| 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. | ||