Fusarium hypocotyl rot

Fusarium hypocotyl rot
Disease and Host. 
Fusarium hypocotyl rot is caused by the fungus Fusarium oxysporum. It occurs primarily on Douglas-fir, and to a lesser extent on Shasta red fir, western larch, sugar pine, white fir, and ponderosa pine, in the Pacific Northwest. Symptoms usually begin to appear after the first period of high temperatures (above 32 degrees C or 90 degrees F) in late June or July. The disease continues to cause damage through August or September. Fusarium hypocotyl rot is the greatest single cause of loss of postemergent seedlings in Pacific Northwest nurseries.
Symptoms.
Random mortality begins in seedbeds in June and continues through October. Mortality is highest in July and early August. Top symptoms are similar to those caused by Fusarium root rot, but careful observations made just as the seedling tops begin to turn yellow reveal a discolored region of dead tissue on the hypocotyl, that portion of the stem between the ground and the two lowest needles, or cotyledons. This discolored region quickly expands, girdling the stem and killing the top while the roots remain healthy. The roots then become discolored and decay in the same way as do roots infected with Fusarium root rot. Seedlings remain erect. White mycelia with pink or orange spore pustules can be seen occasionally on infected seedlings at or above the soil line. The sickle-shaped spores are readily identifiable under a microscope.

Rhizoctonia hypocotyl rot affects the stem of the plant between the cotyledons and the roots (the hypocotyl). The fungal organism, Rhizoctonia, is very common in most soils. Hypocotyl rot affects isolated plants, as opposed to Rhizoctonia bare patch which occurs in patches. Seedlings are infected during emergence and die rapidly, reducing emergence. Any surviving plants will be stunted and prone to other problems such as collar rot. Hypocotyl rot is worse in crops following the pasture phase in a rotation, and is also promoted by early sowing. Affected plants are characterised by red-brown lesions on the hypocotyl or tap root.