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

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.

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.

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.”

• Sacramento Valley Orchard Source
• The Hill