Evaluation of winter hardiness of apple cultivars in the Kuban horticultural zone

The most important property of scion-rootstock combinations of fruit crops, which determines the feasibility of their introduction into production, is winter hardiness. Therefore, the selection of the most promising cultivars for each specific zone for a particular fruit crop is traditionally carried out based on the principle of their higher resistance to adverse weather conditions.

For the normal development of frost resistance, it is necessary to ensure the timely cessation of growth processes and the entry of the plant into a state of organic dormancy. As the ambient temperature decreases, the hardening process begins, which reaches its maximum when plants exit the state of deep dormancy. After this, frost resistance gradually decreases, especially during frequent thaws.

Taking into account the complex of processes occurring in apple trees during the wintering period, as well as the results of experiments, winter hardiness is defined as a complex multi-component property. Based on literature analysis, the authors identify four main effects caused by frost and, accordingly, four components of winter hardiness. Each of them is a response of the apple tree to the action of a damaging factor.

During the years of research (2005–2010) in the Kuban horticultural zone, the following types of frost impact were observed during the cold period: a decrease in air temperature to a mark close to the minimum – type II (2006, 2009, 2010), sharp fluctuations in day and night temperatures – type III (2005, 2007), and recurrent frosts after thaws – type IV (2005, 2009). It is noteworthy that the resistance of a cultivar (scion-rootstock combination) to the effect of a damaging factor in early and mid-winter is determined by the freezing of wood, while resistance to type III frosts is judged by damage to bark and buds. It has also been shown that under the action of recurrent frosts after thaws, both wood, and bark, and buds freeze. In addition, during the study period, spring frosts were observed (2005, 2009), during which flowers and fruit set of fruit crops were damaged.

Periodically recurring decreases in air temperature in January to a mark close to critical cause enormous damage to fruit plantations. Thus, in suburban farms of the city of Krasnodar, after the effect of low temperatures in January 2006 (down to –36 °C), complete death of fruit trees was established on an area of 412 hectares. Freezing of perennial wood was noted on an area of 126 hectares, and 100% death of flower buds of fruit crops on 458 hectares, including apple trees on 303 hectares. As a result, the damage from the complete death of fruit-bearing trees was estimated at 91.1 million rubles. In order to prevent a recurrence of such catastrophic damage caused by frosts in the future, it is necessary to select cultivars that are resistant to adverse conditions of the winter period.

Field surveys conducted after the impact of such low temperatures established that the degree of freezing of trees of scab-immune and resistant apple cultivars varies. Table 2 – Degree of freezing of apple trees on M9 rootstock (Kuban State Agrarian University educational farm orchard planted in 2005, layout

5×2 m, January, 2006)

Cultivar wood, score buds, %

One-year-old trunk Liberty

2 0.5 0.5 (control) Imrus 2 0.5 0.5 Florina

9 1.0 1.0 GoldRush 39 3.0 3.0 Enterprise 15 2.0 2.0 Topaz 16 2.0 2.0

Thus, the GoldRush cultivar suffered most from critical frosts (wood freezing 3 points, bud death 39%) (Figures 15, 16). The trees of Liberty, Imrus, and Florina cultivars were damaged to a lesser extent. Wood freezing was noted in the range of 0.5–1.0 points, and bud death amounted to only 2–9%. An intermediate position regarding the studied indicators was occupied by the Enterprise and Topaz cultivars on M9 rootstock.

From the literature (Tyurina, 1993; Sedov, Zhdanov, Sedova et al., 1989; Savelyev, 1998) it is known that severe freezing and even death of flower buds can be caused by frosts at the end of winter after thaws (the fourth type of frost impact).

In this regard, it was of interest to evaluate the frost resistance of the studied apple cultivars at this stage of overwintering using diagnostic methods. For this, the method of artificial freezing of annual shoots was used (Doroshenko, Lyashok, 1996).

• Figure 15 – Damage to generative buds of the GoldRush cultivar (Kuban educational farm orchard planted in 2005, January 2006)
• Figure 16 – Freezing of GoldRush apple trees in the winter of 2005/2006 (Kuban educational farm orchard planted in 2005, May 2006)

When compiling the freezing program, the nature of temperature changes under natural conditions and the physiological state of the plants at that moment were taken into account. Based on this provision, the freezing of annual shoots was carried out in early February 2007, after a long thaw, at a temperature close to critical during this period (–20 °C).

The fructose coefficient (Kf) was used as a diagnostic criterion, which is the result of dividing the fructose content in the buds of annual shoots after freezing (C2) by the similar indicator before freezing (C1). The relative constancy in the content of soluble carbohydrates (especially fructose) in the buds of annual shoots indicates the frost resistance of scion-rootstock combinations at this stage of overwintering. The results of the experiment are presented in Table 3.

Resistance to winter frosts and temperature fluctuations

When choosing an apple cultivar on M9 rootstock for the Kuban region, it is important to evaluate its ability to withstand not only stable frosts but also winter thaws with sharp temperature fluctuations. During the forced dormancy phase, the stress resistance of fruit buds can be determined by changes in fructose content: in susceptible cultivars, the sugar level rises sharply after freezing. Experiments have shown that in the control cultivar Liberty and the cultivar Imrus, the fructose content in the buds after artificial freezing of annual shoots remained practically unchanged. This indicates that the plants are able to tolerate low temperatures in mid-winter without damage.

In less winter-hardy cultivars GoldRush, Enterprise, and Topaz, the fructose content in the buds after freezing increased 1.84–2.71 times, which indicates stress and tissue damage. The Florina cultivar under similar conditions showed moderate resistance to the effects of cold.

Assessment of frost resistance of apple on M9 rootstock during the forced dormancy phase (experimental accuracy Sx% ≤ 3)

Winter thaws followed by sharp cooling are another serious test for the orchard. Liberty and Florina cultivars on M9 rootstock showed good resistance to daily temperature fluctuations (type III frost impact). Observations in the orchard confirmed the accuracy of laboratory forecasts: after the return frosts in February, bud damage in Liberty, Florina, and Imrus cultivars amounted to only 9–10%. Trees of GoldRush, Enterprise, and Topaz cultivars on similar rootstock suffered more — 25–27% of their buds were damaged.

Protection of fruit set from spring frosts

Spring frosts often damage the already formed fruit set, threatening the future harvest. To identify the most resilient cultivars, apple fruit sets on M9 rootstock were subjected to artificial freezing for 5 hours at a temperature of –3 °C. The results obtained were compared with the actual condition of the trees after prolonged spring frosts in April 2009.

• Viable fruit sets of Liberty, Imrus, and Topaz after frosts in the orchard — 35–50 %
• Preserved fruit sets of the same cultivars under artificial freezing — 35–45 %
• Viable fruit sets of Florina and GoldRush after frosts in the orchard — 20 %
• Undamaged fruit sets of GoldRush after artificial freezing — 15 %
• Undamaged fruit sets of Florina after artificial freezing — less than 25 %

During the tests, the Enterprise cultivar withstood the spring cooling the worst. Its fruit sets suffered more than others, both in the laboratory during artificial freezing and during the real April frosts in the orchard. The Florina and GoldRush cultivars showed moderate resistance to frost. The greatest resilience to spring cold was demonstrated by Liberty, Imrus, and Topaz.

However, in order to recommend cultivars for cultivation in organic orchards of the southern region, it is necessary to clarify the manifestation of their resistance to other abiotic stress factors – drought and high air temperatures during the summer period. Therefore, the need for special research arose.