-- 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.
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.
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.
-- 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.
-- 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.