Volume 5 Issue 7
Lisa Wasko DeVetter, WSU Small Fruit Horticulturist
Matthew Arrington, Ph.D. Graduate Student in Small Fruit Horticulture
The main objective of this project is to test the hypothesis that foliar-applied boron (B) can increase fruit set and resultant yield of highbush blueberry (Vaccinium corymbosum). Specific sub-objectives are to: 1) evaluate the effects of foliar-applied B on measured yield components when applied at low or high rates in the spring or fall, and 2) measure the effects of B application on pollen germination, tube growth, and fertilization of ovules.
In 2014, we began this project with Experiment 1, which was conducted with established ‘Duke’ plants at the Washington State University Mount Vernon Research and Extension Center (WSU-REC). These plants were heavily infected with blueberry shock virus (BIShV) in 2015, which negatively impacted the collection of fruit set and yield data. As a result, we are unable to confidently draw conclusions from this year of the study for that particular experiment. In 2015, Mr. Matthew Arrington began his PhD program in DeVetter’s program and initiated a series of related trials that we are including in this progress report as Experiment 2. Further description about the two experiments are below.
Established ‘Duke’ plants located at the WSU-REC were used for this experiment. Tissue samples were collected in Aug. 2014 and detected nutrient deficiencies were corrected, including B. Soil samples were also collected in Fall 2014. Results from the preliminary tissue and soil sampling are presented in Tables 1 and 2. Correcting deficiencies was an important first step to allow discernment of treatment effects from the potential effects of pre-existing deficiencies within the plants.
Treatments were applied to individual plots consisting of three plants per plot. A total of three blocks with treatments in a randomized order within a block provided the experimental design. A buffer of two plants from the perimeter of the planting and one plant in between adjacent plots was established. Five treatments were applied and varied based on rate and timing of B application. These treatments include: 1) untreated control (no additional B); 2) fall applied B at 8 oz/acre; 3) spring applied B at 8 oz/acre; 4) fall applied B at 32 oz/acre; and 5) spring applied B at 32 oz/acre. The foliar product used for the experiment was BioGro NUE Boron 10% (derived from boric acid). Fall and spring applications corresponded to postharvest/prior leaf senescence and prebloom, respectively. The first cycle of treatment applications began in the fall of 2014 and was carried through in the spring of 2015. Foliar nutrient, soil chemistry, fruit set, estimated yield, and fruit firmness data were collected and are presented below. We would like to note that this study occurred during an exceptionally warm and dry year, which likely influences our results. Mover, the high incidence of BIShV influences the quality of our data, as we had few treatment plants that were not impacted by BIShV. These factors should be taken into consideration when interpreting the data, as results from 2015 may not be applicable to the normal climactic conditions experienced in western Washington.
Foliar and soil test results are presented in Tables 3 and 4. Foliar samples collected across the treatments were within nutrient sufficiency ranges with the exception of sulfur (S) and copper (Cu) (Table 3). Plants treated in the spring with both high and low concentrations of B were slightly below the sufficiency range for S. We do not believe this difference is significant nor related to our treatments. All treated plants were below the sufficiency range for Cu. However, newly published research in Oregon is proposing to lower the sufficiency standards for this micronutrient (Strik and Vance, 2015). Furthermore, we did not observe any symptoms of Cu deficiency. Soil test results are presented in Table 4. There are differences between soil results from 2014 and 2015 (Tables 2 and 4). Many of these differences can be attributed to our fertility regime, which was newly implemented in 2014 (DeVetter’s research program was initiated in 2014). The most notable difference between the years is the increase in organic matter from 2014 to 2015. Sawdust mulch was applied in 2015 and is likely responsible for the increases in organic matter, particularly if sawdust mulch remained in the soil sample. In addition, we began sending our foliar and soil samples to be tested at a new lab (Brookside Laboratories, Inc.; New Bremen, OH) and differences in testing protocols may result in differences in our results between 2014 and 2015.
Percentage fruit set is presented in Fig. 1. Overall fruit set was greater than 60% for all treatments, with the exception of plants treated with B at a low concentration in the spring. Based on our observations in the field, the reduction in fruit set for plants in this treatment was due to high infection rates of BIShV, not treatments. Fruit set was greatest in the control treatment. Fruit set was on average greater in 2015 than in previous years, which is attributed to warmer weather that was more conducive for pollination and fruit set (Strik, 2004). Fig. 2 presents estimated yield, which is determined for each plant using the following equation:
Estimated yield = [average cane #] x [average # fruit clusters/cane] x [average # berries/cluster] x [average berry weight] x [0.0022062] = lbs/plant
The variables of “[average # fruit clusters/cane]” and “[average # berries/cluster]” were determined from two canes per bush
and two clusters per cane, respectively. Average berry weight was determined from a sample of 30 berries per treatment and replication. The value of 0.002205 is a conversion factor to allow data to be presented in pounds. Individual plant yields were low due to BIShV. Similar to the fruit set data, estimated yield was lowest for plants treated with low concentrations of B in the Spring. We again attribute this to plants in this treatment being the most severely impacted by BIShV. Estimated yields were otherwise similar across the treatments. Firmness was also assessed on harvested berries using a FirmTech 2 (Bio Works, Inc., Wamego, KS). No treatment differences were detected and firmness exceeded 200 g/mm deflection (data not presented). Pollen was collected in the spring, desiccated, and stored at -20 °C. Pollen viability tests will soon be underway according to the procedures outlined by Dogterom et al. (2000).