- Fruits: apples, general tree fruits
- Crop Production: cover crops
- Education and Training: display, mentoring, on-farm/ranch research, participatory research
- Pest Management: biological control, compost extracts
- Soil Management: earthworms
[Note to online version: The report for this project includes tables and figures that could not be included here. The regional SARE office will mail a hard copy of the entire report at your request. Just contact North Central SARE at (402) 472-7081 or email@example.com.]
Kura clover was established as a ground cover in two apple orchards and tested for its ability to limit dispersal of Venturia inaequalis spores and thus inhibit apple scab primary infection. The hypothesis was not supported in data obtained by sampling airborne spore levels nor by assays of primary scab incidence on apple leaves. The efficacy of the environmentally benign compounds potassium bicarbonate and a solution of methionine and riboflavin was tested as sprays during the growing season to control apple scab and the sooty blotch/flyspeck complex. Disease suppression to approximately the level afforded by traditional sulfur sprays was observed.
The objectives of our research have focused on identification of integrated disease control strategies that might reduce the need for synthetic fungicide spray applications in commercial apple production. In orchards of the upper Midwest United States, spray campaigns typically begin in early spring to blunt the primary infection phase of apple scab disease, in which ascospores of Venturia inaequalis produced in decaying leaf litter on the orchard floor are the predominant source of infection. Over the course of the 5-6 month growing season, ten or more fungicide applications are commonly applied to control scab epidemics and late-season diseases caused by the sooty blotch/flyspeck (SBFS) complex. We reasoned that an orchard management strategy aimed at limiting initial scab infection in the spring, combined with use of environmentally benign antifungal spray compounds to control scab and other late-season diseases, might significantly reduce the reliance of apple growers on fungicides.
We hypothesized that the presence of a dense foliar canopy provided by a Kura clover (Trifolium ambiguum) ground cover crop would limit Venturia ascospore dispersal and thereby reduce the incidence of primary scab lesions. Accordingly, our approach included seeding Kura clover in portions of two Wisconsin apple orchards in the spring of 1997 and testing its impact on scab epidemiology in subsequent years. The effect of Kura on airborne ascospore levels was determined by placing spore samplers above the clover canopy and comparing spore capture rates there to those obtained by using samplers located in adjacent orchard sections that had not been seeded with clover (existing ground cover, EGC). In addition, the ability of a Kura clover ground cover to reduce initial scab infection was determined by comparing the incidence of primary scab lesions on leaves of apple trees growing in the Kura plots to that observed among leaves situated above the adjacent EGC plots.
Contrary to expectations, experiments conducted at both the Peninsular Research Station in Sturgeon Bay, WI, and the West Madison Agricultural Research Station (WMARS) in Madison, WI, indicated that airborne ascospore counts in Kura plots were not consistently different from counts obtained in control EGC plots.
Furthermore, at neither location did the presence of Kura clover consistently correlate with a significant decrease in incidence of primary scab lesions on young apple leaves. This might be attributable to several factors. One possibility is that the Kura foliar canopy, while obviously denser than that of the grassy EGC, nonetheless does not present a substantial impediment to ascospore dispersal. Alternatively, or in addition, it may be that the dispersal range of ascospores carried aloft from adjacent EGC plots is sufficient to obscure the anticipated effects of the Kura canopy on spore dispersal. Finally, it may also be possible that alternative sources of spring inoculum exist, such as spores that overwinter in trees, that would not be affected by the presence of Kura clover.
Results from experiments to assess the efficacy of environmentally-benign antifungal spray formulations were more promising. Two experimental sprays– a methionine-riboflavin mixture (M-R) and a potassium bicarbonate (KHCO3) formulation– were applied to apple trees in several Wisconsin orchards. Both compounds were consistently as effective as the Captan fungicide positive control regimen in reducing SBFS incidence. Furthermore, our preliminary results indicate that the M-R mixture, when applied in an early-season regimen, is as effective as sulfur treatments in controlling scab disease progression.
Based on those results, a commercial formulation of the KHCO3 spray (Kaligreen) has been selected for testing in USDA-sponsored multi-state trials for control of SBFS. The ability of the M-R spray to control apple scab—as well as SBFS– will be further evaluated.
1. Test the ability of the orchard floor cover crop Kura clover (Trifolium ambiguum) to break the life cycle of the apple scab pathogen (Venturia inaequalis).
2. Test the efficacy of environmentally benign compounds as sprays during the growing season to control apple scab disease.