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

Abstract

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

Parent Publication

• Proceedings of the sudden oak death third science symposium

Citation

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