After just a year of support for this project, we have not yet completed any of our objectives. However, we have set up all of the experiments in the greenhouse and field needed to accomplish those objectives.
A greenhouse trial over the winter months taught us the level of foliarly-applied potassium phosphate needed to maintain proper P nutrition in citrus. A second greenhouse trial was established to determine whether foliarly applied potassium phosphate would decrease citrus levels in phloem compared to the application of calcium phosphate (i.e. rock phosphate). By September of this year, it was clear that just one month of treatment with foliar potassiumphosphate decreased organic acids level in phloem (citrate, malate, and alpha-ketoglutarate) by more than half compared to trees whose roots were treated with calcium phosphate.
The first greenhouse trial taught us the appropriate levels of potassium phosphate to spray on citrus trees in the field. In April 2019, a field trial commenced in a grove o 20-year-old infected trees. There were 10 replicates and four treatments in a randomized complete block design. We spray the trees with 0, 1x, 3x, and 9x the optimal level observed in the greenhouse. Even at 1x, the plants are receiving enough P for flushing and fruit development. The plants are sprayed six times a year including after each flush.
A second field trial was established in August 2019 in the Immokalee area. That trial is the same design as that in Polk County. The idea was to have a trial on the ridge (Polk County) and flatwoods (Collier County) regions of citrus production. In both trials, the outcomes being measured are CLas titer in leaf midribs and leaf area index. Baseline samples were taken prior to the first spray and subsequent sampling will be done at 6, 12, and 18 months after the first spray.
As we wait for field results, we are now testing the effect of a foliar potassium phosphate spray (compared to root-applied calcium phosphate) on CLas titer in graft-infected trees in the greenhouse. We expect our first results from this in six months.
Given the speed with which foliar potassium phosphate can reduce organic acids levels in citrus phloem, we expect to see positive results in the field in reducing CLas titer in the first quarter of 2020. I am pleased to report that this project is working as planned so far. Foliar potassium phosphate does reduce citrate levels and the levels of other organic acids in phloem. As organic acids, particularly citrate, are the preferred carbon source for Liberibacter crescens, we expect this treatment to starve the pathogen.
Our team (Triplett, Vincent, Killiny, and Wang) are working very well together and meet to discuss the project every two weeks.major