Experts Offer Pest-Control Tips for California's Hobby Growers

In California’s olive-grow­ing regions, home orchardists use organic solu­tions to com­bat olive tree pests, dis­eases and com­peti­tors.

“California home olive grow­ers deal with the olive fruit fly and scales of dif­fer­ent kinds,” Javier Fernandez-Salvador, UC Davis Olive Center exec­u­tive direc­tor, told Olive Oil Times. ​“In terms of dis­eases, olive knot, pea­cock spot and soil-borne pathogens such as ver­ti­cil­lium wilt and phy­toph­thora are preva­lent.”

“Fortunately, these issues can be dealt with by using pre­ven­ta­tive prac­tices and organic solu­tions,” he added.

According to the University of California’s Division of Agriculture and Natural Resources (UC ANR) home orchard guide, hobby grow­ers can proac­tively approach by under­stand­ing which olive vari­eties are best suited for home gar­dens and land­scap­ing in California.

The olive tree is an ever­green best suited to areas with hot and dry sum­mers and mild but cool win­ters. Certain vari­eties offer superb olives for eat­ing, while oth­ers tend to be favored for olive oil.

Success for home orchardists starts with select­ing the best olive trees to grow in their loca­tion. Certain vari­eties per­form best in dif­fer­ent micro-cli­mates, and mak­ing the cor­rect selec­tion pre­vents some prob­lems before they ever begin.

The UC ANR high­lights top table olive vari­eties such as the Ascolano, Manzanillo and Sevilliano. Manzanillo trees are most sus­cep­ti­ble to cold snaps, pea­cock spot and olive knot.

Arbequina, Frantoio, Leccino, Maurino and Mission are the top choices for pro­duc­ing olive oil in the Golden State.

Many nurs­eries in California offer a vari­ety of olive trees and work closely with hobby gar­den­ers to develop a com­plete grow­ing plan. This includes select­ing trees well-adapted to spe­cific cli­mates, con­di­tions and nat­ural topog­ra­phy.

With care­ful selec­tion, home gar­den­ers can man­age ongo­ing needs such as irri­ga­tion, fer­til­iza­tion and prun­ing for suc­cess­ful growth.

“Don’t over­wa­ter,” Fernandez-Salvador said. ​“A lot of the prob­lems with olive trees start with over­wa­ter­ing. Olives behave dif­fer­ently than other fruit trees, so it is essen­tial to have a plan for water­ing appro­pri­ately to avoid root rot and other com­mon issues.”

The sec­ond big tip he offered home grow­ers: ​“Keep small orchards man­aged tech­ni­cally,” he added. ​“This enables you to mon­i­tor the tim­ing of irri­ga­tion and fer­til­iza­tion and address issues proac­tively.”

Fernandez-Salvador also advises ask­ing for help and advice before a prob­lem takes root. ​“Learn about issues and take care of them proac­tively,” he said.

Ellie Andrews, a UC ANR spe­cialty crop advi­sor, rec­om­mends hob­by­ists uti­lize the many online resources avail­able through UC ANR, includ­ing olive pro­duc­tion and inte­grated pest man­age­ment.

According to Fernandez-Salvador, pre­ven­tion meth­ods and organic prac­tices can pro­vide solu­tions for the main com­peti­tors, pests and dis­eases fac­ing California olive grow­ers.

New olive trees are espe­cially sen­si­tive to com­pe­ti­tion from weeds, with weed man­age­ment best prac­tices chang­ing over time. In the early years, many pro­duc­ers pre­fer to hand weed. After trees are estab­lished, reg­u­lar mow­ing keeps the orchard weed-free.

Olive knot is one of the most per­va­sive bac­te­ria California pro­duc­ers – hobby and com­mer­cial – face.

“Olive knot is a canker-like form of growth that dis­torts a lot of tis­sue in the tree,” Fernandez-Salvador said. ​“It can cause sys­temic dam­age, so it is impor­tant to pre­vent it. In terms of oil qual­ity, olive knot can con­t­a­m­i­nate the oil and greatly reduce pro­duc­tiv­ity.”

“Prevention is the best action,” he added. ​“As olive knot tends to occur dur­ing rainy and cold con­di­tions, hob­by­ists need to san­i­tize all equip­ment and avoid prun­ing in rainy, cold weather,” he added.

While olive trees tend to have fewer issues than other fruit trees, home grow­ers may be shocked when an olive tree’s leaves sud­denly turn yel­low.

This issue is typ­i­cally related to water­ing, so a fixed sched­ule can help deter­mine whether over-water­ing is the cause. If over-water­ing is not the cause, yel­low leaves may indi­cate nitro­gen defi­ciency.

McEvoy Ranch, one of California’s largest pro­duc­ers, demon­strated that a few doses of a nitro­gen-heavy fer­til­izer, such as fish emul­sion, may help resolve the issue.

Globally, the olive fruit fly is one of the tree’s most per­ni­cious pests. Native to the Mediterranean, the inva­sive species has posed a prob­lem for California olive grow­ers since 1998. Biological con­trol meth­ods require pre­cise tim­ing and ideal envi­ron­men­tal con­di­tions.

The adult olive fruit fly is rarely seen as it lays eggs under olive skin. The lar­vae feed on the olive flesh. Visual signs include brown tracks and tun­nels. According to Fernandez-Salvador, fruit dam­aged by fruit flies is sus­cep­ti­ble to rot and can drop pre­ma­turely.

“Fruit fly dam­age can cre­ate havoc with olive trees,” he said. ​“If the lar­vae come out and eat the fruit, it cre­ates space, and the fruit rots from the inside out and fer­ments. Even two to five per­cent dam­age may affect the qual­ity of oil. It can affect its shelf life and may present as defects in the olive oil.”

The fly can ruin an olive crop, so hobby farm­ers resort to var­i­ous organic options for con­trol­ling it. Trapping includes mul­ti­ple meth­ods, such as ball traps, Olipe traps, McPhail Traps, and sticky traps. Spraying may con­sist of appli­ca­tions of kaolin clay or spin­osad.

Peacock spot (Spilocaea oleaginea) is another com­mon olive tree issue in California caused by a fun­gus. The olive leaves turn yel­low with black spots, and the lesions can spread, espe­cially in bad weather. If not addressed, all the leaves can fall off.

The agron­o­mists at McEvoy Ranch use an organic fixed cop­per full canopy spray imme­di­ately after the har­vest to pre­vent the fun­gus from set­ting in.

A sec­ond mid-win­ter appli­ca­tion may also be help­ful, as the fun­gus thrives in wet envi­ron­ments. McEvoy Ranch noted that annual prun­ing offers a nat­ural approach to increas­ing air cir­cu­la­tion by open­ing the canopy.

Another fun­gus, ver­ti­cil­lium dahliae, can cause olive leaves to turn brown but remain on the trees. This con­di­tion is known as Verticillium wilt. Site selec­tion is crit­i­cal to pre­vent this con­di­tion. Ideally, olive trees should be in fields not planted with night­shades or other ver­ti­cil­lium-sus­cep­ti­ble crops, includ­ing cot­ton.

The University of California offers offi­cial guide­lines for pest mon­i­tor­ing tech­niques and man­ag­ing pests with or with­out pes­ti­cides. In addi­tion, home orchardists may want to con­sult with nurs­eries and local mas­ter gar­dener pro­grams for input.

For instance, in Sonoma, the Sonoma Master Gardeners offer a free ser­vice to help grow­ers iden­tify olive tree prob­lems and respond proac­tively.

Andrews advises new olive pro­duc­ers to ​“con­sult UC ANR resources such as the organic olive pro­duc­tion man­ual, talk with expe­ri­enced grow­ers and con­sul­tants, con­nect with olive grower net­works, and attend train­ing and work­shops.”

“I wish more home grow­ers knew how to pre­vent prob­lems when start­ing,” Fernandez-Salvador added. ​“I encour­age home gar­den­ers and small-scale pro­duc­ers to take advan­tage of edu­ca­tional resources to learn about grow­ing olive trees organ­i­cally.”

• University of California Agriculture and Natural Resources