Agronomic factors affecting blueberry fruit quality in northern highbush blueberries
Note Number: AG1422
Published: December 2010
Updated: May 2013
Highbush blueberries are perennial, long lived, deciduous, woody shrubs, native to America, Asia and Europe. They require 6-8 years to reach full production and are 1.5-3.5m high at maturity. They belong to the family Ericaceae, which includes plants such as cranberry, azalea, rhodendron and heather. Like other ericaeceous plants, blueberries thrive in acidic soils and do best when the pH is between 4.5 and 5.5.
Blueberry production is a long term enterprise. Consequently it is recommended that good site preparation commence at least a year prior to planting to eliminate perennial weeds, build soil organic matter to help ensure adequate drainage, pH and nutrients. Cover crops can be sown to protect the bare ground, such as, annual cereal grains, grasses and legumes which can be turned in prior to planting to build organic matter.
Blueberries are a shallow rooting plant, with most of the roots in the upper 30 cm of soil. Blueberry roots lack root hairs but the roots are associated with mycorrhizal fungi in the soil and these fungi facilitate the absorption of nutrients by the plant. The plants require an open, porous soil high in organic matter, but low in nutrients. These factors are important to take into account when considering management practices that may affect quality
The consumer purchases fruit according to its appearance, aroma, a perception of taste and shelf life. But it is flavour that brings them back for more. Key parameters of blueberry quality assessment include:
- Intensity and uniformity of blue colour
- Sweet/acid balance
- Shelf life
Breeding programs focus on taste as well as other factors, in relation to production and yield but genetics is only part of the story affecting fruit quality. Management issues such as irrigation, plant nutrition, pruning and other factors will affect fruit quality and good agronomic practice is essential to maximise yield and quality. Extreme weather events, late frosts and other factors outside the growers control will also affect quality but management is essential to reduce the impact of these events.
It is important to select the most suitable varieties for a given location. Breeding programs and genetics are critical in developing new varieties which will behave differently in various locations under different irrigation and nutritional practices. Consequently variety selection will be a critical starting point.
Climatic factors can have a range of affects on fruit quality with respect to damage and shelf life while optimum growing conditions are required to maximise fruit quality.
Climate and temperature is a prime factor when selecting a suitable variety to plant. The effect of local microclimate will have a role when deciding how to plant the crop, choosing layout, aspect and variety. The Northern Highbush blueberry has a minimum requirement of 850 to 1,000 chilling units (CU) for shoot growth and flowering while the southern types require only 350-650 CU.
The optimum temperature for northern highbush blueberry growth, fruit set, size and ripening is 20°C-26°C during the day and 16°C at night. As daytime temperatures increase to 30°C, leaf photosynthesis can decrease by as much as 47% in some varieties. Soil temperatures below 8°C or above 20°C will limit root growth, with the optimum soil temperatures for root growth being from 14° C to 18° C.
Late spring frosts can also damage flowers and fruit and it is important to avoid planting in areas where late frosts may occur after budburst. Fruit set is very sensitive to frost damage. To reduce the impact of late frosts, rows can be planted down the slope to allow air drainage and a north easterly aspect is desirable.
Fruit quality can be adversely affected by high temperatures, particularly during harvest. High temperatures (day or night) during harvest will decrease berry firmness and storage life. Shade netting and good irrigation practices can help to reduce the impact of temperature extremes.
Summer rainfall (or overhead irrigation) can affect fruit quality, particularly approaching harvest. Rain can remove the bloom leaving marks on berries reducing fruit quality and may cause berry splitting making the fruit unmarketable. Improper use of overhead irrigation can have the same effect. This is a factor that needs to be considered when producing in areas of high summer rainfall. Summer rainfall and increased humidity may also lead to increased disease pressure from diseases such as grey mould (Botrytis).
Areas or regions prone to hail are another issue to consider when planting an orchard as hail can not only ruin the current crop but also damage shoots and reduce the following year's crop. Appropriate netting may also be of benefit and reduce the impact of hail.
Orchard aspect is an important consideration for blueberry production. Aspect will affect factors such as chilling requirement, berry quality and yield. Altitude will also have a major influence on berry production with higher altitudes being more susceptible to late frosts but also having the advantage of cooler and less extreme summer temperatures.
A north east facing slope, encourages cold air to move away, decreasing risk of frost damage in spring, dries the dew for earlier morning picking in summer and reduces the intensity of afternoon sun.
In warmer production areas a more southerly aspect may be important when considering orchard establishment as the intensity of heat from western facing exposed sites could be an issue in maintaining good berry quality.
When considering aspect it is also important to consider prevailing winds as blueberry bushes are sensitive to hot and cold wind. Blueberry plants will thrive better when sheltered from hot northerly winds. Growers should consider establishing windbreaks early in the life of a blueberry orchard and positioning of the windbreak will depend upon aspect and the prevailing winds. Good air flow is also critical for the control of fruit rots such as Botrytis and assists with disease control, so a windbreak needs to allow good airflow as well as protection.
When considering the establishment of windbreaks it is essential to consider that windbreaks will also require management and avoid affecting the crop by shading or reducing water availability.
Planting density will have an impact on productivity and quality. Higher density plantings may yield more fruit but, size and quality may be affected. Higher density plantings will allow less air movement and have the potential to result in higher disease pressure, particularly in humid environments. Strik and Buller (2002) reported larger berries at wider plant spacings, when evaluating Bluecrop (high chill). A density of 4,300 plants /Ha (at 75cm between plants and row width 1.5m) may be the optimum spacing without affecting fruit size.
Blueberries prefer nutritionally "impoverished" acidic soils, that are well aerated, well drained, uncompacted, at a low pH and high in organic matter. However, low pH soils may cause problems with aluminium toxicity. Where the soil type is unsuitable plant growth will be weak, production low and fruit quality will be affected. Where pH is too low or too high in soils nutrients will be tied up making them unavailable to the plant. Soils with pH greater than 5.5 may result in plants showing symptoms of chlorosis and iron deficiency leading to reduced growth and yield while if soils are very acid soils, pH 3.5 - 4.0, plant growth will also be affected. Low pH soils should be limed prior to planting.
Netting can be used to protect plants from bird and hail damage, but will also modify the climate. Netting of some type is essential for bird control depending on the size of the production area. If birds are left unchecked, the birds may rapidly reduce fruit quality down to processing grade and reduce total yield.
Research into the use of exclusion netting has shown that black netting slowed down fruit development and delayed harvest between 5-10 days (Lobos et al, 2008). The effects of netting on fruit quality are variable and more research is needed to clarify benefits and optimum requirements for instalment and removal.
Vaccinium plants like most plants of the family Ericacea are colonised in nature by Ericoid endomycorrhizae. These fungi aid the plant to absorb nutrients such as, organic nitrogen, phosphorus and iron that are less available in low pH soils and overcomes the lack of root hairs for the plant.
Research indicates plants "infected" with mycorrhizae produce more canopies and have lower nitrogen requirements from added fertilisers. Fungal strain also influences total plant growth, number and length of shoots and height of the bush. However, the amount of infected roots and the effect of the mycorrhizae are dependent on a range of factors, including; type of soil, pH, quality and content of organic matter and soil moisture.
Blueberries require less nutrients compared to other fruit crops and excess fertilizer can have a detrimental effect. Excess nitrogen will inhibit the mychorrizal population and may lead to excessive vegetative growth, nutrient problems, restricted flower bud formation and delayed fruit maturity. Leaf analysis is an accurate method of determining nutrient requirements and should be taken yearly. Leaf samples should be taken in early January using the youngest full size leaf from fruiting shoots.
Fertiliser should be broadcast evenly under the entire canopy as there is very little lateral nutrient translocation within the plant. Applications should be split with half applied before bud break and the rest 6 weeks later. If plants are drip irrigated then fertiliser can be applied through the drip line during irrigation. Nitrogen fertiliser in the ammonium form is preferred to the nitrate form.
The type of mulch may also contribute to the nutrient input, however if sawdust or wood chips are used as mulch, note that this may "tie up" nitrogen in the short term as bacteria use nitrogen to break down the mulch, reducing short term availability. Consequently fertiliser rates may need to increase by up to 50% in the short term if using wood chips. Excessive or late fertilisation with nitrogen may force late season growth that is susceptible to frost damage and reduces flower bud development for next year's crop.
Calcium is critical to fruit growth and quality, but is often low in the acid soils in which blueberries grow. Research has concluded that foliar application is likely to have little benefit. To maintain calcium supply to the plant it is essential that irrigation application is even and that plants are not stressed, as this will affect calcium uptake.
This is a critical management practice. Annual pruning will maintain blueberry plant vigour and productivity, and influence fruit size and quality.
Blueberries are an understorey species and light interception and distribution within the canopy greatly affects fruit quality (sweetness and flavour) and quantity through the leaf to fruit ratio and flower bud formation. Pruning also affects fruit quality by aiding air movement, solar penetration which will improve fruit quality and reduce disease incidence.
The main time to prune is in winter when the plant is fully dormant. Carbohydrates produced in late autumn will have had sufficient time to move into the roots and crown for storage. At this time damaged wood can be identified and the structure of the bush developed with removal of older canes and excessive growth.
The first year canes called "whips" are unbranched and do not produce much fruit in the first year of growth, but they are essential for future crops. The second and third year canes have laterals with good vigour and many strong fruit buds. They are the best fruiting wood. Fourth or fifth year canes and older wood with small, weak laterals and few fruit buds per lateral, are not productive and should be removed.
Weak bushes require more pruning than vigorous bushes as pruning stimulates vegetative growth. Weak twiggy wood should be removed as it produces few shoots and flowerbuds and small berries. By 7 years of age, plants should have 10 to 15 canes of different ages.
Summer pruning does not replace winter pruning and consists of branch removal during the period of active growth in spring and summer. The main objective is to cut back vigorous shoots, eliminate apical dominance and stimulate lateral bud break during the same season. If plants overbear, berry size will be reduced. Fruiting canes should be pruned back to remove one third of flower buds.
Pruning in early autumn late summer can have the effect of delaying bloom the following spring and stimulates new growth which may affect yield and fruit size the following season.Irregular pruning results in erratic yields from year to year and with very large bushes with individual canes competing for light and a higher number of older canes. Weak, twiggy wood generally has fewer flower buds and produces smaller berries
Is a critical component of fruit production and must be included by growers in management strategies to ensure good yields. Vaccinium flowers are adapted to vibratile (Buzz) pollination, a specialised foraging behaviour by bees, such as bumble bees and many indigenous bees, but not honey bees. A diverse group of native bees visit Vaccinium species and their importance to fruit production is only just beginning to be recognised. Areas surrounding orchards provide important habitat to pollinators. Blueberries growing on old heath lands often have access to the native pollinators adapted to the Ericacaea flower.
However, the native pollinator population is rarely abundant enough to pollinate a large commercial planting. Habitat around the orchard requires a succession of flowering plants for native bee populations to thrive. It is important to use 'bee-friendly' practices to protect and encourage bee activity.
The use of pesticides during flowering should be avoided particularly during daylight hours or use pesticides that are safe to bees. The blueberry flower is not a favourite flower for the honey bee, but it is still an effective pollinator. Bees can be added at around 2-5 hives per ha and should be introduced at 5% flowering (not before) to improve pollination.
Pollination affects yield and fruit size with good pollination increasing fruit set, seed number, berry size and faster, more evening ripening. While some varieties such as Bluecrop can self pollinate other varieties such as Northland require cross pollination with another species, for these varieties a suitable pollinator is essential.
Some factors influencing pollination outside the control of the grower include:
- Temperature, for honeybees cannot forage at temperatures below 12.7°C.
- Flower age at pollination is important because of high flower densities and relatively short bloom duration for the plant requires that there is a suitable number of pollinators available. If pollination does not occur within three days after the flower opens, fruit set is unlikely.
- The quality of fruit is a function of the number of ovules fertilised, so the number of pollen grains transferred is critical.
Given the importance and range of factors outlined above it is essential to ensure adequate pollination to achieve good yield and fruit quality.
Blueberry plants are shallow rooted and the use of mulches can have a range of benefits. Mulching can have an affect on plant health and contribute to plant size, berry yield, soil moisture and reduce frost damage and daily temperature fluctuations. The use of degraded woodchip may improve soil properties in the long term, especially in low fertile soils improving organic matter and soil drainage. The use of mulches will aid water retention in the soil, protect the roots from extremes in temperature and aid in weed control.
Mulches may influence root zone temperature both positively in summer, but negatively in winter/spring. The use of mulch will also break down into a possible nutrient source, conserve soil moisture and can reduce the water requirement by up to 20% through the increase in organic matter.Inter-row swards can be used to control weeds, and will shade soil from the sun, aid in organic matter accumulation and plants such as clover will help fix nitrogen. Valuable inter row crops to consider include clover, vetch and oats however these crops do not establish well on acid soils. There is potential for inter-row swards to compete for nutrients and water but the blueberry roots will not grow much beyond the row space so the impact should be limited.
Mulches have a number of benefits but there are also have some negative impacts depending on the type of mulch. There may be short term reduced availability of nitrogen and high levels of manganese in organic mulches such as pine sawdust, manure and pine needles and cultivar sensitivity may contribute to manganese toxicity in blueberries. Some composts and manures may have a high pH or salt content and these would not be recommended for blueberry production.
Irrigation management is critical to fruit quality and plant growth due to the shallow rooted nature of blueberries. Consequently a constant and even supply of soil water is required for blueberry plants. Blueberries will require around 25 mm per week of water during the growing season in addition to the replacement of soil evaporation.
There are several critical periods during which water stress significantly impacts on yield/quality. The first 2 weeks after petal fall, 2 weeks prior to and after harvest. Water stress will slow fruit growth and the skin becomes less elastic making the skin more prone to splitting during a following rainfall event or high humidity. Without rain or irrigation the fruit will shrivel. Water stress after harvest, reduces flower bud induction and consequently next seasons yield.
A continuous supply of water throughout the life of the plant is essential for quality fruit. Inadequate irrigation will affect the current season's crop reducing yield and quality and reduce the flowering/fruiting potential for the following season.
Note that blueberries are very sensitive to salts and before using water of unknown quality a water test should be carried out. This will particularly be the case if ground water is to be used.
It is important to keep the entire root system moist and cool. Water and nutrients do not easily translocate from one side of plant to the other and consequently if using drip irrigation a double line drip system is recommended. Alternatively microjets or low level sprinklers are effective for these systems will wet a greater area and support a larger root zone. This will be particularly relevant in high rainfall areas where irrigation is required less often and the roots will be more widespread.
The use of overhead irrigation has some benefits such as, an aid to frost protection and cooling under hot conditions, but there is potential to mark the fruit due to droplets affecting the bloom and due to increased humidity in the canopy there is the potential for increased disease pressure. Management of overhead irrigation will be more critical.
Good irrigation management and scheduling is essential for good yield and fruit quality. It is important not to over or under-irrigate. The use of moisture monitoring systems and scheduling irrigation is essential.
Irrigation can be scheduled using a range of methods. Either by estimating evapotranspiration (total water loss from the plants and soil by transpiration and evaporation) data calculated from weather stations or via the use of evaporation pans to determine the amount of replacement water needed.
Other methods are based on monitoring soil moisture levels using tensiometers (simple and easy to use but require constant maintenance), resistance blocks and capacitance sensors (such as an Environscan® system) to name a few. For more on the irrigation method best suited to your needs consult an irrigation specialist.
Pests and disease
Damage by insects and birds can degrade fruit quality and will down grade fruit significantly. Bird control can be achieved by the use of bird netting or other forms of netting. This can be a major cost but the impact if these pests are not controlled can be significant. Crop monitoring is also essential to identify what pests and beneficials are present, whether adequate control is being achieved by beneficial organisms, or to determine if chemical control of pests or diseases is required.
Several diseases will directly affect fruit quality and shelf life such as Alternaria, Colletotrichum and Botrytis but other diseases which affect the plant will also negatively impact on yield and fruit quality. It is essential monitor plants regularly for pests, diseases and beneficial organisms.
Highbush Blueberry Production - A Pacific Northwest Extension Publication, PNW 215, Oregon State University, Washington State University and University of Idaho. Reprinted September 2006. Editor Bernadine Strik, Extension berry crops specialist, Oregon State University.
Lobos et al (2008), Physiological Response of Vaccinium corybosum 'Elliot' to Shading Nets in Michigan", Proceedings of 9th International Vaccinium Conference. Proceedings of 9th International Vaccinium Conference (2008) Acta Horticulture 810 March 2009. Editor K.E. Hummer USDA-ARS, NCGR Corvalis, Oregon, USA. Strik, B and Buller, G (2002) Improving yield and machine harvest efficiency of 'Bluecrop' through high density planting and trellising. Acta Hort. 574:227-231.
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