May 8, AppleTalk Conference Call

AppleTalk Conference Call Summary
Tuesday, May 8 2018, 8:00 – 9:00 AM
Guest speaker: George Sundin, Professor of Plant Pathology, Michigan State University
Presenter: John Aue, Threshold IPM
Moderator: Peter Werts, IPM Institute of North America; questions or comments, pwerts@ipminstitute.org

May 8th Call Stream: CLICK HERE

Regional update
Tree phenology around the state has made astounding progression from green tip last week on lots of orchards to tight/open cluster and even bloom toward the Wisconsin-Illinois border this week – movement that has typically taken up to a month in past years. Growers are reminded to record their green-tip date to help predict and track the primary ascospore maturity and primary according to the model in your local NEWA station. Looking forward, petal fall dates should also be recorded for plum curculio management.

Degree-Days (base 50°F) since January 1, 2018

Rain in the forecast for much of the remaining week paired with moderately cooler temperatures will offer prime conditions for apple-scab development, and warmer temperatures on Monday will likely induce fire blight activity later in the week. Most of Wisconsin has reached 120-130 degree-days (DD), base 50°F, not far off from where we were last year. Historically, first generation codling moth biofix has occurred around 300 DD (typically May 15th-22nd), but can occur as early as 180 degree-days. Growers should hang codling moth traps sometime over the next week to ensure the recommended period of 10 days lies between hanging and first-observed flight.

Growers who normally monitor redbanded leafroller (RBLR) should make sure these traps are hung, as the first flight usually begins around 75 DD. We can also expect to see obliquebanded leafroller overwintering larvae emerge at 105 DD, kicking off the start of the spring-leafroller complex (which also includes spring cankerworm and green fruitworm), so growers can begin to look for these larvae in tree terminals and developing fruit buds.

Early season disease management with Dr. George Sundin, Professor of Plant Pathology, Michigan State University
Has Bitter-Rot Resistance to QoIs (strobilurins) been documented in Michigan orchards?
Bitter rot has not been a big problem in Michigan, so many of those tests have not been done. Growers may have shifted to using strobilurins for summer-disease management, e.g., bitter rot, black rot.

What is the prevalence of non-cedar apple rust pathogens in Michigan orchards?
2017 was the worst year for rust in Michigan since 2002; however, George suspects this is a “once-a-decade” problem. Growers shouldn’t be concerned this year as pressure is expected to return to normal.

Do low-temperature thresholds for powdery mildew induced winter bud mortality in apples follow a bell-shaped curve or a steep cut-off?
While there isn’t substantial data on the topic, George speculated that a steeper model makes more sense because powdery mildew infected buds are already damaged by the infection and may not survive temperatures below -15°F. Since mildew requires a living bud to survive, the pathogen will die with the infected bud. George could only find one paper that addressed this issue from a group in Oregon. The group blasted buds with cold temperatures, noting increased mortality between 2°F and -7°F; however, the experiment was artificial and completed in a lab.

Are there cool-weather canker-forming fungi in the Upper Midwest? (e.g. Anthracnose spread in Western Washington State.)
There was a significant outbreak of the anthracnose fungus on LindaMac and RubyMac in Michigan five to eight years ago, but the problem faded without definitive reason. Pressure from these canker fungi is unlikely at this time, but copper application at bud break or in response to a hail-damage event might be a good idea even if fire blight is under control.

Fire blight
Is there a low-temperature threshold for E. amylovora in overwintering fire blight cankers? Might this vary with canker location, i.e. first-year wood versus main trunk?
George does not believe a low-temperature threshold for E. amylovora exists. Fire blight cankers survive very well within the protective exopolysaccharide sugar solution that encases the bacteria, and George had even stored cells in lab at -80°C in this solution with no noticeable death. Additionally, bacterial populations are so high within cankers that even a 1000-fold decrease wouldn’t prevent a rebound to critical mass to form bacterial ooze.

If growers were unable to apply copper at bud-break, is there any rate or formulation, e.g. low rate of Cueva that could be applied closer to bloom with less risk of fruit russeting? How well will this work on scab?
At this point, tree phenology is too far along to support any rate of copper application. Reduced rates have been tried, but russeting always a risk. Copper will be most effective against cankers when available both now and later, making Cueva (copper octanoate) a bad fit due to its high solubility/wash-off rate. In this case, the best coppers are fixed coppers like Kocide (copper hydroxide) that are less soluble. On non-bearing trees, however, Cueva can be used to protect against shoot blight for a quick, focused kill just before a rain event. Some growers hoping to maximize growth on non-bearing trees will apply the lowest rate identified in the extended-spray schedule throughout the season, rather than using the more expensive Cueva.

Is our rapid apple growth (two weeks from bud-break to bloom) likely to be reflected in bacterial growth within cankers?
Bacterial growth may not have caught up to typical growth during bloom, but there isn’t a uniform connection between tree phenology and bacterial growth. The link is more temperature-dependent. With a lot of warm days preceding bloom, there will likely be a lot of inoculum around. Even one oozing canker out of 1000 is still enough inoculum for insects to establish the disease on flowers.

Do you have an updated opinion on Actigard efficacy?
Actigard (acibenzolar-S-methyl) works by activating the tree’s resistance response to ward off infection. As consistent results of strong performance continue, Actigard is liked more and more for shoot blight treatment at 1-2 ounces per acre. Actigard can be applied to treat blossom blight at 20% bloom but is not as effective in blossoms due to the huge number of bacterial cells in these infections (over 10 million). Streptomycin applied during bloom is a better option. To restrict canker progression in young trees, apply Actigard at a high rate directly onto cankers and 1-2 feet above and below. For more information, see this article by Ken Johnson at OSU, https://www.goodfruit.com/fire-blight-management/. For trees with cankers established on the central leader, tree removal is advised over aggressive pruning.

What are the best Apogee application timing and rates for shoot-blight as opposed to vegetative growth management?
With some orchards losing as many as 3000-4000 trees, avoiding infection of the central leader is a serious priority. The growth inhibiting plant-growth regulator, Apogee (prohexadione calcium) is the best material for shoot blight control since Apogee induces the trees to produce a physical barrier (thickened cell walls) to fire blight infection. Furthermore, the bacteria won’t develop resistance to this method of control (physical barrier), and Apogee applied at this timing and rate won’t significantly impact growth. The “Apogee effect” will eventually wear off and allow growth to resume in July. Phil Schwallier, Michigan State University – Extension, has noted similar growth in Apogee treated and untreated trees. If the infection happens in hot, dry conditions, e.g., July, infections will generally move slowly enough for shoots to be pruned out, and fire blight will be under control.

In terms of the timing, young trees should be treated at king bloom petal fall and three times at two-week intervals. After the third spray, need for a fourth application can be determined after reassessment – hot and dry conditions probably won’t require a fourth spray. It takes between ten days and two weeks for treatment to take effect, and then it last for a while. When selecting application rates, a window of 3-4 oz. per acre with enough volume of water to get the coverage for adequate absorption. In the worst cases of infection such as blossom blight occurring at king bloom petal fall, the full volume can be used at that time.

Apple scab
With NEWA sites forecasting 30-50% ascospore maturity during bloom this Sunday, is scab ahead or behind tree phenology?
Scab was slightly ahead this year after a significant infection caused by rain last week. While NEWA only had 3% at the time of infection, spore captures at MSU suggested a significantly higher spore release. Upcoming rain events in the forecast will release plenty of spores.

Does color intensity of secondary lesions reflect pathogen virulence or other metrics of its success?
Color intensity probably won’t have as much of an effect on virulence as the actual number of spores-per-lesion. Lesions will typically contain thousands upon thousands of spores, but that number is probably insignificant because any spore-containing lesion poses a substantial risk.

Are biofungicides capable of impacting overall incidents throughout the season for organic producers?
George has only tested a few products without much success. Even with conventional fungicides, apple scab is very difficult to control. However, the scab in some organic orchards never looks as bad as it does in a conventional orchard.

If properly used, are new SDHI fungicides like Aprovia more likely to have a longer “effective” life than older materials within the same class either by modes of action or metabolic cost to the pathogen for resistance development?
All SDHIs should provide excellent control of scab because they each target the same three-protein complex in the fungi known as the “SDHI pocket”. While all SDHIs are subject to resistance risk, each active ingredient within this class of fungicides attacks this protein in a different way, so resistance to one does not necessarily mean resistance to all. George thinks Fontelis (penthiopyrad) is most at risk for resistance, but on an equal level suggests a mutation for resistance to Aprovia (benzovindiflupyr) isn’t out of the question. To hedge against resistance, do not exceed two applications of an SDHI for primary scab use.