recommendation when using the combination of pre-mix SDHI products?  Often lower rates are used in pre-mixed products, e.g., Luna Sensation and Merivon, and growers need to use the full rate.  Luna Tranquility is SDHI plus Scala (pyrimethanil).  Scala is weaker as temperatures warm up, one difference is that the SDHI active ingredient is higher in Luna Tranquility.

The resistance to strobilurins is qualitative, meaning the pathogen is either susceptible or resistant and increasing the rate will not overcome resistance and the sensitivity does not decrease once the mutation happens.  If there is no resistance, strobilurins can still be used, and should be used preventively.  Considering the number of years we have been using them, it is a huge risk to use post-infection.  In practical terms, growers have about 20-25 applications of a strobilurin.  If a material like Flint (trifloxystrobin) has been used post-infection, each post infection application could significantly accelerate resistance.

With quantitative resistance, i.e., resistance in DMIs, we can incrementally increase the rate to overcome resistance and can kill isolates which are resistant.  Eventually the orchard will become less sensitive to the higher rate over time and eventually the sterol inhibitor will quit working.  In Michigan and New York growers no longer expect DMIs to work, but as resistance is developing we can still get control.  In a year where scab is not bad, we can still get excellent control, even if scab resistance is developing.

Strobilurins first became available in Michigan in 1999 and had been used post infection heavily in 1999 and 2000.  In 2008 MI had region wide control failure on McIntosh from Sovran (kresoxim-methyl), but could have been Flint.  Some had used these materials alone and others with 3lbs of an EBDC.  Most of the orchards sampled observed almost 100% resistance.  This likely means resistance began to develop two years previously and when enough isolates of the spore are resistant and present, the failure occurred.

Growers in Michigan do not expect any control from Rally (myclobutanil), but new SIs, e.g., Inspire Super (difenoconazole + cyprodinil ) and Indar (fenbuconazole), will still control fungicides which are resistant to Rally (myclobutanil).  Last year in MI growers used Fontelis and this could be mixed with Flint or an SI plus a protectant, e.g., captan or an EBDC.  Keep spray intervals tight when we are using half rate of EBDC and SDHI.

In terms of SI resistance, silver or green tip copper spray could help reduce some resistance in the population.

Is powdery mildew resistance to sterol inhibitors and strobilurins known to be either qualitative or quantitative?

Resistance in mildew should be the same as in V. inaequalis, e.g., resistance to an SI is quantitative and qualitative for a strobilurin.

Could one or two applications of three pounds of sulfur between tight cluster and second cover effectively overcome SI or strobilurin resistant powdery mildew, if used in two successive seasons (in addition to SDHI’s and/or SIs or strobilurins)?

Sulfur will provide broad spectrum control of powdery mildew.  Sulfur will also affect DMI resistant strains of powdery mildew.

Powdery mildew prefers actively growing shoots and should be controlled before tight cluster to prevent further infection.  Once powdery mildew begins sporulating at tight cluster, 1% survival will still generate enough spores to cause further infections.

If powdery mildew was present last year, temperatures of ten below are required to kill buds.  Those infected shoots will be there next year and we will see it again.  The protectant fungicides for scab, i.e., captan and EBDCs have no impact on mildew.

We have seen more powdery mildew over the last five to six years.  Growers should also emphasize management in non-bearing and immature blocks.  Powdery mildew is a good traveler, where as scab does not travel very far.  Infections can continue to spread on young trees that will be vigorous through the season.

Powdery mildew cannot be grown in the lab and therefore cannot screen for resistance.  Fungicide trials are an option, but can’t isolate and grow it to test for resistance in the same way we can for scab, cherry leaf spot and brown rot.  Therefore, knowledge on powdery mildew resistance remains limited.


Is there historical data or research that suggests a short pre-bloom period influences E. amylovora’s significance in that same year? 

This year Michigan had a very short and warm pre-bloom period.  Bacterium is activating but expecting minimal problems in Michigan this year.

Cold weather during bloom is perfect for preventing blossom blight infections.  Temperatures in the 50’s during bloom are good for controlling fireblight.  We are too early to know what the bloom weather will be.  Last year was very hot before bloom, but then it got cold and that helped prevent fireblight.  Once it warms up, bacterium is still there and could cause shoot infections, i.e., growers could escape blossom blight, but still get shoot blight.

Weather conducive for shoot blight include temperatures in the 70’s or higher.  Highs in the 90s is too hot.  Any microscopic injuries at the fruit tip can cause infection.  It only takes a few bacterial cells to cause the infection and think at bloom a blossom could have 1 million bacterial cells.

Apogee could be applied at King Bloom petal fall to slow growth of shoots.

Several growers want to use a peroxide product for fireblight control during bloom.  If coverage is excellent after significant bacterial growth, but prior to rainfall, and grower repeats application after 24-36 hours of rapid population growth in E. amylovora, what are their chances of avoiding blossom infections? 

There is no evidence of streptomycin resistance in our region and a recent survey in Wisconsin and Illinois has not found resistance.  From the stand point of resistance management, we want to kill the pathogen and streptomycin is the only one which kills the pathogen.  If you don’t use it more than 3 times during a bloom period, resistance risk is low.

Oxidate (hydrogen peroxide) is a contact sterilant and will have some effect on killing fireblight, but will readily dissipate.  Nothing is going to kill one million cells on a flower.  Just think the soap commercial if we kill 99.9% of million cells, we still have 1000 fireblight cells left and they will just grow to a million.

Where there is marginal to high fireblight risk streptomycin is the best material.  When lower risk or moderate-risk pressure is present, Serenade (Bacillus subtilis) and oxytetracycline may be an option.