Ask Your PCA: How do I deal with phytophthora issues in prune trees?

Issue Date: August 11, 2021
Mateo Marquez PCA, Integral Ag Services, Durham
Mateo Marquez

Phytophthora infection in prunes is typically a below-ground infection of the roots. It usually occurs when the roots have been weakened, and typically it is associated with very wet periods of flooding or saturated soils that weaken the tree and cause asphyxiation of the roots.

Phytophthora infections can occur above ground, too, especially on the trunk if the standing water is above the soil surface and on the trunk.

Phytophthora spores are waterborne, so either flooding events or surface and river water that has phytophthora spores can spread the disease. Compromised trees are more susceptible to phytophthora infections, so it's important to reduce tree stress, starting with mitigating against extended periods of saturated soil.

Irrigation can also be problematic if there are areas where water is standing for long periods of time, as excess soil moisture is essentially a flooding event. It can stress trees, and the roots can become weakened, allowing the pathogen to invade the weakened tissue.

Best management practices begin with prevention. Start before planting and assess whether the site is prone to flooding. If it is, take measures to mitigate against the potential for saturated soils by breaking up hardpan that will trap moisture in the root zone. Another option is building bigger berms in case of a flooding event. A bigger berm effectively increases the depth of the root zone, and it will put a higher percentage of the roots in soil that is not saturated, assuming the berm isn't completely submerged.

After a flood event, either pump or divert the water, but do everything possible to limit the amount of time the soil is saturated.

Chemical treatments such as Ridomil are commonly used. Also, phosphites are helpful in assisting plants in defending against phytophthora infections, but prevention before planting and rootstock selection are the most effective management tactics.

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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