California’s Wet Winter Leaves Groves Susceptible to Disease, Waterlogging

With El Niño increasingly predicted to last through June, experts advise on how to keep olive tree roots dry to avoid damage from Phytophthora and waterlogging.
An atmospheric river on January 4, 2024 (Photo: NASA Earth Observatory)
By Thomas Sechehaye
Feb. 15, 2024 22:15 UTC

The El Niño cycle bring­ing wet weather to California is one of the strongest such cycles on record, accord­ing to researchers from the University of California – Los Angeles (UCLA).

Their asser­tions are cor­rob­o­rated by the National Oceanic and Atmospheric Administration’s cli­mate pre­dic­tion cen­ter, which also reported a 62 per­cent chance El Niño would con­tinue from April through June with his­tor­i­cally strong con­di­tions early in the year.

The total amount of rain in a year is not the most impor­tant fac­tor for Phytophthora or water­log­ging in an orchard; more impor­tant is the dura­tion of stand­ing water in the orchard- Jaime Ott, orchard sys­tem adviser, University of California

Record-shat­ter­ing rains poured over sec­tions of California this week, with rain­fall totals as high as ten inches (25 cen­time­ters), bring­ing wide­spread flash floods. As atmos­pheric rivers pound California, olive grow­ers face the chal­lenge of poten­tial dis­eases and prob­lems that may ensue.

From my under­stand­ing, we are a bit under aver­age for rain­fall so far, though February is pre­dicted to be wet,” Jaime Ott, an orchards sys­tem advi­sor for Northern California at the University of California Cooperative Extension, told Olive Oil Times.

See Also:Controlling Black Scale in California Groves

Still, California olive grow­ers are con­cerned about wide­spread wet­ness result­ing in Phytophthora and water­log­ging. Excessive water – caused by rain, flood­ing or overir­ri­ga­tion – presents an ongo­ing chal­lenge for olive orchards.

Phytophthora is a com­mon par­a­site that causes root and root crown rot in olive trees. The mold species – respon­si­ble for the potato blight that caused the Great Famine of Ireland – takes advan­tage of sat­u­rated soil con­di­tions.

Trunk infec­tions can lead to poor vigor, leaf yel­low­ing and sud­den tree col­lapse or death. Cultural prac­tices that avoid pro­longed and fre­quent soil sat­u­ra­tion can help min­i­mize dis­ease losses.

The Phytophthora infec­tion is caused by an aggres­sive pathogen which may be present in water or soil and which infects trees by trav­el­ing through water in the soil,” Ott said.

It causes roots to die and can cause dead tis­sue, called a canker, on the crown or trunk,” she added. Basically, the roots drown and die, and the plant declines because it can no longer take water up through the dead roots.”

Meanwhile, water­log­ging is a phys­i­o­log­i­cal prob­lem when the top two feet (60 cen­time­ters) of a tree’s root zone is sat­u­rated with water for an extended period. Waterlogging can result in leaf yel­low­ing, poor vigor, defo­li­a­tion and pos­si­bly tree death.

The total amount of rain in a year is not the most impor­tant fac­tor for Phytophthora or water­log­ging in an orchard; more impor­tant is the dura­tion of stand­ing water in the orchard,” Ott said.

Standing water is influ­enced by the amount and dura­tion of rain­fall, soil drainage con­di­tions, water pen­e­tra­tion abil­ity and cul­vert or ditch main­te­nance qual­ity.

My under­stand­ing is that olives are sus­cep­ti­ble to both infec­tion by Phytophthora species and dam­age from water­log­ging, though I don’t think Phytophthora is con­sid­ered a major dis­ease of olives,” Ott said.

While Phytophthora and water­log­ging are sep­a­rate prob­lems, their symp­toms are very sim­i­lar, and they can be chal­leng­ing to tell apart when exam­in­ing an unhealthy tree.

If a tree is in stand­ing water or water-sat­u­rated soil, it can be affected by Phytophthora, water­log­ging, or both,” Ott said. Careful soil water man­age­ment is the best foun­da­tion for con­trol of Phytophthora root and crown rot.”

Once the tree is infected with Phytophthora, the symp­toms can progress even with­out excess water. However, water­log­ging symp­toms typ­i­cally improve once excess water is gone.


Cultural prac­tices that avoid pro­longed and fre­quent soil sat­u­ra­tion, espe­cially near the tree root crowns (where the trunk begins), help min­i­mize losses.

The University of California’s inte­grated pest man­age­ment pro­gram rec­om­mends plant­ing trees on berms, short­en­ing irri­ga­tion times and improv­ing soil water pen­e­tra­tion and drainage to reduce losses caused by Phytophthora.

They addi­tion­ally advise not to place irri­ga­tion equip­ment too close to the trunk of trees; the goal is to pro­vide acces­si­ble water to the root sys­tem with­out over­wet­ting the soil around the root crown.

The University of California also rec­om­mends that grow­ers avoid plant­ing olive trees in low areas and heavy clay soils.

I would add that it’s worth check­ing the drainage ditches around your orchard and mak­ing sure they are clear and can move water effec­tively away from it,” Ott said.

She advised that orchard plan­ning is crit­i­cal for peo­ple and com­pa­nies plan­ning to plant new olive trees.

Our best pre­dic­tion is that the weather will become more vari­able and extreme in the future: droughts will be more severe, and wet win­ters will be wet­ter, and we are more likely to yo-yo’ between too wet and too dry, with very lit­tle just right,” Ott said.

Plan your orchard accord­ingly: plant on berms to get your trees up above poten­tial flood­ing, main­tain veg­e­ta­tion on your orchard floor to improve water pen­e­tra­tion and reduce runoff and ero­sion dur­ing heavy rain,” she added.

Keep in mind that Phytophthora and water­log­ging are two dis­tinct prob­lems,” Ott con­cluded. Do what you can to design around extreme weather so you aren’t scram­bling when it hap­pens.”


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