Phytophthera fungus in Texas oaks

Are your oak trees defoliating, with black stains on the trunks and frothy, bleeding cankers?

Another of the most overly misdiagnosed and misunderstood fungal problems associated with oak trees in the Austin area is the commonly misspelled phytophthera spp. This fungus, of which several types affect many hardwoods and plants, can be quite disruptive to many oak trees. Phytophthera is found in all soils, but becomes a pathogen when stress conditions detrimental to overall root health persist.

In other areas of the country, and often in the Austin area, this fungus attacks oak trees with ‘wet feet’. Oak trees like well-drained soils. After rainfall, the soils dry back out and the roots of healthy oaks enjoy healthy oxygen distribution throughout the root zone. Conversely, in poorly drained soils, marshy anaerobic conditions persist. These constantly wet, marshy areas are bad for live oaks, and are breeding grounds for many fungal pathogens and for the phytophthera fungus.

In the Austin area, we generally see the most severe phytophthera infestations in new developments where entire root zones are buried by fill soil. The worst type of fill soil is clay fill. Clay particles are extremely fine, and subsequently permeation is very poor. Poor permeation due to high clay content keeps soils damp. This is exacerbated by changes in grade typically associated with new developments. Where water could previously run off and away from tree roots, it now persists for very long periods, sometimes constantly.

Another factor can be long wet weather periods. In years such as 2007, we experienced almost 70 inches of rain in the Austin and surrounding areas. In shady back yards, this cool wet summer period was associated with phytophthera infestations in certain back yard oaks where constant shade and lack of air circulation caused some soils to remain wet for up to 6 months. This led to phytophthera conditions associated with the bleeding cankers and roof loss. Some live oaks lost larger scaffold limbs associated with this disease.

Lastly, severe drought stress can also cause this disease to manifest in Austin area oak trees. Until I moved to the Central Texas area 12 years ago, I was not familiar with this disease relative to drought stress, as it was always associated with poor drainage in Northern California where I was previously working. Although it is typically not fatal under these conditions, the trees are weakened due to drought and unable to fight it off themselves. It is generally considered to be more of a secondary pathogen under these conditions.

In order to treat this disease, drainage conditions have to be set right as soon as possible. Water needs to be diverted from the root zone. ALL fill soils need to be either kept off the roots completely or tree wells need to be built prior to grade change. With respect to drought, root zones need to be mulched and watered, and often compaction may need to be addressed by fertilization/deep watering to break up the soil, and help invigorate roots, or in some cases even radial trenching can be done.

Lastly, we generally apply several fungicides to work either directly or indirectly on the fungus. Often multiple applications are necessary. It is also worth noting that controlling this fungus with respect to post oak infestations is a much slower process and may take more applications. Post oaks have tyloses; pitted, compartmentalized wood cells making the wood non-porous to pathogens, and therefore fungicide control is also slower and more difficult.