IPM Update--Apples and Strawberries


Fire blight. Blossom blight and shoot blight have been reported in apple orchards in southwest and central Iowa during the past 2 weeks, but the incidence of these phases of fire blight appears to be low. We can be grateful for the cool prebloom and bloom period conditions, because they helped to suppress activity of the fire blight bacterium, which prefers average daily temperatures above 60o F. Because of the cool weather, the MARYBLYT program did not advise spraying during the bloom period in several orchards even when rain or heavy dew had occurred.

Should you prune out fire blight strikes as they appear in the orchard, or wait until the dormant season? Fire blight experts agree that the response should depend on the situation. Some examples:

SCENARIO 1 - The strikes are on young trees of susceptible cultivars and/or rootstocks. This scenario demands prompt pruning of strikes as soon as they appear.

SCENARIO 2 - The trees are older, but there are only a few strikes per tree, and there are susceptible cultivars in the orchard. This one is a judgment call; it's advisable to prune promptly if you can make time to do it.

SCENARIO 3 - There are abundant strikes (dozens to hundreds) on every tree of a susceptible cultivar. Paul Steiner, the developer of the MARYBLYT model, advises growers to "go fishing" if this occurs. In other words, hold off pruning until the dormant season.

Any summer pruning should use Steiner's "ugly stub" method. This means to cut back into last year's wood, leaving a 4-inch stub. Come next fall, winter, or early spring (pre-bud break), these stubs will be easy to spot and prune out. It is Steiner's conviction that sterilizing pruning shears is a waste of time, even in summer, because the fire blight bacterium is present beneath the bark of an infected branch well below the visible boundaries of a strike; in other words, it will reinfect pruning cuts regardless of whether the cutting surfaces are sterilized. The ugly stub method assumes that a small area of blight will reappear at the stub, but will not progress much in this older wood, and can thus be cleaned up when the stubs are taken out.

A reminder: if strikes appear, don't fight them with streptomycin sprays. It will do no good, and will hasten the development of strep resistance in the orchard.

Codling moth. Pheromone trap captures continue to vary quite widely from orchard to orchard. For instance, the number of captures in one orchard is less than 10% the number in another orchard just one mile away. The reason for such a sharp difference isn't always obvious, but it translates into a far different risk of codling moth damage. It reinforces the need to trap moths on an orchard-by-orchard basis.

Modern IPM methods for codling moth predict codling moth development by counting up degree-days after a "biofix" date, which is set when the first few moths are captured. Insecticide sprays are timed not only by trap captures, but also by degree-day thresholds (typically 250 and 1260 degree days after biofix), which coincide with insecticide-vulnerable stages of successive codling moth generations. That's all well and good, but what happens if trap captures are so few that biofix never arrives, or arrives weeks or even months after other orchards in the area? Other IPM programs around the country circumvent this problem by assigning a biofix date that fits the average timing of biofix recorded in the nearby orchards.

Insecticides are then applied at 250 and 1250 degree days after this date. But if capture numbers continue to be low (averaging less than five male moths per trap per week), no additional sprays would be recommended between 250 and 1250 degree days or after 1250 degree days. Are you thoroughly confused now? Call Donald Lewis for clarifications and additional details.


Tarnished plant bug (TPB). Harvest of June-bearing cultivars has started in southern and central Iowa. At this point, we have discontinued scouting for TPB nymphs, for two reasons: 1) the vast majority of berries have passed the point of development (bloom or immediately post-bloom) at which noticeable TPB damage can occur; and 2) insecticide applications during harvest become more difficult for growers due to preharvest restrictions and simply lack of time.

This article originally appeared in the June 16, 1993 issue, pp. 1993 issue, pp. 94-95.


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