Yellow patches are formed on the large leaves of Arisarum by a species of parasitic Alga, Phyllosiphon, which lives in the mesophyll. Many tropical leaves are spotted yellow by epiphytic Algae—e.g. Cephaleuros.
It must be noticed that many fungi produce yellow spots or flecks in the earlier stages, which turn brown or black as the fructifications appear, e.g. Dilophia graminis, Rhytisma acerinum.
The yellow-spotted leaves of Farfugium grande (Senecio Kaempferi) are so like those of Petasites attacked with Aecidium in its early stages, that an expert might be deceived until the microscopic analysis was completed.
Red spots, varying from rusty or foxy red to bright crimson, are the symptomatic accompaniment of several fungi, the former often characterising the teleutospore or aecidium stage of Uredineae—e.g. Aecidium Grossulariae—the latter sometimes indicating the presence of Chytridiaceae.
Red spots are also caused by Gloeosporium Fragariae on Strawberry leaves, Polystigma rubrum on Plums.
Crimson spots on Apple and Pear leaves are also due to Phytoptus: they turn brown later.
Brown spots or flecks, varying in hue from dull slaty brown to deep red browns, are a common symptom of Fungus and Insect diseases, the colour often indicating the death of the tissues, rather than any special peculiarity of the action of the parasite. Good examples are furnished by the Potato-disease, and by Peronospora viticola, Sphaerella vitis and other disease-fungi of the Grape Vine. The teleutospore stage of many Uredineae also occurs in deep brown spots.
Black spots and flecks are exceedingly common symptoms of the presence of fungi, e.g. Fusicladium on Apples and Pears, and the pycnidial and ascus stages of many Ascomycetes—e.g. Phyllachora graminis. The teleutospore stages of species of Puccinia, Phragmidium, etc., are also so deep in colour as to appear almost black.
Scab on Pears is due to the presence of Fusicladium, which indurates the outer skin of the fruit causing it to crack under pressure from within, and to dry up, the deep brown to black patches of fungus persisting on the dead surface.
Black spots on grasses and sedges are caused by Ustilagineae, and are commonest in the grain, the soot-like powdery spores (Smut) being very characteristic. Ustilago longissima induces black streaks on the leaves. Many of these fungi cause distortions or pustules on leaves and other organs.
