The significant plant growth regulator used on citrus is gibberellic (GA3), or ProGibb®. One big reason is that it’s a critical tool for harvest management because citrus fruit color isn’t an indicator of ripeness. A navel orange, for example, can be perfectly ripe internally, but the rind might still be green. By spraying ProGibb, a grower can delay rind aging on a certain block, allowing for a more orderly harvest, while also increasing the integrity of the rind and reducing rind disorders (rind staining, water spotting, puffy rind, sticky rind).
ProGibb also prevents creasing in a variety of citrus crops. Creasing occurs when the rind tissue stretches after cell division in the albedo skin layer (white tissue under the rind) has stopped, but cell division in the flavedo layer (orange tissue) and epidermis continues. The stretching ability of the albedo’s cell walls slows down and it splits. Unfortunately, handling increases this splitting or tearing. ProGibb makes cells more elastic. It can complement other measures to stop creasing such as proper irrigation, correct nutrient levels (Ca, K, Zn, N), nematode and other pest control, reduced compaction and pruning.
Growers can also use ProGibb to delay harvest, essentially storing their fruit on the tree until a better market window opens. But it is important to wait until all the fruit is uniformly orange, or the crop can color unevenly. Growers should never spray mottled fruit, or the ripe fruit will retain the mottled color. If the grower waits until the green changes to orange, the resulting fruit could be too old and pithy.
The timing of the ProGibb spray will vary depending on the variety, environmental considerations, etc., but to delay rind aging and reduce creasing, application is generally made prior to color break, as early as the “golf ball stage” in some varieties.
In a great example of return on investment, California naval orange growers can make a single application of ProGibb in early September that allows the fruit to maintain quality all the way through June. Growers can spray part of their groves to allow a late harvest, and harvest non-treated blocks early. Fruit supply to the market is steady and prices to growers are better. If a grower can make an extra 50 cents per box by harvesting good quality fruit later, at 1,500-carton production per acre, he would be able to realize an extra $750/acre for a treatment that costs a fraction of that.
Another huge benefit: ProGibb increases fruit set and yields on seedless navels and Valencias as well as on Clementines and tangerine hybrids. This benefit is particularly noticeable on tangerine hybrids with pollination problems such as Orlando, Robinson, Minneola and Sunburst.
In lemons and limes, ProGibb reduces the quantity of small tree ripe fruit, produces a desirable harvesting pattern, improves peel integrity – and actually, overall fruit quality and value.
On grapefruit, ProGibb can delay problems associated with rind aging (such as puffiness, softening, and orange coloration), prevent preharvest drop of mature fruit, increase peel strength, produce a more orderly harvest pattern and reduce water loss during storage.
ProGibb can also provide benefits when applied to citrus after harvest. Sour rot, Geotrichum candidum, is a serious disease that can destroy lemons in storage. ProGibb applied with the storage wax can delay the senescence of the rind, allowing the lemon to be naturally less susceptible the disease. On lemons and other citrus, ProGibb applied with the final shipping wax will slow changes in rind color, allowing the fruit to retain their natural bright color longer.
Girdling vs. ProGibb
ProGibb offers a number of benefits over girdling to increase fruit set. Both are effective, but girdling is labor intensive, while ProGibb only requires just a tractor spray. Timing of use of the PGR is helpful also – at 100% petal drop rather than in August. It’s a simple part of a grower’s spring program. With girdling, trees face the possibility of phytophthora, and the long-term effect of the practice of girdling in general is still unknown.