Fire Blight Risk Remains High

Monday afternoon, May 12, 2014:  Warm weather and predicted showers continue to favor fire blight.  A strep spray should have been applied sometime on Saturday or Sunday to all orchards that are at high risk for fire blight  (i.e., trees <6 years old, active fire blight in the orchard or adjacent orchards in either of the last two years, apple cultivars that are highly-susceptible to blight, all pear cultivars).

Unfortunately, if we get any rain at all in the next few days, orchards that were sprayed with strep this past Saturday or Sunday (May 10 or 11) will need a second strep application either tomorrow (Tues.) or within 24 hours after the start of any rain. Orchards sprayed with strep today (Monday) will probably need a second application on Wednesday or Thursday.  The need for a second application was verified using both the MaryBlyt and the Cougar Blight (NEWA) models for predicting fire blight.

It may seem counter-intuitive to apply strep a second time, just two or three days after the first spray.  The rationale for the second spray is that, with warm weather, many new blossoms have opened since the first spray was applied. Bacteria deposited on those new flowers by pollinators or by splashing rain reach disease-causing thresholds very quickly in warm weather.  The only at-risk orchards where a second spray may NOT be needed are orchards where all flowers were already open when the first spray was applied.

For orchards where fire blight has been causing persistent losses, or in younger orchards that had blight last year,  Apogee can be tank-mixed with streptomycin to further reduce risks from fire blight.  Apogee does not control blossom blight, but orchards treated with Apogee when shoots are one to four inches long (i.e., right now) are much more resistant to secondary spread of fire blight. Thus, Apogee provides a second level of defense for situations where the streptomycin sprays are not 100% effective.  Anyone with at-risk orchards who has not yet applied a strep spray this year should definitely consider applying Apogee along with streptomycin prior to the next rain event.  Remember that Apogee must be applied with a water conditioner if you have hard water, and remember that Apogee should NOT be applied to Empire (see the product label!).

Finally, remember to omit Regulaid (or other adjuvant) that is commonly used to enhance uptake of strep if your strep spray will be tank-mixed with chemical thinners during late bloom or at petal fall.

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Preventing Brown Rot Blossom Blight

Thursday, 8 May, 2014:  The fungi causing brown rot blossom blight invade stone fruit flowers during wet weather when stone fruits are in bloom. A modest fungicide program is usually sufficient to protect stone fruit flowers, but under unusually favorable conditions, trees may require several fungicide applications over the 7-14 days when stone fruits are blooming, including sprays at petal fall and/or shuck split.

Most of the brown rot in eastern United States is caused by Monolinia fructicola and causes the disease known as American brown rot (ABR).  However, Monolinia laxa is a different, less-common brown rot pathogen that has recently become more prevalent in some cherry orchards, especially in Michigan where it caused major losses in some tart cherry blocks in 2013 (see MI story). The brown rot disease caused by M. laxa is called European brown rot (EBR). Differentiating between EBR and ABR in the field is almost impossible because the symptoms are very similar.  However, EBR often causes spur and twig die-back in addition to blossom blight, whereas that phenomenon is less common with ABR.

Several factors have favored the appearance of M. laxa in recent years.  First, European red-fleshed tart cherry cultivars such as Balaton and Danube appear to be especially susceptible to  M. laxa. Where these cultivars are included in cherry plantings, they may play a role in harboring inoculum that can then spread to other cultivars.  Second, chlorothalonil (Bravo and generics) is reasonably effective for protecting against blossom blight caused by ABR under modest pressure, but it is NOT effective for protecting flowers from blossom blight caused by EBR.  Third, EBR can infect flowers at somewhat lower temperatures than those needed for significant levels of ABR infection.

Cherry growers whose plantings include European tart cherry cultivars may want to take a pro-active approach toward EBR by including Rovral in at least one fungicide spray during bloom. (Efficacy of Rovral against EBR has not yet been verified, but I am assuming that it will work because of its broad activity against this kind of fungi, and it will certainly provide better control of ABR than what is provided by chlorothalonil.) Rovral can be applied only twice per year, and it cannot be applied after petal fall.  It is the preferred alternative to chlorothalonil in blossom blight sprays because it is a totally different chemistry than the DMI, QoI (strobe), and SDHI fungicides that are important products for preharvest brown rot control. Thus, using Rovral during bloom  will not generate any selection pressure for resistance to products such as Orbit, Indar, Gem, and Pristine that really should be reserved for use as  preharvest brown rot sprays.

EBR seems to be a problem primarily on tart cherries, so Rovral will be most useful on cherry blocks, although it would also be an excellent choice for peaches, nectarines and apricots when conditions favor severe blossom blight infection.  Chlorothalonil is still the preferred fungicide for plums/prunes during bloom because it is the only product that is highly effective in preventing black knot.  Rovral will NOT protect against black knot.

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Fire Blight (not yet!)

Thursday noon, May 8, 2014: With apples and pears in bloom and several days of intermittent rainfall, many folks are probably wondering about the risk of fire blight.  I just finished running the MaryBlyt model using weather data from the Highland-HVL NEWA weather station along with forecasted temperatures from Accuweather for the next 7 days.

The MaryBlyt model indicates that we will not accumulate enough degree hours (DH) to trigger blossom blight until Sunday, May 11, if current forecasts hold.  Even on Sunday, the accumulated DH will barely reach threshold levels required for infection.  HOWEVER, if forecasts hold, then by next Tuesday we will have accumulated enough heat to be at 2.5 times threshold levels, and that level of risk has the potential to result in extremely serious blossom blight infections. (Click to see the MaryBlyt output, including forecasted temperatures through May 15).

Note that MaryBlyt is a different model than Cougar Blight.  The NEWA website uses Cougar Blight and therefore may differ slightly from the projections provided by MarbyBlyt.  I prefer results from MaryBlyt which was developed using more east coast data, but either model is acceptable.  For those who wish to run MaryBlyt using their own weather data, you can access the free software at http://www.caf.wvu.edu/kearneysville/Maryblyt/index.html.

Streptomycin protects only those blossoms that are open at the time of application.  As a result, there is no benefit from applying strep sprays today or tomorrow so as to get the jump on protection for Sunday. Strep sprays applied today or tomorrow will be a total waste of money and WILL NOT be adequate to protect trees on Sunday because a large number of new flowers will be opening over the next few days.  Thus, if current forecasts hold, the best time to spray strep (weather conditions permitting) will be Saturday evening or Sunday morning.  Even applications made within 24 hr after infection event are better than those made two or three days too early because strep has about 24 hr of post-infection activity and all flowers open during the infection period are accessible for spray coverage in post-infection sprays.  However, we generally prefer sprays applied just ahead of infection periods to those applied after infection periods because bad weather after an infection period may limit spray coverage.

Within the MaryBlyt model, infection can occur only when four criteria are all fulfilled simultaneously:
1. There must be open flowers.
2. Accumulated degree hours (base 65 F.) since the flowers opened must total almost 200.
3. Mean temperature for the day must be at least 60 F.
4. There must be moisture (rain, dew, sprays) on the day of infection or the prior day to allow the bacteria to move from the stigma down into the nectaries at the base of the flower where infection occurs.

The model incorporates other complex factors to account for the effective “lifetime” of a flower after it opens along with some other details, but the four items listed above are the key factors.  Given our current temperatures, Items 2 and/or 3 are missing because temperatures are too cold.

Organic growers have have a new option for controlling blossom blight this year:  Blossom Protect (BP) is a biocontrol fungicide that has worked pretty well in tests on the west coast.  Unlike the case with streptomycin, however, BP absolutely must be applied as suggested on the label, with the first application at about 10% bloom and additional applications at roughly 40, 70, and 90% bloom.  The biocontrol organisms in BP require time to become established on the flowers before the fire blight pathogen arrives.  Thus, it cannot be applied at the last minute prior to potential infection periods as we attempt to do with streptomycin.  As noted on the label, application of BP after mid-bloom may, in some years, contribute to fruit russetting on russet-susceptible cultivars such as Golden Delicious.

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Mark Wild Cherry Hosts for X-disease Now

Tuesday, May 6, 2014: In the Hudson Valley and southern New England, the time when sweet cherries are blooming is a critical time for managing X-disease.  During cherry bloom, seedling Mazzard cherry trees growing in hedgerows and woodlots adjacent to stone fruit orchards can be easily detected because of their showy white flowers (Fig. 1). Seedling sweet cherry trees often become established via seeds carried by birds from cultivated cherry trees.  These seedling trees can harbor X-disease and provide an inoculum source for commercial peach, nectarine, and sweet cherry orchards.  While the seedling Mazzards are in bloom as they are right now, they should be marked for removal, and removal should be completed before June 1.

Do not confuse Mazzard seedling trees with shadbush (Amelanchier species) that are also blooming right now.  Shadbush trees have a more wispy twig structure and the trunks on shadbush lack the large horizontally oriented lenticels common to trunks of Mazzard seedlings (Fig. 2).  When removing potential hosts for X-disease, it is important to note that wild black cherry (Prunus serotina) is NOT a host for X-disease and need not be removed from orchard perimeters to control X-disease.  Black cherry blooms later in the spring (Fig. 3) and has a much different flower structure than Mazzard seedling trees (Fig. 4).

X-disease is one of the most difficult diseases to manage in peach, nectarine, and sweet cherry plantings in the Hudson Valley and southern New England. The pathogen that causes X-disease is a phytoplasma that can live only in  phloem of infected plants or inside one of its insect vectors.  Several leafhopper species are able to acquire the phytoplasma from infected chokecherry (Prunus virginianae) or sweet cherry trees and then transmit the pathogen to orchard trees. Transmission occurs during summer, and insecticidetreatments are not very effective for preventing transmission if infected chokecherry or sweet cherry trees are present within 500 feet of susceptible stone fruit crops.

Peach and nectarine trees are considered dead-end hosts for the disease, apparently because peaches and nectarines fail to develop the high titers of phytoplasma in the phloem that allow efficient acquisition of the pathogen.  However, some experienced peach growers feel that peaches and nectarines may still contribute to slow spread of the disease even though such transmission has never been documented in controlled trials with insect vectors. Infected chokecherries and sweet cherries have phloem cells that are uniformly filled with the X-disease phytoplasma, and leafhoppers feeding on these trees can readily acquire the organisms and later transmit them to orchard trees.

The only control for X-disease is vigilant removal of wild hosts (chokecherry and Mazzard seedling trees) and regular removal of diseased sweet cherry trees as soon as symptomatic trees are diagnosed in sweet cherry plantings.  X-disease in cultivated sweet cherries is best diagnosed during cherry harvest when diseased trees can be detected by their failure to ripen fruit on infected limbs (Fig. 5).

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Protecting Pollinators during Bloom

Monday, 5 May, 2014:  Several recently published studies have reported that some fungicides may interact with other agrochemicals in ways that make them harmful to bees.  Initial reports were sketchy. In their May 1st issue, Good Fruit Grower  published a short article that I wrote expressing concern about the need for better information on exactly which fungicides might contribute to bee mortality. (See pages 43-44 at http://read.dmtmag.com/i/298602).

Now David Biddinger and Kathleen Demchak at Penn State have responded with an excellent article detailing what is currently known about interactions of various pesticides (including fungicides) with various species of bees.  Check out their article at this link.

At Cornell, Bryan Danforth and his students are attempting to assess the abundance and importance of native pollinator species in agriculture. Check out their project at http://www.northeastpollinatorpartnership.org.

Understanding how pesticides and other ag practices impact orchard ecology should enable us to do a better job of reducing negative impacts from the pesticides that are needed to protect crops from insects and diseases.

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Fungicide Strategies after the Rain

Thursday morning, May 1, 2014:  Rains over the past two days exceeded 2 inches and have almost certainly removed all protection from fungicides that were applied prior to the rain.  Orchards that were sprayed after Saturday morning of last week (let’s call them Case-A) should have been well protected through the major scab infection period that we just experienced.  In these orchards, one could presumably continue with protectant-only programs, although personally I would hedge my bets and include one of the more powerful fungicides in my next spray, both to pick up any scab that has been missed and to protect against powdery mildew.

Orchards that were last sprayed prior to Saturday morning, April 25, (Case-B orchards) almost certainly need a spray that will provide post-infection activity. In Case-B orchards where DMI fungicides are still effective, the best option will be an application of either Inspire Super-plus-mancozeb or Rally-plus-mancozeb sometime between now and sunrise on Sunday morning.  I suggest that this follow-up spray with a DMI can be as late as Sunday morning because even weathered fungicide residues in Case-B orchards probably protected trees through the first 12-15 hr of the Tues-Thurs rain event. The DMI fungicides will provide 96 hr of post-infection activity, therefore cleaning up infections back to Wed morning if applied before sunrise on Sunday morning.

In Case-B orchards where Rally (or Nova, Rubigan, Vintage, etc.) have failed to provide scab control in the past, options are more complicated because other fungicides will provide only 48 to 72 hr of post-infection activity. The upper limit for reach-back may be closer to 48 than 72 hr for this event because of the warm temperatures today. If one assumes that post-infection sprays in Case-B orchards should reach back to dawn on Wed morning when the second big batch of spores were released and fungicide residues may have been depleted, then the deadline for getting that spray applied will be sometime between dawn and noon on Friday morning.

For these Case-B orchards, I would suggest that mancozeb plus one of the new SDHI fungicides may be the best option at this time.  The SDHI fungicides include Fontelis, Luna Tranquility, and Merivon in New York (except for Long Island where none of these are registered). Add Luna Sensation to the list of available SDHIs in other states. The SDHI-mancozeb combination will take care of scab that may be resistant to other chemistries and will provide excellent control of both scab and mildew, along with good control of rust diseases, which are also active at this point.  Other options for 48 hours of post-infection activity include Vangard (alone or as included in Inspire Super), Scala, and Syllit, but none of those products except Inspire Super will control rust or mildew.  Flint and Sovran may also provide 48 hr of post-infection activity, but their reach-back activity begins to fade as the scab populations shift toward resistance to these products, and I therefore consider them unreliable for post-infection activity.

Where post-infection activity is needed, excellent coverage is extremely important because the product being applied must hit all of the leaves where scab infections have been initiated.  Post-infection sprays do not have the benefit of redistribution from leaf to leaf via rain as occurs when protectant fungicides are applied ahead of infection periods.  Therefore, post-infection sprays are best applied during periods of low wind. Forecasts suggest that low-wind conditions (<5 mph) will persist from 9 PM tonight until 6 AM tomorrow and again from 9 PM on Friday to 10 AM on Saturday.

Finally, I want to emphasize that, based on the RIMpro apple scab model that was developed in Europe (and that I am evaluating this year), the Tues-Thurs infection period will probably turn out to have been the most important scab infection period of the year.  Significant infection periods are also predicted for next week, but they are likely to be less severe than the one we just experienced.  Thus, fungicide sprays applied today through Sunday will be critical both for their reach-back effects against scab and rust, and for protecting against additional scab infection periods forecast for next week.

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Scab Warning

Sunday evening, April 27:  Weather forecasts for the coming week vary, but at least one of the forecasts is suggesting that we may have more than 3 inches of rain in the Hudson Valley between Tuesday morning and Thursday evening.  The apple growth stage in the Hudson Valley and ascospore maturity (based on several different models) are such that unprotected orchards could sustain very severe scab infections if the forecasts are correct.

The dilemma is that many orchards were probably sprayed last Thursday or Friday, but the protection from those sprays has been compromised both by the two-thirds inch of rain that occurred early on Saturday morning and by additional leaf expansion since then.  If we receive more than an inch of rain between Tuesday and Thursday, then  trees sprayed prior to Saturday morning (26 April) will likely run out of protection before the wetting period ends.  Those using a protectant fungicide program will want to have a fresh cover of mancozeb plus captan in place before the rains begin on Tuesday whereas others may opt for one of the protectant fungicides (captan or mancozeb) plus one of the fungicides that is absorbed into leaf tissue:  Flint, Sovran, Fontelis, Merivon, Luna Tranquility, or Inspire Super. Those who cannot re-apply fungicides to all of their acreage before rains begin on Tuesday should focus on recovering their most scab susceptible cultivars and/or blocks known to have higher levels of inoculum from last year.

Although some of the NEWA stations suggested that we had a scab infection period over the weekend, I suspect that risks from that potential infection period were rather low, especially where the rain started after dark on Friday night. At this time of year few ascospores will be released at night, and the wetting periods of the weekend were generally too brief and cold to add up to an infection period if night time hours are omitted where events began after dark.

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Brown Rot & Black Knot Warning on Stone Fruit

Thursday, April 24: The weather forecast is suggesting that we may have a 24-hr wetting period  with mean temperatures slightly above 50 F from 7 PM on Friday through at least 6 PM on Saturday.  The forecast is for scattered showers, so whether or not we get enough wetting to allow infection by fungal diseases remains to be seen.  Nevertheless, it would be prudent to apply a fungicide tomorrow morning on any stone fruits that are in bloom so as to prevent development of brown rot blossom blight.  At the Hudson Valley Lab, apricots and some cultivars of Japanese plums are now in bloom (see photo below).

Japanese plums are generally less susceptible to black knot than are common European plum cultivars such as ‘Stanley.’  Nevertheless, considering how devastating black knot can be if it is not controlled, plums that are at white bud or in bloom should be protected with chlorothalonil prior to the rains predicted for Friday night, especially if there is any history of black knot infection in the blocks in question.  As always, read the product label before making any fungicide applications.  Some, if not all, of the chlorothalonil labels specify the crops may not be retreated at intervals of less than 10 days.

14-04-24 Early Golden plum buds-pollinator DSC_0008‘Early Golden’ plum  in bloom at the Hudson Valley Lab at noon on April 24.

                      14-04-24 Brown rot mummies prune-plums DSC_0011 14-04-24 Black knot on prune-plums DSC_0015
Brown rot mummies (left) and black knot (right) in a plum tree.  As noted in a previous post, both should be removed before rains begin tomorrow because they will supply inoculum for initiating disease during the coming season.

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Apricots at Full Bloom

Tuesday, April 22: Apricots at the Hudson Valley Lab were in full bloom this afternoon.  Trees on top of the hill behind the lab showed very little bud damage, but other apricots planted in a colder area on the back side of the hill had hardly enough flowers to set a crop.

I’m fascinated by the fact that apricots in our orchard often appear to attract a wider range of pollinating insects than we see in other tree fruit crops. Perhaps that occurs because apricots are the first trees to come into bloom, the insects are hungry, and we have relatively few apricot trees, thereby forcing the pollinators to congregate on the few available trees.   Today apricot trees were buzzing with honeybees, bumble bees, and at least two or three other kinds of pollinators.  Neither we nor any close neighbors maintain honeybees, so we presume that the honeybees we see at this time of year, before nearby orchardists bring in rented hives for their apples, are probably wild bees that live in the woodlots or abandoned orchard land that  surrounds our research orchards in Highland, NY.

.Apricot pollinators
Apricot pollinators (enlarged)
(click on link for larger photos)

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Avoid Adding Insult to (Frost) Injury

16 April 2014:  Low temperatures early this morning (Wed) varied by region, but ranged from 28 to as low as 23 across the Hudson Valley. More freezing temperatures are predicted for early Thursday morning.  Green tissue in apple buds that have been frozen is especially susceptible to uptake of spray materials for a day or two after being frozen because ice crystals that form in the green tissue create microscopic injuries in plant cells that serve as entry points for anything that is applied to the injured leaves. The injuries created by freeze damage will heal as leaves grow, but applications of pesticides should be delayed as much as possible during the day or two immediately following sub-freezing temperatures.

Products such as oil, copper, and captan can be especially toxic if absorbed into leaves, but I suspect that many other fungicides can also exacerbate freeze-injury if they are applied immediately after leaves thaw.  Of course, the worst-case scenario is to spray trees BEFORE buds thaw because the melting ice crystals will create mini-vacuums that will actively pull the the spray material into the leaf tissue.  (Remember, ice occupies more volume that the equivalent amount of water!)

Even in the absence of any spray-exacerbated injury, freeze-damaged cluster leaves are likely to show crinkling and other malformations as they expand. However, the severity of that injury will be much greater if damaged leaves are sprayed too soon after a freeze.  At the recent ISHS Symposium on Physiological Principles and Fruit Production that was held in Geneva, several speakers noted that cluster leaves provide much of the energy required for flowering and fruit set. Therefore, minimizing injury to cluster leaves is important for maximizing productivity.

Most areas of the lower Hudson Valley (Ulster/Dutchess Counties and south) had at least a light Mill’s apple scab infection period on Tuesday, and in some areas the temperature and duration of wetting was sufficient to generate a moderate infection period.  Obviously, apple scab can also damage cluster leaves, so orchards that were unprotected during Tuesdays infection period may need a post-infection spray no later than about noon in Friday.  However, given the freeze-damage that will be present in cluster leaves, I would avoid applying captan, oil, or copper in any sprays applied Thursday or Friday.

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Spring Checklist for Stone Fruit Disease Control

Tuesday, 15 April:

  1. Brown rot: Remove last year’s brown rot mummies from trees:  Even though pruning on stone fruits is often delayed until after bud break (especially on peaches and nectarines), it is important to walk through stone fruit blocks and remove any brown rot mummies that remain attached to or suspended in the branches because these mummies will supply an abundance of inoculum for brown rot blossom blight if they are still present when trees begin to bloom.  Controlling blossom blight can be very difficult if mummies are present and warm rains occur during bloom.  If the brown rot mummies are removed during winter or early spring, they can usually be left in the sodded row middles between trees where they will rapidly break down before bloom.  However, when brown rot mummies are removed just before bloom, they should be collected and removed from the orchard to ensure that they can’t produce inoculum that will be blown into trees.
  2. Black knot: Scout plum and tart cherry blocks and perimeter hedgerows for black knot. Prune out all of the knots and remove them from the vicinity of the orchard.  Black knot infections in the trunk or major limbs should be carved out with either a sharp knife or by shaving them out with a chainsaw.  The black, knot-like structures must be removed from the orchard because they will release massive numbers of ascospores beginning when trees are near white bud.  One or two knots left in a plum orchard can produce so many spores that even the best fungicide program will fail to prevent new infections. Black knots growing on wild Prunus in hedgerows can also generate inoculum for orchards and should therefore be removed.
  3. Peach leaf curl:  Peach or nectarine blocks that did not receive a leaf curl fungicide at leaf-drop last fall should be protected with a leaf-curl spray at this spring. Copper, chlorothalonil (Bravo and generics), and ferbam are preferred materials, but only copper will provide the added benefit of suppressing the bacteria that cause bacterial spot.  Leaf curl sprays are especially important on peach/nectarine trees that had no crop last year because the fungicides applied during summer to control brown rot also help to prevent establishment of the leaf curl fungus. Blocks with no fruit generally do not receive fungicide sprays during summer, so leaf curl can be especially severe in those blocks the following year if leaf curl sprays are omitted.  Although leaf curl sprays in spring should be applied at bud swell, adequate suppression of leaf curl is often possible even if the leaf curl spray is applied after bud swell.  However, be aware that copper applied after green tissue is evident can severely damage the exposed tissue.
  4. Bacterial canker in sweet cherries is best suppressed with copper sprays applied at leaf drop in autumn, but applications at dormant or bud swell in spring may also be beneficial.  If buds are already swollen, then use the lower label rates of copper so as to avoid the possibility that high levels of copper residues will be redistributed to flowers that will be opening in several weeks.
  5. Apricots should be given a dormant spray of copper at high label rates to suppress both bacterial spot and bacterial canker.  No antibiotic sprays are registered to suppress bacterial spot on apricots, so a spring copper is especially important on this crop to suppress inoculum levels.  The bacterial canker pathogen can become systemic in apricots and will  kill young trees and major scaffold limbs on older trees.  To minimize the opportunity for bacterial canker to gain entry, I suggest that apricots should never be pruned in spring.  Instead prune apricots on hot dry days immediately after bloom (i.e., as the first step in crop load adjustment) and again after harvest.  The pathogen that causes bacterial canker (Pseudomonas syringae) is a cool weather pathogen whose populations tend to collapse when temperatures warm into the 80’s.
  6. Be careful in siting new orchards: When planting new peach and nectarine trees, remember that most cultivars developed in California are extremely susceptible to bacterial spot.  Planting these super-susceptible cultivars next to other more resistant cultivars may put all of your peach/nectarine/apricot/plum plantings at risk of damage from bacterial spot because the super-susceptible cultivars will harbor inoculum that will then spread to other more resistant crops and cultivars.  If you can’t resist planting California varieties, at least plant them on a different part of your farm away from other stone fruits where they will be less likely to act as a “typhoid Mary” for the rest of your existing stone fruit plantings.
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Clarification on Scab Spores Ready to Go

3 PM, Mon, 14 April: My previous message on apple scab ascospore assessments contained some confusing language as it relates to the ascospore maturity model on the NEWA website.  In that posting, I stated “the NEWA model is predicting actual spore discharge, not percentage of mature spores, which would be much higher.”  That statement was incorrect because the NEWA model does, in fact, predict ascospore maturity even though the spore “maturity” predicted by NEWA is much different from spore “maturity” as assessed by squash mounts.

The NEWA model is based on information first published by Gadoury and MacHardy in the early 1980s and was derived and validated primarily by measuring spore discharge following various degree-day accumulations, although they also used some squash mount data. Many spores that appear morphologically mature in squash mounts fail to discharge in the next rain because they actually are not physiologically mature enough to be ejected (see Plant Disease 76:277-282 [1992]).  In fact, at the Hudson Valley Lab we usually found that no significant spore discharge actually occurred under field conditions until our squash mount assessments showed about 12-15% of asci with morphologically mature spores.

To sum up, the percentage of mature ascospores predicted in the NEWA model is an estimate of mature spores likely to discharge in the next rain, which contrasts with the fact that most spores assessed as “morphologically mature” in early-season squash mounts are NOT yet mature enough to discharge.  My short-cut approach to this longer explanation was simply to state that the NEWA model predicts discharge so as to avoid confusion with the concept of morphological maturity as depicted in squash mounts.  I should have stated instead that “the NEWA model is predicting physiologically mature spores ready for discharge in the next rain, not the percentage of morphologically mature spores as assessed in squash mounts, which would be much higher.”

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Scab Spores Ready to Go!

Noon, Mon, April 14: Early-season apple cultivars at the Hudson Valley Lab reached the green tip bud stage (50% of fruiting buds showing green tip) on Sunday, April 13.  Temperatures over the past three days were 5 to 8 degrees higher than last Friday’s forecast had suggested, and the predicted high for today is 80 F.  Forecasted temperatures for the rainy period that will be arriving tomorrow (Tuesday) have also been increased slightly, with all indicators suggesting that we will sustain a moderate scab infection period with perhaps 20 hr of leaf wetting and a mean temperature of 53 to 56 degrees.

Apple leaves collected this morning from an abandoned orchard just north of the Hudson Valley Lab provided a mean discharge of 208 spores in our shooting tower, well above the 50-60 spores that we have traditionally considered an economic threshold for our spore tower discharge tests.  Thus, our spore assessments suggest that apple scab ascospores will be discharged during the coming rain, and that orchards with carry-over inoculum will be at risk of infection if they are not covered with fungicide before the rain begins. The apple scab model in the NEWA system (http://newa.cornell.edu/index.php?page=apple-diseases) predicts that 3% of the total season’s ascospores will be released on Tuesday.  Note that the NEWA model is predicting actual spore discharge, not percentage of mature spores, which would be much higher.

Of particular concern is the fact that daytime temperatures for Tuesday are predicted to be in the low 60’s.  Research conducted in various locations has shown that spore release during the first three to five hours of a wetting period is roughly twice as great when temperatures are above 50 F as compared to temperatures in the low 40’s.  The total number of spores released at lower temperatures may eventually be the same as at warmer temperatures, but the more rapid release at warmer temperatures means that a lot of spores have more time to be blown around, find green tissue, and initiate infections before the temperatures drop and/or the wetting period ends.

Orchards that are not sprayed with fungicides before Tuesday’s rain may benefit from post-infection fungicides applied on Wednesday or Thursday.  Vangard, Scala, and Syllit are the preferred post-infection fungicides for this stage of bud development, and all three of them will all provide 48 to 72 hours of post-infection activity counting from the start of the wetting period.  Post-infection activity may be limited to 48 hours if temperatures remain warm, but 72 hr of post-infection activity is possible with lower temperatures, such as those predicted for Tuesday evening, Wednesday, and Thursday.

The disadvantage of post-infection spraying is that windy weather often follows spring rains, thereby making it difficult to achieve good spray coverage. Also, one cannot depend on rains to redistribute fungicides applied in post-infection mode.  Finally, whereas Syllit (dodine) at 1.5 pt/A applied in combination with mancozeb will provide good protection when applied ahead of rains, the original research on dodine conducted in the 1960’s showed that the equivalent of 3 pt of Syllit/A is required for efficient post-infection activity.  With Vangard and Syllit, however, research trials at the Hudson Valley Lab showed the same level of post-infection activity against scab when we used the lower label rates as when we used the higher label rates for those two products.  Thus, whereas Syllit must be used at higher rates when applied in post-infection timings, there is no need to use higher rates of Vangard or Scala.  All three of these products should always be applied in combinations with mancozeb in early-season sprays.

 

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Apple scab warning

2 pm Fri, April 11:   Leaves collected this morning from an abandoned orchard just north of the Hudson Valley Lab showed a wide variation of ascospore maturity with most pseudothecia of the dozen examined still showing little or no spore differentiation. However, two pseudothecia from one leaf had roughly 20% mature spores.  No ascospores were released in the tower shoot.  Based on these observations, I am confident  that no scab spores will be released in rains tonight or tomorrow.

High temperatures in Highland for today through Monday of next week are predicted (by Accuweather) to be 65, 66, 70, and 74 with lows on Sunday and Monday nights predicted to be 52 and 50.  Currently Accuweather is predicting rain will begin about 3 AM on Tuesday with a strong potential for 24 hr of leaf wetting.  At noon today, I noted the first hint of green in buds of McIntosh, Jerseymac, and Ginger Gold in our research orchards at the Hudson Valley Lab.

If the weather forecasts are correct, we will almost certain be at full green-tip by the time rains begin on Tuesday of next week. I am fairly certain that the warm weather, possibly with a few drizzles, between now and Tuesday will allow enough additional spore maturation to ensure some spore release with the rains next Tuesday.  Thus, I believe that by next Tuesday there may be a significant risk of scab infection for orchards that had carry-over inoculum from last year.  That risk will be much higher if temperatures during the rains next Tuesday are in the mid-50’s rather than in the low-40’s as commonly occurs with early-season rains.  Given the current forecast, the mean temperature for next Tuesday from 6 AM (when the first spore release might occur with the coming of daylight) until 8 pm that evening will be 56 F.  After than the temperature will drop, with rains ending after midnight, but with a strong likelihood of continue wetting until at least 6 AM.  The mean predicted temperature for that potential 24-hr wetting period is currently 50.6 F.

Mike Biltonen of Apple Leaf will be making another scab assessment on Monday morning, and I will be running my own test on Monday as well.  Furthermore, the weather forecast may change to reduce the duration of rainfall and/or the predicted temperatures, either of which would significantly reduce the risks of a major scab infection period on Tuesday.  Nevertheless, it would seem prudent to plan on having a fungicide applied before Tuesday morning in orchards that have carry-over scab from last year.  Orchards that had no leaf or fruit scab last year should not be at risk from this first infection period. However, a warm rain with a wetting duration of 24 hours will provided a good test for the hypothesis that any given orchard was “clean” last year.  We rarely get significant green-tip scab infections when temperature during the first infection period is in the 40’s, but I can recall several years when warm rains at green-tip allowed for massive infections in unsprayed blocks.

Last year the weather forecasts for the lower Hudson Valley for the period between green-tip and mid-bloom were consistently inaccurate and resulted in scab sprays being applied repeated for predicted wetting events that never developed into infection periods.  Thus, the predicted green-tip wetting period for Tuesday may still fizzle, but those who need several days to cover orchards (or to clear brush before they can spray) have now been given fair warning!

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Fungicides and Copper Sprays for 2014

Mon, 7 April: Because deep snow interfered with winter pruning in many orchards, it is likely that many folks will still be contending with brush from recent and on-going pruning when the first sprays will be needed (perhaps by the end of this week).  Nevertheless, it will be critical to get sprays applied on-time to minimize the potential for season-long disease problems that can evolve if early-season sprays are delayed in high-inoculum orchards.

For our most recent thoughts on scab control strategies, see our 24 March 2014 article in Scaffolds.  For relevant comments on the use of copper at silver tip and green tip, see the 25 March 2013 article in Scaffolds.

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