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Oak Pests - A Guide to Major Insects, Diseases, Air Pollution and Chemical Injury

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Importance. -- Soil-inhabiting fungi such as Fusarium, Cylindrocladium, Rhizoctonia, Pythium and Phytophthora species cause heavy losses (25 to 50 percent) in pre- and post-emergence seedlings. Fungi attack the young developing radicles killing seedlings 30 to 45 days after the seedlings emerge. Cylindrocladium spp. are the most important pathogens in hardwood nurseries.

Identifying the Fungus. -- The pathogens involved are minute and can be identified only by the use of a microscope. Tentative identification can be made by cultural characteristics.

Identifying the Injury (figure 71). -- The first symptom is failure of seedling emergence. The seed may rot or seeds may have a dead or damaged radicle (pre-emergence damping-off). Seedlings may remain stunted.

Biology. -- The fungi are mostly soilborne and in the absence of a host remain inactive as chlamydospores or sclerotia. The presence of host roots stimulates the fungus, which grows over roots and penetrates the epidermis and cortex. Cylindrocladium spp. also produce airborne conidia which may cause leaf spots and defoliation.

Control. -- Grass cover crops will reduce the inoculum potential; however, clover and other leguminous cover crops increase the pathogen. Do not apply nitrogen fertilizer until 45 days after the seedlings emerge.

Figure 71

Figure 71. -- Seedling root disease causing stunting of seedlings.
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