Jia FENG, Li LI, Ya LI, Lei YANG, Hui DONG, Li YANG

Shijiazhuang Institute of Pomology, Hebei Academy of Agriculture and Forestry Sciences, Shijiazhuang 050061, China

Abstract Continuous overcast and hazy days are common for the winter in Hebei Province, these disastrous weathers have tremendous negative effect on the production of protected strawberry cultivation, for leading to the problems such as degrading photosynthesis of leaves, poor pollination and fertilization, more malformed fruits, low percentage of fertile fruits, delayed maturation of fruits, degraded quality and yield, and aggravated diseases.In view of the disastrous weathers and problems, the authors put forward the measures for the management of protected strawberry cultivation in continuous overcast and hazy days, to provide technical support for strawberry farmers.

Key words Protected strawberry cultivation, Continuous overcast, Hazy days, Control measures

1 Introduction

China is the world largest strawberry producing country, ranking the first in both growing area and yield[1], the protected cultivation of strawberry has increased consecutively in past years.There are always continuous overcast days in autumn and winter in north China[2-4], which has a great impact on the protected production, and is an important factor causing yield and quality degradation.In the continuous overcast days, greenhouse temperature at daytime is usually below 15 ℃, the humidity above 90%, influencing the strawberry production badly.In recent years, the continuous overcast and hazy days occurred in higher frequencies, directly causing damages of protected strawberry cultivation.Particularly, the overcast and hazy days over 1 month from December 2012 to January 2013 resulted in the poor pollination, more malformed fruits, low percentage of fertile fruits, delayed maturation of fruits, degradation of quality and yield, aggravated diseases and so on because of the continuous low temperature and insufficient sunlight.This paper analyzed the causes of these negative impact, proposed corresponding measures, to provide references for the prevention and control before occurrence of bad weathers.

2 Causes of overcast and hazy days influencing protected strawberry cultivation

2.1 Weak sunlight leading to low temperature in the facilityIn winter, sunlight is the major source of heat in the facility.Temperature in the facility is naturally low because of continuous overcast and hazy days, and weak sunlight.When the overcast days last for 5 to 7 d, heat accumulation in the greenhouse is insufficient, daytime temperature in the greenhouse is below 15 ℃, night temperature below 5 ℃, resulting in poor pollination and fertilization, and more malformed fruits.Moreover, continuous overcast days leads to low soil temperature below 10 ℃, and the slow root system activities, influencing the absorption of nutrients and plant growth negatively.

2.2 Weak sunlight influencing photosynthesis of strawberry plantsSunlight is extremely insufficient in continuous overcast and hazy days, photosynthesis of strawberry plant leaves degrades greatly, and the nutrient accumulation decreases, as a result, leaves grow big but thin with pale colors, plants grow taller but weaker, the yield declines greatly, and the plants are more vulnerable to the diseases.

2.3 High air humidity inducing plant diseasesContinuous overcast and hazy days cause high air humidity in the greenhouse, the relative air humidity even reaches 100%.In sunny days, ventilation helps dehumidify the greenhouse, but in overcast and hazy days, ventilation cannot reduce, but even increases the air humidity inside.The over-high air humidity induces the occurrence of various diseases, such as gray mold, red infarction and red stele root rot, therefore reduces the yield of strawberry.

2.4 High air humidity leading to physiological nutritional deficiencyIn continuous overcast and hazy days, high air humidity in the greenhouse inhibits the transpiration of strawberry plants, blocks roots’ absorption of water, and also nutrients such as K, Ca, Mg, Zn,etc., but in fact, there are possibly sufficient nutrients in soil.

3 Control measures for overcast and hazy days

3.1 Rolling up the cotton cover insistentlyThe cotton cover should be rolled up at noon in case the temperature is not too low, so that strawberry plants can receive some scattered light, which will raise temperature inside and increase the yield.Meanwhile, a reflective curtain can be hung in the rear part of the greenhouse to increase the light and temperature.After the long continuous low-temperature, overcast and hazy days, the cotton cover cannot be totally rolled up in sudden sunny days, the cover can only be curled up partly to avoid quick temperature rise and strong sunlight, and then totally rolled up after strawberry plants adapting to the temperature rise, moreover, ventilation is very important in case of high temperature.

3.2 Keeping warm using multilayered coverWaterproof plastic film is necessary to cover the cotton cover or quilt to keep warm and make snow cleaning easy; or wheat straws or bran can be used to cover the ground, prevent heat loss and raise the soil temperature; if possible, the entire greenhouse can be equipped with double-layered or even triple-layered plastic film to keep warm.

3.3 Watering reasonablyGreenhouse in overcast and hazy days has low temperature and weak sunlight, too much water will lead to high humidity.So the plants should be watered reasonably to control soil and air humidity.If necessary, the frequency and amount of watering must be properly controlled, control of watering can enhance air permeability of soil, help prevent leaf yellowing, and root rot.

3.4 Enhancing sunlightSunlight in continuous overcast and hazy days is particularly weak, and weak sunlight is an important factor restraining the yield and benefit of protected strawberry cultivation.The sunlight can be strengthened in the following ways:(i)Hang a 1.5-m wide reflective curtain on the north rear wall inside the greenhouse, it can greatly enhance sunlight in this area, and consequently strengthen photosynthesis of the strawberry leaves.(ii)100-W incandescent bulbs can be installed as the light source, 600-750 bulbs per hectare, to enhance the illumination for 5-6 h daily from 17:00 to 22:00, which will greatly increase the yield and reduce the quantity of malformed fruits.(iii)The specialized Led fill-in light for strawberry growing can be used to supplement the light.(iv)Greenhouse film should be cleaned frequently to facilitate sunlight going through the film into the greenhouse, which will greatly increase the sunlight and air temperature inside.

3.5 Raising temperature properlyTemporary heating facilities can be used to keep the daytime temperature around 20 ℃ and the lowest night temperature around 5 ℃.In continuous overcast and hazy days, the greenhouse temperature, particularly night temperature must be controlled properly, because plants have greater respiration and more consumption in case of high night temperature, but the plants are not able to produce sufficient nutrients for the night consumption even if the curtains are curled up.Heating is usually adopted at night, and short-time ventilation is necessary at noon.If the plants are still small, it is better to install small plastic tunnels inside the greenhouse to reduce the soil temperature drop, and relieve the withering of plants after clearing up.

3.6 Applying supplementary pollinationIn case of the continuous overcast and hazy days during the flowering season, supplementary pollination can be applied, such as bumblebee supplementary pollination and artificial supplementary pollination.The supplementary pollination helps reduce the percentage of malformed fruits, improve the marketability of strawberry.When bumblebees do not come out from their nests in case of low temperature during the overcast and hazy days, ventilating fans can be used at noon with higher temperature to realize the artificial supplementary pollination.

3.7 Reducing humidityAccording to the outside temperature, small gaps can be opened to ventilate from 12:00 to 14:00, 5-10 min each time, 2-3 times in total, to reduce the humidity inside.When relative air humidity within the greenhouse is controlled below 75%, the germination of most fungal spores can be effectively controlled, so that the occurrence of fungal diseases will be avoided.As the air humidity grows too high, wheat straws or bran about 20-25 cm thick can be used to pave among the rows, which can absorb abundant humidity in the air, and also absorb more heat at daytime and consequently raise the night temperature within the greenhouse.

3.8 Applying foliage top dressingRoot systems of the plants fail to absorb sufficient nutrients such as K, Ca, Mg and Zn during the continuous overcast and rainy days, thus foliage top dressing can be applied to supplement the nutrients that leaves need.Concentration of the liquid manure should be 600-1 000 times.

3.9 Preventing diseasesStrawberry plants are more vulnerable to diseases because of the low temperature, weak sunlight and high humidity in the continuous overcast and hazy days, particularly powdery mildew, gray mold, leather rot.Powder can be sprayed from inside to entrance above the plants, to prevent and control diseases and also reduce greenhouse humidity.Moreover, smoke agent such as procymidone can be burned at night when the greenhouse is sealed to smoke the greenhouse, once every 7-10 d, to prevent many diseases.

3.10 Management of strawberry plants for the sudden clearing up after the long overcast daysThe cotton cover is curled up for a short time every day during the overcast and hazy days, after clearing up, the strong sunlight and drastic temperature rise after curling up the cover will facilitate the transpiration greatly, but root systems will still absorb water slowly, so leaves and plants will wither.Therefore, after clearing up the cotton cover should not be curled up totally at once, which enables plants to adapt to the environment, and avoid damages.In case of withering, the cover must be put down immediately, and curled up again as the leaves restore growth, and then put down again in case of withering, and curled up again until plants restore growth.After repeating this process for several times, plants will need shorter time to restore growth, and finally adapt to the temperature.If plants wither severely, clean water can be sprayed on the leaves to relieve the water shortage of leaves, which helps reduce the withering period.Without suitable measures, the plants will wither truly and it will cause serious damages.