Our on-farm IPM trials have ended for June-bearing strawberries, but are continuing for apples. Some preliminary findings:
Tarnished plant bug (TPB). The 3 cooperating growers saved an average of 1 insecticide spray using the IPM scouting program (2 sprays vs. 3 for their conventional program). Incidence of harvested fruit with apical seediness (evidence of TPB feeding injury) was 6.5% for the IPM treatment and 5.2% for the conventional spray program; essentially no different. Incidence of damage ranged from 4 to 10% of the berries. Factors that may have influenced these injury levels include torrential rains, which may have washed off some insecticide residue, and abundance of weeds (the 10%-injury field had a very large population of weeds).
Gray mold. The 2 growers who were able to participate saved an average of 1/2 spray this season by using the IPM program of bloom sprays only. Incidence of gray mold on fruit was negligible on both farms.
Fire blight. Using the MARYBLYT program in 1993 did not result in fewer streptomycin sprays for the 8 participating growers; in each case, an average of 2 sprays was applied during the bloom period. The timing of the sprays was different than in a traditional program, however. Because the period of shoot elongation has been greatly prolonged by our continual rains, we are still evaluating incidence of blossom and shoot blight in these orchards. In general, fire blight incidence around the state is light, but a few orchards suffered moderately severe outbreaks. The most extensive outbreak of shoot blight occurred in an orchard in which highly susceptible varieties were pruned heavily and late (well after bud break); this pruning encouraged rapid shoot growth, which raised the risk of fire blight. As you may recall, the shoot blight phase occurs when the fire blight bacterium invades wounds made on expanding shoots by insects with piercing-sucking mouthparts, such as aphids and leafhoppers.
Codling moth AND fire blight. In the same orchard, we also noticed a possible fire blight risk associated with using IPM for codling moth control. Because moth captures were low, the grower did not apply any insecticide for about a month after the 250 degree-day spray. Unfortunately, shoot blight incidence (percent trees with strikes) was about twice as high (roughly 50% vs 25%) in the IPM block vs. the standard-practice control block. This result, although it is not replicated, raises the possibility that the codling moth program may leave other insects uncontrolled and, therefore, may not be suitable for highly fire blight-susceptible blocks during the period of shoot elongation, especially when other risk factors (such as late, heavy pruning and a history of fire blight in the orchard) are present. In a "normal" year (remember those?), however, shoot elongation ceases by the end of June, and the risk of shoot blight plummets at the same time, so insecticide sprays for codling moth after that time could be based on an IPM program (population monitoring plus degree-day models). We need more data before drawing any firm conclusions, but this experience suggests caution in using IPM for timing insecticide sprays during the shoot elongation period if the orchard or block is at high risk for fire blight due to genetic susceptibility, pruning practices, or both.
This article originally appeared in the July 14, 1993 issue, p. 116.