AppleTalk Conference Call Summary

Tuesday, May 6th, 2014, 8:00 – 9:00 a.m.
Presenter: John Aue, Threshold IPM.
Moderator: Peter Werts, IPM Institute of North America; questions or comments, Guest Speaker: David Rosenberger, Cornell Hudson Valley Fruit Lab

May 6th Call download: Click Here

David Rosenberger, Professor Emeritus, Cornell University Department of Plant Pathology and Plant-Microbe biology, will join us to discuss early season apple-scab management and other diseases of tree fruit.

Tree phenology and early season insects
Trees in southern Wisconsin or further south and west, are at half-inch green or tight cluster and may be nearing pink by the end of the week.  Now is the time to apply oil or an insecticide for San Jose scale (SJS). When difficult spraying conditions are present, i.e., persistent cool and wet weather, oil for SJS and mites works best when there has been warm temperatures for 12 to 24 hours prior to spraying. Delayed-dormant oil applications smother insects and will be the most effective on SJS and mites during the warmer weather. Note: dormant oil only works when it is wet and if spraying at night or early morning consider the temperature. Warm temperatures after you spray will not be beneficial for the application since it has done its job once it has dried.

If you are using an insect growth regulator, e.g., Esteem (pyriproxyfen), it is most important to have calm wind conditions, if these ideal temperatures cannot be met. Esteem needs to penetrate into the woody areas of the tree and will be absorbed by SJS and will have a greater effect on the scale, if it is warmer or cooler than desired when spraying.

Green fruitworm
Green fruitworm (GFW) are hatching in southern Wisconsin.  Scout for GFW by inspecting the outer leaves of fruit clusters prior to or at tight cluster. Small GFW larvae can be found in the folds of these outer leaves. The larvae can be identified by the three lines that run down their back. Scouting will serve as an early indicator of GFW pressure in the orchard this year.

Pollinators
Native pollinators and managed honey bees are already foraging at the edges and in the woods. If pre-bloom insecticides toxic to pollinators are applied, applications should be made when pollinators are less active, including at night or early in the morning. Even with the cool weather, John has been surprised by the significant numbers of mason bees foraging in orchards in Southern Wisconsin.

Note: How to Reduce Bee Poisoning from Pesticides, http://www.step-project.net/NPDOCS/PNW%20591.pdf, outlines how to select pesticides least toxic to bees.

Q&A with guest speaker, David Rosenberger, Hudson Valley Fruit Lab
Is Syllit (dodine) more of an antisporulant than an eradicant?
Syllit does not have a 96 hour reach back, such as DMI fungicides, e.g., Rally (myclobutanil), Vintage/Rubigan (fenarimol), do, but acts as an antisporulant that will slow down fungal growth in leaves. In high inoculum orchards conidia can overwinter in bud scales and targeting those orchards with Syllit in addition to an EBDC, e.g., Dithane/Penncozeb (mancozeb), could help reduce conidia. Note: even in orchards that do not appear to have high inoculum levels conidia can still be present in bud scales and can be suppressed if targeted by copper or an EBDC before conidia get wet by dew.

During the April 29 call, a grower asked about the effectiveness of Syllit (dodine), when applied with an EBDC, e.g., Polyram (metiram) or Penncozeb (mancozeb), to eradicate an infection in cool temperatures.

David Rosenberger mentioned Syllit redistributes quiet well. It is possible that it does have increased uptake later in season but in New York Syllit has traditionally been a product that growers tank mix for green tip and half-inch green sprays because it seems to work much better in cold weather than the demethylation-inhibitor (DMI) fungicides, e.g., Topguard (flutrialfol) or Rally (myclobutanil), and also does not pose absorption issues. Syllit has traditionally been used early because it provides no control of mildew and rust, when they are of no concern early in the season. Syllit should also be tank mixed with a protectant fungicide, as it does not protect against infections.

How successful are conidia at overwintering in cold weather?
David is unaware of any of studies done on this and suspects apple scab conidia are not affected by cold weather, when compared to powdery mildew spores. Research conducted on powdery mildew has shown that it is the buds which die, not the fungus, and there is not a lot of basis to say that fungus is compromised by cold temperatures.

Powdery mildew and black rot
Powdery mildew does not pose the same level of problem in our region as it does in warmer climates, though there has been an increase in pressure over the last five to ten years. This could be partially attributed to moving away from the use of DMIs. We know that powdery mildew overwinter on the buds and are more susceptible to winter kill. David noted that Bob Spotts of The Ohio State University wrote an article in Scaffolds, between 1974-1976, that showed temperatures at -10° F will eliminate over 95% of overwintering powdery mildew. Even in this scenario, there are still buds that may survive and 100% control with cold temperatures is unlikely. This year we may expect lower powdery mildew pressure, however certain varieties highly susceptible to powdery mildew, e.g., Ginger Gold and Jersey Mac, may still have issues. David suggests we created a habit of delaying mildew sprays with DMIs and we can not do that with newer chemicals that do not have great post infection activity.

Growers with extremely susceptible cultivars may want to make a fungicide application on powdery mildew by pink. Waiting until petal fall may be too late where DMIs have developed resistance.  Keep in mind older SDHI fungicides like Fontelis (penthiopyrad) will control mildew, but will not offer the post-infection activity that can be achieved with newer SDHIs,e.g., Luna Sensation (fluopyram, trifloxystrobin) or Merivon (fluxapyroxad, pyraclostrobin) and QoIs (strobilurins),e.g. , like Flint (trifloxystrobin) or Sovran (kresoxim-methyl).

Mitigating the potential to develop scab resistance to our systemic fungicides with higher rates of EBDCs is an option growers ask about. David suggests using 3lb rate of an EBDC with systemic fungicides as a protective program instead of post infection. Even if a 4.5-6lb rate of EBDCs plus Fontelis, SDHI, are applied after a rain event and infection, only the Fontelis will provide post-infection eradication. The bottom line is all fungicides should be applied ahead of rain. A higher rate is the best anti-resistance strategy but a lot of it comes down to the money. Once scab gets out of control it can take several years to bring your orchard back into control and it is probably more economical in the long run to spend the money and be diligent.

Should we expect to see more issues with mummies by cutting back on Captan and other products after petal fall?
Fruitlet mummies in trees are very complex and are not necessarily a black rot host or an indication of the presence of black rot. Some studies have shown that larger mummies may contain more black rot spores. The presence of black rot may depend more on the spray program from the previous year. The number of mummies left in tree may depend on the weather conditions during June drop and if a chemical thinner was used at this time. Warm and sunny conditions may influence drops, if a good abscission layer is formed allowing the fruit to drop.  In cool cloudy weather the abscission layer may fail to form. Larger mummies may result from the fruit growing a little larger before the chemical thinner was applied or began working.

Black rot infections which cause frogeye leaf spot can happen as early as green tip, but the majority occur at tight cluster to pink and do not show up until after petal fall. Frogeye leaf spot is an indicator of black rot but the legions do not sporulate and will not contribute to greater infections. If you see a lot of frogeye leaf spot while scouting it is a good indicator that the fungicide you were using at that time is not going to protect the fruit because it is not controlling black rot.

Fungicide recommendations for black rot
Weakest
– 3lb rate of Dithane, Manzate or Penncozeb (mancozeb)
– Older DMIs like Rally (myclobutanil), Vintage/Rubigan (fenarimol)
Works well
-Topsin M (thiophanate-methyl) in summer spray, QoI, such as Flint (trifloxystrobin) and Pristine (boscalid,pyraclostrobin) and Captan (captan). Note: Captan has low rainfastness
-Captan effective in all rot especially for bitter rot control, Topsin M is not effective in bitter rot control
Unknown efficacy
– Indar (fenbuconazole) or Inspire Super (difenoconazole, cyprodinil)

Bitter rot
What suggestions do you have for managing bitter rot? In the last six years we have begun to see bitter rot in unthinned Honeycrisp or orchards that have late-season hail injury.
David advises growers who have forecasts of 93° F temperatures for three days in a row to heavily irrigate before the hot weather to reduce increased stress. Most bitter rot in New York develops on fruit in damp weather and will eventually produce orange-colored-spore ooze, with a distinct saucer or sunken shape legions. David is increasingly convinced that bitter rot develops in Honeycrisp when there is hot temperatures 93-95° F and higher, in conjunction with high sunlight and humidity. Bitter rot occurs when the trees become drought stressed and close their stomata, stopping evaporative cooling. With stomata closed and faced toward the sun fruit temperatures can be thirty degrees higher than air temperature and will become defenseless against bitter rot. Once spores are present in a cluster of fruit it can spread rampantly. Drought stress on the trees can also result from over cropping.

David notes there is no research on the use of kaolin products to reduce fruit temperatures and the amount of sunlight hitting the apple. One drawback with kaolin is the difficulty for kaolin to weather out of the deep stem bowl common in Honeycrisp and may make fruit less appealing for fresh-market sales.

Apple scab resistance to strobilurins (QoI) and SDHI fungicides
Would applying two SDHI sprays, Fontelis or Merivon, at pink and bloom and then not using them again and switching to two QoI sprays, Flint and Sovran, the next year delay or prevent resistance?
Resistance is very complex, no one has all the answers and it is safe to say that using them half as often should theoretically lengthen their effectiveness. This assumes your neighbor is not using them and already developed resistance.

Luna Sensation and Merivon also contain a strobilurin or QoI and when those products are used in an orchard that has developed QoI resistance we are unsure of the effects. It is unknown whether it adds to the selection pressure just having it in the mixture or if having the QoI and SDHI combination cancels out the selection. No one has said this is the best way to use it.

David would prefer to see people limit applications of two sprays per year and alternate chemistry during the year if you are going to use QoI and SDHI in multiple sprays. One possibility would be to use Fontelis or Luna Sensation, which does not have a QoI in two pre-bloom sprays and apply the QoI, such as Flint, later in the season if needed. That way you will have only two applications of a QoI and two additional SDHI, such as Fontelis or Luna Tranquility (pyrimethanil, fluopyram), in the season.

What options do we have to manage secondary scab?
If a grower has scab out of control in summer and are using a QoI, strobilurin, late in the season, e.g., Flint (trifloxystrobin) or Prisitine (boscalid, pyraclostrobin), or a DMI, like Inspire Super (difenoconazole, cyprodinil), all of these are adding selection pressure for resistance to Venturia inaequalis. If DMIs are still effective, they are the best products to use at petal fall or first cover because you get powdery mildew and rust control and are effective antisporultants that will suppress apple scab. Two SDHI, e.g., Fontelis, sprays prebloom one year and two QoI/SDHI sprays pre bloom the next year is a good idea. Careful rotation of fungicide classes is essential and not repeatedly using one product is the goal.

Tank mixing Captan at petal fall and possible phytotoxicity
David suggests avoiding Captan when mixing too many chemicals in the tank because mixes can result in injury at petal fall and first cover from to much adjuvant for the young leaves to handle. He speculates another issue may be the quality of newer formulations of Captan especially the ones being made in China.

To help prevent phytotoxicity moderate what you put into tank mix and watch the weather. If you have clear sunny weather during applications there will be no problem of phytotoxicity. The problem is cloudy wet weather, when there is not cuticle, and Captan is applied and then temperatures spike that is when you can have problems. Leaf edge burn can also occur from applying NAA under certain conditions, or even Captan, under slow drying conditions when leafs are first unfolding and possible necrosis moving into leaf tissue as cuticle is developing.