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United States Department of Agriculture

Agricultural Research Service

Title: Environmental parameters affecting inoculum production from lilac leaf pieces infected with Phytophthora ramorum

Author
item Shishkoff, Nina

Submitted to: Sudden Oak Death Science Symposium
Publication Type: Proceedings
Publication Acceptance Date: April 14, 2008
Publication Date: April 14, 2008
Citation: Shishkoff, N. 2008. Environmental parameters affecting inoculum production from lilac leaf pieces infected with Phytophthora ramorum. Phytopathology. 96:S107 . Sudden Oak Death Science Symposium.

Interpretive Summary: 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 to determine the effect of moisture on propagule production, infected pieces of lilac leaf were placed on the surface of pots containing healthy lilac plants that were kept constantly moist or watered twice a day. Propagule production from leaf pieces declined significantly less steeply under the constantly moist conditions. Overall, 28% of plants exposed to moist conditions developed root infections, while only 6% exposed to trickle irrigation did. However, these infections were slight: in infected plants, only .05-3.8% of root segments. In another experiment to look at inoculum production under more typical nursery conditions, infected leaf pieces were placed on the surface of pots and watered 1, 2, or 3 times a day. Propagule production declined significantly slower in pots watered 3 times a day. After a month, the number of infected roots detected was low, from 0.6-1.8%. In a third experiment to look at the effect of different amounts of infected tissue, 0, 2, 4, 8, or 16 leaf pieces were placed on the surface of pots kept constantly moist or watered twice a day. After a month, no root infection was seen in pots watered twice a day, but was observed in pots kept constantly moist, from 38% in pots with 2-4 leaf pieces, 62% in pots with 8 leaf pieces, and 75% in pots with 16 leaf pieces. As with previous experiments, the percent root infection was low: from 0.6-13.6%. The effect of temperature was studied by placing leaf pieces on the surface of pots kept at 10C, 15C, 20C, and 25C. Practically no propagules were seen after day one, but at day one, significantly more propagules were produced at 10C. Infected leaf pieces were also placed in vials of sterile distilled water at 10C, 15C, 20C, and 25C. Different propagules were observed at different temperatures. At 10 or 15 C, propagules were predominantly zoospores, while at 20 or 25C, they were predominantly sporangia. These results confirm the importance of fallen leaves as inoculum producers under greenhouse conditions and also confirm that cool moist conditions are more conducive to inoculum production that warm dry conditions. Although lilac roots did become infected from exposure to these fallen leaves, the amount of infection was very slight after 1 month under greenhouse conditions. Root infections have been shown to spread to above-ground plant parts in Rhododendron, so they may be significant in the disease cycle even in low amounts. Inoculum produced from fallen leaves might also be spread in irrigation splash and runoff.

Technical Abstract: Because fallen leaves infected by Phytophthora ramorum 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, a 6.5 cm2 piece of infected lilac leaf was placed on the surface of potting mix in pots containing healthy lilac plants. The pots were kept constantly moist or trickle-irrigated twice a day. Inoculum from leaf pieces was quantified over time by shaking them in a known volume of sterile distilled water, then plating aliquots on PARP selective media. At the end of the experiment, root samples were plated on PARP media. A mixed model regression analysis was run on the first five data points, showing that propagule production declined over time but declined significantly less steeply under the constantly moist conditions. Overall, 28% of plants exposed 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 pots containing healthy plants and watered 1, 2, or 3 times a day. After a month, root samples were taken. Regression analysis on the first five data points showed that propagule production declined significantly slower in pots watered 3 times a day. Infected roots were observed in all treatments, but the degree of infection was low, from 0.6-1.8%. In a third experiment 0, 2, 4, 8, or 16 leaf pieces were placed on the surface of pots containing healthy plants kept constantly moist or watered twice a day. After a month, root samples were taken. Root infection was observed in pots kept constantly moist, from 38% in pots with 2-4 leaf pieces, 62% in pots with 8 leaf pieces, and 75% in pots with 16 leaf pieces. The effect of temperature was studied by placing infected leaf pieces on the surface of potting mix in pots containing healthy plants kept at 10C, 15C, 20C, and 25C and watered twice daily. An analysis of variance was run on data from day one, showing significantly more propagules produced from leaf pieces at 10C. Infected leaf pieces were also placed in vials of sterile distilled water at 10C, 15C, 20C, and 25C. Different propagules were observed at different temperatures. At 10 or 15 C, propagules were predominantly zoospores, while at 20 or 25C, they were predominantly sporangia. Greater numbers of propagules were recovered from 10 and 15C treatments than from 20 and 25C treatments. These results confirm the importance of fallen leaves as inoculum producers under greenhouse conditions and also confirm that cool moist conditions are more conducive to inoculum production that warm dry conditions. Although lilac roots did become infected from exposure to these fallen leaves, the amount of infection was slight after 1 month under greenhouse conditions.

Last Modified: 8/21/2014
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