A test system to quantify inoculum in runoff from Phytophthora ramorum-infected plant roots

Authors: Shishkoff, Nina

Submitted to: Sudden Oak Death Science Symposium
Publication Type: Abstract Only
Publication Acceptance Date: 7/15/2009
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Foliar hosts of Phytophthora ramorum are often susceptible to root infection, but the epidemiological significance of such infections is unknown. We used a standardized test system to study inoculum in runoff from root-infected Viburnum tinus cuttings. Viburnum were inoculated by pouring a sporangial suspension (500 sporangia/mL) into pots, waiting 24 hours, and then transplanting the carefully washed plants into 2 x 2 inch pots with a plastic mesh insert in the bottom for easy drainage. Runoff was sampled weekly over 50 days, and inoculum measured as colony-forming units (CFU) per mL of runoff or as CFU/pot. Peak inoculum production was observed at 1-3 weeks after inoculation, with counts of 28-330 CFU per pot; the total inoculum recovered over the 50 days, averaged over three trials, was 641 CFU/pot. To look at the effect of root age on amount of runoff, cuttings were rooted two weeks apart and inoculated when the youngest cuttings first produced adventitious roots; runoff was collected every three days for 16 days. More inoculum was produced on young (48-56 days) and medium-aged (62-70 days) cuttings compared to older (79-86 days) cuttings (17,000-21,000 CFU/gram dry root weight and 2900 CFU/g dry wt, respectively) in 3 trials. When infected cuttings were incubated at different temperatures for 16 days, 31,000 CFU/g dry wt were produced on plants incubated at 15 C, compared to 23,000 CFU/g dry wt at 20 C and 6,360 CFU/g dry wt at 25 C (average of 2 trials). When four species of Camellia grown from seed were compared to Viburnum cuttings, root-infected Viburnum tinus gave off the most inoculum (760 CFU/pot over 16 days, or 14,500 CFU/g dry wt). We also compared the amount of inoculum in runoff from P. ramorum-infected Rhododendron cuttings to that of cuttings infected with P. cactorum and P. citricola, finding little difference (3464 CFU/ g dry wt for ramorum, 1176 for cactorum, and 5559 for citricola; average of 3 trials). These results suggest the importance of infected roots in the life cycle of the pathogen, since root infection is difficult to detect, but inoculum in runoff might result in spread of the pathogen.