What organism is known to penetrate root tissue and cause galls or knots?

Nematodes are microscopic, worm-like organisms that can cause significant damage to the root systems of plants. They enter the root tissue and can induce the formation of galls or knots, which are abnormal growths that can disrupt the normal function of the host plant. These galls often result from the plant's response to the feeding activity and chemical secretions of the nematodes, leading to a distortion of the root structure and impairing the plant’s ability to absorb water and nutrients effectively.

In contrast, while mites and aphids are pests that can damage plants, they primarily target leaves and stems rather than penetrating root tissues. Grubs, on the other hand, typically refer to the larval stage of certain beetles and affect different parts of the plant, usually above ground or just beneath the soil surface, but do not specifically induce galls on roots in the same way that nematodes do. This distinction highlights the unique role nematodes play in causing galls or knots on plant roots.