Mary Sutton, University of Georgia (UGA) assistant professor and citrus Extension specialist, recently wrote about the causes of alternate bearing in Georgia groves. Alternate bearing is a common problem in many mandarin varieties.
A slightly edited version of Sutton’s report follows.
CARBOHYDRATES
During photosynthesis, leaves produce sugars that are either exported and utilized for growth or stored as starch in the leaves for later utilization. For this reason, leaves are considered a source of carbohydrates while the organs that import carbohydrates for utilization (new flush, roots and fruit) are considered carbohydrate sinks.
The movement of carbohydrates from source to sink is critical for all new growth. However, carbohydrates are a limited resource so there is a lot of competition among the different sink organs for those carbohydrates. Prioritization of one sink type comes at the expense of another. So, for example, in a heavy crop load, carbohydrates are being invested in fruit production rather than into vegetative flush. While this will result in higher yields at harvest, it also means there’s less new wood available for flowering in the subsequent season.
In addition, when growth slows during the winter, carbohydrates will continue to accumulate during the winter to support flowering in the subsequent season.
Altogether, when carbohydrate reserves have been severely depleted during a heavy year and fruit production was prioritized over vegetative flush, this limits the number of flowers that can be produced the following year. This ultimately results in a low-yield year. Therefore, at its core, alternate bearing patterns can be explained by fluctuating carbohydrate availability.
HORMONAL BALANCE
Gibberellic acid (GA) is the major hormone that controls flowering. GA promotes vegetative growth over reproductive growth. GA is produced by mature fruit and exported to the surrounding buds to inhibit floral induction. This is why you rarely see flowers on branches that produced a lot of fruit the year before.
GA levels have to drop in order for flowering to proceed. Harvesting too late or having fruit still on the tree during flowering can greatly reduce the number of flowers produced.
NATURAL PATTERN
Alternate bearing is a natural phenomenon in many citrus species. The difference in production between on and off years can vary from variety to variety. For example, sweet oranges rarely see strong alternate bearing patterns while mandarins are more prone to them.
ENVIRONMENTAL STRESS
Extreme environmental conditions can cause damage to trees and trigger a low production year. This can trigger a strong alternate bearing pattern if the following year is unusually high. The alternate bearing pattern currently being observed in Georgia is likely the result of this.
Many trees sustained significant damage during the December 2022 freeze. As a result, production was low during the 2023 season as trees prioritized vegetative growth over fruit production. The 2024 season was then abnormally large for a lot of folks and likely depleted carbohydrate reserves. The fruit set for the current (2025) season is low as a result.
Learn more about the outlook for the 2025 cold-hardy citrus crop here.
Source: UGA
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