Which leaf features can help identify a plant disease?

Examining leaves is a primary step in diagnosing plant disease. Identifying specific features helps distinguish between infectious pathogens and environmental stressors.1

Key leaf features used in diagnosis include:

• Spots and Lesions: The color, size, shape, and margins of spots or blights are often indicative of specific fungi or bacteria. Bacterial infections often appear as dark, water-soaked spots, sometimes surrounded by a lighter-colored ring called a halo.1
• Color Changes: Yellowing, green and yellow mottling, and vein clearing are common symptoms, particularly in viral diseases.
• Deformations and Malformations: Leaves may appear curled, misshapen, or stunted. Viral infections can cause leaf distortion or reduced development between veins, while insect injury often manifests as holes, stippling, or downward curling.12
• Necrosis: Sudden death of tissue, such as leaf spots, ring spots, or streaks, indicates necrosis.2
• Signs of Pathogens: Beyond symptoms, diagnosticians look for "signs," such as visible mold growth or fungal fruiting bodies appearing as dark specks on the leaf surface.1

It is important to note that similar symptoms can arise from environmental factors, such as nutrient imbalances, chemical injury, or extreme weather, rather than a pathogen.1