Understanding Crop Load and Alternate Bearing

By Fernando Alferez and Muhammad A. Shahid

Citrus trees bloom profusely. It is estimated that a healthy adult tree can produce more than 200,000 flowers every year. This number is much higher than that of fruits the tree can maintain and develop. Normally only between 1% to 5% of flowers set fruit that develops, matures and can ultimately be harvested.

In an endemic HLB environment, predicting future crop yield is even more challenging. Environmental and climatic conditions also play a role in determining crop load. In Florida, alternate bearing, a main determinant of yield, may occur throughout the whole state or in different areas asynchronously. The main driver for this phenomenon is climate, as adverse environmental events such as freezes or drought may cause a severe crop reduction in a given year.

While sweet orange varieties may show some mild alternate bearing in response to severe environmental stress, alternate bearing in mandarins is a major problem around the world. As mandarin cultivation expands in North Florida and southeastern states, alternate bearing may become a challenge in years to come.

Alternate bearing is characterized by irregular production between successive seasons. Excessive production in one year (on year) is followed by a year with very low or even no production (off year).

ALTERNATE BEARING INDEX

The severity of alternate bearing may vary over time and among different citrus varieties. The severity of alternate bearing can be estimated through the alternate bearing index (ABI). The ABI helps growers quantify the severity of these fluctuations and compare the bearing habits of different cultivars, such as satsuma mandarins, navel oranges and other citrus varieties.

The index is calculated using yield data collected over several consecutive years by comparing the differences in yield between each pair of years. Monitoring the ABI can help growers evaluate the effectiveness of crop load management practices, including fruit thinning, pruning, irrigation, nutrient management and the use of plant growth regulators. All of these practices can reduce alternate bearing and improve long-term orchard productivity.

FLOWER INHIBITION

It is important to remember that in a citrus tree, next season’s crop load is determined well before bloom. The main cause of alternate bearing is flower inhibition. For instance, if harvesting is delayed in early varieties such as satsumas, clementines or navel oranges in an on year, alternate bearing will be promoted. An off year will follow, due to less availability of carbohydrates to sustain new flower differentiation and fruit set. A rule of thumb for these varieties is that in general, the larger the crop is in one given season, the less flowers will be produced in the following season, alternating yields over the years.

CLIMATE

Climate seems to play an important role in alternate bearing, acting as a trigger. Unfavorable weather, such as temperature stress (i.e., freezes) may reduce flowering and/or fruit set, resulting in a light crop. This enables large carbohydrate reserves that are not going to be utilized as almost no fruit is developing in that given year. These reserves will be available the following season to support a heavy crop. Conversely, a heavy crop may deplete carbohydrate reserves in the tree, which will not be able to support a big crop next year (Figure 1).

THINNING

The commercial goal should be to maintain a consistent crop every season by eliminating or alleviating alternate bearing. Crop load can be increased in a predicted off year by increasing flowering and fruit set. Conversely, crop load can be reduced in an on year by decreasing flower formation or reducing the number of fruits. In general, the latter is easier than the former, and some management practices can be adopted to this end, thus breaking the cycle depicted in Figure 1 and alleviating alternate bearing. Management practices include pruning, girdling and thinning.

Since pruning and girdling are not recommended under HLB, the focus should be on thinning. The aim of fruit thinning is to reduce competition between fruitlets for carbohydrates and increase the leaf-to-fruit ratio. This may allow fruit remaining in the tree to reach a larger size as there is more leaf area per fruit supplying each fruit with carbohydrates. Thinning can be performed manually or chemically.

Hand thinning is the manual removal of fruit. In some parts of the world, hand thinning has been used in mandarin varieties such as Murcott but is very expensive and labor intensive. In Florida, hand thinning has been practiced to a limited extent historically. Hand thinning should start as soon as natural June drop has finished and should end before summer flush. This would avoid some inhibition of summer and fall budbreak, reducing chances of a decrease in flower buds.

Chemical thinning agents are mostly synthetic auxins. As with other plant growth regulators, synthetic auxin application may result in disparate effects that depend on different factors, including:

• Concentration of the natural auxin in the tree, which varies during the year
• Type of auxin applied
• Concentration of the auxin
• Time of application (as this will interfere with the developmental stage of the fruit)
• Carbohydrate status of the tree
• Tree cultivar
• Weather conditions

Although timing of application may vary, in general, chemical thinners are applied at the end of the June drop. This is important because applying thinners too early may result in excessive fruitlet and leaf drop. Auxins applied too late may be ineffective. In Florida, the main auxin used has been naphthalene acetic acid (NAA), with very good results in Dancy and Murcott and observed cumulative return cropping. In other countries such as Uruguay, there is ample experience in the use of NAA in Owari satsumas and Okitsu mandarins.

THE PATH FORWARD

The Southeast is emerging as a new citrus-growing region, particularly in North Florida, South Alabama and Georgia, which comprise more than 15,000 acres of fresh-market citrus. In addition, citrus under protective screen is a commercial reality in Central Florida, with more than 1,000 acres. Both production systems will benefit from advanced knowledge on crop load management to maximize productivity, fruit quality and economic returns.

The expansion of the varietal portfolio and the possible availability of new chemistries warrant further investigation on fruit thinning and crop load management adapted to regional conditions. Particular emphasis should be placed on managing alternate bearing in fresh market cultivars such as satsuma mandarins (Owari), UF 950, Shiranui and Bingo, where excessive crop loads can reduce fruit size, impair return bloom and contribute to year-to-year yield fluctuations.

Optimizing crop load management will also help reduce fruit puffiness (a major factor affecting marketability) in satsumas while improving fruit size and uniformity in cultivars such as Bingo. Developing integrated crop load management strategies through optimized thinning practices, plant growth regulators, pruning, irrigation and nutrient management will enhance yield consistency, fruit quality, marketable yield and long-term orchard productivity.

Fernando Alferez is an associate professor at the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) Southwest Florida Research and Education Center in Immokalee, and Muhammad A. Shahid is an assistant professor at the UF/IFAS North Florida Research and Education Center in Quincy.