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American chestnut is one of hundreds of plant species plagued by root rot caused by Phytophthora cinnamomi. Phytophthora root rot is thought to have contributed to chestnut dieback prior to the arrival of chestnut blight, and it may now present a serious limitation to establishment of blight-resistant hybrid chestnut. We manipulated soil compaction and moisture to evaluate the effect of soil physical factors on incidence of Phytophthora root rot on American chestnut seedlings. Seedlings were grown under three watering regimes, two soil compaction levels and two fungicide levels. Increased soil moisture enhanced seedling growth in loose soil, but irrigation did not impact seedlings growing in compacted soils. Seedling mortality was greatest in wet, compacted soils. Disease incidence was highest in the wettest soils, irrespective of compaction level. Fine root necrosis and Phytophthora infection occurred on 58 and 24 …