Plant Growth Regulators in California Citrus

University of California Riverside Extension subtropical horticulturist Mary Lu Arpaia recently discussed the uses of the plant growth regulators 2,4-D, NAA and ethylene in California citrus. Her June 9 presentation was part of the Citrus Research Board Webinar Series.

2,4-D

One use of 2,4-D in California is to increase fruit size of navel oranges, grapefruit, mandarins and mandarin hybrids. For that use, Arpaia advised:

• The dosage for orange and grapefruit is dependent on fruit size but is limited to fruit less than .75 inch for oranges and 1 inch for grapefruit.
• Timing for mandarins is 21 to 35 days after 75% petal fall.
• One application is allowed per cropping cycle.

2,4-D can also be used pre-harvest for fruit drop control of navel and Valencia orange, grapefruit, lemons, tangelos and other citrus hybrids and tangerines. Arpaia offered the following advice:

• This use is limited to one application.
• Do not apply within seven days of harvest.
• Avoid applying to trees less than 6 years old.
• Application dosage and timing depend on type of citrus.
• Avoid high temperatures.

The third use for 2,4-D in California citrus is for lemon button retention during storage. 2,4-D can reduce Alternariastem end rot during long-term storage. Arpaia said it can be applied mixed in with storage wax or following final water rinse. 2,4-D may delay coloration of fruit but is not as effective as gibberellic acid, she noted.

NAA

NAA (1-Naphthaleneacetic acid) is a synthetic auxin used preharvest in California for chemical thinning. Arpaia advised growers to only use NAA when they want to thin a heavy crop to increase fruit size. The treatment can have variable results and should be avoided when the temperature is above 85 degrees, she said. Use is limited to one application when fruit are 5 to 20 millimeters in diameter. Apply NAA before the end of June drop, she advised.

ETHYLENE

Ethylene is used postharvest to enhance coloration of navel oranges, lemons (no storage), mandarins and Valencia oranges (regreened).

Arpaia’s recommendation was to use one to five parts per million for one to four days with high humidity at approximately 68 degrees.

During her presentation, Arpaia also addressed the use of the plant growth regulator gibberellic acid; learn more here.