Fruit IPM Update

The continued wet weather favors outbreaks of the major foliar and fruit diseases on apples and strawberries. Insect pests are becoming active, too.

APPLES

• Apple scab. This is shaping up to be a "high risk" spring for scab infection, with numerous infection periods. Nevertheless, the IRS (Integrated Reduced Spray) program from Cornell University will give excellent control of primary scab. This four-spray program specifies sprays of a SI fungicide (such as Nova or Rubigan) at tight cluster, pink, petal fall, and first cover (7 to 10 days after petal fall). The SI fungicide should be tank mixed with a contact fungicide (such as captan, mancozeb, metiram, etc.) for the latter two to three of these applications. If scab is well controlled during the primary infection period (lasting through about second cover), and other local sources of scab, such as abandoned apple orchards and unsprayed and scab-susceptible crabapple trees, the risk of fruit infections later in the season is much lower than if primary infections are common.
• Fire blight. Apple growers using the MARYBLYT program in cooperation with ISU have applied streptomycin for blossom blight control between one and three times so far this season. Delay in the bloom period due to the cool weather, coupled with frequent rain, have created very favorable conditions for blossom blight outbreaks, particularly in orchards with a history of fire blight (even if only a few strikes). The bloom period is the primary infection phase of the disease; the cankering of twigs, branches, and rootstocks that can appear later usually results directly from infections that began during bloom. DO NOT APPLY STREPTOMYCIN MORE THAN FOUR TIMES DURING BLOOM, because more frequent application raises a serious risk of developing streptomycin-tolerant strains of the fire blight bacterium.
• Cedar-apple rust. Galls of this fungus on cedar trees are fully expanded now and are actively releasing spores. Make sure your fungicide program includes a fungicide with good activity against rust (see May 5 issue of HHPN, p. 60).
• Codling moth. A sufficient number of codling moths have been captured in pheromone traps in all seven cooperating apple orchards in central Iowa to initiate biofix. The cooperators are continuing to take daily maximum and minimum temperatures; an insecticide spray will be applied for codling moth control at 250 degree-days after the biofix date.

STRAWBERRIES

• Tarnished plant bug. Bloom is just getting underway on June- bearing varieties in central Iowa, so the tarnished plant bug can become active about now. Feeding injury by this insect during bloom and early stages of fruit development results in small, button-shaped berries and loss of fruit size and yield. On May 17, IPM scouts found tarnished plant bug nymphs in a commercial strawberry field in central Iowa, but the number of nymphs was too low to advise application of an insecticide. The threshold for making an insecticide spray is 0.5 nymphs per flower cluster.
• Gray mold. This disease, causing fruit rot and development of fuzzy, brown coatings of fungal spores on infected fruit, is favored by the prolonged cool, wet conditions we have been experiencing. The IPM program we are using with on-farm cooperators, developed by Ohio State pathologist Mike Ellis, specifies fungicide sprays (e.g., Ronilan or Rovral) at very early bloom (5 to 10% of flowers open) and again at full bloom, but no sprays close to or during the harvest period. A third spray is sometimes applied if wet weather persists during the late phase of bloom. This program has given excellent control of gray mold, because research has shown that the highest-risk period for infection is the bloom period.

Tarnished Plant Bug

This article originally appeared in the May 19, 1993 issue, pp. 1993 issue, pp. 74-75.