Final answer:
Fungi are the disease-causing pests that grow as long filaments and invade the plant, leading to various plant diseases and can significantly affect the health of crops.
Step-by-step explanation:
The disease-causing pests that grow as long filaments and invade the plant are fungi. These pathogens can be detrimental to plants, absorbing nutrients and causing diseases that damage or even kill the host plant. Various types of fungi, including biotrophic fungi like powdery mildews and rusts, exist that feed off of living plant tissue and produce structures called haustoria which penetrate plant cells. An example of a significant agricultural disease caused by fungi is wheat rust. Fungal diseases can be quite serious, and management of these diseases is critical for the health of crops.