Phosphite can be applied to trees via stem injection, bark or foliar spray, and soil drench. In tests conducted to date, soil drench applications have not been effective against P. ramorum canker (Garbelotto and others 2007). Foliar applications, which are widely used to control root-rotting Phytophthora species, have shown relatively short-lived activity against P. ramorum canker in greenhouse tests on oaks. Foliar applications are also difficult to execute on large trees and may require high volumes of spray. Low volume aerial application of phosphite by helicopter to mature tanoaks has been tested in Oregon (Kanaskie and others 2010). Although these tests showed that aerial phosphite application suppressed P. ramorum in treated trees to some degree, infections were not completely prevented. This study did not provide conclusive results on aerial phosphite application for controlling SOD, but results were not overly promising.
Phosphite applied via stem injection or bark spray application reduced P. ramorum canker size on potted coast live oaks that were artificially inoculated (Garbelotto and others 2007, Garbelotto and Schmidt 2009). The effect appeared to last at least 18 months after treatment. In addition, branches cut from coast live oaks after treatment with a bark spray application developed smaller lesions than branches from nontreated trees when inoculated in the laboratory with P. ramorum (Garbelotto and Schmidt 2009). Based on these results, injection and bark spray application are currently the preferred methods for applying phosphite to oaks and tanoaks. Each method has advantages and disadvantages (table 3-1). In general, injection is more suited for treating small numbers of trees.
Table 3-1—Comparison of injection and bark spray methods for phosphite application
|
Stem injection |
Bark spray application |
Equipment |
Need injection equipment. Most injector types also require a motor and drill. Cost variable, depending on type of injection equipment used. More injectors may reduce application time, but increases equipment cost. |
Need sprayer, wand, and nozzles. Cost variable depending on equipment used (backpack vs. tank sprayer). |
Material required |
Lower amount of phosphite required – minimum of about 1.75 ml formulated product (45.8% potassium phosphite) per inch stem diameter |
Higher amount of phosphite required – minimum of 31 ml of formulated product per cm stem diameter. Surfactant (Pentrabark®) also required. |
Efficiency of uptake |
Except for leakage from injectors, all of applied product is introduced into tree. |
Significant losses of material via overspray and runoff during application process. Of the material deposited on the trunk, a relatively small percentage may be absorbed by the tree. |
Translocation |
Most or all of the chemical introduced into the tree’s xylem. Phosphite moves up to the tree canopy before being translocated back to the bark of the lower trunk. Some injectors apply chemical to the cambial zone, where direct uptake into the bark is possible. |
Material is absorbed directly through bark into the phloem and may be moved upward and downward. |
Application speed |
Varies with injection equipment and tree size, but slower than spraying, especially if many trees are being treated. More suitable for small numbers of trees. |
Relatively fast. Because of fixed setup and cleanup time, generally more efficient to treat multiple trees at a time. Suitable for treating large numbers of trees. |
Frequency of application |
Current recommendation is a 1.5-2 year retreatment interval. |
Current recommendation is two treatments in first year and annually thereafter. |
Accidental inoculation via contaminated equipment |
Pathogen could be accidentally introduced into trees via drilling and injectors (depending on type used) when moving from asymptomatic infected trees to noninfected trees unless equipment is cleaned and disinfested. |
Negligible risk of cross-contaminating trees via spraying process. |
Nontarget effects |
Wounds created on trunk may provide entry point for insects and pathogens, potentially including P. ramorum. Multiple treatments will result in a large number of injection points. Injected solutions can kill phloem and xylem tissues if too concentrated. Injection sites may ooze for up to several years |
Spray solution kills mosses on bark surface. Overspray can cause browning (necrosis) of sprayed foliage of some species. |