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Healthy Fruit 1997 Vol. 5:2

Apr 16, 1997

Growth Stage. . . Progress Blessedly Slow

McIntosh at 1/4 inch green in earliest areas, silver tip or dormant in later areas. The cold temperatures appear to have left apples untouched, peaches maybe lightly damaged, just enough to help thinning.

Scab Update

In order to get apple scab, three things have to come together: green susceptible tissue, apple scab spores, and a wetting period long enough for those spores to grow. Now, before you grab for the remote because you've heard all of this until your ears bled, let me tell you the details have changed quite a bit over the past few years. In some cases, this may change the way you should approach scab management.

For example, at present we have a relatively high percentage of mature scab spores. Normally at green tip, about 5% to 10% of the season's scab spores are mature. This year, about 15% to 25% of the spores are mature. So we have plenty of mature inoculum. That is part of the inoculum story

The other component of inoculum is the amount of overwintered scab that is present in or near the orchard. Bill MacHardy and Dave Gadoury have called this the potential ascospore dose, or PAD. Methods for estimating PAD are in the New England Apple Pest Management Guide, but you had to have done the evaluation last fall. The best you can do at this point is roughly estimate inoculum. Most well­managed commercial orchards, even last year, did not have scab. In these orchard, inoculum is low. In blocks where you saw some scab, then levels may be relatively high. In these blocks, the combination of alot of inoculum maturing early could result in an unwelcome surprise later on in the spring.

What about the green tissue? Well, no areas in Massachusetts are beyond 1/4 inch of green tissue. That means that the target area on the apple tree is quite small, compared to what it will be at tight cluster through bloom. The chances that spores will hit buds at green tip are relatively low, and for the next week target area will probably stay minimal to non­existant across the state.

The real changes in understanding scab development have come in the area of the infection process. The wetting periods and Mills Period charts we are familiar with have come under intense scrutiny in New Hampshire and New York. Dave Gadoury and Bob Seem recently summarized this work in Scaffolds. Two points stand out.

Ascospore discharge is suppressed at night. This suppression is not absolute. About 5% of the available ascospores will be discharged during darkness when rain begins at night. That's 5% of the spores mature at that time, NOT 5% of the total spore load for the season. Whether or not this small percentage is a threat depends upon PAD. In an orchard where PAD counts were made, scab would have to be present on at least 53 leaves of 600 terminal shoots (and there are about 10 to 20 leaves per shoot) in order for the 5% release at night to be a problem.

It takes less time for scab infections to occur than was originally thought by Mills, particularly at lower temperatures. The original Mills table isn't used in our New England Guide. For 3 years we have been using the criteria developed by MacHardy and Gadoury, which significantly reduces the hours of wetting needed for infection. For comparison, look at the table below.

With cold rains predicted for the end of this week, should a fungicide be applied? The only argument for applying a fungicide now is that there is a large amount of mature inoculum. The tissue area on the trees is low, and it will probably take at least a day of wetting for an infection to occur assuming temperatures remain at 40 F or below. Probably only "high­inoculum" blocks need to be treated at this time.

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
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

It would probably be best to wait and see. If there is an infection period, then in blocks where there is alot of scab inoculum from last year should be treated. A pair of applications of Nova or Rubigan would cover the infection. The first application can be put on up to 4 days after the start of the infection period, and the second approximately 10 days later.

If you don't use Nova or Rubigan, wait and see is a riskier strategy. The post­infection activity of the protectant fungicides like captan or mancozeb is much shorter, on the order of a day. That's why they're called protectants. Then you have to take your best guess about the rain and the amount of inoculum in the orchard.

Still Early for Tarnished Plant Bug.

A few days this past week were conducive to the early stages of TPB flight activity. Low numbers of TPB have been found on knee­high, sticky white rectangle traps at Rice's Orchard in Wilbraham. This is traditionally one of the earliest orchards for TPB emergence; these captures were recorded at 1/4" green.

It is still very early in the plant bug season, and sticky white rectangle traps placed at knee height (no higher) in the tree periphery will provide an accurate reflection of the population of active TPB. These traps should be in place no later than this week to give a good measure of TPB numbers.

Leafminers Active in Early Orchard

The situation for leafminer is very similar to that of tarnished plant bug, although in Wilbraham, LM captures on red rectangle trunk traps were greater than 1 adult LM per trap this week. All leafminers observed on the traps were alive, suggesting that this was the first period of LM activity for the year. Nearly all orchards are later than this site, and there is time through this week to hang monitoring traps for leafminer. We recommend using sticky red rectangle traps, stapled to the south side of the tree trunk, with the top of the trap at knee height. As with TPB, we recommend the use of 4 traps per 8 acre block, and both types of trap should be in place no later than green tip to provide an accurate assessment of the potential for the development of damaging populations.

For both tarnished plant bug and leafminer, no pest control decisions need to be made until tight cluster at the earliest. In next week's issue, we will include updated thresholds for treatment of both pests.

Time to Treat Pear Psylla

There has been abundant psylla egglaying observed in the past couple of days in early and mid­developing orchard blocks. Now is the time to begin 2 to 3 consecutive oil sprays on pears as an egglaying deterrent and egg suffocant. Oil is the only material for insect control which we recommend for pre­bloom use on pears. The only constraining factor to this method is the potential for cold weather at night. We do not advise applying an oil spray if the temperature is expected to drop below 40F during the night following the oil spray or on the second night after application. Low temperatures following an oil application can cause serious injury to developing tissue.