Understanding Citrus Root Mass Issues Evan G. Johnson Citrus Growers Institute April 8, 2014
Outline Early root infection and root loss Implications of root infection Early and late phases of HLB associated root loss When does it occur? How much loss? What causes root loss? Root growth or root turnover Implications for root health management
Presymptomatic root loss is linked to root infection 2.5 2.5 Root density (g l -1 ) 2.0 1.5 1.0 0.5 a 41% b Root density (g l-1) 2.0 1.5 1.0 0.5 a b b 0.0 Healthy Infected 0.0 Healthy Root Whole tree 2/14/2013
HLB root infection affects canopy Root infection Can affect canopy health before canopy infection Root health is compromised without visible symptoms Root health is correlated to yield in phytophthora and diaprepes How does the root infection affect canopy infection? 2/14/2013
Roots are an HLB reservoir Log leaf Las g -1 10 8 6 4 2 0 Pruning 0 2 4 6 8 10 MPI Low root bacteria Log leaf Las g -1 10 8 6 4 2 0 Pruning 0 2 4 6 8 10 MPI High root bacteria Pruning 2/14/2013
Roots are an HLB reservoir Bacteria Phloem flow No pruning Pruning 2/14/2013
HLB root infection Root infection Can t prune HLB out of trees Root infection acts as reservoir for bacteria to infect new flush, especially after heavy pruning Prevent rootstock suckers if tree is removed How does root infection affect root health? 2/14/2013
Early and late phases of fibrous root loss Early root loss Symptomless Tree Occurs before leaf symptoms 30 50% root loss Root infection Root density (g l-1) 2.5 2.0 1.5 1.0 0.5 a b b Late root loss 70 80% root loss 0.0 Healthy Root Whole tree Canopy thinning
Healthy What does this mean? Symptomless Infected Thinning Full roots 30 50% root loss 70 80% root loss
A closer look at thinning Woody branches visible Symptomatic new flush Leaf drop and dieback
What is causing root loss Traditionally thought to be carbohydrate starvation Phloem plugging in canopy and trunk blocks sugar transport Dependent on root infection Phloem plugging in roots?
Early root loss precedes carbohydrate starvation Root density (g l-1) 2.5 2.0 1.5 1.0 0.5 a b b Starch (mg g -1 ) 20 15 10 5 A A B 0.0 Healthy Root Root and Leaf 0 Healthy Root Root and Leaf Location of Bacterial Infection Location of Bacterial Infection
Fibrous roots lack phloem plugging Roots Leaves HLB P X X P P X Healthy P X P P X X
Root loss summary Early root loss 30 50% root loss before leaf symptoms Not sugar starvation Not phloem plugging Dependent on root infection Late root loss 70 80% loss as canopy thins Sugar starvation? Not phloem plugging
What is causing early root loss? Healthy fibrous roots are replaced every 9 12 months Faster turnover of existing roots? Not starvation or plugging? Reduced root growth? Changes in hormone levels may inhibit replacement of old fibrous roots
Measuring Root Growth Take to lab for processing Install Root Cages Under Tree Collect Root Cages After 2 months Soil Cores at time of cage harvest
Measuring Root Growth Root Cage New Growth Root Density Soil Cores
Root Growth is not inhibited Root growth (g dry weight) 5 4 3 2 1 Fall Root Flush From Sept to Nov 2013 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Root density (g dry weight/l soil) *Similar pattern Less root growth Nov 2013 to Jan 2014 Root density (g dry weight/l soil) Root growth (g dry weight) Root Density 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0 1 2 3 4 5 6 Disease severity Root Growth 5 4 3 2 1 0 0 1 2 3 4 5 6 Disease Severity
Root Growth is not inhibited Presumed Healthy New Growth Root Density Declining trees
Conclusions What we know 30 50% root loss before leaf symptoms Management considerations Increasing root growth is not likely to help 70 80% loss as canopy thins Root growth is maintained in HLB affected trees Even declining trees Faster root turnover Need to encourage root longevity Minimize stress on roots Encourage optimal environment for roots
Search for solutions Rootstocks Do new rootstocks maintain larger root systems? Can they delay early or late phase root loss? Seasonality When is root health management most important for HLB affected trees?
Acknowledgments Lab members Riham Selim Mathew Rousey Peace River Packing CRDF Jim Graham s lab Diane Bright Kayla Gerberich Tony McIntosh Jian Wu