Scaffolds

March 27, 1995                     Volume 4                     Geneva, NY
Index:Diseases | Chem News | Other

COMING EVENTS

                                                     43F       50F
Current DD accumulations (Geneva 1/1-3/27):          111       49
                       (Highland 1/1-3/27):           89       35

 

Coming Events: Ranges: Green fruitworm peak flight 64-221 19-108 Redbanded leafroller 1st catch 32-480 5-251 Spotted tentiform leafminer 1st catch 73-433 17-251 Rosy apple aphid nymphs present 91-291 45-148 McIntosh at green tip 24-161 4-74

PHENOLOGIES: Apple (McIntosh), Geneva and Highland: Silver Tip Pear, Cherry, Peach, Plum: All dormant

PEST FOCUS (Geneva) Pear Psylla eggs found, 3/21 (Mike Dunham) Green Fruitworm - 1st catch, 3/27


ASCOSPORES, ONE LAST TIME

By:(Wayne Wilcox)

As many of you know, my commodity responsibilities changed during this past winter (good-bye tree fruits, hello grapes). Furthermore, (a) our Department's efforts to refill my old position are on hold due to the NYS budget situation; and (b) Ron Nevill, who has done the hands-on work to provide weekly ascospore maturity counts from Geneva over the last 10 years, is on disability leave for an indefinite period of time. Therefore, we will not be providing these weekly counts during the coming season, although Dave Rosenberger will continue to do so for selected locations in eastern NY.

For the last several years, I've tried to discuss the limitations of our weekly counts, some perspectives on interpreting them, and how they can be estimated more quickly and locally using a simple degree day model. Lacking alternatives, this will be the year to put the model to the test. Below is a reworking of last year's article on the subject, with some additional discussion of the research that Dave Gadoury and Bob Seem in our Department have been conducting on ascospore maturity and discharge.

Two main points: (1) The simple degree day model used to estimate ascospore maturity (see below) is at least equal to, and probably better than, the method we have traditionally used. It's also a lot faster, and can be run by anyone with access to daily minimum and maximum temperatures in their region. Furthermore, this model can be used to estimate spore maturity in local areas, where tree and pathogen development may be (and probably are) different than in Geneva. (2) Ascospore maturity determinations are relatively meaningless if you don't know what the seasonal inoculum potential is in your individual orchard(s). For instance, seasonal inoculum doses in commercial orchards commonly range between 100 and 100,000 ascospores per square yard of orchard floor. Admittedly, orchards in the upper ranges don't stay commercial for very long, but even "clean" orchards can easily vary between the 100 and 1,000 spores/square yard level. Thus, a soaking rain will cause an equal release of 50 spores/square yard when the 1,000-level orchard is 5% mature or when the 100-level orchard is 50% mature.

In my opinion, we all spend too much time worrying about whether 5% or 25% of the spores are mature, and not enough time putting these percentages into context. The widespread green tip infections that occurred in 1993 are a perfect example: they occurred when a very small percentage of the spores were mature, but the 1992 Summer of Rain had given us an abnormally high base level to start with. Remember, 5% of a lot still can be more than 100% of a little.

Nevertheless, ascospore maturity data can be useful, particularly when growers or advisors learn, through experience, how to interpret them for their own particular orchards or regions. Below, I've summarized the data from our ascospore maturity determinations at Geneva over the last 7 years. These include the percentage of asci APPEARING to be "mature" using our traditional microscopic examinations and those that had already discharged their ascospores. Also included are the data from our artificial ascospore discharge tests, expressed as we reported them each week (spores/LP field, 1 hr shoot) and as a cumulative percentage of the season's total through the date in question. Finally, for each assessment date I've also provided the number of degree days (base 32F) accumulated after the first date of McIntosh green tip. The purpose of providing the degree day accumulations is to validate the usefulness of the following degree-day model.

In the early 1980's, Dave Gadoury and Bill MacHardy provided data from New Hampshire showing that the first truly mature ascospores are ready to shoot at McIntosh green tip, and that the seasonal spore load continues to mature at a rate that can be described according to a degree-day model, using 32F as a base. So, to run the model, you "start the clock" on the day that approximately half of the McIntosh fruit buds show green tip, then keep track of the accumulated degree days above 32F (DD32). That is, for each day, DD32 = average temp (which is calculated as the max + min temps, divided by 2), minus 32. Example: if high temp = 60 and low = 40, then DD32 = 18, i.e., 50 (the average temp) -32 (the base temp). According to the model, spore maturity progresses slowly for the first 175-225 accumulated degree days after green tip, then increases rapidly. How does this relate to traditional spore maturity counts?

                                     ASCOSPORE DISCHARGE TEST
          PHEN  DD32     % OF ASCI     Spores/LP  Cumula-
YEAR DATE (a*)  (b*) "Mature" Dischrgd  field     tive %
'88  4/14  GT     0      5      <1       6          1
     4/21  QIG   52     11       1      11          3
     4/28  HIG  123     13       2       4          4
      5/5  TC   212     14       3      29         10
     5/20  BL   583     22      37      83         24
     5/26  PF   764     18      48     118         45
      6/2       999     18      71      94         63
      6/9      1184     10      84     154         90
     6/16      1414     10      90      73        100

 

'89 4/21 GT 0 2 0 0 0 4/27 QIG 61 3 0 0 0 5/5 HIG 188 7 1 27 5 5/11 TC 283 13 4 20 8 5/18 P 425 47 8 149 34 5/26 BL 684 46 26 230 73 6/1 PF 865 39 45 96 90 6/7 1073 11 86 52 98 6/16 1350 2 98 9 100

'90 3/29 GT 0 6 0 0 0 4/5 GT 55 16 1 13 1 4/12 QIG 96 31 2 28 4 4/19 HIG 151 25 5 35 8 4/26 TC 287 37 7 64 14 5/4 BL 545 - 7 217 36 5/10 BL 656 45 23 214 57 5/17 PF 810 31 42 24 60 5/24 954 25 56 112 71 5/24 1152 19 71 86 79 6/8 1399 13 82 178 97 6/14 1586 9 91 35 100

'91 4/4 GT 0 9 0 0 0 4/11 HIG 189 33 5 44 4 4/18 TC 268 21 3 90 12 4/25 TC 348 22 6 84 20 5/2 P 529 30 27 114 31 5/9 BL 662 31 43 146 44 5/16 PF 865 29 50 150 58 5/23 1076 33 47 243 80 5/30 1362 28 62 146 94 6/6 1607 16 79 72 100

'92 4/16 ST - 12 0 0 0 4/24 QI 75 27 <1 30 3 4/30 HIG 141 38 5 46 8 5/6 TC 262 45 5 108 20 5/15 BL 481 66 6 177 40 5/21 BL 637 46 47 221 64 5/28 PF 803 13 78 201 86 6/4 937 17 75 58 93 6/12 1228 2 97 66 100

'93 4/15 ST - 10 0 0 0 4/22 QIG 74 19 <1 3 <1 4/29 HIG 156 31 3 14 2 5/6 P 342 70 8 140 17 5/13 BL 547 42 29 251 45 5/27 PF 839 68 23 198 66 6/3 1003 39 57 179 86 6/9 1157 8 89 122 100 '94 4/21 GT 55 9 <1 1 <1 4/28 ETC 228 23 <1 18 3 5/6 EP 367 30 5 45 9 5/12 P 498 38 25 81 20 5/19 BL 604 33 32 196 47 5/25 PF 784 38 36 122 64 6/3 15 62 118 81 6/9 12 79 140 100


a* PHEN = Approximate phenological stage of McIntosh trees at Geneva: ST = silver tip, GT = green tip, QIG = 1/4" green, HIG = 1/2" green, TC = tight cluster, P = pink, BL = bloom, PF = petal fall b* Accumulated number of degree days (base 32F) since McIntosh green tip.

SO WHAT DOES IT MEAN? Three major points stand out:

WHAT DOES IT MEAN FOR 1995? Accumulated number of degree days since green tip and tree phenology will probably give you as much useful information as our traditional determinations did in the past. The 1994 season was a fairly light one in MOST orchards, so early infection periods should be much less important ON AVERAGE than in years following a bad scab season. Growers planning to use an SI fungicide program in orchards that had little scab last year should be able to delay their first spray until tight cluster without unpleasant surprises. Most primary inoculum should be expended before first cover (although there are MANY good reasons to include an SI through the first cover spray). Growers who had scab last year shouldn't be playing games this year. Same old stuff.

AND WHAT ABOUT DISCHARGE OF ASCOSPORES AT NIGHT? For many years, it has been known that the discharge of scab ascospores is severely suppressed during darkness under most conditions. Some time ago, this knowledge led workers in other parts of the world (e.g., Australia, South Africa) to propose that when a rain begins at night, the hours of leaf wetness that occur between the start of the rain and daylight can be ignored when determining the potential for a primary infection period. In 1989, Bill MacHardy and Dave Gadoury published a paper based partly on data from New Hampshire, in which they also proposed adopting this practice.

The reasoning here is simple: leaf wetness doesn't matter if there are no ascospores around to cause an infection, so don't start the infection period clock until the inoculum is present. Doing so has no effect on the ultimate calculation of many infection periods (e.g., those that start during the day, or rains that begin at night but continue for a long time into the next day), but sometimes it can have a major effect on the interpretation of rainy periods. For instance, on April 24 last year, a rain started in Geneva at 9 PM and leaves remained wet until 8 AM Monday morning, giving us 11 hr of leaf wetness at an average of 58F. According to traditional interpretations, this constituted a light Mills infection period; however, if we wait until 7 AM to start the clock, we get an insignificant 1 hr wetting period.

So, which one's right? Unfortunately, biology is seldom as black-and- white as we'd like, so it often comes down to playing the odds. Published studies and recent work by Dave Gadoury and Bob Seem provide some basis for assessing these odds: Even though ascospore release is severely suppressed at night, it isn't eliminated. When four rainy periods during the month of May were analyzed in New Hampshire, 96-97% of the ascospores released were shot during the day and only 3-4% at night. Whether or not 3-4% is significant depends on how much you're starting with, that is, what's the seasonal overwintering inoculum potential from last year's scabby leaves and how many of those are mature. This is the same dance as described above: 3-4% of a lot is still a lot, 3-4% of a little may not be important (e.g., 3-4% release of the mature spores during a night rain early in the seaon, when only 10% of the seasonal crop has matured, translates to 0.3-0.4% of the seasonal spore load released at that time. But how big IS the seasonal spore load?).

My own opinion is that we still don't know it all, but if you're going to disregard the hours of wetness between the start of a nighttime rain and the next morning, do so only in a low inoculum orchard. Other factors in this equation are relative cultivar susceptibility (Macs or Goldens?), where we are in terms of general ascospore maturity, whether "extenuating circumstances" apply (if it hasn't rained for 3 weeks previously, spores are more likely to release no matter what), and the specific characteristics of your fungicide program (an SI program that will suppress "surprises"? Enough contact fungicide residue still on the tree to take care of a few spores that might slip through?). Finally, remember that there are a lot of different programs for controlling apple scab, and this doesn't have to get any more complicated than you want it to be.

ROLL OUT THE BARREL

By:Art Agnello

For those of us who believe in the "conservation of weather" theory, it doesn't seem strange for us to have been compensated for last year's cold winter temperatures with relatively moderate ones this year. The possible downside is that, while we're busy feeling grateful for the balmy breezes of December and January, we're likely to overlook the possible beneficial effect those 1994 sub-zero readings had on certain pests like European red mites and pear psylla. Some people have been speculating whether this past winter was a bit too kind to our 6- and 8- legged friends, and whether they might end up being in rare form once the spring starts in earnest. Obviously, overwintering mortality is likely to be a bit lower during a winter that doesn't have many extreme low-temperature periods. However, as always, the overall potential for troublesome populations is probably more dependent on the spring weather conditions -- how quickly it gets warm, how much rain we receive, how many up-and-downs there are, etc.

This doesn't change the importance of acting with timeliness in the matter of applying the first sprays, which often contain oil. Fortunately, the relative lack of snow accumulation over the past few months has left orchard floors that may be in somewhat better than normal condition for early tractor work, especially if our current weather trends continue. As in past years, it's especially important to prepare beforehand for the early season applications most important to your operations, by having in mind not only your intended strategy but also a fallback alternative, just in case all the sprays don't get on as intended. The following information is a review of previous years' guidelines for early season oil applications:

PEAR PSYLLA

Early oil sprays can be very beneficial against pear psylla until the swollen bud stage, not to kill the adults, but to inhibit their egg- laying activity. This approach is particularly advisable in years when their early activity is drawn out because of persistent cool and wet spring weather. The basic strategy is to buy some time before applying an insecticide spray. Oil rates depend on when you start: If your buds are at the dormant stage, one spray of 3% oil, or (to hedge your bets against uncertain weather trends) two of 2% through green cluster are recommended; if you start at swollen bud, one spray at 2% or two at 1% up to white bud should be adequate for this purpose, especially if applied as soon as the psylla become active (50F or above). This will also give some red mite control at the same time.

A few points regarding contact materials for psylla at white bud: First of all, Mitac can be used nearly anytime during the season, of course, and its label no longer restricts its use to two applications (of either formulation) per season, but we still feel it may be advisable to "save" it for postbloom and use something else at white bud, if you so elect. Pears remain off the Morestan label for New York growers (although it remains available for use on apples). Both Mitac and the pyrethroids have been a little sporadic in their effectiveness against postbloom psylla populations recently, although the variability in their performance makes it difficult to generalize about possible causes. Research and grower experience tells us that resistance to pyrethroids exists, that it is highly localized (your psylla may be resistant, but not your neighbor's), and that cross-resistance may or may not be involved (what happens with Asana may not necessarily happen with Ambush or Pounce). Mitac apparently did a good job last season, although the weather's influence should not be overlooked here (ideal for spring oil sprays, and not so favorable for late summer resurgence). Agri-Mek is available again this year (see "Chem News") at a recommended application timing of 10-15 days after petal fall, combined with keeping an eye on August populations.

Other options may be worth noting. M-Pede insecticidal soap can provide some suppression when used in a seasonal spray program, starting at swollen bud. Keep in mind that uniform drying conditions are required to prevent droplet residue on fruit, and that it is solely a contact material with virtually no residual activity. Last year, I looked at a program of 3-4 sprays of UltraFine oil starting at petal fall in some grower orchards. Both the 1% and 2% rates did a good job against the light to moderate infestations they had, but we weren't able to verify the long-term effectiveness against heavier populations. The grower elected a mid-July Agri-Mek spray in the one orchard where threatening numbers eventually developed. This should still be considered an experimental approach, good against low populations, and on the expensive side.

EUROPEAN RED MITE

Our preferred approach to early season mite (and to a lesser degree, San Jose scale) control in apples continues to be a delayed- dormant spray of petroleum oil from green tip through tight cluster, to conserve the efficacy of the contact miticides we still have. Technically, we have been advising that it is possible to get good control of overwintered eggs using 2 gal/100 at the green tip through half-inch green stage, or 1 gal/100 at tight cluster; this advice assumes ideal weather and excellent coverage. As we all know, oil applications don't always live up to our expectations, not only because of weather and coverage problems, but also because proper timing is difficult. That is, we have seen mites start to hatch when the trees are at solid tight cluster, so naturally the oil loses its ability to smother anything when it reaches only up to the kneecaps. To be practical, you'll probably be better off if you do the following:

Good coverage of the trees is essential to take advantage of oil's potential efficiency; this in turn requires adequate spray volume delivered at an appropriate speed. Experience and research show that a 1X concentration (300 gal/A) is clearly preferable; however, if all other conditions are perfect (weather, speed, calibration), then 3X, or 100 gal/A, is the highest concentration that MIGHT be expected to give acceptable control at any given time. Some growers have concentrated more than this to save time and the hauling of extra water, with no problems; you might get away with it, but it doesn't really pay to take the risk.

SAN JOSE SCALE

This may be one of those rare pests that is on the decline in N.Y., but for those blocks still affected, a 2% oil treatment at half- inch green will control the nymphs, and is the preferred treatment if no other problem insects need to be controlled. Combining the oil with an insecticide is usually not more effective in this case than using the oil or insecticide alone. If you choose not to use oil against the scale nymphs, or have rosy apple aphid or other early season insects to be controlled, an insecticide would be more appropriate. For both of these pests, Lorsban 4EC or Supracide 2EC have proven very effective during the green tip to tight cluster stage. Check the opening buds for infestations of rosy apple aphid; treatment would be advisable upon finding one colony per 100 clusters.

CHEM NEWS

On Friday, March 24, the NYS DEC gave Provado 1.6F a label for use to control postbloom populations of green aphids, leafminers and leafhoppers in N.Y. apples. A future issue will provide some suggestions for the effective use of this product, based on Harvey Reissig's field trials.

THIS JUST IN - N.Y.'s Section 18 request for the use of Agri-Mek in pears has been granted for 1995. We'll print the provisions of this label (which is virtually identical to last year's) next week.



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Scaffolds is published weekly from March to September by Cornell University -- NYS Agricultural Experiment Station (Geneva), and Ithaca -- with the assistance of Cornell Cooperative Extension. New York field reports welcomed. Send submissions by 3 p.m. Monday to:

Scaffolds Fruit Journal
Editors: A. Agnello, D. Kain
Dept. of Entomology, NYSAES
Geneva, NY 14456-0462
Phone: 315-787-2341 FAX:315-787-2326
E-mail: art_agnello@cornell.edu

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