GREEN FRUIT DROP IN ‘DRAPER’: A QUICK FIX?
University of Fraser Valley
Figure 1. A cluster that’s lost fruit to the "Drop of Death".
Photo E. Gerbrandt
This article is a reprint from the BCBC Newsletter, February 2015
Defining Green Fruit Drop
Most growers in southwestern BC and northwestern Washington who have invested in plantings of ‘Draper’ know what is meant when phrases like “the ‘Draper’ drop”, “premature fruit drop” and “green fruit drop” come up in conversation. This is because they’ve seen it first-hand in the field. In Oregon and other regions, few who have heard of this disorder have seen it in person as its occurrence has not been reported in other areas where ‘Draper’ is grown. Likewise, to this author’s knowledge the scientific literature has yet to describe this condition in ‘Draper’. This condition, specific to Washington and BC, can be described as: fruit setting and developing normally until just before turning blue when suddenly, and seemingly inexplicably, ten or twenty (or thirty or forty!) percent of the green fruit drop off the bush over the course of a week or two.
To give the condition a name, we will use the name green fruit drop (GFD). A first distinction is that GFD is different from the premature blueberry fruit drop that was associated with a virus seen in ‘Bluecrop’ by researchers at the United States Department of Agriculture. This virus affects individual plants, causes loss of virtually all fruit and spreads throughout a field progressively. GFD has only been observed in ‘Draper’ and does not manifest like a disease caused by a spreading pathogen or pest. Rather, across the region, it is seen in some fields and not in others, occurring to a varying extent within and between fields and years while not being exclusively linked to a particular soil type or soil nutrient deficiency. In fact, it can occur to a great extent in one field and not at all in a field just down the road with few differences in field management to explain the disparity.
A second distinction is that GFD is different from what occurs when a blueberry flower receives inadequate pollination: dropping small, red, disk-shaped fruit after petal fall but before they begin to size. The fruit that drop in GFD have received adequate pollination, containing normally developing seeds that should be able to provide fruit tissues with an adequate stimulus for development. Rather than lacking in seeds, the dropped fruit are brown inside, being partially decomposed. In comparison, the fruit that remain on the bush and make it to harvest are perfectly normal to all appearances.
Determining a Cause
As to the cause of this disorder, early guesses from several local experts pointed toward a calcium deficiency because of the similarity of symptoms to calcium disorders in other crops, such as bitter pit in apple. Further support for this idea was provided by the observation that GFD was worse in fields with greater plant vigour, which indicates an interaction with nitrogen as seen in blossom end rot in tomato. Without attempting an exhaustive (exhausting?) explanation of the physiological and genetic reasoning underlying such a hypothesis, the following is a description of the results of two preliminary trials that were conducted in 2014. The primary purpose of these trials was to find a “quick fix” solution to the problem while a secondary outcome was to understand the cause of the disorder.
In Abbotsford, BC small five-plant plots were used in two replicated field experiments in an established field of ‘Draper’ that is severely affected by GFD. In the first experiment, manure was applied to the row and four treatments were applied: a no calcium control was compared with calcium chloride (8,300 ppm calcium), calcium carbonate (400 ppm calcium) and calcium thiosulphate (450 ppm calcium) products. The widely varying concentrations of these products were based on the different rates recommended by the manufacturers for each product. In the second experiment, no manure was applied to the row, but a range of experimentally applied nitrogen rates were combined with calcium treatments. Via split application in May and June, 0, 50, 100 and 150 lbs of nitrogen were applied in the form of granular ammonium sulphate. For the 0 and 150 lb rates, all four calcium treatments were compared while only the control calcium treatment was compared for the 50 and 100 lb rates of nitrogen.
Summary of Experimental Results
In the row with manure, the percent fruit dropped was not significantly reduced by the calcium thiosulphate product, but it was reduced by the calcium carbonate product by approximately a third and by the calcium chloride product by approximately 50%. In the row without manure, the percent fruit dropped was most dramatically reduced by the calcium chloride product under both the high and low nitrogen rates while being more moderately reduced by the calcium carbonate and thiosulphate products. The reduction in fruit drop in response to calcium treatments was more dramatic for the low nitrogen rate, which confirmed the observation that there is an association with plant nitrogen status and, therefore, vigour. Since the percent fruit dropped from each bush was negatively correlated with estimated yield for each bush, it was clear that the treatments that reduced drop were also effective in increasing yield. This is an important distinction because it shows that the calcium treatments were actually correcting a deficiency rather than causing the early abortion of flowers/fruit due to phytotoxicity (i.e., damage to plant tissues) or changing the balance of reproductive to vegetative growth. It is important to note that the calcium chloride product, which was applied at a much higher than the other products and contains chlorine to which blueberry plants are sensitive, did result in a considerable amount of phytotoxicity in the form of leaf burning.
Do we have a Quick Fix?
These trials indicate calcium deficiency as the probable cause of GFD. Most importantly, foliar applications can be used to correct the condition. By way of explanation, high vigour due to standard and excessive nitrogen fertilization seems to make the condition worse because the vegetative shoots pull too strongly at the plant’s available calcium to permit sufficient allocation to the developing fruit. How ‘Draper’ can be managed for high productivity without GFD reducing yields is a question that is yet to be answered. Therefore, another couple years of experimental research is required to determine the best rates, dates and number of applications of calcium products to properly correct the apparent calcium deficiency in ‘Draper’ grown in southwestern BC and northwestern Washington. As well, a field-based survey across BC, Washington and Oregon will be used to fully understand the condition and provide diagnostic tools for its correction.
In the meanwhile, a “quick fix” can be recommended to growers with plantings of ‘Draper’ that suffer from GFD. It is generally recommended that growers work with their consultants to employ calcium foliar sprays during late bloom and the early stages of fruit development to increase fruit calcium and prevent GFD. These applications should be made with caution as products such as calcium chloride pose the risk of phytotoxicity and the negative effects of phytotoxicity could out-weigh the benefits of their application if applied at the wrong rate or date (e.g., burning of flowers and early fruit causing a reduction in yield). Therefore, applications should be made at modest rates and under appropriate weather conditions to increase the chances of a net benefit of increased harvestable yields. In addition, expensive calcium products are unlikely to outperform the basic calcium chloride product used in this study because this form of calcium is taken up by plant tissues with relative ease. Application of calcium to the soil is also unlikely to have any effect on GFD as the plant does not actively take up calcium from the soil until after the time when the calcium deficiency is thought to originate. In fact, most of the fields that are severely affected by GFD also have high levels of soil calcium. Until more research can be presented after the 2015 season, good luck in the season to come!
Participating growers; Mark Sweeney (BC Ministry of Agriculture); Tom Baumann and Zach Fleming (Expert Agriculture Team Ltd.); Karina Sakalauskas and Michael Dossett (BC Blueberry Council); Lisa DeVetter (Washington State University); and Bernadine Strik (Oregon State University).
Figure 3. Cross-section of affected fruit.
Photo E. Gerbrandt
Figure 2. Dropped fruit.
Photo E. Gerbrandt
Funding Provided By: