PESTS: Pathology

Plant pathogens include fungi, bacteria, viruses, viroids, and phytoplasms. These microscopic organisms cause crop diseases when they come into contact with the plants under environmental conditions that favor the pathogen while making the host more susceptible to attack. Crop damage caused by pathogens often looks similar to symptoms related to abiotic stresses such as water or nutrient deficiencies.

Tri-Tech’s Pest Control Advisors and Certified Crop Advisors are trained to differentiate between pathogen induced disease and abiotic issues so that they can prescribe the appropriate mediation strategy.

See the table below to learn about symptoms and signs associated with different pathogens.

The Disease Triangle represents the interaction between the pathogen, a susceptible host, and environmental conditions favoring infection. Plant disease development requires interaction between all three components. If any one is absent, disease will be avoided.

Asian Citrus Psyllid: Vector of HLB, The Citrus Greening Disease

Botrytis on Strawberries

Pathogen Descriptions and Damage Symptoms



Crop Damage


Microscopic, tubular filaments called hyphae that grow into a network of mycelium in the soil and in plants. Large masses of mycelium as well as fruiting bodies and spores can be seen without a microscope.
Leaf spots; leaf, fruit, or tuber scabs; leaf, flower, branch, or twig blight; cankers; dieback; stem rots; soft or dry rots; stunted growth.


Microscopic single celled organisms, less 0.002mm. Bacterial cells can be spherical, ellipsoidal, spirals, rods, or even filamentous. Bacteria can only survive in moist environments, usually in water films lining soil pores or plant roots.
Leaf spots; cankers; galls; discoloration; malformed leaves and fruit; slimy ooze on plant surface


Submicroscopic parasite made of genetic material protected by a layer of protein. Viruses are too small to be detected by most common agricultural lab methods, so they must be identified based on symptoms.
Discoloration; malformed plant parts; necrosis; stunted growth; low crop yield.
Source: Flint, Mary Louise. 2012. IPM in Practice: Principles and Methods of Integrated Pest Management. University of California, Agricultural and Natural Resources. Second Edition
The descriptions and damage symptoms listed in the table above are extremely general and non-exhaustive. Each pathogen represents a broad range of organisms with different physical appearances and effects on plants.
Also, please note that most bacteria and fungi found in soil are beneficial plant symbionts, and only a few groups are pathogenic.