a) Insects:

• Insects carry plant pathogens either externally (epizoic) or internally (endozoic).
• They can disseminate bacteria, fungi, viruses, mycoplasmas, spiroplasmas, rickettsia, etc.

Fungal diseases:

• The spermatial oozings at the mouth of spermagonia in the ascomycetes attract various type of insects, flies, pollinating bees and wasps which play a dual role, viz., pollination and transmission of plant pathogens.

Bacterial diseases:

• The fire blight organism (Erwinia amylovora) and citrus canker bacterium (Xanthomonas axonopodis pv. citri) are transmitted by flies (bees) and ants and the later by leaf miner respectively.

Viral diseases:

• Both Aphids (Aphididae) and leaf hoppers (Cicadellidae or Jassidae) in the order Homoptera contain largest number and the most important insect vectors of plant viruses.

Certain species of mealy bugs and scale insects (Coccoidae), whiteflies (Aleurodidae) and tree hoppers (Membracidae) in Homoptera also transmit virus diseases