Purpose: Investigate the effects of underground pests on the severity of HLB in citrus trees co-infected with citrus nematode (Tylenchulus semipenetrans) or burrowing nematode (Radopholus similis) compared to HLB alone.Progress Summary: We confirmed that the roots of all of the trees treated with the two types of nematodes were successfully colonized and samples have been taken to examine the damage. Damage to the cortex of fibrous roots is the most common visual symptom (by staining and microscopy). Citrus Nematode Exp 1 – The experiment was established and primary infections with HLB and citrus nematode were confirmed. We had a low number of HLB trees test positive so we regrafted the HLB treatment trees in September. Samples for a second PCR is scheduled for January2021. The trees that received citrus nematode are yellowish with thin foliage, and trees on Swingle rootstock are more vigorous. The population density of the citrus nematode has increased from a moderate level of >680 in summer to >2100 juvenile and male nematodes per 100 cm3 soil in October, which now exceeds the damage threshold. As a consequence the 200 plants (10 treatments, 20 reps) were randomized in a blocked design for growth until the trial terminates in Spring 2021. Burrowing Nematode Exp 2 – 60 Val trees (15 trees x 4 TR) were established. Leaf samples were taken to confirm HLB infection (9 months). Burrowing nematode infection will be measured from soil core/root fragment samples in January 2021.Burning Nematode Exp 3 – UFR rootstocks planted in sandy soil, inoculated with burning nematode – these plants look very bad compared to non-inoculated controls. The roots have been recovered, washed and stained, and examined under the microscope. These roots show evidence of cortex damage to the fibrous roots. Root samples from the same plants were taken for metabolite analysis (by GC-MS), currently stored at -80 ºC until analysis.Future rootstock evaluations – USDA in Ft. Pierce confirmed that they will send us seeds for germination, to evaluate their susceptibility to nematodes during the next year. These include US-802, 812, 897, 942, 1283, 1284, 1516. These will be added to our evaluation effort, as it seems they have not been screened for their tolerance/susceptibility to nematodes.