Leather rot. Recent heavy rains over some parts of Iowa will probably result in an increase in leather rot incidence in fields with a) poorly drained soil, b) thin mulch, or c) both of the above. The off-purple color of ripening berries, the acrid smell, and the bitter taste are indicative of leather rot-infected berries. Some people claim that a single infected berry can taint an entire batch of jam, and I'll verify that the taste of a fresh, infected berry tends to linger in the memory. If you are hard-hit by leather rot this year, make plans to increase your mulch cover next season. If you see puddles of standing water in your field for several hours after a rain, it may be helpful to tile the field or to switch to better-drained ground.
Tarnished plant bug. At harvest, watch for the button berries that indicate injury by tarnished plant bug nymphs. The injury actually occurred as the result of feeding activity at the bloom stage. Therefore, bloom is the critical time to scout for this insect, and to apply insecticides if captures exceed the threshold of 0.5 nymphs per cluster.
Codling moth. Captures of codling moths in pheromone traps in cooperating orchards this month have generally been higher than in 1993. The reason isn't known for sure. If there is effective residual of a labeled insecticide on the fruit, however, even a high rate of moth activity in the orchard will not translate into a high rate of fruit injury. This is not a recommendation for a preventive insecticide program, but rather to monitor moth numbers with the help of inexpensive, easy-to-use pheromone traps. As we found last year, you can't use the orchard down the road or in the next county as a reliable indicator of codling moth pressure in your orchard; we sometimes found 50-fold differences in codling moth captures in orchards that were just a few miles apart!
Fire blight. The shoot blight phase of this disease has begun to appear on susceptible varieties, such as Jonathan. In one orchard, the blighted shoots were traced back to the existence of holdover cankers on large limbs. Even a few active cankers can give rise to plenty of fire blight bacteria to be spread throughout the orchard by insects and by wind. It is certainly possible that the high winds we experienced back during bloom could have spread strands of the pathogen from oozing cankers. Once probing insects such as leafhoppers and aphids start to feed on the tips of expanding shoots, they create wounds that are invaded by fire blight bacteria hanging around on the surfaces of the shoot. The result is the curled "shepherd's crook," blackening, and withering of the shoot.
This brings up the perennial late spring/early summer question about fire blight: to prune, or not to prune? Late spring/early summer pruning is advisable if the number of strikes per tree is low enough to be removed without an unrealistically large commitment of labor, if the trees are less than 8 to 10 years old (and therefore vulnerable to loss of major limbs), and/or if the varieties and/or rootstocks are highly susceptible to fire blight. Paul Steiner, the developer of the MARYBLYT system for fire blight management, advises orchardists to "go fishing" if the number of strikes is too high to be removed as they appear. Assuming you do decide to remove strikes at this time of year, cut below the visible canker but don't worry about sterilizing your shears between cuts, because Steiner has shown that this will not prevent the pruning stub from becoming infected. Steiner advocates the "ugly stub" technique for spring/summer pruning, which means that cuts are made into two-year-old wood and a stub at least 4 inches long is left above the next branch intersection. What happens next is that a small, confined canker redevelops at the pruning stub but doesn't progress further. The stub must then be removed during the dormant season, when its shape helps it be noticed by the pruning crew. To make the stub even more conspicuous in winter, carry a spray can of orange paint and mark each stub as you do your summer pruning.
This article originally appeared in the June 17, 1994 issue, pp. 1994 issue, pp. 92-93.
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