By Natalie Willis, Reporter, Valley Ag Voice
An unusual disease outbreak is afflicting California almonds on top of an already tumultuous market. Phytophthora — a soilborne microorganism that causes root and crown rot on tree bases — was detected in mid-February of 2023 and has since spread across the state.
According to Florent Trouillas, UC Cooperative Extension Specialist in fruit and nut pathology, heavy rainfall and mounting El Niño conditions may bring a recurring outbreak of aerial Phytophthora, casting serious implications on the almond industry.
Once established it can infect a wide variety of crops, but it mainly impacts California almonds. One species of the disease — Phytophthora syringae — is of particular concern due to the unprecedented outbreak last winter fueled by the atmospheric rivers.
While the disease is rare, Trouillas noted that outbreaks of the disease are generally associated with El Niño years, and recent and persistent rain across the state should put growers on alert.
“It was found statewide – meaning in every almond-producing county – and disease incidence in orchards ranged from 10% of the trees infected to 75%,” Trouillas said.
The disease does not kill the tree, rather it causes branch dieback which results in additional work and expense. When coupled with already high labor costs, the expense of treating Phytophthora becomes a daunting financial feat.
According to research from the 2023 outbreak, the Phytophthora syringae strain attacks cuts caused by pruning, effectively infecting young shoots on almond trees without any wounds.
“This was really the first time we had seen widespread evidence of infection on the twigs,” Trouillas said.
Thus, growers are advised to prune in dry weather as pruning shortly after rainfall increases the likelihood of infection. Cankers and gumming are characteristics of an infected tree, with the pattern and color of the gum balls being essential to properly diagnosing this Phytophthora strain.
If gum balls are found, generally in mid-February, growers can contact their local UC Cooperative Extension advisor to confirm if the tree is infected.
“It is super critical for growers that, whenever they see gumming, not to assume that it is this aerial Phytophthora because there are many other diseases that can cause gumming on the tree,” Trouillas said.
Upon a confirmed diagnosis, growers may be advised to apply a compound to control the infection and be provided with various curative treatment options.