Phytophthora/Diaprepes complex

Phytophthora/Diaprepes complex

Diaprepes root weevil

Foot rot Diaprepes damage PD complex

Infection follows damage by larval feeding on the root bark

Tolerant Swingle replants perform in Diaprepes groves on suitable soils for the rootstock (Polk Co.) Swingle replant thriving Cleopatra mandarin dying

Phytophthora spp. coincident with Diaprepes (PD complex) in marginal soils requires different resistant rootstocks to Phytophthora Southport Two case studies 1) Susceptibility of rootstocks to the PD complex in poorly drained soils of moderate to low ph Swingle does OK, Cleo does poorly (P.nicotianae) 2) High ph, bicarbonate soils that exacerbate populations Swingle does poorly, Cleo does OK (P. palmivora) Vero Beach

Southport Less tree decline from PD complex on res Swingle,C 32,C 35 vs susc Cleo & C 22 and/or with DRW control Tree decline (% incidence) 70 60 50 40 30 20 10 Diaprepes control Without control Linear regr lines C-32 citrange C-32 citrange Swingle C-35 citrange Cleopatra C-22 citrange Cleopatra C-22 citrange y=9.76+0.92x r 2 =0.65 y= -4.51+0.63x r 2 =0.78 0 C-35 citrange Swingle 10 15 20 25 30 35 40 45 50 Phytophthora (% incidence) Insect control further reduces decline

Vero Beach Flame grapefruit trees 2 yr after infestation with Diaprepes & Phytophthora Swingle US 802 (Pummelo x trifoliate)

Diaprepes IPM In Florida Groves http://www.crec.ifas.ufl.edu/extension/diaprepes/index.shtml http://www.crec.ifas.ufl.edu/extension/diaprepes/key.shtml

Citrus roots, HLB, Water Quality, & Phytophthora

Current situation and questions about HLB expression in Florida Situation HLB incidence approaching 100%, especially in young groves Crop estimate continues to decline >70% reduction from pre HLB preharvest fruit drop occurred on all varieties in all regions

Bacterial infection of the phloem causes carbohydrate disruption and fruit starvation About two months before harvest sugars move from the leaves into the fruit such that the Brix and the sugar:acid ratio increases In HLB trees starch accumulates in the leaf cells and disrupts the chloroplasts (leaf mottling) Movement of sucrose from the leaves to the fruit through the phloem is reduced Lack of carbohydrate supply causes fruit to starve and release prematurely

Fruit drop is related to early stage root infection by Liberibacter asiaticus that results in loss of fibrous roots Healthy Symptomless Infected Thinning Full roots 30-50% root loss 70-80% root loss

Root loss big picture Early stage root loss (30 50%) Late stage root loss (>70%)

New root growth is not inhibited HLB affected trees: sign that declining HLB tree are in a survival mode Presumed Healthy New Growth Root Density Declining trees

HLB reduces fibrous root lifespan Root growth is stimulated by Liberibacter infection Root turnover must be increased Healthy fibrous roots are replaced every 9 12 months What is the lifespan of Liberibacter infected roots?

Root Growth and Loss a visual example HLB Inoculated PCR (-) Healthy October 2015

HLB Inoculated PCR (-) Healthy November 2015

HLB Inoculated PCR (-) Healthy January 2016

HLB Inoculated PCR (+) Healthy February 2016 Root Growth Stimulated

HLB Inoculated PCR (+) Healthy June 2016 Root Dieback Begins

HLB Inoculated PCR (+) Healthy July 2016

HLB reduces fibrous root lifespan Root growth is stimulated by Liberibacter infection Root lifespan is decreased From 9 12 months 4 months Large investment in roots with minimal return Costs carbohydrate that would feed leaves and fruit Contributes to 30 40% fruit drop

What do these findings tell us about managing HLB roots? What we know Root growth is maintained on HLBaffected trees Priority as tree declines is to grow new roots HLB promotes faster root turnover Root replacement is expensive and reduces fruit production and retention Management considerations Stimulating extra root growth is not likely to help and may increase roots at the expense of fruit!! Better to encourage root longevity and function in water and nutrient uptake Minimize root stress in the wetted zone where 80% of the fibrous roots are located

Is there rootstock resistance/tolerance All rootstocks tested can be infected with Las in the roots Do any rootstocks have differential disease response Root longevity Root density (nutrient and water uptake)

Two rootstocks differed from Swingle 5 4 3 2 1 Fibrous root density (g dry weight/l soil) Jan13 Feb13 Mar13 Apr13 May13 0 Swingle Orange 19 Orange 4 Changsha + 50-7 Cleo + Carrizo Healthy Orange 4 (UFR 2) Healthy Swingle Jul13 Sep13 Nov13 Jan14 Apr14 Jul14 Sep14 Nov14 Feb15 May15

Differential rootstock responses to HLB Genetic capacity for improved root health Needs to be bred into more and better rootstocks What does this mean for yield? Preharvest fruit drop

HLB Fruit Drop Swingle UFR-4 UFR-2 Changsha + 50-7 Cleo + Carrizo 180 160 140 120 100 80 60 40 20 0 Fruit Drop (fruit/tree)

Current HLB root loss model HLB on most rootstocks Bacteria move to roots Water or temperature stress Carbohydrate stress Inoculated by psyllid 30 50% root loss Sectored symptoms and leaf drop General leaf drop Severe root loss and rapid disease spread HLB without root loss Bacteria move to roots Symptoms spread Symptoms more severe Symptoms spread Inoculated by psyllid No root loss Sectored symptoms and leaf drop Resilient to water stress Root starvation begins Significant root loss

Soil ph and well water quality affect root health and HLB disease expression Microjet irrigation concentrates fibrous roots in the wetted zone Some groves (e.g. fresh fruit blocks) have history of dolomite liming for control of copper toxicity Common condition: ph > 6.5 in wetted zone is associated with well water high in bicarbonate (>100 ppm) and > HLB expression (i.e. fruit drop) Bicarbonate reduces root uptake of Ca, Mg, K, Fe (e.g. high Ca in soil/moderate levels in leaves) Groves with bicarbonate stress are experiencing > deterioration in fibrous root density, lifespan and function in root uptake Rootstock sensitivity: Swingle > Carrizo > Sour orange > Cleopatra

Percentage of carbon species in solution for various ph ranges: H 2 CO 3 = carbonic acid, HCO 3 = bicarbonate, and CO 3 2 = carbonate

Comparison of 39 groves in Highlands and Desoto Co with varying liming history and irrigation from deep vs. shallow wells Data from Davis Citrus Management Shallow wells Well water ph 9 8 7 6 5 4 3 ButlerN ButlerS Reedy Wal Hugh Weig Col Ruvin Parker Cole Mama Web18 Steph Water 1 McCann Con Glen MaxS Web19 Rhapp Water 3 Coke Cham MaxN Smith Water 2 Three Sweet Hen Swan Moody Bent Sachs 0 50 100 150 200 250 Ten Ken Deep wells Well water bicarbonate

Lower root density is related to well water ph>6.5 (r 2 = 0.50 ) and soil ph>6.2 (r 2 = 0.25) 1.0 Well water ph 1.0 Soil ph in the wetted zone Fibrous root density (mg/cm 3 ) 0.8 0.6 0.4 0.2 0.0 Fibrous root density (mg/cm 3 ) 0.8 0.6 0.4 0.2 0.0 3 4 5 6 7 8 9 Well water ph 4.5 5.0 5.5 6.0 6.5 7.0 7.5 Soil ph

Relationship between soil ph status, root mass density and change in yield from 2011 13* Grove status No. of blocks surveyed Root mass density (mg/cm 3 ) Change in block yield from 2011 13 Low ph stress Ridge High ph stress Ridge High ph stress Flatwoods 14 0.6 Increased 6% 10 0.4 Decreased 3% 13 0.2 Decreased 20% *Yield data kindly provided by Davis Citrus Management

Health and fruit drop for Valencia/Swingle trees (2012 13) Soil acidification appears to reduce fruit drop in groves with high bicarbonates in the water (2013 14) ph 6.4: Fruit drop minimal ph 7.2: Fruit drop resulted in early harvest

Combination of HLB and bicarbonate stress increases susceptibility to root pests and pathogens Increases root damage caused by Phytophthora interaction with HLB Increases populations of nematodes and reduces control with nematicides High ph reduces entomopathogenic nematodes (EPNs) that control larvae of Diaprepes Burrowing nematode interaction with HLB

Statewide Phytophthora counts have resurged in 2014 in response to a doubling in root mass compared to 2013 Less fruit drop in 2015?!? Average Dry Root Mass 1.2 P. nicotianae (propagules/cm 3 ) 25 20 15 10 5 0 2008 2009 2010 2011 2012 2013 2014 YTD Root dry weight (g)/20 cores Propagules/g of root mass 1.0 0.8 0.6 0.4 0.2 0.0 50 40 30 20 10 2013 2014 YTD Average Disease Load 2013 2014 YTD Based on 2400+ soil samples and 750+ root samples statewide Data courtesy of John Taylor, Syngenta Crop Protection 0

How well fungicides control infection and increase root growth depends on whether the roots are infected with Las Root infection with Pn Fibrous root dry weight 40 8 Root infection (%) 30 20 10 Control Aliette Ridomil Fibrous root dry weight (g) 6 4 2 Control Aliette Ridomil 0 HLB + HLB - 0 HLB + HLB -

Manage soil stresses first Balanced, lower and more frequent application of water and nutrients to the reduced root system ( spoon feeding ) Reduce soil ph/bicarbonate stress to sustain root function in nutrient uptake and root longevity To assess bicarbonate stress : check soil ph (wetted zone) test well water for ph, bicarbonates, salinity, cations, anions Water conditioning: Inject N furic acid or sulfuric acid (40%) to reduce irrigation water to ph 6.5 or lower (more cost) Soil conditioning: broadcast sulfur in wetted zone to reduce soil ph

Acidification of the soil and/or water reduces root zone ph and promotes release of Ca, Mg and K for root uptake Water conditioning Faster, lower soil bicarbonate Injection of N furic acid or sulfuric acid (40%) Soil conditioning Slower, high soil bicarbonate 300 lbs/treated acre of Tiger 90 sulfur lowered soil ph in 9 months Valencia/Swingle 10 yr old Sulfur ph Root density (mg/cm 3 ) No 6.4 1.1 Yes 5.9* 1.4* * Significant difference P < 0.05

Manage root pest and pathogens after correcting water/soil stresses Phytophthora, nematodes, weevils should be managed more aggressively to sustain root health details in FCPMG www.crec.ifas.ufl.edu/extension/pest/ Phytophthora count >10 20 propagules/cm 3 recommend rotation of fungicides: Aliette/phosphite after spring shoot flush Mefenoxam after spring early summer rains begin Aliette/phosphite after midsummer shoot flush Mefenoxam after fall shoot flushes Remember root flushes follow shoot flushes

Questions? contact Jim Graham jhgraham@ufl.edu