Trichomes (Plant Hairs)
Gail Ruhl, Sr. Plant Disease Diagnostician, Purdue University
Have you ever noticed the fuzzy growth on the underside of an oak or sycamore leaf and wondered what was wrong with the tree? Fuzzy mats of hairy growth on the underside of tree leaves are often mistaken for a plant disease or insect problem. In actuality, the whitish-tan fuzzy growth is a part of the plant known as trichomes. Wikipedia defines a trichome as a small hair or other outgrowth from the epidermis of a plant, typically unicellular and glandular. Trichomes may provide greater surface area, and create a sunlight- or wind-deflecting blanket. Thick mats of trichomes on leaves can actually help a plant control its temperature. A carpet of fuzz on a leaf's underside can reduce a plant's water loss through evaporation. When viewed with magnification, trichomes can be seen to come in many forms including straight, branched, star-shaped, and tufted.
For more information on Plant Hairs, see Backyard Nature
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Trichomes on oak leaf
Close up of trichomes
Trichomes on oak leaf. The round ‘shot-hole’ type symptoms are produced by the oak shothole leafminer