Causes of Deformed Strawberry Fruit in Greenhouses, Prevention and Control Methods, and Whether They Are Safe to Eat
Growing strawberries in greenhouses is a good way to increase yield and income. But in practice, plants often bear many abnormal fruits that are too thin, too plump, curved, flat in shape, or uneven on the surface, known as deformed fruit. What causes deformed strawberry fruit in greenhouses? How can it be prevented and controlled? Are they safe to eat?
1. Choosing good varieties is the most economical and effective measure for controlling the occurrence of deformed fruit
Under the same growing conditions, varieties such as Zhangji, Allstar, Toyonoka and Tianbao have a deformed fruit rate more than 30% lower than varieties such as Shuofeng and Shuomi.
Varieties with excessively tall inflorescences are also prone to producing deformed fruit because fruit set is inconsistent, causing uneven nutrient distribution. For example, varieties such as Chunxiang and Jinxiang have taller inflorescences and therefore produce more deformed fruit.
Practice shows that cold-tolerant varieties, because of their strong resistance to low temperatures, greatly reduce the probability of deformed fruit occurring. For example, cold-tolerant varieties such as Gorella and Changhong have few deformed fruit.
Varieties with poor disease resistance are susceptible to disease during the flowering period, which leads to an increase in deformed fruit. For greenhouse strawberries, choosing varieties that tolerate low temperatures, have a short dormancy period, can flower normally under low-temperature conditions, have a relatively high self-pollination rate, and produce large fruit with good shape, early maturity, disease resistance, high yield and good quality is an effective measure for controlling the occurrence of deformed fruit and improving economic returns.
2. Adjust the greenhouse temperature properly. Too low a temperature is an important cause of deformed fruit
Strawberries like a warm, cool climate; they are not resistant to severe cold or heat. When the temperature reaches 5°C, they begin to sprout and grow, and the most suitable temperature for growth and development is 20°C-25°C; below 10°C and above 30°C are both unfavorable to their growth and development. Temperature requirements differ at different stages: before bud emergence, 25°C-28°C during the day and 15°C at night; from bud emergence to flowering, 22°C-26°C during the day and 10°C at night; after flowering and fruiting, 15°C-25°C during the day and no lower than 8°C at night. During the flowering and fruiting period, if the temperature is too low, it will seriously affect growth and development, leading to poor pollination and the appearance of deformed fruit.
In northern China, from December to early March of the following year, the climate is severely cold and greenhouses are often hit by cold waves. This period is exactly the flowering and fruiting period of greenhouse strawberries, and cold waves have a great impact on strawberry growth and development.
If the greenhouse is poorly constructed, the lighting angle is inappropriate, or the insulation facilities are inadequate, it is very easy for the room temperature to fall below 5°C, which readily leads to more deformed fruit.
For this reason, the north wall of the greenhouse should be thickened as early as possible. Before a cold wave arrives, add 4 layers of kraft paper quilting under the straw matting, which can raise the room temperature by 4°C; build a small arch tunnel over the strawberry beds, cover the arch tunnel with a 90-100 cm wide plastic film, and bury the lower end of the film into the soil and press it firm, which can raise the temperature inside the small arch tunnel by 3°C; installing an electric stove or heating wire in the greenhouse gives an even better warming effect. These insulation measures can ensure that the winter greenhouse temperature stays above 5°C.
3. Doing pollination well is a key measure for reducing the occurrence of deformed fruit
In the severe cold season, even if the room temperature is above 5°C, strawberries often bear deformed fruit because low temperatures cause poor pollination. In the cold season, the outdoor temperature is low, the greenhouse vents are small and opened for short periods, the interior is in a windless state, and it is cold outside the greenhouse with no insects entering, so strawberries cannot rely on wind or insects for pollination.
Poor pollination is another important cause of deformed fruit. The countermeasure is to do pollination well. There are mainly three pollination methods: First, plant pollinator varieties. This is a simple, easy, effective measure that yields twice the result with half the effort for preventing deformed fruit. The method is to interplant 5%-10% of a variety with abundant pollen among the main variety.
If the main variety is Hokowase, pollinator varieties such as Chunxiang or Toyonoka can be interplanted. Second, artificial pollination. Strawberry flower buds generally take 4-5 days from flowering to petal fall.
During the flowering period, after 10 a.m. each day, when there is no water on the leaves and flower buds, use a soft brush for artificial pollination, pollinating once every other day. Third, bee pollination. Bee pollination gives good results. One week before strawberry flowering, place one box of bees (about 5,000 bees) in each greenhouse.
Pesticide spraying should be stopped 15-20 days before introducing the bees. Bee pollination is not only better than artificial pollination, but can also save a large amount of pollination labor.
Flowers that have been pollinated fade quickly and can drop one day later. Rapid petal fall also has the effects of reducing nutrient consumption, concentrating nutrients for fruit set, fast fruit development, larger fruit, higher yield, and maturity 7 days earlier. According to the author's experiments, after pollination the fruit weight of the entire inflorescence can increase by 22%-28%, and the probability of deformed fruit is reduced by about 40%.
4. Water imbalance is another important cause of deformed fruit
Strawberries are very sensitive to water; they fear both drought and waterlogging. Soil that is too dry or too wet in the greenhouse will cause an imbalance between water and nutrient supply and demand, leading to poor fruit development and the formation of deformed fruit.
Because strawberry roots are relatively shallow, mainly distributed in the top 20 cm of soil, they are not drought-tolerant. In addition, the room temperature is relatively high and soil moisture evaporation is large, so drought is very likely to occur.
When soil moisture is insufficient, water should be applied in time. However, because the greenhouse is ventilated little in the cold season and moisture indoors is not easily lost, waterlogging damage is likely to occur, so flood irrigation should not be used; instead, water should be applied frequently in small amounts.
Using subsurface irrigation under plastic film can both meet the strawberry's water needs and avoid excessively high air humidity in the greenhouse. When greenhouse air humidity is too high, it easily induces disease; it can also hinder anther dehiscence, leading to poor pollination and the production of deformed fruit.
Choose long-life drip-free greenhouse film, which can effectively reduce water droplets and prevent droplets from wetting the stigma and affecting pollination. After entering the flowering period, watering should be appropriately controlled so as not to affect pollination and fruit set. From fruit set to maturity, water demand increases, and a small amount of water should be applied once every 5-7 days so as to keep the soil constantly moist.
5. Improper fertilization is also a key factor leading to deformed fruit
Greenhouse strawberries have a long fruiting period and a relatively high demand for the three elements nitrogen, phosphorus and potassium. Insufficient organic fertilizer, excessive application of nitrogen fertilizer, or improper fertilization can very easily lead to excessive vegetative growth of branches and leaves, causing poor ventilation and light penetration in the plants and worsening the occurrence of deformed fruit. On the basis of heavily applying organic fertilizer, attention should be paid to increasing the application of phosphorus and potassium fertilizers, and ammonium bicarbonate should not be applied as a top dressing.
Top dressing should follow the principle of “apply little but often, and closely combine with watering.” From closing the greenhouse to strawberry bud emergence, top dress generally once every 15 days, applying 15 kg of ternary compound fertilizer per mu; from bud emergence of the terminal inflorescence to before fruit harvest, top dress once every 20 days, applying 20 kg of standard compound fertilizer per mu; after the first harvest peak, top dress once every 30 days, applying 15 kg of compound fertilizer per mu.
After dissolving ternary compound fertilizer in water, using the hole-punching top-dressing method has prominent advantages such as saving fertilizer, being simple and convenient, and having a quick fertilizer effect, and is especially suitable for greenhouse application.
After the first batch of fruit is harvested and when the second full flowering period begins, spray the leaves with 0.3% potassium dihydrogen phosphate solution once every 5-7 days, for 2-3 consecutive sprays, with good results.
People often say that hollow strawberries should not be eaten because they have been treated with swelling agents, and that deformed strawberries are also caused by hormones and should not be eaten. Is this true?
The problem of deformed fruit is relatively complex; environmental factors such as low temperature can cause strawberry fruit deformation. An experiment once showed that if the seeds on a strawberry are affected by low temperature, it will lead to poor development of the strawberry; if they are removed during the strawberry's growth process, it will also cause the strawberry to become deformed.
In addition, during flower development, the growing points may increase in number due to external factors such as temperature changes and merge together to become a laterally growing structure. This phenomenon is called fasciation.
But these deformed fruits are all safe to eat, and some deformed fruits are even sweeter and more fragrant.