The Efficacy of Various Preventive Soil-Applied Potassium Phosphite Products Against Foliar <i>Phytophthora</i> Infections on <i>Rhododendron</i> over Time

Abstract Background Phosphite products have a history of use as fertilizers and fungicides. In contrast to conventional fungicides, phosphites work both directly and indirectly by activating plant defense mechanisms and are proven to be effective against Phytophthora root rot and canker diseases. There are numerous phosphite products on the market labeled as fungicides or fertilizers, but comparative studies on their efficacy and duration of impact are scarce. Methods We compared the efficacy of commercially available phosphite products against foliar infections of Phytophthora nicotianae on Rhododendron spp. The products were labeled as fertilizers or fungicides and formulated as soluble concentrates, granules, or slow-release tablets. In 2 separate trials, preventive applications were made to the root zone of containerized and field grown Rhododendron spp. Induced resistance was assayed by measuring lesion size following inoculation of detached leaves over time. Results In the containerized greenhouse study, all phosphite products suppressed lesion development starting as early as 1 week post-treatment and suppression was sustained for 8 to 12 weeks, resulting in significantly reduced lesion area compared to inoculated, non-treated controls. In the field trial, Reliant L (fungicide) and Reliant Dry Phite 28G (fertilizer) suppressed cumulative lesion area 3 weeks post-treatment with effects persisting 6 and 8 weeks, respectively. All products suppressed cumulative lesion area at 4 and 6 weeks post-treatment except the phosphite tablet (fertilizer). Conclusions Whether labeled as a fertilizer or fungicide or formulated as a liquid or granule, soil applications of phosphite products provided prolonged, systemic protection against foliar Phytophthora spp. infections in Rhododendron spp. to varying degrees.