Refinement of Non-Antibiotic Spray Programs for Fire Blight Control in Organic Pome Fruit

Fire blight-susceptible, certified organic pome fruit is produced currently on 9,000 ha in the Pacific Northwest region of the United States with acreage continuing to expand in spite of a 2014 prohibition on antibiotics as allowable materials for infection suppression. Non-antibiotic practices for fire blight suppression mirror conventional management, but the full bloom to petal fall period when antibiotics are typically sprayed for fire blight control continues to receive research scrutiny owing to drawbacks and weaknesses of alternative materials. As solitary treatments, effective non-antibiotic materials (e.g., a yeast biocontrol, soluble coppers, and alum) raise the risk of a crop-value reducing, phytotoxic response termed fruit russeting. Conversely, materials with less russeting risk (e.g., Bacillus-based biorationals) are less effective for fire blight control. Spray programs using a combination of materials applied from mid-bloom to petal fall have the potential to provide high levels of protection with reduced russeting risk. In orchard trials, the effects of non-antibiotic spray programs on epiphytic population size of Erwinia. amylovora in flowers, yeast biocontrol population size, floral pH, infection suppression, and fruit russeting revealed strategies for sequencing sprays of non-antibiotic materials. The yeast biocontrol, Blossom Protect (Aureobasidium pullulans), sprayed at 70% bloom was an important contributor to fire blight suppression as was the soluble copper material, Previsto, when applied at full bloom. Choice of material for the petal fall spray timing was important to fruit russeting risk but apparently less important to overall infection incidence. Consequently, treatment programs of Blossom Protect at 70% bloom, a soluble copper at full bloom, and a Bacillus-based biorational at petal fall best balances the quality of infection suppression with risk of fruit russeting.

Evaluation of Blossom Thinning Spray Timing Strategies in Apple

In the eastern USA and several other apple-growing regions, apple blossom thinning using lime sulfur is a relatively new crop load management strategy. This study sought to evaluate how application timing of lime sulfur + stylet oil blossom thinning sprays would influence thinning efficacy and crop safety of ‘Gala’ apples. This project occurred at two locations in the USA, Winchester, Virginia, and Mills River, North Carolina, during the 2019 growing season. Two main timing strategies were assessed: (1) model-guided sprays with the pollen tube growth model (PTGM), (2) fixed spray intervals with the first spray applied at a specified percentage of open bloom (20% vs. 80%), and the second spray applied at a reapplication interval (48 h vs. 72 h). Model-guided and 20% open bloom + 48-h treatments reduced fruit set and increased fruit weight, diameter, and length at both sites. Treatments with a delayed first spray at 80% open bloom or a more extended second reapplication of 72 h were generally ineffective. There was no conclusive evidence that lime sulfur + stylet oil blossom thinning spray timing influenced russet incidence/severity or leaf phytotoxicity. This study demonstrated that effective and safe blossom thinning can be obtained from applying two sprays at 20% open bloom and 48 h thereafter or using the PTGM.

Low Effectiveness of Prosulfocarb and Mesosulfuron-Methyl + Iodosulfuron-Methyl against Vulpia Myuros

Due to natural tolerance to most widely used herbicides for grass weed control, prosulfocarb as pre-emergence or early post-emergence herbicide and mesosulfuron + iodosulfuron as post-emergence herbicide are the mainstays of any chemical control program for Vulpia myuros in Denmark. However, farmers often report variable efficacy of these herbicides on V. myuros compared to other grass weeds. Dose–response experiments were conducted to evaluate the performance of prosulfocarb and mesosulfuron + iodosulfuron on V. myuros. Prosulfocarb was sprayed at different plant growth stages to study the influence of plant growth stage on the performance of prosulfocarb on V. myuros in comparison with the more susceptible grass weed species Apera spica-venti. Doses causing 50% reduction in response variable (ED50) were estimated from the dose–response analysis. The ED50 values revealed a higher tolerance of V. myuros to prosulfocarb and mesosulfuron + iodosulfuron than A. spica-venti. The relative difference in the effectiveness of prosulfocarb between V. myuros and A. spica-venti was constant among plant growth stages studied. The highest levels of V. myuros control were achieved when prosulfocarb was sprayed pre-emergence (BBCH 00), while the control substantially declined at later growth stages. The results from the current study document the tolerance of V. myuros to prosulfocarb and mesosulfuron + iodosulfuron and highlight the importance of optimization of prosulfocarb spray timing for achieving maximum control of V. myuros.

Enhancing Zinc Accumulation and Bioavailability in Wheat Grains by Integrated Zinc and Pesticide Application

Incorporating foliar zinc (Zn) spray into existing pesticide application is considered highly cost-effective to biofortify wheat (Triticum aestivum) with Zn. However, the effectiveness of this combined approach in terms of Zn enrichment and bioavailability in grain and its milling fractions is not well examined. Two-year field experiments were conducted in 2017 and 2018 with three sets of foliar applications (nil Zn as control, foliar Zn alone, and foliar Zn plus pesticides) at the anthesis, milk stage, or both. Compared to the control, grain yield was not affected by foliar Zn application alone or combined with pesticides, while the Zn concentrations and bioavailability substantially increased in the whole-grain, bran, and flour irrespective of spray timing. Yield losses by 28%–39% (2018 vs. 2017) led to 7%–18% and 18%–38% increase of Zn density in grain and flour, respectively. Further, such negative responses were uncoupled by foliar spray of Zn or Zn plus pesticides, and absent from the control plants. Nonetheless, grain Zn biofortification was achieved in both low- and high-yield plants with either Zn spray alone or combined with pesticides. Together with the enhanced Zn bioavailability in grain, bran, and flour, the effectiveness of this combined strategy is validated to biofortify wheat with Zn.

Validation of a Florida Strawberry Anthracnose Fruit Rot (AFR) Warning System in Iowa

Field validation of a disease-warning system for strawberry anthracnose fruit rot (AFR), caused by Colletotrichum acutatum sensu lato, that was originally developed for use in Florida was conducted in Iowa from 2012 to 2014. Day-neutral strawberry (cv. Tristar) was artificially inoculated with the pathogen at the start of the fruit maturation period. A factorial combination in replicated trials of two spray timing methods (the Florida warning system and prescheduled, calendar-based timing) and two fungicides (captan and pyraclostrobin) was compared, along with a nonsprayed control. The calendar-based and warning system-based treatments provided statistically equivalent control of AFR incidence compared with the nonsprayed control, and the warning system treatments required an average of 1.7 fewer fungicide sprays annually than the calendar-based treatments. Further analysis of the field data suggested that the warning system might underestimate AFR risk under high disease pressure; in these circumstances, a lower action threshold value may need to be adopted. Overall, these results indicated that the Florida warning system can be valuable for helping Midwest strawberry growers control AFR with less reliance on fungicide sprays, but it may require modification to account for periods of high inoculum pressure, subject to results of further field trials.

Control Recommendations for Black Medic (Medicago lupulina) Based on Growth and Development in Competition with Strawberry

Strawberries are an important horticultural crop in Florida. Black medic is among the most problematic weeds within the production system. To better coordinate control measures, black medic growth and development while in competition with strawberry was studied. Twelve plants were randomly selected at each of four field sites in Hillsborough County, FL, in 2014. Plants were repeatedly measured over the growing season for stem length and number of primary branches, flower buds, flowers, and seed clusters. Growing degree days (GDD) were calculated (Tbase=0 C) starting from the hole-punch application of the plastic mulch (October 8, 2014, to October 10, 2014) from weather station data generated from the Florida Automated Weather Network. Strawberry height and width increased consistently across all sites, but black medic growth and development varied considerably. Strawberry suppressed black medic growth up to 1,805 cumulative GDD at three of four sites where black medic remained beneath the strawberry canopy. After 1,805 GDD, the black medic stems still remained below but experienced exponential growth for total stem length and, in turn, flower buds, inflorescence, and immature seed clusters. Ideal clopyralid spray timing based on susceptible plant size was 890 to 1,152 GDD. Optimal hand-weeding time frames would likely occur as the plant stems expand beyond the strawberry canopy (to improve visibility) and before flower production to prevent seed return to the seedbank. First seed production was observed at 1,200 GDD at the earliest site and between 1,966 to 2,365 GDD across all the other sites. Overall, consistent trends were observed across sites, but between-site variability was observed that could not be accounted for by differences in temperature.