Updated February 2022. Originally posted October 18, 2023

Jaime Ott, UCCE Tehama, Shasta, Glenn, and Butte Counties

We have had bountiful rain this winter, and orchards with standing water were a common sight. Excessive water, caused by rain, flooding, or even overirrigation, is a challenge for orchards and can cause two very different problems: waterlogging and Phytophthora infection.

The Bottom Line

• Take photos/notes of flooded sections of your orchard. Even with no standing water, dig down with a shovel or auger to monitor soil saturation in the top 2 feet.
• Check stem water potential using a pressure chamber before starting irrigation. Beginning irrigation too early in the season will compound the problem of excess winter water.
• Symptoms of Phytophthora infection and waterlogging overlap. Work with your farm advisor or PCA to submit samples and determine if you are dealing with Phytophthora.
• For waterlogging: the only solution is to remove the excess water (or wait for it to dry out).
• For Phytophthora: Phosphonate applied as a foliar spray in the fall can protect trees for up to 5 months. Mefenoxam (Ridomil Gold), Oxathiapiprolin (Orondis), and Fluopicolide (Presidio) are effective treatments, but expensive. Other fungicides will not work. See 2022 Fungicide Efficacy and Timing for more information.

For the future

• Plant trees high (on mounds, islands, or berms) to keep the root crown out of saturated soil.
• Maintain your drains and ditches to move water out of the orchard.
• If flooding is an issue in your orchard, plant on rootstocks tolerant of wet feet such as Krymsk 86 or Rootpac-R (Rootstock Comparison Chart).

The Context

Waterlogging is a physiological problem which happens when the soil in the root zone of the tree (the top 2 feet or so) is saturated with water for extended periods of time. Roots need oxygen to survive, but saturated soil cannot hold much oxygen. Eventually the roots suffocate and die: fine roots die first, but larger roots can be affected if the saturated conditions persist for long enough. Trees are most sensitive to excess water when they are actively growing and the weather is warm. Waterlogging can lead to leaf yellowing, poor vigor, lack of fine roots, defoliation, and (over an extended period) tree death. These symptoms will generally improve once the excess water is gone and the tree has had a chance to regrow the fine roots that it lost.

Phytophthora infection, on the other hand, is caused by an aggressive pathogen taking advantage of soil saturation. When orchards are flooded, or soil is saturated with water for more than 24 hours, any Phytophthora living in the soil can produce spores which sense and swim towards a tree, where they initiate infection. Symptoms of Phytophthora infection depend on where the tree has been infected. Root infections lead to leaf yellowing, poor vigor, lack of fine roots, and eventual tree death. Trunk infections lead to leaf yellowing, poor vigor, amber-colored gumming on the trunk, and sudden tree collapse after bud push or when the weather warms up (see photos below). Under normal circumstances trunk infections will start below the soil line, but if an orchard has been exposed to flooding, infections can start as high as the water level was on the trunk. Once the tree is infected, symptoms can progress even without the presence of excessive water.