Phosphonate injection treatments to help prevent sudden oak death infection of susceptible oak species (coast live oak; California black oak, Shreve’s oak, and canyon live oak), if applied in the fall (after October 30th ), need only to be applied once every other year (biannual frequency). If your first phosphonate treatment is in the spring, then treat again in the fall (after October 30th) and then treat once every other year.
If applying phosphonate as a bark application in conjunction with the surfactant PentraBark®, then one fall treatment each year is the prescription. If you apply it the first time in the spring, apply again in the fall after October 30th and then apply again in the fall of each year (after October 30th).
When looking at SODmap, you may wonder if P. ramorum-positive trees are close enough to pose a threat to trees on your property. Use the free App SODmap mobile to get the answer. Just tap the App risk function and it will tell you whether infected trees trees are within 200 m (high risk) or 1,000 m (moderate risk). If infected trees are further than 1,000 m, risk is typically low. Alternatively, you can use Google Earth, tapping the icon with a “ruler” on the top bar. Click your mouse on the closest infected tree and then move the mouse to your property. Be careful to actually move the mouse all the way to an oak tree within your property. The mouse should be drawing a line when you go from the infected tree to your oak. When you click the mouse again on top of the oak on your property, the ruler will tell you the distance. The following is suggested:
1- As the crow flies, measure from any infected tree to any oak of interest.
2- Repeat the measurement process using 3 different infected trees that appear to be close to your property.
3- If distance is slightly higher than 200 m (high risk) or 1,000 m (moderate risk), but your property is downhill and/or downwind, you may still consider your oaks to be at risk.
When treating oak trees with phosphites to help prevent SOD, it may be beneficial to apply the topical treatment to bark that is dry. Rain occurring after the treatment (even immediately after) does not affect the application. If injecting phosphites, it may be better to avoid rainy days, as lower evapotranspiration is often documented for trees in such conditions. Lower evapotranspiration rates may result in poor tree absorption of phosphites.
Note: Topical treatments are most effective when applied in the fall, from November to early December. Injections should only be performed from fall to early winter.
Injections may be better for large trees with a regular shape, while topical applications may be better for large, irregular, gnarly trees.
In order to obtain maximum efficacy with oak phosphonate treatments, minimum temperatures should not be below 50 ºF. Maximum daytime temperatures should be higher than 60 ºF, but not exceed 85 ºF.
Based on the study published by the UC Berkeley Garbelotto lab in California Agriculture (http://nature.berkeley.edu/garbelotto/downloads/calag2009.pdf), azomite and/or lime washes alone are not effective in controlling SOD.
While phosphonate injections to help control SOD are now recommended once every 2 years, spray applications should still be applied to the bark of oak trees annually.
If SOD phosphite treatments are being applied by injection November 1st – December 15th, data has shown that treating every 2 years is sufficient. When preparing the injections, be sure to use the most current dosage recommendations as they have recently been updated. Current recommendations can be found at http://nature.berkeley.edu/garbelottowp/?page_id=2345. The new dosages are significantly more diluted than the original label dosages, yet are just as effective and cause almost no damage to the wood (the original label dosages do cause some wood damage).
If SOD phosphite treatments are being applied using a topical bark application, treat every 1 – 2 years. While data has shown that topical applications last for 18 months, fall treatments are the most effective. Therefore, in a county known to be infested, treat trees in riparian areas, within 20 miles of the coast, or within 2 miles of redwood forests every year. Treat trees in drier areas (of infested counties) every 2 years.
Phosphonate is only intended to be used on trees that do NOT have sudden oak death (SOD). Once a tree is infected, it is of no use. It helps to boost the tree’s own immunity system against the pathogen. So, if a tree is very susceptible to P. ramorum (the pathogen that causes SOD), then it may still get infected and die, even with the phosphonate treatment. Really, the phosphonate just helps increase the odds that a moderately susceptible tree will be able to ward off infection. So, it is an odds game and you are just increasing your odds of success. Variables that contribute to success include tree susceptibility, disease pressure in the area, hosts present in the area, and environmental conditions.
Research has identified coast live oak individuals that may be somewhat resistant to SOD, possibly because of the presence of some chemicals in the bark and phloem. However, there is no information yet on how genetics and the environment may affect this trait.
Tanoak has some mother trees that produce SOD-tolerant seedlings and some that produce seedlings that respond very well to phosphonate treatments. These tanoak families are at UC Berkeley, but are not currently commercially available.