Gibberellic Acid Has Multiple Uses in California Citrus

There are numerous preharvest and postharvest uses of gibberellic acid (GA) in California citrus, a University of California Riverside Extension subtropical horticulturist reported during a June 9 webinar. Mary Lu Arpaia’s GA discussion was a major portion of her broader topic of plant growth regulators in California citrus. Her presentation was part of the Citrus Research Board Webinar Series.

WHAT GA WORKS FOR

Arpaia cited the following uses of GA in California citrus:

• Delay rind aging and reduce physiological disorders of navel orange. Arpaia’s advice was to apply GA about two weeks before color break to delay coloration of peel and reduce peel issues such as rind staining, water spotting, sticky rind, peel softening, and puff and crease. Puff and crease is a physiological disorder in which the spongy white layer of a citrus rind (albedo) separates from the flesh and the outer peel.
• Delay creasing and peel aging of Valencia orange. Target fruit from August to October, Arpaia suggested.
• For tangerine hybrids and grapefruit/pummelo, GA can delay disorders associated with rind aging, puffiness and softening as well as increase peel strength. GA use may reduce preharvest drop of mature fruit in grapefruit/pummelo, Arpaia said.
• Decrease rind aging, yellowing and coloration in lemons and limes. Apply when fruit are approximately half- to full-size but still full green, Arpaia advised.
• Delay chlorophyll breakdown/coloration in lemon storage. Arpaia suggested applying GA to fruit prior to storage.
• Increase fruit set and yield in Clementine mandarin. Arpaia said one to three GA applications are allowed from early bloom to up to four weeks after petal fall. She said to allow for up to three days between applications. She added there is strong evidence that GA efficacy is crop load-dependent and that crop load should be considered when using GA to increase mandarin fruit set or fruit size.

APPLICATION ADVICE

Arpaia offered several strategies to maximize the benefit of GA in California citrus:

• The best time to spray GA for prolonging rind quality is late-September to mid-October (about two weeks prior to color break).
• Growers need to follow up with a 2,4-D spray to stop drop.
• The best GA uptake is with a slightly acidic spray.
• Good spray coverage is essential to maximize benefits.
• Leaf and fruit drop may occur after a GA spray. That can happen when trees are sprayed within one to two weeks of a narrow-range oil spray.
• Avoid treating stressed trees.
• Do not apply GA once fruit are turning color; wait until fully colored. But if sprayed too late, GA can negatively impact the following year’s crop.

During her presentation, Arpaia also addressed use of the plant growth regulators 2,4-D, NAA and ethylene. 

Read a January report about the use of GA in the Southeast’s cold-hardy citrus region here.