Introduction

At the ISU Horticulture Farm, we compared the disease-warning system to a conventional fungicide-spray schedule over 3 growing seasons (1997-1999). All trees in the test block received a conventional protectant fungicide program through first cover. In all years, incidences of sooty blotch and flyspeck were rated 1 week before harvest on a subsample of 50 fruit per tree.

Treatments

1. 2nd-cover spray (Benlate + Captan) delayed until 175 wet hours after 1st cover, followed by sprays every 10-14 days until harvest (Hartman, 1996).
2. Same as Treatment 1, except a 225-hr threshold was used between 1st- and 2nd-cover sprays.
3. Conventional spray timing (Benlate + Captan every 10-14 days from 1st cover to harvest)
4. Unsprayed control (no fungicides after 1st cover).
5. In 1998 and 1999, we used SkyBit data in the 175-hr threshold system as well.

Results

The warning system using the 175-hr wetness threshold saved 1 to 4 sprays/yr, with no more sooty blotch and flyspeck than the protectant-schedule control (Table 1). This result makes it clear that the warning system, under the conditions tested, can be both effective and economically attractive. Based on these results, at an estimated cost of $20/acre/spray, and assuming a mean savings of 2 sprays per year on 20 acres, a grower could save $800/yr in spray costs without added risk of losses due to sooty blotch and flyspeck. Assuming that it cost $100 per year to obtain the weather data, either from a sensor or from SkyBit Inc., the net savings in this example would still be $700/year. The success of the 225-hr wetness threshold in 1997 suggests that there is potential for additional spray savings, especially in relatively dry growing seasons.

In 1998 and 1999, the 175-hr threshold was used in conjunction with wetness data estimated for the ISU Horticulture Farm by SkyBit Inc. Timing fungicide sprays according to this so-called “site specific” weather data resulted in disease control equivalent to that in the protectant-schedule control, and saved 1-2 sprays/yr (Table 2). However, the SkyBit data directed one more fungicide spray per year (assuming one fungicide spray every 10-14 days) than when the warning system was operated with data from the Wetness / Temperature Logger located under a tree in the test block. This occurred because SkyBit estimated considerably more hours of wetness than actually occurred beneath the canopy of an apple tree. SkyBit estimates, provided for a 1 km2 area, could not account for the sheltering effects of the canopy, which reduce the duration of dew and light rain periods. To use SkyBit or similar estimates more effectively for under-canopy wetness estimation, it will be necessary to use empirical models to calibrate to this microenvironment. A 2-year modeling effort to accomplish this aim for apples is currently underway at the ISU Horticulture Farm. Once reasonably accurate calibration models are developed, this service will offer major advantages to growers, because they will be able to utilize weather-based warning systems without the labor and inconvenience associated with do-it-yourself weather monitoring, and at a comparable or slightly lower cost than on-site monitoring.