Plant Disease Symptoms and Signs: A Diagnostic Guide

Symptoms vs. Signs: The Key to Diagnosis

The most common mistake in plant diagnosis is confusing symptoms with signs. To successfully identify a pathogen, you must understand the distinction between these two concepts.

Symptoms

Symptoms are the plant's reaction to the pathogen. They are the internal or external morphological and physiological abnormalities expressed by the host. Because the host acts as a selective medium, the specific type of symptom can help narrow down the possible causes. Common categories of symptoms include:

• Galls: Abnormal growths or swellings, such as crown gall caused by bacteria or nematodes.
• Blights: Rapid progression of chlorosis (yellowing), necrosis (death), and total tissue death.
• Wilts: The drooping or loss of turgidity in plant parts, often caused by pathogens blocking water transport (e.g., Fusarium wilt).
• Cankers: Decayed or dead sunken areas on stems, branches, or trunks.
• Spots: Localized lesions on leaves or fruit, such as tan spots on wheat.
• Rots: Soft, decayed tissue, often seen in roots or fruit.

Signs

Signs are the actual structures or products of the pathogen itself. While symptoms tell you the plant is sick, signs often tell you who is making it sick. Signs are often microscopic, such as bacterial cells or fungal spores, but some can be seen with the naked eye.

Example: Strawberry Grey Mold

• Symptom: The fruit becomes soft, rotten, and mummified.
• Sign: A visible mass of grey powdery spores (Botrytis cinerea) coating the fruit.

Pathogen Types and Disease Classification

Once symptoms and signs are identified, pathologists use this information to classify the disease and identify the responsible pathogen. Plant diseases are often named based on their most prominent symptoms, such as "Citrus canker" or "Tar spot of maple".

The biological cause of the disease can vary widely across different kingdoms of life. While fungi are responsible for approximately 85% of all plant diseases, many other organisms are involved:

The Process of Disease Diagnosis

Effective diagnosis requires a systematic approach. Because "symptoms tell us little about a disease" on their own—for instance, many different pathogens can cause similar-looking spots—a pathologist must combine multiple pieces of evidence:

1. Observe Symptoms: Note the pattern of chlorosis, necrosis, or morphological changes.
2. Search for Signs: Use tools like hand lenses, stereoscopes, or compound microscopes to look for spores, bacterial ooze, or mycelium.
3. Analyze the Host: Consider the plant species and the environment, as the host's reaction is a key part of the diagnostic puzzle.

By integrating the observation of symptoms (the plant's response) with the detection of signs (the pathogen's presence), you can move from simply seeing a "sick plant" to identifying the specific etiology of the disease.