Propagule Production by Phytophthora ramorum on Lilac (Syringa vulgaris) Leaf Tissue Left on the Surface of Potting Mix in Nursery Pots.

Lilac leaf tissue infected with Phytophthora ramorum was placed on top of potting mix in pots and exposed to different watering regimes or different temperatures to determine if it could serve as a source of inoculum. If pieces of infected leaf were placed in pots containing healthy lilac plants kept under constantly moist conditions or under twice-a-day trickle irrigation for 1 month, inoculum production from infected tissue declined for the first 4 days but declined significantly less steeply under constantly moist conditions. At the end of the experiment, 28% of plants exposed to moist conditions developed root infections, whereas only 6% exposed to trickle irrigation did. If infected leaf pieces were placed on the surface of potting mix in pots containing lilacs and watered for 5 min one, two, or three times a day, inoculum production in the first 4 days declined but declined significantly more slowly in pots watered three times a day. If 0 to 16 leaf pieces were placed on the surface of potting mix in pots containing healthy lilacs under constantly moist conditions, leaf number significantly influenced the incidence of root infection. The effect of temperature was more difficult to quantify. At 10 or 15°C, propagules included zoospores whereas, at 20 or 25°C, they were predominantly sporangia. These results confirm the importance of detached leaves as inoculum producers under greenhouse conditions.