Phytophthora root rot was recently diagnosed in a container nursery. The media was overly wet, pots were in a shaded area, and water was collecting in the area. The following is information on controlling Phytophthora root rot in the nursery.
Prevention - Select nursery sites with well-drained soils. Phytophthora root rot in nurseries can be greatly reduced by improving soil drainage and limiting irrigation. Do not transplant seedlings from infested nurseries into disease-free ones. Avoid using diseased trees for mulch because chlamydospores or oospores may survive in this host material for several years. Avoid movement of equipment between infested and noninfested areas. Diseased seedlings should be lifted separately and destroyed by fumigation or burning. Any effort to save healthy looking trees within diseased areas will result in the spread of the fungus to other areas of the nursery and to outplantings. After working in infested areas, clean equipment thoroughly with steam or an equivalent method.
Chemical - Fumigate soil with methyl bromide and chloropicrin mixture to reduce damage. Unfortunately, fumigant penetration in heavy soils is often inadequate. Metalaxyl, a selective systemic fungicide, is registered as a seed-bed soil drench for seedlings of Fraser fir and Douglas-fir. Two annual applications of metalaxyl are effective in controlling Phytophthora root rot.
Phytophthora root rot progressing up the stem of a seedling.
Information from http://www.forestpests.org/nursery/phytophthora.html