Drought in southern Brazil: Considerations and technical indications for apple tree management

The cultivation of apple trees in southern Brazil occurs under climatic conditions described, according to the Köppen climate classification, as humid subtropical climate or mild temperate climate. In this region, annual rainfall volumes are greater than 1.700 mm. 

The southern region of Brazil has a high volume of annual rainfall, but the precipitation is not well distributed over the months, with dry conditions occurring between the months of November and March. This condition is more pronounced in years when the “La Niña” phenomenon occurs, characterized by anomalies in the Surface Temperature of the Pacific Ocean, which affects the volume of rainfall in the southern region of Brazil. In 2021/22 we are under the influence of this phenomenon (which is occurring for the second year in a row), which is affecting the region's climate conditions, causing prolonged drought. This condition has generated water deficits in the soil that affect plant development and productivity, generating losses in several production chains. For the apple production chain, the consequences of drought are manifested in adult orchards and in orchards in formation.

In the main apple producing region in RS, data compiled by Embrapa show that rainfall volumes in recent months are well below the historical average, generating a deficit of 293 mm, which represents 60% of the normal volume for the region. 

Considering the development rate of apple fruits, adequate water supply throughout the cycle is essential for good quality productivity. It should be noted that the availability of water for plants from December onwards will be essential for obtaining larger-caliber fruits, as well as for the absorption of nutrients, aiming for high fruit quality. Therefore, water restriction in the spring and summer of 2021 is being decisive in reducing vegetative growth and fruit development in adult apple orchards, especially after mid-December. This situation is more evident in places with soils with a shallow surface horizon or with a significant presence of gravel, whose water retention capacity in the soil is lower, which in certain situations can cause adult plants to bite.

drought effects

In young orchards, especially in plantings carried out in 2021, the commitment to the growth of branches is high, with a complete stoppage in the growth of shoots due to the intensity of the drought evident in this cycle. The significant reduction in rainfall could have a significant impact on apple orchards established this year, as in addition to the delay in vegetative growth and necrosis of root tissues, high plant mortality may occur in certain locations, leading to increased costs due to the demand for seedlings. to reestablish the desired number of plants per hectare in each orchard.

Water restriction is being decisive in reducing vegetative growth and fruit development in adult apple orchards in the spring of 2021, especially after mid-December. In apple orchards in full production, typical symptoms of water deficiency are observed, such as cessation of branch growth, cessation of fruit growth and wilting of leaves at times of highest temperature during the day. In certain situations, where the soil's water retention capacity is lower, the symptoms are more severe, with high leaf abscission and senescence, fruit wilting and branch necrosis. In some situations, the mortality of adult plants is observed, given the severity of the water restriction during this period.

In the last ten days of December 2021 and in the first days of 2022, there was a significant increase in daily temperatures, exceeding 30 °C in some locations. The high temperature, associated with high solar radiation intensified the frequency of fruits with sun damage to the epidermis, especially in fruits located in the most exposed portions of the plant canopy. In leaf tissues, in addition to the possibility of necrotic portions, symptoms of nutritional deficiency can be observed, especially of the nutrient potassium. 

For early cultivars, such as Condessa and Castel Gala, in addition to Eva clones, water stress has accelerated fruit maturation, with increased pre-harvest abscission rates. As for Gala clones, in most orchards, water stress does not significantly advance apple maturation. In situations of greater water restriction, symptoms of fruit dehydration and yellowing are evident.

Orchard management under water stress conditions

Alternatives to minimize these effects in new orchards without irrigation systems involve the periodic supply of water through spray tanks adapted to supply water in a localized manner in the canopy projection area, mainly in areas of the orchard where soils have a shallow surface horizon or with significant presence of gravel. For plantings carried out in the spring of 2021, frequent applications of water are suggested (in the same way as described above), but throughout the planting area. For the portions of the orchard with plants with more advanced symptoms of water deficit (withering of leaves and fruits, yellowing and senescence of leaves, necrosis of branches) all fruits must be removed from the plant, in order to minimize the possibilities of plant mortality.

Considering the high evapotranspiration observed in adult orchards, combined with high temperatures and low relative humidity at certain times of the day, applications of foliar fertilizers are not recommended during these dry periods, as in addition to the low efficiency in nutrient absorption, due to the microclimatic conditions in the Inside the orchard, damage to fruits and leaves may occur, resulting from the rapid concentration of fertilizer salts immediately after application.

Phytosanitary applications and phytoregulators for pre-harvest use must be carried out at times with mild temperatures and relative humidity greater than 50%, aiming to increase technical efficiency. It is worth paying attention to this aspect, as in the month of December/2021 several days were recorded with relative humidity levels below 30% in the afternoon, generating high evaporative loss that can minimize the contact time of agricultural pesticides on the surface foliar, which may result in loss of efficiency.

Considering the high fruit load evidenced in part of the orchards in the main producing regions, and considering the possibility of water restrictions for the months of January and February, the readjustment of the fruit load may be carried out by removing fruits of lesser commercial interest ( less color, smaller size and presence of defects in the fruit epidermis). It is noteworthy that continued water restriction may partially compromise the development of the red color of the epidermis and, mainly, the average fresh mass of the fruits, depending on the magnitude and duration of the period of water stress. Splitting the harvest, with the removal of more advanced ripening fruits, followed by the use of phytoregulators to delay maturation may be an alternative to be intensified in this cycle, aiming to increase the average size of the fruits. However, this management must be prioritized in orchards located in regions with deeper soil, and with the possibility of rain occurring during the harvest period, so that the improvement in fruit caliber is effectively obtained.