(58) Suppression of Phytophthora cinnamomi Activity on Rhododendro × PJM `Elite' by Two Compost-amended Container Media under Two Irrigation Regimes and Nursery Conditions

Abstract

Composted municipal biosolids were incorporated into a potting mix containing sphagnum peat and sand (1:1 by volume) at rates of 0%, 25%, and 50%, by volume. A second medium was prepared by mixing cow manure compost in the same basic mix at rates of 0%, 10%, and 25% by volume. Each mix was inoculated with P. cinnamomi colonized millet seed at a rate of 200/200 cc of compost-amended media. The potted plants were placed outdoors under nursery conditions 14 July 2003. One half of the plants were irrigated every day, except when natural precipitation occurred; the other half was watered once each week. Soil water potential of all treatments was measured daily with tensiometers. Plants were harvested on 18 Aug. and 21 Oct. 2003, when the experiment was terminated. Frequent rainfall during the period prior to the first harvest masked any impact that the irrigation treatments may have had on disease suppression. Even so, three compost treatments proved successful in suppressing disease activity. Between the first and second harvest dates rainfall was significantly less frequent; thus, differences in P. cinnamomi activity between the wet and dry regimes was noted at the 21 Oct. harvest. Under the dry regime, all inoculated compost treatments, except the 25% municipal biosolid compost, exhibited disease suppression based on root symptom severity and percentage of root infection. Suppression based on shoot symptoms and percentage of shoot loss was evident only in the 50% and 25% biosolid and cow manure composts, respectively. Under the wet regime, only one treatment exhibited suppression of disease activity. All compost treatments held more water particularly at lower moisture tensions. The presence of more water would tend to favor more disease activity and not suppression.