Scaffolds Fruit Journal, Cornell University NYSAES

Diseases | Insects | Credits


Volume 6, No. 4								April 14, 1997


COMING EVENTS

                                                     43F       50F
Current DD accumulations (Geneva 1/1-4/14):         127        51
                       (Highland 1/1-4/14):         181        71

Coming Events:                              Ranges:
Redbanded leafroller 1st catch                    32-480     5-251
Green fruitworm peak flight                       64-255    19-108
Spotted tentiform leafminer 1st catch             73-433    17-251
Rosy apple aphid nymphs present                   91-291    45-148
Pear psylla 1st nymphs                           111-402    55-208
Green apple aphid present                        127-297    54-156
Oriental fruit moth 1st catch                    129-587    44-338
Pear thrips in pear buds                         137-221    54-101
Spotted tentiform leafminer 1st oviposition      141-319    48-154
Obliquebanded leafroller larvae active           149-388    54-201
European red mite egg hatch begins               157-358    74-208
McIntosh at half-inch green                      112-221    54-101

Phenologies: (Highland): Apple (McIntosh) - Early Tight Cluster
                         Pear (Bartlett) - Late Bud Burst
              (Geneva):  Apple (McIntosh) - Early Green Tip
                         Pear, Cherry, Plum - Swollen Bud
                         Peach - Quarter-Inch Green

PEST FOCUS
   Highland: 1st Spotted Tentiform Leafminer trap catch 4/11.
             1st Green Apple Aphid on apple.
             1st Apple Rust Mite observed.

TRAP CATCHES (Number/trap/day)
Geneva:   
                                   4/3    4/7   4/10   4/14
Green Fruitworm                      -    0.2*     0      0
Redbanded Leafroller                 -      -      0      0
Spotted Tentiform Leafminer          -      -      -      0

Highland (Dick Straub, Peter Jentsch):
                                   4/3    4/7   4/14
Green Fruitworm                      -    0.1*   0.4
Redbanded Leafroller                 -    0.4*   2.4
Pear Psylla (eggs/bud)               -    1.4*   3.1
Spotted Tentiform Leafminer          -      0    0.8*
Oriental Fruit Moth                  -      -      0
                                                       * = 1st catch


Diseases

MILLS REDUX: The Latest Revisions of the Infection Period Table for Primary Apple Scab

by David M. Gadoury & Robert C. Seem
Plant Pathology, Geneva

THE ORIGINAL MILLS TABLE
The Mills Infection Period Table has been used throughout the world for over 40 years. This table (in the unlikely event that you've forgotten) provides an estimate of the number of hours of continuous leaf wetness required for infection by ascospores at temperatures from 42 to 78F, and formed the basis of disease forecasting system that is still in use today. The essential elements of Mills' system were:

  1. a table for predicting three levels of infection based upon temperature and duration of leaf wetness, and
  2. a rule that secondary infection by summer spores (conidia) requires only two-thirds the time required by ascospores.

The original Mills Table is rarely used today. The most common version was modified sometime in the 1980s by Alan Jones at Michigan State University.

Many orchard and lab studies, both in the US and abroad, have indicated that ascospores can infect in less time than indicated by the original Mills Table, or by Jones' revision. Furthermore, none has supported Mills' claim that conidia require one-third less time than ascospores. The original table was also vague at low temperatures, saying only that infection at less than 42°F took "more than 2 days". Finally, the three levels of infection were often misused as forecasts of scab severity following rain.

THE NEW INFECTION PERIOD TABLE
The new infection period table is the result of several years of collaborative research involving Bill MacHardy (Univ. New Hampshire), David Gadoury (long ago at UNH, now at Geneva), Arne Stensvand (once at Geneva, now in Oslo, Norway), and Bob Seem (at Geneva since day 1). The new table is based upon a more thorough understanding of the biology and epidemiology of the apple scab pathogen than was available to Mills in 1944. In 1989, MacHardy and Gadoury reduced the times reported by Mills by three hours at all temperatures. The new infection times were supported by previous orchard studies in several countries, and by subsequent work by MacHardy in New Hampshire. This table is now used in several countries.

Despite the revisions made over the years, previous infection period tables lacked data to adequately support the times given for infection at temperatures below 42°F. Mills original table stated simply that infection at 42°F or below took "more than 2 days". Jones modified the infection times only between 37 and 46°F. Based on our most recent studies, we now know that infection can take place in as little as 30 hours at 37°F. That's 18 hours LESS than the time in Mills original table, and 11 hrs less than in Jones revision. These are not trivial differences. The table presented here also appears in the 1997 Cornell Pest Management Recommendations for Tree Fruit Production. Above 43°F, the table uses the times recommended by MacHardy and Gadoury from 1989, and at lower temperatures the times were based upon experiments by Stensvand. Leaf attacked by scab

WHY USE THE NEW TABLE? Because the earlier tables underestimate the time required for infection at all temperatures, and grossly so at 34-46°F. Granted, that's a rather narrow temperature range, but infection periods in that range occur all too frequently between green tip and pink. At 70°F, there is only a 3-hour difference between the new table and the original (6 vs 9 hrs). Following most rain events, that difference may not effect a spray decision. However, at 37°F, the difference is 11-18 hrs (30 vs 41-48 hrs).

WHY DOES THE NEW TABLE HAVE ONLY ONE LEVEL OF SEVERITY? Because factors other than the length of the wetting period are far more important in determining severity of scab. The actual amount of scab that develops following a rain event is determined by:

  1. the actual quantity of ascospores available (PAD, or potential ascospore dose; the number of ascospores/sq. ft. orchard floor),
  2. the density of the tissue exposed; i.e.,the size of the target,
  3. the susceptibility of the target to infection, and
  4. the presence or absence of secondary (conidial) inoculum.

Increasing the length of a wetting period above the minimum required for infection may result in a 2- to 6-fold increase of scab. Don't worry about this relatively minor impact (length of the wetting interval), and ignore the much larger (>100 X) combined impacts of target size, target susceptibility, and inoculum dose. That's why the new infection period table describes only the minimal conditions required for infection. After these minimum conditions are satisfied, factors other than continued rain will determine the amount of disease that develops.

Consider this: severe scab will follow "light" Mills periods when PAD or secondary inoculum is high and the stage of tree development maximizes target size and target susceptibility (early tight cluster to bloom). Conversely, when inoculum dose is low and trees are at green tip, "severe" Mills periods may not result in ANY detectable scab infection. As you are painfully aware by now, there are few low-inoculum orchards this year. But next year.......

TROUBLED BY NOCTURNAL EMISSIONS? MacHardy and Gadoury developed guidelines on how to deal with night-released ascospores in 1989. Ascospore discharge is suppressed by darkness. This suppression is not absolute. About 5% of the available ascospores will be discharged during darkness when rain begins at night. Whether or not this small percentage constitutes a threat depends upon PAD. The threshold value of PAD that is now used to delay spraying beyond green tip is 53 infected leaves/600 shoots. At or below this level of PAD, the night-released ascospores will not cause a sufficient number of infections to warrant spraying.

Now, having said all that, *using the new infection period table is independent of day and night release of ascospores*. The new table was developed because primary infection occurs in less time than indicated in earlier tables. Because of the region-wide prevalence of high overwintering inoculum levels in 1997, the errors in the earlier tables are likely to be especially significant this year. It would be a mistake to stick with the older tables this year under the assumption that it was "less risky".

CAN SECONDARY INFECTION OCCUR IN LESS TIME THAN PRIMARY? Probably not. In fact, most studies indicate that the ascospores have a slight edge in the race to infect. On the other hand, conidia can disperse more rapidly following rain. Our studies indicate that the race is, for practical purposes, dead even. So, you can (and should) use the same table for both primary (ascsospore) and secondary (conidial) infection.

Table 1. Minimum times required for infection by ascospores of Venturia inaequalis according to Mills (1944), Jones (1980), and the New Infection Period Table of MacHardy and Gadoury (1989) as amended by Stensvand, et al, (1997).

_______________________________________________________________________
              Minimum hours of leaf wetness required for infection
              ____________________________________________________
Temperature      Mills      Mills revised by Jones      New Table
___________      _____      ______________________      _________

   34             >48             48                       41
   36             >48             48                       35
   37             >48             41                       30
   39             >48             33                       28
   41             >48             26                       21
   43              25             21                       18
   45              20             17                       15
   46              19             16                       13
   48              15             15                       12
   50              14             14                       11
   52              12             12                        9
   54              12             11.5                      8
   55              11             11                        8
   57              10             10                        7
   59              10             10                        7
   61-75            9              9                        6
   77              11             11                        8
________________________________________________________________________


Insects

PEAR PSYLLA CONTROL WITHOUT AGRI-MEK

by Dick Straub
Entomology, Highland

Because the NYS-DEC has requested substantive human safety provisions above and beyond those of the federal label, there is a high probability that Agri-Mek will not be available this season for use on pear (or apple) in New York State. Although the performance of Agri-Mek has varied in New York among seasons and locations, the results were generally quite good and this insecticide/miticide has been the standard for psylla management. If Agri-Mek is not available, in most pear orchards there remain two options: Mitac and Provado.

Fortunately, Mitac is still effective in many orchards. This contact material has been around for some time and most growers are aware of how to use it and the results to expect. Results from our research trials have shown that two applications (total of 3 lb a.i./Acre), as needed, usually maintain populations below damaging levels. It is a decent rescue material in situations where previous treatments of something else have failed. Mitac is restricted to two applications per season.

Because Agri-Mek has been the standard, usage of Provado (imidacloprid) has not been extensive. Provado is absorbed into the leaf and therefore acts primarily as a stomach poison. The label allows for two applications at 20 oz/acre, to be applied postbloom as needed. Provado has been included in our evaluations during the past six years. A summarization of the data suggests that an application at petal fall, followed by a second application within 14 to 28 days, works well against low to moderate populations. However, during two years with extremely high populations (1994 & 95), this two-application scheme did not work - but then, neither did anything else! Provado appears to be suitable for rescue situations.

Pear psylla
Pear psylla nymph enclosed by its honeydew droplet
Pear psylla nymph
Pear psylla "hard shell" (fifth instar) stage nymph

History verifies that pear psylla can be a difficult pest to manage. Obvious reasons include: the earliness with which they appear (in Hudson Valley, 1st adult - 25 March; significant oviposition - 5 April); high reproductive potential; preference for new growth, of which most pear orchards have an abundance; the protective barrier provided by honeydew; and the relative "toughness" of the hardshell stage. Good timing, thorough coverage, application(s) of dormant oil to delay egg laying and a relatively low incidence of water sprouts will make the management task easier.

It should be noted that neither Mitac nor Provado affect plum curculio, codling moth, oriental fruit moth or leafrollers. A note of warning - in most years, we have seen the development of inordinately high pear rust mite populations in plots where either of these materials were used.

THE BUGS OF APRIL

by Art Agnello & Harvey Reissig
Entomology, Geneva

Early season arthropod pests, that is, those present before bloom, generally fall into two groups - the mites, and everything else. Generally speaking, everyone has to do something about their mite populations, but not everyone has to do something about the rest of the potential pests, or at least usually not all of them; these include rosy apple aphid, tarnished plant bug, and spotted tentiform leafminer. We've already given our waffled and highly conditional assessment of mite management this season, and it's not going to get much clearer until we hear from the regulatory camp, so let's leave bad enough alone for the time being and move on to the insects.

Fortunately, all three of these management issues converge at about the time of pink (along with half a million other things, but you're tough, we know, otherwise you wouldn't be growing apples). The whole key behind a pink strategy boils down to setting priorities, or else call it degrees of nervousness, and this is something for which most growers show an intrinsic talent. Everyone has their own version of this process, all of which stem from a familiarity with one's own orchards - does a given block have a history of or sensitivity to a specific pest? Start with your knowledge of the block, apply a sampling procedure where appropriate, and make a management decision.

In our opinion, the most crucial of the pest decisions to be made at pink has to do with rosy apple aphid (RAA), because this is the last defining period for a successful rosy management option. Although RAA feeds mainly on apple foliage, causing leaf chlorosis and curling, its saliva is also translocated to nearby fruits, which become bunched, stunted, and malformed. RAA will attack all apple varieties, but varieties such as Cortland, Monroe, R.I. Greening, Ida Red, and Golden Delicious are particularly susceptible, and those in the McIntosh family are relatively tolerant. As with most aphids, this species has a complex life cycle, starting with black eggs that overwinter on twigs, in bud axils, and in bark crevices. The eggs develop into solitary, wingless "stem mothers", who then give birth to living young, most of whom are also wingless. RAA nymphs are visible beginning at about tight cluster but are most easily observed at the pink bud stage. Aphids at pink

Our control recommendations for RAA cover the period from 1/2-inch green to the pink bud stage, using any of a number of materials: Thiodan, Lorsban, Lannate, Vydate, Supracide or Asana, listed roughly in order of increasing harm to beneficial mites. Pink applications of any of these products do a better job than an earlier spray. This is an observation resulting from the fact that, in those cases where aphid populations have built up during early summer on vegetative growth inside the canopy, a pink spray will have done a more effective job of reducing populations than an earlier treatment at half-inch green. From the standpoint of management practicality, it is therefore easier and more natural to consider the need for aphid control at the time of the pink spray.

Because RAA populations are highly variable, it is important to assess their densities before making a treatment. In past surveys, approximately 50% of the orchards sampled have ended up requiring treatment. If you are inspecting fruit clusters for STLM eggs at pink anyway, it is not much more trouble to note the presence of RAA nymphs or damage at the same time. We recommend, however, that a few more clusters be checked for RAA than are required for STLM sampling. Try to select 10 from the interior canopy area of each of 10 trees distributed throughout the block. RAA nymphs are of course present at pink, and large enough to see without difficulty, but they do occur on the same tree and in the midst of colonies of green apple aphids, which are not usually a problem until the summer.

To distinguish among the species, you can use leaf damage as a cue, as well as the insects' color. RAA nymphs are usually pinkish, sometimes varying to a light brown, slate gray, or greenish black, and the body is covered with a whitish mealy coating. Most importantly, they have pronounced cornicles ("tailpipes"), and long antennae (more than half the body length). Green apple aphid nymphs are clearly green, and without the whitish cast. Their cornicles are little more than buttons, and the antennae are clearly less than half of the body length. Also, aphids found inside curled or distorted leaves at pink are almost always Rosy Apple Aphids. If you find ONE infested cluster (1%, or stop as soon as you find one), we would advise including an RAA material in your pink spray; this threshold may be a little conservative for people who are skilled at finding the aphids.

What else is happening at pink (mites aside)? STLM is laying eggs, but most orchards don't suffer too greatly from 1st brood leafminer, and even if so, a sequential sampling plan can be used to classify STLM egg density at pink or of sap-feeding mines immediately after petal fall (see pages 90, 94 or 96 in the Recommends). Treatment is recommended if eggs average 2 or more per leaf on leaves 2, 3, and 4 of a fruit cluster at pink, or if sap-feeding mines average 1 or more per leaf on these leaves at petal fall. Sampling can be completed in approximately 10 minutes. In recent years, only 1 out of 6 sampled orchards have required insecticide treatments to control first-generation STLM populations. Vydate at pink or Lannate at petal fall have been our standard recommendations for this pest; however, now we also have the petal fall option of Provado, which will add to the leafhopper control if you don't use enough Sevin at thinning to do an adequate job.

Leafrollers are also out there, but only part of the population is active at this time, so it's better to wait for petal fall to address this one. Tarnished plant bug is the only player left, and you're going to have to decide for yourself whether it's a major concern to you. We have seen few orchards in western N.Y. where TPB control is warranted (slightly more in the Hudson Valley), simply because the most effective treatment to use is a pyrethroid, which

  1. wipes out predator mites, and
  2. still rarely lowers TPB damage enough to be economically justified.

If you elect an Asana spray at pink for plant bug, you'll take care of rosy apple aphid (and STLM) at the same time; if rosies are your primary concern, scout for them first, and use Lorsban or Thiodan if you find any.


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
Department of Entomology, NYSAES
Geneva, NY 14456-0462
Phone: 315-787-2341
FAX:315-787-2326
E-mail: ama4@cornell.edu


Photographs courtesy of New York State Integrated Pest Management Program


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