TABLE 6.1–Classes of host susceptibility to Arceuthobium
Class Susceptibility Infection level

I Principal* >=90%
II Secondary 90-50%
III Occasional 50-5%
IV Rare <=5%
V Immune† 0%

*Except for young trees, infection levels usually approach 100% within 6 m of a seed source.
Trees are not infected even where the dwarf mistletoe is common.

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