Plant fungus is a common pathogen that can quickly compromise the health of various species. Visually identifying these pathogens is the first and most practical step in managing plant health. Recognizing the distinct appearance of different fungal diseases allows for a targeted response. Fungi often leave behind physical evidence of their presence, which helps distinguish them from other plant issues.
General Visual Indicators of Fungal Activity
Before specific fungal signs appear, plants often exhibit general symptoms signaling a major health issue. One of the earliest signs is chlorosis, where leaves develop a mottled or patchy yellowing. This discoloration occurs because the fungus interferes with the plant’s ability to produce chlorophyll, which is necessary for photosynthesis.
Another common symptom is unusual wilting, even when the soil contains adequate moisture. This happens when the fungus attacks the plant’s vascular system or roots, preventing water uptake and translocation. Chronic fungal infections also lead to stunted growth or a general decline in vigor. These broad indicators prompt a closer inspection to differentiate a fungal infection from issues like insect damage or nutrient deficiency.
Fungi That Appear as Surface Coatings or Dust
Some fungal diseases manifest as distinct coatings directly on the plant’s surface. Powdery mildew is frequent, characterized by white or grayish patches that look like talcum powder dusted onto the upper surface of leaves, stems, and buds. This appearance results from the fungus growing a network of thread-like structures called mycelium and producing countless spores on the host’s exterior. Powdery mildew generally prefers moderate temperatures and dry leaf surfaces, often thriving in high humidity.
Downy mildew is visually similar but distinct, appearing as a fuzzy or felt-like growth with a grayish-white to purplish tint. Unlike powdery mildew, downy mildew develops almost exclusively on the underside of the leaves. The corresponding upper leaf surface shows pale green or bright yellow, angular spots. The pathogen causing this disease is technically an oomycete, or water mold, which requires cool temperatures and the presence of free water on the leaf surface to initiate infection.
Sooty mold presents as a superficial black, charcoal-like film covering leaves and stems. This mold is not parasitic to the plant tissue but grows on honeydew, a sticky, sugary substance excreted by sap-sucking insects like aphids or scale. A heavy coating blocks sunlight, reducing the plant’s ability to photosynthesize and indirectly leading to yellowing and decline. The presence of this dark coating strongly indicates a secondary insect problem that needs to be addressed.
Fungi That Cause Defined Spots and Lesions
Fungi can cause localized necrosis, resulting in clearly defined areas of dead tissue. Fungal leaf spots are common, appearing as small, circular lesions that are often necrotic (brown or black). They sometimes feature a dark, raised border with a lighter-colored center. Their distinct margins help distinguish them from irregular discoloration caused by environmental stress. If the infection is severe, these individual spots can merge, creating larger, irregular blotches that cause the entire leaf to die and drop prematurely.
Rust fungi are named for their characteristic visual sign: the formation of pustules resembling specks of rust-colored dust. These blister-like structures, which can be yellow, orange, brown, or black, are masses of spores that easily rub off when touched. Rust pustules are most frequently found on the underside of leaves, though corresponding pale spots may be visible on the upper surface. The color of the spore mass is a direct sign of the specific fungal species causing the infection, with the orange or reddish-brown hue being the most common.
Anthracnose is a term for diseases that produce irregular, dark, and often sunken lesions on leaves, stems, flowers, and fruits. On leaves, it appears as expanding blotches that follow the leaf veins, giving a scorched or blighted appearance. When affecting fruits and stems, the fungus creates characteristic sunken cankers, which are often dark brown to black. This infection is destructive to new growth, causing young leaves to become curled and distorted.
Fungi Affecting Plant Structure and Roots
Some dangerous fungal infections occur below the soil line or attack the plant’s main structural components, often remaining hidden until the plant is significantly compromised. Root rot, caused by various soil-borne fungi, becomes apparent above ground through severe wilting and yellowing, even when the soil is moist. Inspecting the roots reveals the true problem: healthy white and firm roots are replaced by soft, mushy, dark brown or black tissue that sloughs off easily. The root’s inability to absorb water and nutrients leads to above-ground decline, mimicking drought stress symptoms.
Fungi that cause stem blights and cankers create localized, dead, and often sunken areas on the main stem or branches. A canker is a necrotic lesion on woody tissue that can girdle the stem, effectively cutting off the flow of water and nutrients to the plant parts above the lesion. These sunken areas may be discolored and sometimes show signs of fungal structures. Examples include a white, cottony mycelium or small, dark, hardened masses called sclerotia. The result is a sudden dieback or collapse of the branch or upper portion of the plant, which indicates that the structural integrity has been compromised.