The strawberry harvest is underway in central and southern Iowa.
Spongy berries. One problem that surfaced last week in many central and western Iowa fields: seedy, spongy-textured berries that seemed to ripen prematurely. Some growers mistakenly diagnosed the problem as leather rot (pathogen: Phytophthora cactorum). After consulting with Dr. Gail Nonnecke, small-fruit specialist in the ISU Horticulture Department, and Dr. Marvin Pritts of Cornell University, the problem was traced to cold- temperature injury. The problem originated back in early May, as the first flush of blooms and small fruit appeared. Temperatures in the 30's and 40's during an unusually cool week did not kill young berries, but resulted in cell damage. The damage resulted in production of ethylene, which caused the berries to ripen before they were fully expanded. The cell damage also changed the turgor of the berries, resulting in the spongy texture. The good news is that the problem should disappear once the initial flush of berries is harvested, because later-developing fruit was not exposed to the cold period.
Tarnished plant bug. In our on-farm trials around the state, numbers of tarnished plant bug in growers' fields seem to be higher than during the 1993 season. TPB injury to berries shows up as "button" berries; that is, berries with shortened, stubby, seedy tips. Buttoning is often severe enough to make the berries unacdcepatable for either pick-your-own or prepick use. The IPM strategy we are following recommends application of an insecticide when counts of TPB nymphs are 0.5 per cluster (average of at least 10 clusters sampled in each of 3 rows in the field). Some growers have been advised to apply two sprays during the preharvest season.
Leather rot. Several cases of leather rot have been diagnosed this season. Infected fruit or entire clusters may turn brown and wither up while still green. Ripening fruit turn a faded purple at the infection site. A good symptom for field diagnosis is the acrid, bitter smell of an infected, ripening berry. The smell is hard to describe, except to say it's unforgettable! Biting into an infected berry (not for the faint- hearted) results in a taste that very unstrawberry-like! The closest comparison might be rancid motor oil. Disgusting though it may bem this technique is very accurate in diagnosing the presence of leather rot. The disease can devastate a planting under favorable conditions, which are prolonged rainfall, a heavy (high clay content) soil, poor drainage, and inadequate mulching. You can avoid a great deal of risk from leather rot simply by doing a very thorough job of mulching before the season begins. The pathogen splashes onto the fruit in soil particles, so if the fruit are protected from the soil by mulch the damage will be light or zero. Additional mulch benefits include weed control (which will control TPB populations), moisture conservation, and a more picker-friendly environment. Ridomil is a fungicide that is labeled for leather rot control, but its effectiveness will be disappointing unless cultural preventives, especially mulching, are also used.
Fire blight. The risk of the blossom blight phase of fire blight was relatively low during the bloom period this year due to cool weather. However, shoot blight can still occur as insects feed on expanding shoots, creating wounds through which the fire blight bacterium can enter and create the infamous "shepherd's crook" symptom of shoot blight. It's important, therefore, to maintain a protectant schedule of insecticide sprays on fire blight-susceptible blocks through at least second cover.
Codling moth. Our on-farm trials are beginning around the state. Captures of male codling moths in pheromone traps have been reported from several orchards. Cooperators will be advised to apply a insecticide spray whenever the average number of moths captured per trap per week is five or more.
ISU is also conducting a demonstration trial in a Nebraska orchard, using mating disruption for codling moth control. A 4- acre block in the National Arbor Day Orchard near Nebraska City is being used to test this new method, which aims to flood the orchard with pheromone and thereby prevent moth mating and egg- laying (and thus prevent fruit damage, too). Mating disruption has proven to be commercially viable in several states; we will keep you updated on the progress of this trial.
This article originally appeared in the June 8, 1994 issue, pp. 1994 issue, pp. 87-88.