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

Author: Shishkoff, Nina;

Date: 2008

Source: In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M., tech. coords. 2008. Proceedings of the sudden oak death third science symposium. Gen. Tech. Rep. PSW-GTR-214. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 449-450

Publication Series: General Technical Report (GTR)

   Note: This article is part of a larger document. View the larger document

Description: Leaves with lesions caused by Phytophthora ramorum Werres, de Cock & Man in?t Veld often drop off infected plants. Because fallen leaves might serve as sources of inoculum both for the above-ground tissues of host plants and for their roots, this study quantified the inoculum produced by such leaves on the surface of pots when exposed to different watering regimes or different temperatures. In one experiment3, a 6.5 cm2 piece of infected lilac leaf was placed on the surface of potting mix in pots containing healthy lilac plants (Syringa vulgaris L.). The pots were then watered using a leaf hygrometer to ensure constant moist conditions, or using trickle irrigation for five minutes twice a day. Pots were incubated under greenhouse conditions. Leaf pieces were assayed at 0, 1, 2, 3, 4, 7, 10, 14, 18, and 22 days by shaking them in a known volume of sterile distilled water, then plating 0.5 ml aliquots on PARP selective media. This allowed the number of propagules produced by each leaf piece to be calculated. At the end of the experiment, four soil cores were taken from each pot and root segments in them were washed and plated on PARP media. A mixed model regression analysis for repeated measures over time was run on the first five data points, showing that propagule production declined over time for the first four d (P=.0001) from approx. 120,000 sporangia/leaf piece at time 0 to almost nothing by day 4, but declined significantly less steeply under the constantly moist conditions (P=.0009). After the first two weeks or so, the leaves in the overhead misted treatment began to give off a new pulse of propagules; examination of plates showed that initially, propagules were predominantly sporangia, but as the leaf decayed, sporangia were replaced by chlamydospores released from disintegrating tissue. When lilac plants in the pots were examined at the end of a month, there was one infected leaf observed in the moist treatment in one replicate. Overall, 28 percent of plants exposed under moist conditions developed root infections, while only 6 percent of plants exposed to trickle irrigation did; a chi-square test comparing infection by treatment showed this to be significant at P =0.07. However, these infections were very slight: in infected plants, only .05 to 3.8 percent of root segments plated yielded colonies of P. ramorum. In another experiment3, infected leaf pieces were placed on the surface of potting mix in pots containing healthy lilac plants and watered using overhead irrigation for 5 minutes 1, 2, or 3 times a day. Leaf pieces were assayed at 0, 1, 2, 3, 4, 5, 7, 10 and 14 days. After a month, root samples were taken. A mixed model regression analysis was run on the first five data points, showing that propagule production declined with time (P<0.0001) but declined significantly slower inpots watered three times a day (P<.001). Infected roots were observed in all treatments (5 percent in pots watered once a day, 10 percent when watered twice a day, and 20 percent when watered three times a day), but a chi-square test to see if watering frequency had an effect of root infection was not significant. Again, the actual number of infected roots detected was very low, from 0.6 to 1.8 percent. In a third experiment3 0, 2, 4, 8, or 16 leaf pieces were placed on the surface of soil kept moist with overhead misting or watered using trickle irrigation twice a day. After a month, soil cores were taken from each pot and root segments in them plated on PARP media. No oot infection was seen in pots with trickle irrigation. Root infection was observed in pots kept constantly moist, from 38 percent in pots with 2 to 4 eaf pieces, 62 percent in pots with 8 leaf pieces, and 75 percent in pots with 16 leaf pieces. A chi-square test to see if number of leaf pieces influenced frequency of root infection was significant at P= 0

Keywords: Phytophthora ramorum, Syringa vulgaris, lilac

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Shishkoff, Nina 2008. Environmental parameters affecting inoculum production from lilac leaf pieces infected with Phytophthora ramorum. In: Frankel, Susan J.; Kliejunas, John T.; Palmieri, Katharine M., tech. coords. 2008. Proceedings of the sudden oak death third science symposium. Gen. Tech. Rep. PSW-GTR-214. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. pp. 449-450

 


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