Plant Stress and Phytophthora ramorum Infection Dr. Rick Bostock Department of Plant Pathology University of California, Davis COMTF Annual Meeting June 8-11, 2010
Root stress predisposition to Phytophthora diseases Episodes of root stress (water deficit, anoxia, salinity) induce susceptibility to Phytophthora root and crown rots Stress levels occur commonly in agriculture Common responses to these stresses systemic elevation of abscisic acid (ABA) Phytophthora cryptogea vs. chrysanthemum stressed non-stressed Water stress predisposition to Phytophthora in safflower (courtesy J.M. Duniway and J.D. MacDonald) Control Salt Control Salt non-inoculated inoculated
Phytophthora ramorum and nursery ornamentals Role of root infections in disease cycle? (Jennifer Parke, Nina Shishkoff) Problem of cryptic, or asymptomatic, infections Episodic root stress and Phytophthora soil moisture extremes and P. cryptogea in Rhododendron Blaker and MacDonald (1981) Phytopathology 71:831
Nursery ornamentals, root stress and P. ramorum 0.5x Hoaglands ± 0.2 M NaCl / 0.02 M CaCl 2 16 h 0.5x Hoaglands ± zoospores 1-3 wk evaluate Rhododendron hybrid Cunningham s White Viburnum tinus cv. Spring Bouquet Control Salt Control Salt Control Salt non-inoculated inoculated inoculated 1 week post-inoculation Roubtsova and Bostock (2009) Plant Disease 93:912-918
% stem lesioned Predisposition in Rhododendron to P. ramorum 100 80 60 control salt ns * * ns 40 20 0 1 week 2 weeks 1 week 2 weeks 2 x 10 2 zoospores/ml 2 x 10 3 zoospores/ml infections in nonstressed plants catch up to stressed inoculum density is important to see predisposition effect Roubtsova and Bostock (2009) Plant Disease 93:912-918
Stem lesion length (cm) Predisposition in Rhododendron to P. ramorum Hoaglands + zoospores 24 h fresh 48 h 24 h Hoaglands fresh 1-3 wk Hoaglands ± salt Hoaglands evaluate 7 6 5 control salt * ns 4 3 2 1 0 10 2 zoospores/ml 10 3 zoospores/ml a postinfection stress episode can significantly enhance disease and symptom development Roubtsova and Bostock (2009) Plant Disease 93:912-918
duration & intensity Abscisic acid in plant stress responses and disease abiotic stress ABA s role in disease resistance and susceptibility of increasing interest ABA osmoadaptation, water balance altered gene and protein expression impacts on other hormone networks (JA, SA, CK, ET) altered disease resistance threshold for predisposition
Other systems under study for predisposition research Salt predisposition in tomato Phytophthora capsici interaction Noninoculated Inoculated control Salt no salt salt no salt salt noninoculated inoculated w/ P. capsici Arabidopsis and Phytophthora capsici. ABA ABA can substitute for the root stress
Number living plants Tomato seedling survival curves 6 hour episode of salt stress sufficient to predispose 45 40 35 30 25 20 15 10 5 0 24 h salt 6 h salt 1 h salt Control 0 12 24 36 48 60 72 84 96 108 120 132 144 156 Time after inoculation (h) a a b b DiLeo et al (2010) Phytopathology in press
Number living plants Tomato seedling survival curves stress effect on resistance can persist for up to 24 hr 45 40 35 30 25 20 15 10 5 0 24 h recovery 4 h recovery 2 h recovery Control 0 12 24 36 48 60 72 84 96 108 120 132 144 156 Time after inoculation (h) a ab ab b DiLeo et al (2010) Phytopathology in press
(+)-ABA nmoles/g d wt. Necrosis (Abs 405 nm) Tomato ABA mutants (flacca, sitiens) response to salt and P. capsici WT - salt 0.08 0.07 0.06 0.05 * * n.s. control salt 0.04 flacca - control and salt WT - control 0.03 0.02 n.s. 0.01 salt Time (hrs) 0.00 RR NY flc sit ABA increases rapidly in wild type roots during salt stress but not in ABA mutants ABA mutants (flc, sit) are not as predisposable as wild type (RR, NY) DiLeo et al (2010) Phytopathology in press
% stem lesioned ABA stimulates ramorum blight development in Rhododendron 100 90 80 70 60 50 40 30 20 10 0 salt ABA P. ramorum P. ramorum + ABA P. ramorum + salt 7 14 21 Days after inoculation ABA treatment partially mimics the effect of salt of disease development Roubtsova and Bostock (2009) Plant Disease 93:912-918
duration & intensity ABA pathway mutants abiotic stress Signal interactions hormone mutants Impact of plant activators (Actigard) on ABA osmoadaptation, water balance ABA Stress tolerance and anti-death genes Infection biology (chemotaxis, invasion) threshold for predisposition
Zoospore motility choice assay chemoattraction of root exudates droplet of zoospores microcapillary tube with root exudate Petri dish exudate from: salted roots control roots
Implications Brief episodes of abiotic stress impair basic disease resistance mechanisms in roots and shoots Root stress impact is on host physiology and pathogen behavior Mild episodic stress can affect inoculum thresholds necessary for disease extent and significance of root infections in various hosts consistency and reliability of pathogenicity tests for host resistance efficacy of chemical treatments to manage disease
Current research Rhododendron and P. ramorum Assess other abiotic factors N rates, chilling, waterlogging, water deficit for capacity to stimulate soilborne and cryptic infections. Does root stress reduce fungicide efficacy? Can plant activators mitigate abiotic stress effects on ramorum blight development?
Acknowledgements Tatiana Roubtsova Matt Pye Matt DiLeo John Duniway Jim MacDonald Dave Rizzo Plants Hines Nurseries, Inc. Monrovia Nursery Co. Support from USDA Exotic/Invasive Pests and Diseases Program, USDA NRICGP, UC Agricultural Experiment Station, and USFS (new)