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Volume 7, No. 3                                                 April 6, 1998


                                                     43F       50F
Current DD accumulations (Geneva 1/1-4/6):           178       102
                    (Geneva 1997 1/1-4/6):           110        42
                (Geneva "Normal" 1/1-4/6):            81        34
                       (Highland 1/1-4/6):           281       148

Coming Events:                              Ranges:
Pear psylla 1st oviposition                       25-147      1-72
Redbanded leafroller 1st catch                    32-480     5-251
Green fruitworm flight peak                       64-255    19-108
Spotted tentiform leafminer 1st catch             73-433    17-251
Pear psylla nymphs present                       111-402    55-208
Oriental fruit moth 1st catch                    129-587    44-338
Obliquebanded leafroller larvae active           149-388    54-201
European red mite egg hatch                      157-358    74-208
McIntosh at tight cluster                        188-279    68-138

Phenologies (Geneva): Apple (McIntosh) - Half-Inch Green
                            (Red Delicious) - Half-Inch Green
                      Pear (Bartlett) - Bud Burst
                      Sweet Cherry (Darrow) - Bud Burst
                      Tart Cherry (Montmorency) - Swollen Bud
          (Highland): Apple (McIntosh) - Tight Cluster
                      Pear (Bartlett) - Green Cluster
                      Plum (Stanley) - Green Cluster
                      Apricot - 50% Petal Fall
                      Peach - Full Bloom

TRAP CATCHES (Number/trap/day)
                                  3/23   3/27   3/30    4/2    4/6
Green Fruitworm                      -      -      -    0.7*   0.6
Spotted Tentiform Leafminer          -      -      -    0.2*     0
Redbanded Leafroller                 -      -      -    0.2*     0

Highland (Dick Straub, Peter Jentsch):
                                  3/23   3/27   3/30    4/6
Pear Psylla (eggs/bud)               0    3.0    9.7    5.5
Green Fruitworm                      -      0    3.0*   0.1
Spotted Tentiform Leafminer          -      0    0.1*   0.1
Redbanded Leafroller                 -      -      -      -
                                                            * 1st catch
   Geneva: Green Fruitworm flying.
 Highland: 1st Tarnished Plant Bug onbserved on apple, 3/31
           1st Pear Psylla nymphs observed on pear, 3/31



byDave Rosenberger
Plant Pathology, Highland

Apple scab ascospore counts as determined from squash mounts:

Date     Location      Immature   Mature  Discharged   Shoot  
4/1   Ulster (Highland)  65%       33%       2%         432      
3/30     Saratoga        58%       41%       1%          52
3/31     Peru            88%       11%       0%          93     
4/6   Ulster (Highland)  54%       41%       5%       >1,000

Leaves collected in Saratoga with trees at half-inch green and in Peru with trees at green-tip showed advanced spore maturity for both locations. The numbers of spores trapped in the spore tower discharge were relatively low for the Saratoga sample because the leaves tested had relatively few pseudothecia compared with the leaves from Highland and Peru. The tower discharge was higher than expected for the spore maturity levels observed in the Peru sample, probably because of a high degree of variability in spore maturity among various leaves in the sample.

The first scab infection period in the lower Hudson Valley occurred April 1-2, began at 2:30 pm, and involved 23 hours of wetting at 53F. Early blooming apple varieties were at the early tight cluster bud stage. Most commercial growers either applied mancozeb before the rain or used SI-mancozeb combinations after the infection period.


byDave Rosenberger
Plant Pathology, Highland

The "Mills Table" has been a standard tool for determining apple scab infection periods since it was first published in 1944. Dr. Mills made careful observations that allowed him to show the relationships between temperatures, duration of leaf wetting, and development of apple scab infections. The biology of the apple scab fungus hasn't changed much since 1944, but various researchers have suggested ways that the Mills Table could be improved. The latest changes are evident on pages 41-43 in the 1998 Pest Management Recommendations for Commercial Tree-Fruit Production. The reasons for the changes are explained on page 41 in the "Recommends".

The old Mills table showed different time requirements for initiating light, moderate, and heavy scab infections. The 1997 Apple Scab Infection Table presents only a single value for hours of wetting that are required to initiate infections. The old Mills table also showed differences in wetting requirements for primary and secondary infections, whereas the 1997 Scab Infection Table does not differentiate between primary and secondary infections. The hours of wetting required for infections are considerably shorter in the 1997 Scab Infection Table than those indicated for primary infections in the original Mills table (see Table 1 below).

Since wetting requirements for scab infections are shorter in the 1997 Scab Infection Table than those indicated for primary infections in the original Mills table, does that mean that Dr. Mills was wrong? No, he wasn't wrong. Dr. Mills developed his infection period table based on observations in the field. Under field conditions, he noted that infections caused by conidia (secondary infections) often occurred with shorter wetting periods than did infections caused by ascospores (primary infections), and he also noted that severity of scab increased as duration of the wetting periods increased. Thus, his observations integrated the hours of wetting required for infection, risk factors associated with longer wetting periods, and inoculum level differences between primary and secondary scab into a single table of predicted apple scab infection periods (i.e., the original Mills Table).

The 1997 Apple Scab Infection Table has been simplified to show only the minimum wetting periods required for infection by either ascospores or conidia. The 1997 table does not address levels of scab severity that may result after the minimum requirements for infection have been met. Factors that affect the severity of apple scab infections include duration of the wetting period, inoculum levels within the orchard, and amount of susceptible tissue present on the tree at the time of the infection period.

Scientists now have solid data showing that ascospores and conidia require the same number of wetting hours to cause infections. However, the severity of apple scab that results from infection periods of the shortest duration is often quite different for infections caused by ascospores and infections caused by conidia. A few primary scab infections in an orchard will produce conidia that outnumber the ascosporic inoculum by several orders of magnitude. As inoculum loads increase, probabilities increase that observable infections will occur within the minimum wetting periods identified in 1997 Scab Infection Table.

When only a few spores are present (e.g., only ascospores in orchards with low to moderate levels of overwintering scab), then the risk of infection during the minimum wetting period listed on the 1997 Scab Infection Table is actually quite low. The low risk does not occur because ascospores require more time to cause infection. Instead, the low risk results from the fact that with a small number of ascospores and a minimal-wetting infection period, most ascospores will be "stranded" in mid-air as the wetting period ends. Very few of the ascospores will land on susceptible tissue during the minimal-wetting infection period. It is this "probability factor" associated with low inoculum levels that caused Mills to conclude that ascospores required longer wetting periods than did conidia. Similarly, the probability of infection increases as the duration of the wetting period increases because more spores have time to "find" susceptible tissue. Again, this observation led Mills to propose varying levels of infection for varying durations of wetting at any given temperature.

Apple growers who prefer a scab infection table that also provides severity predictions may wish to refer to the 1980 revision of the Mills Table developed by Al Jones in Michigan (Table 1). The original Mills Table was not very accurate at temperatures below 48 F. The 1980 revision by Jones provides better predictions for low temperatures. Anyone using either the original Mills Table or the revision by Jones should recognize that these tables may occasionally fail to predict ascospore-initiated infections resulting from short wetting periods in high-inoculum orchards. Also, be aware that the shorter infection times indicated in the 1997 Scab Infection Chart must always be used as soon as conidia are present in an orchard.

Table 1. Minimum times required for infection by ascospores of Venturia inaequalis according to Mills (1944), Jones (1980), and the 1997 Infection Period Table in the Cornell "Recommends".


           Minimum hours of leaf wetness required for infection

                         Mills as revised by Jones

            Original    Light    Moderate     Heavy        1997 Scab
Temperature   Mills   Infection  Infection  Infection   Infection Table

34           >48          48        72          96          41
36           >48          48        72          96          35
37           >48          41        55          68          30
39           >48          33        45          60          28
41           >48          26        37          53          21
43            25          21        30          47          18
45            20          17        26          40          15
46            19          16        24          37          13
48            15          15        20          30          12
50            14          14        19          29          11
52            12          12        18          26           9
54            12          11.5      16          24           8
55            11          11        16          24           8
57            10          10        14          22           7
59            10          10        13          21           7
61             9           9        13          20           6
63-75          9           9        12          18           6
77            11          11        14          21           8


by Dave Rosenberger
Plant Pathology, Highland

For many years, apple growers in New York and New England have used estimates of apple scab ascospore maturity as one of the "risk factors" to consider when timing early season apple scab sprays. Information on ascospore maturity was traditionally generated by removing pseudothecia (the globular spore sacs) from overwintered apple leaves and examining the contents of the pseudothecia under the microscope. Each pseudothecium is filled with elongate, finger-like structures call asci, and each ascus eventually contains eight ascospores. Early in the season, many of the asci appear only partially developed and no ascospores are present. As the season progresses, the asci fill with spores and the ascospores gradually develop their characteristic shape and olive-green color. At this point, the spores are termed "mature" even though some additional time is required before they will actually discharge in a rain.

By examining squashed pseudothecia beneath a microscope, skilled observers can count how many of the asci have colored spores and thereby determine the proportion of asci with mature spores. Based on many years of experience, we know that ascospores begin discharging in significant numbers after 15-17% of the asci contain mature spores. Generating ascospore maturity data by counting ascospores requires 2-3 hours of skilled technical time for each count that is generated. Counts taken on the same day can be quite variable depending on the exact location of the leaves sampled and the skill of the individual making those counts.

Now ascospore maturity can be accurately estimated using a degree-day model developed by Gadoury and MacHardy in 1982. The model is presented in table format on Page 43 in the 1998 Pest Management Recommendations for Commercial Tree-Fruit Production. The table shows the number of spores that are actually mature and *ready to discharge in the next rain*. Thus, spore maturity predicted by the model will be lower than "maturity" as determined by squash mounts because squash mounts count all of the colored spores as mature even though some of them will not discharge in the next rain. Apart from the fact that the definition of "maturity" is different for squash mounts and for the maturity predictions provided by the degree-day model, the two systems provide equally accurate estimates of ascospore maturity. In fact, the degree-day model will almost certainly prove more accurate than squash mount data collected by a novice observer.

The spore maturity table in the Cornell "Recommends" has a column showing the 90% confidence intervals for the predicted ascospore maturity levels. Most growers can just ignore this confidence interval information, but a short explanation follows. The 90% confidence interval was generated by statistical analysis of the data used to create the table. The 90% confidence interval means that if the degree-day model were compared to actual spore trap data for 100 different seasons, then mean percentage of mature spores as determined by spore trapping would lie between the bounds of the 90% confidence interval in 90 of the 100 seasons. In 10 of the 100 years, the spore trapping data might show that maturity was slightly outside of the 90% confidence interval. The confidence intervals for the spore maturity model range from 0-7% when 1% of ascospores are mature on up to 21-80% when 50% of ascospores are mature. The confidence interval becomes wider toward the middle of the season because degree-days are a less accurate predictor in the middle of the season than toward the beginning and end of the season. The wide confidence intervals could be interpreted as an indication that the model is not very accurate. However, as stated earlier, careful comparisons have shown the model is just as accurate as an experienced observer doing squash mounts. The only difference is that we never reported confidence intervals without squash mount data!

The ascospore maturity model provides a basis for predicting when the supply of ascospores is exhausted. The predicted end of the primary scab season will occur considerably earlier than the traditional dates that have been determined using squash-mount data. The earlier dates predicted by the model are justified based on spore trap data showing that few spores are generally released after petal fall even when there are still significant numbers of ascospores in the leaf litter.

Even though ascospores may be depleted before petal fall, growers should not eliminate petal fall and first cover fungicide sprays! The period from bloom through first cover is the period of highest risk for apple scab because terminal leaves are developing very rapidly, fruit are highly susceptible, and secondary spores are becoming available during this time period if primary scab was not controlled perfectly. The petal fall and first cover fungicide sprays are critical for protecting fruit until the effectiveness of the prebloom sprays can be fully assessed. If no primary scab lesions are visible after the first cover spray has been applied, then further protection against apple scab should not be necessary.

Despite availability of the ascospore maturity model, we have continued to make manual squash mount assessments in eastern New York so that we can compare manual counts with the model predictions. In some seasons, the squash mount assessments show slight variations in spore maturity at green tip that would not be noticed if we used only information from the model. This year, for example, a larger-than-usual number of spores were "mature" or colored at green-tip. However, within several days, counts as determined from squash mounts (and adjusted for differences in the meaning of "maturity") were similar to those generated by the model.



by Art Agnello
Entomology, Geneva

We get the feeling (mainly from the apricots and forsythia blooming everywhere we look) that there may not be too much early season oil use this year. In fact, it might be better if maybe we just got the whole season over with as quickly as possible, harvest in July and take the rest of the year off. However, in keeping with Paul Chapman's perennial assessment of NY weather as 'unusual, as usual', we now hear casual mention of snow forecast for later in the week, so it's anyone's guess how many blocks may actually hold still long enough to consider some prebloom oil sprays.


To review our oil basics for pears, early applications can be very useful against pear psylla until the swollen bud stage; it doesn't kill the adults, but it does interfere with their egg-laying activity.

Pear psylla eggs.

Granted, psylla adults have probably been out sporadically since the warm spells of mid-February. However, it's a sure bet that they're not finished yet. The strategy behind this approach is to delay the timing of any insecticide spray until as late as possible before bloom. Oil rates depend on when you start: If your buds are at the dormant stage, one spray of 3% oil, or 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.


Despite the newly acquired prebloom miticides that are now available, a delayed-dormant spray of petroleum oil from green tip through tight cluster can be a preferred approach for early season mite control to conserve the efficacy and help lessen the likelihood of resistance to our contact miticides. 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, however, the ideal is a rarity under NY spring conditions.

Tight cluster.

During a year like this one, when many trees have already bolted straight through all those early stages, the common wisdom is that tree development may actually get ahead of the mites. That is, mite eggs could conceivably be pretty far from hatch even if your Macs are well into tight cluster. For this reason, it would be a good idea to keep the oil rate fairly high (no lower than 1 1/2%) even for sprays made just before pink. Once pink color starts to show of course, you're pretty much out of the oil option unless you want to risk burning some petals. As always, be wary of sudden frosts, and try to time your oil sprays at least 48 hours before and after the occurrence of any freezing temperatures.

On a resistance management note, most growers have at least some blocks that received an application of Apollo or Savey last year, and got good control with this approach. Because of this season's compressed time scale, you might consider two alternatives:

Chemical News


On March 26, the EPA granted a specific exemption under the provision of FIFRA Section 18 for the use of tebufenozide (Confirm) to control obliquebanded leafroller (OBLR) on apples in NYS. Under this specific exemption, Confirm 2F, manufactured by Rohm & Haas, may be applied in a maximum of 4 applications, at a rate of 18.0 fluid oz. product in a minimum of 100 gallons of water per acre per application (0.28 lb a.i.). A pre-harvest interval of 14 days is required. According to manufacturer recommendations, one application is advised at petal fall for control of the overwintered brood, and a maximum of 3 applications against the 1st summer brood larvae are recommended at the following timings: 200-300 DD, 500-600 DD, and 800-900 DD (base 43F) after the first sustained moth catch.

General Information


by Art Agnello
Entomology, Geneva

To help supply you with the resources necessary to be well-informed as you begin the season, here is a current listing of all the tree-fruit publications that we can determine are available from the various Cornell agencies:

Sources from which publications are available are indicated by the following code letters:

(C)  Resource Center-GP, Cornell Univ., 7 Business & Technology Park, 
     Ithaca, NY  14850  Tel: 607/255-2080  FAX: 607-255-9946
(G)  Communications Services Bulletins, Jordan Hall, N.Y.S. Agric. Expt. 
     Sta., Geneva, NY  14456  Tel: 315/787-2249
(N)  Northeast Regional Agric. Engineering Serv., Coop. Ext., 152 Riley-
     Robb Hall, Ithaca, NY  14853-5701  Tel: 607/255-7654  
     FAX: 607-254-8770
(P)  Photo Lab, Barton Laboratory, N.Y.S. Agric. Expt. Sta., Geneva, NY  

Tree Fruit Disease and Insect Fact Sheets 
(Cost for individual Fact Sheets is $1)
102FSTF         Tree Fruit Fact Sheet Set  $22.  (C)
                Corresponding Slide Sets $25.  Individual Slides $5. (P)
102GFSTF-D3     Fire Blight.  1994.
102GFSTF-D4     Powdery Mildew of Apple.  1980.
102GFSTF-D5     Cedar Apple Rust.  1981.
102GFSTF-D6     Black Knot of Plum.  1992.
102GFSTF-D7     Phytophthora Root and Crown Rots.  1992.
102GFSTF-D8     Cherry Leaf Spot.  1993.
102GFSTF-D9     Apple Scab.  1993.
102GFSTF-D10    Brown Rot of Stone Fruits.  1993.
102GFSTF-D11    Sooty Blotch and Flyspeck.  1994.
102GFSTF-D12    Perennial Canker.  1995.
102GFSTF-I1     Pear Psylla.  1978.
102GFSTF-I2     Codling Moth.  1996.
102GFSTF-I3     Plum Curculio.  1980.
102GFSTF-I4     Green Fruitworm.  1980.
102GFSTF-I5     Obliquebanded Leafroller.  1980.
102GFSTF-I6     Peachtree Borer.  1980.
102GFSTF-I8     Apple Maggot.  1991.
102GFSTF-I9     Spotted Tentiform Leafminer.  1980.
102GFSTF-I10    European Red Mite.  1980.
102GFSTF-I11    Rosy Apple Aphid.  1980.
102GFSTF-I12    San Jose Scale.  1980.
102GFSTF-I13    White Apple Leafhopper.  1980.
102GFSTF-I14    Dogwood Borer.  1985.
102GFSTF-I15    Cherry Fruit Fly and Black Cherry Fruit Fly.  1988.
102GFSTF-I16    Woolly Apple Aphid.  1988.
102GFSTF-I17    Oriental Fruit Moth.  1988.
102GFSTF-I18    Beneficial Insects.  1989.
102GFSTF-I19    Redbanded Leafroller.  1989.
102GFSTF-I20    European Apple Sawfly.  1991.
102GFSTF-I21    Tarnished Plant Bug.  1991.
102GFSTF-I22    Comstock Mealybug.  1991.
102GFSTF-I23    Predatory Mites.  1995.
102GFSTF-I24    American Plum Borer.  1997.
102GFSTF-M1     Meadow Vole and Pine Vole.  1980.

IB 112     Training and Pruning Apple Trees.  1986.  Reprinted 1992.  
           $3.50.  (C)
IB 219     Orchard Nutrition Management.  1991.  $4.  (C)
IB 221     Predicting Harvest Date Windows for Apples.  1992.  $4.75.  
IB 227     Economics of Apple Orchard Planting Systems.  1992.  $3  (C)
IB 231     Biology and Management of Apple Arthropods.  1993.  $5.50  
IB 236     Wildlife Damage Management in Fruit Orchards.  1994.  $4.75  
IB 237     Pollination and Fruit Set of Fruit Crops.  1995.  $3.  (C)
IB 242     Integrated Weed and Soil Management in Fruit Plantings.  
           1998.  $7.00  (C)
IPM 207    Apple IPM: A Guide for Sampling and Managing Major Apple 
           Pests in New York State.  1993.  (IPM Manual)  $10.  (C)
Video.     Simplified Insect Management Program: A Guide for Apple 
           Sampling Procedures in New York.  1989.  Rental $20, Purchase 
           $29.95.  (C)
AF         Apple Facts-Varieties of Commercial Interest: Braeburn.  
           1993.  $0.50  (G)
AF         Apple Facts-Varieties of Commercial Interest: Gala.  1993.  
           $0.50  (G)
AF         Apple Facts-Varieties of Commercial Interest: Fuji.  1993.  
           $0.50  (G)
AF         Apple Facts-Varieties of Commercial Interest: Gingergold.  
           1993.  $0.50  (G)
AF         Apple Facts-Varieties of Commercial Interest: Jonagold.  
           1997.  $0.50  (G)
FLS 50     Green Fruitworms.  1974.  $1.  (G)
FLS 53     Empire, a High Quality Dessert Apple.  Reprinted 1992.  $1.  
FLS 58     Growth Stages in Fruit Trees - From Dormant to Fruit Set.  
           1976.  $2.  (G)
FLS 92     Biology and Control of Cytospora Fungi in Peach Plantings.  
           1982.  $0.30  (G)
FLS 95     Blister Spot of Apple.  1982.  $0.30  (G)
FLS 108    Diagnostic Keys for Diseases of Apple, Peach and Cherry.  
           1984.  $0.50  (G)
FLS 116    Chemical Thinning of Apples.  1986.  $0.50  (G)
FLS 117    Peach and Nectarine Varieties in New York State.  1986.  
           $0.50  (G)
FLS 118    Preventing Decomposition of Agricultural Chemicals by 
           Alkaline Hydrolysis in the Spray Tank.  1986.  $0.50  (G)
FLS 119    IPM in New York Apple Orchards - Development, Demonstration, 
           and Adoption.  1987.  $0.50  (G)
FLS 123    Basing European Red Mite Control Decisions on a Census of 
           Mites Can Save Control Costs.  1988.  $0.50  (G)
FLS 124    Insects Associated with Apple in the Mid-Atlantic States.  
           1988.  $1.  (G)
FLS 127    Sweet and Tart Cherry Varieties: Descriptions and Cultural 
           Recommendations.  1989.  $0.75.  (G)
FLS 128    The Effects of Ground Cover Manipulations on Pest and 
           Predator Mite Populations on Apple in Eastern New York.  
           1989.  $0.50  (G)
FLS 133    Northern Lights Apple.  1990.  $0.50  (G)
FLS 134    Royal Empire Apple, a Sport of Empire.  1990.  $1.  (G)
FLS 139    A Method to Measure the Environmental Impact of Pesticides.  
           1992.  $1.  (G)
FLS 140    Royalton Black Sweet Cherry.  1993.  $1.  (G)
FLS 141    Hartland Black Sweet Cherry.  1993.  $1.  (G)
FLS 142    Fruit Pest Events and Phenological Development According to 
           Accumulated Heat Units.  1993.  $1.  (G)
FLS 143    Sampling Second Generation Spotted Tentiform Leafminer.  
           1993.  $0.50  (G)
FLS 145    Minimal Processing of New York Apples.  1995.  $2.  (G)
FLS 146    Small Scale, Sustainable, IPM and Production Systems for 
           Apples in Romania.  1996.  $2.  (G)
FLS 147    Fortune Apple.  1995.  $1.  (G)
Sch 6      Phytophagous and Predaceous Mites on Apple in NY.  1980.  
           $0.50  (G)
Sch 36     Biology of the Codling Moth in Hudson Valley Orchards.  1989.  
           $0.50  (G)
SpR 55     Proceedings, Brown Rot of Stone Fruit Workshop.  1985.  $0.50  
SpR 57     1985 Processed Apple Products Workshop.  1985.  $0.50  (G)
SpR 65     Processed Apple Products Workshop.  1992.  $1.  (G)
SpR 67     Juice Technology Workshop, Oct. 1993.  $10  (G)

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:

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Photographs courtesy of New York State Integrated Pest Management Program