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Additional key words: Phytophthora cinnamomi, Arctostaphylos myrtifolia, potassium phosphite, phosphonate, chemical control
This 2016 report presents results on experimental phosphite treatments to protect native stands of Arctostaphylos myrtifolia from root rot caused by the introduced pathogen Phytophthora cinnamomi. Treating plants at two-year intervals with a foliar application of potassium phosphite at 12.4 kg/ha using a standard spray volume of 300 L/ha provided good control of disease. P. cinnamomi propagule density was lower at the disease front of treated plants compared to nontreated plants, but levels of inoculum would be sufficient to allow disease to progress if phosphite treatment was discontinued. This application rate was also effective in reducing mortality among seedlings which had started to grow in old mortality centers.
We initiated a series of experiments to find an effective phosphite dose using ultra low volume (ULV) applications. Due to the amount of liquid used for standard rate applications, a successful protocol for ULV treatment is necessary to allow for application by air or in off-road situations that commonly exist in Ione manzanita habitat. ULV treatments use approximately one tenth the spray volume of standard volume sprays, i.e., about 30 L/ha, and therefore require a large increase in the concentration of the spray solution. Microplot tests indicated that phytotoxicity could develop after ULV applications of 12.4 kg/ha. To reduce the likelihood of phytotoxicity, initial ULV treatments were applied at 8 and 10 kg/ha. However, these lower applications rates showed inadequate levels of disease suppression. We initiated a second set of studies using split ULV applications, i.e., two applications separated by a period of 4-6 weeks, which allowed us to double overall application rates (to 16 and 20 kg/ha) while minimizing phytotoxicity. Initial evaluations indicate greater disease control at 20 kg/ha compared to 16 kg/ha, but repeat applications and further evaluations are needed to determine whether ULV applications will adequately suppress disease progress if applied at two-year intervals.