This proposal is aimed at following previous work in CRDF-710 and CRDF-818 with a series of precise experiments that will: 1. Elucidate the nature of the HLB signal(s) 2. Provide additional evidence on its transmission in terms of movement across tissues and between trees though underground organs. 3. Determine the progression of physical symptoms from its inception. 4. Examine the in-tree variation in CLas titer. All experiments were completed and final testing done in December. 1. To test for he unlikely, but increasing, possibility that HLB is transmitted by extracellular vectors, we isolated DNA from HLB leaves and inject these into 2 year old Valencia trees. The trees are being kept in a greenhouse and are under observation. Trees continue growing normally. Trees tested in September 11, one tree tested HLB+, though a high PCR value. Trees were retested again late December and all tested HLB-. 2. Experiments for objective 2 are well under way. Two trees (one healthy and one HLB+) were root grafted in three different locations and placed in special pots large enough to accommodate the 2 trees (5 pairs). The trees were placed in a greenhouse and kept under observation. PCR analyses were conducted once more in September 2016. At this time, 3 out of the 5 pairs of the initially healthy trees tested positive, although clear visible symptoms were not evident in all cases. By the end of December, 4 of originally healthy trees tested HLB+. One remains under observation. 3. Grafted trees with HLB material are being monitored weekly using Narrow-band imaging under polarized illumination. Although we continue to have issues with the background, we have established a standard curve and a correlation relationship between starch levels, PCR values, and polarized light readings. 4. Trees have been grafted for a substantial amount of time and some are showing HLB symptoms. However, given that analysis of this objective destroys the trees, only trees with clear symptoms are tested. PCR analyses was conducted in a total of three trees that showed symptoms and tested HLB+. In any of the trees, there was correlation between PCR values and leaf position. These possible results prompted the design of a new system for HLB determination with a much higher level of precision.