Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2009
Publication Date: May 1, 2009
Citation: Shishkoff, N. 2009. Propagule production by Phytophthora ramorum on infected leaves of lilac (Syringa vulgaris). Plant Disease. 93:475-480. Interpretive Summary: Phytophthora ramorum causes symptoms on a variety of plant hosts. It was first observed on European nursery stock and in California forests in the mid-1990s; it has now been reported from many countries in the European Union, and has spread in natural areas of California and Oregon. The major means of spread outside the infected regions in the US has been through infected nursery stock. Because fallen leaves might serve as sources of inoculum, this study quantified inoculum produced by such leaves on the surface of pots when exposed to different watering regimes or different temperatures, and demonstrated that inoculum could be produced in significant amounts on fallen infected lilac leaves. Inoculum from fallen leaves continued to be produced for up to 22 days, depending on environmental conditions. Moisture was clearly an important factor, but it was more difficult to gauge the importance of temperature. Root infection was observed in lilacs exposed to fallen infected leaves, but the amount was quite low after 1 month under our greenhouse conditions. Root infections have been shown to spread to above-ground plant parts in Rhododendron, so even a small amount of infection might be significant in the disease cycle in some hosts. Inoculum produced from fallen leaves might also be spread in irrigation runoff, and infest soil under pots in nurseries. Best management practices for containerized plant nurseries include enjoinders to remove and destroy plant litter and also to water sparingly and in the early part of the day, to reduce the threat of ramorum blight in a nursery setting.
Technical Abstract: Leaves with lesions caused by Phytophthora ramorum often drop off infected plants. Because fallen leaves might serve as sources of inoculum, this study quantified inoculum produced by such leaves on the surface of pots when exposed to different watering regimes or different temperatures. In one experiment, pieces of infected lilac leaf were placed on the surface of potting mix in pots containing healthy lilac plants. The pots were then kept under constantly moist conditions or under twice-a-day trickle irrigation for one month. Leaf pieces were assayed periodically and the amount of inoculum produced from them was quantified. Propagule production declined over time for the first four days but declined significantly less steeply under the constantly moist conditions. At the end of the experiment, 28% of plants exposed to infected leaf tissue under moist conditions developed root infections, while only 6% exposed to trickle irrigation did. In another experiment, infected leaf pieces were placed on the surface of potting mix of pots in pots containing lilacs watered using overhead irrigation for 5 min 1, 2, or 3 times a day. Propagule production in the first four days declined with time but declined significantly slower in pots watered 3 times a day. Infected roots were observed in all treatments, but a significant effect of watering frequency was not observed. In another experiment 0, 2, 4, 8, or 16 leaf pieces were placed on the surface of potting mix in pots containing healthy lilacs kept moist with overhead misting or watered using trickle irrigation twice a day. After a month, root samples were taken. Root infection was observed in pots kept constantly moist, and the number of leaves significantly influenced the amount of root infection. The effect of temperature was more difficult to quantify, either with infected leaf pieces in pots or in vials of water kept in controlled-environment chambers set at 10C, 15C, 20C, and 25C. Different propagules were observed at different temperatures. At 10 or 15 C, propagules included zoospores, while at 20 or 25C, they were predominantly sporangia. These results confirm the importance of fallen leaves as inoculum producers under greenhouse conditions.