Amaya Atucha
Extension Fruit Crop Specialist, UW-Madison
August 2019 Newsletter

One of the most important factors affecting fruit growth rates is temperature. The number of days with moderate temperature (60-85 deg F) is the best predictor of fresh fruit mass accumulation. What this means is that the higher the number of days the temperature stays in this range in the period between fruit set and harvest, the larger the fruit will be. This is because the rate of photosynthesis in cranberry leaves is the highest when temperatures are in the mid-70s, which means that in that range of temperatures, more carbohydrates (sugars) are being produced through photosynthesis in the leaves, and those carbohydrates are the ones that will make the fruit grow. When temperatures are over 85 deg F during the fruit growing period, the production of carbohydrates is lower, resulting in smaller fruit, which basically means as seasons get warmer and we experience more days with temperatures over 85 deg F, especially in July and the early part of August, we will struggle to achieve bigger fruit size. The same is true with cooler temperatures (< 60 deg F) during the beginning of fruit development in late June and early July that will result in smaller fruit, or when we have an early fall and fruit stop gaining fresh weight because the rate of photosynthesis decreases and fewer carbohydrates are being produced.

         Which brings me to my next topic, over-fertilizing or late season fertilizer application will not compensate for small fruit. I often hear that growers want to “pump up” berries with fertilizer to get them to size, but it is not the nutrients in fertilizer that will make the fruit grow, but the carbohydrates produced through photosynthesis in the leaves that are translocated into the developing berries. So why do cranberries need to be fertilized with nitrogen (N), phosphorous (P), and potassium (K)? In the case of N, this element is used to produce amino acids, which are the building blocks to assemble proteins. Proteins are essential to capture the energy from the sun through photosynthesis, resulting in the production of carbohydrates that will support fruit growth. The N is not needed in the fruit, but in the leaves for the production of carbohydrates to grow the fruit. Previous research studies have shown that in a bed producing 300 bbl/a only 15 to 20 lb N/a is removed by the fruit. In the case of P, this element is key to the production of the ATP molecule that carries energy in plants. P is part of the cell membrane and has an important function of transferring carbohydrates in the chloroplast to the cytoplasm, where they will be used for growth. Several studies have shown that reductions in P fertilization have no impact on yield or fruit size. I highly encourage growers to cut production costs by reducing their p fertilization, as long as tissue tests are in the normal range. Previous research studies have shown that in a bed producing 300 bbl/a only 3 to 6 lb P/a is removed by the fruit. In the case of K, this element is not part of proteins or the cell me membrane, and does not have a direct role in metabolism. The main function of K is water movement in the plant, as it is key to the opening and closing of the stomata in leaves. Stomata are small openings or pores that allow gas exchange in the plant; water vapor leaves the plant and CO2 enters. Many growers believe K makes fruit bigger, but this is not true. Uptake of most of the nutrients that go to the fruit occurs in the first stages of fruit development (fruit set to pea-size berry).  After that, it is just carbohydrates that make the fruit grow. K is different from N and P in that more K is removed by the fruit, and in the case of a bed producing 300 bbl/a of fruit, and estimated 30 to 35 lb/a of K (or 34 to 40 lb/a of K2O) is removed by the fruit.

         Since we are talking about K, I would like to briefly comment on the topic of K fertilization in August and September for bud set. There is no research supporting that K fertilization in late summer to early fall is needed or beneficial for bud set. K fertilization at any point during the growing season is made to support growth, yield, and overall plant health, not for bud setting. Most likely, the notion of applying K in the fall to help set buds comes from the recognition that growth in spring is supported by stored nutrients, and thus, K fertilization in fall specially in the case of low K in tissue and soil would hel p next year’s initial growth. However, in beds that have received adequate K during the growing season and have tissue and soil K in the normal ranges, there is no need to apply K in the fall.

In summary:

·      One of the most important factors affecting fruit growth rates is temperature, and the number of days with moderate temperature (60 to 85 deg F) is the best predictor of fruit size.

·      Carbohydrates (sugars) make berries, NOT fertilizer nutrients!

·      The goal of monitoring nutrient levels in plant tissue and soil is to apply enough nutrients so that it is never a limiting factor for yield and growth. Applying for fertilizer than that is a waste!

·      There is no research supporting the notion that fall K fertilization helps set buds.