- Author: Elizabeth Fichtner
- Contributor: Therese Kapaun
California nut growers have benefited from the use of phosphite products, marketed and sold both as fertilizers and EPA-labeled fungicides. These products offer plant protection from root and crown diseases caused by a group of pathogens known as Oomycetes. Dr. Elizabeth Fichtner, UCCE Tulare County, has utilized the plant pathology laboratory at Lindcove Research and Extension Center to identify new root diseases of pistachio and document the efficacy of phosphite products in maintaining plant health when challenged with soilborne pathogens.
The results of Dr. Fichtner's studies demonstrate that foliar phosphite application two weeks prior to pathogen introduction may protect pistachio rootstocks from symptoms of root rot. However, the combination of experimental data and observations gained by farm calls with pistachio growers and PCAs suggest that pistachio rootstocks may be susceptible to foliar burn resulting from phosphite use. In greenhouse studies, foliar phosphite application (1.14% active ingredient) on UCB-1 clonal rootstock resulted in foliar burn and plant dieback, however, plants recovered from the foliar burn and maintained root mass comparable to plants grown in the absence of the pathogen.
In April 2015, Yelena Martinez, an undergraduate chemistry major at College of the Sequoias, initiated a study to demonstrate the symptoms associated with foliar phosphite application. Yelena's work demonstrated that phosphite application (0.68% and 1.14% active ingredient) may alter leaf morphology, with affected leaves exhibiting a kidney shape. One month after phosphite application, the new leaves emerging from the plant's apical meristem exhibited normal leaf morphology. Similar studies are planned to assess the impact of phosphite on plant growth and development during the summer season when plants may encounter more environmental stress than in the spring study.
The results of these projects demonstrate the value of phosphites for pistachio rootstock protection from soilborne pathogens and the potential for plants to overcome and outgrow phytotoxicity resulting from foliar phosphite application.