Host Plant

A host plant is any plant species that a particular pest or pathogen can successfully infect, feed upon, and use to complete its life cycle. Understanding host relationships is fundamental to managing tree health threats because it determines which trees are at risk from specific pests and diseases and informs management strategies like species diversification.

Host specificity varies enormously among tree pests and diseases. Some are highly host-specific: the emerald ash borer attacks only ash trees (Fraxinus species), and Dutch elm disease primarily affects elms. Others have broad host ranges: Armillaria root rot can attack virtually any woody plant, and Japanese beetles feed on over 300 plant species. Primary hosts are species where the pest thrives and reproduces most successfully, while secondary hosts may support the organism but less favorably.

The concept of host range has critical implications for urban forest planning. Overplanting any single tree species creates a monoculture vulnerable to devastation by a host-specific pest. American cities learned this lesson painfully when Dutch elm disease swept through streets lined exclusively with American elms. Modern urban forestry guidelines recommend that no single species should constitute more than 10% of the urban tree population, no single genus more than 20%, and no single family more than 30%.