Rust infection on the top side of the leaf (right side of image) can be subtle while structures produced on the lower leaf surface (left side of image) or fruit are obvious.
Jay W. Pscheidt, 2019.
Rust on the fruit of this native Black Hawthorn.
Jay W. Pscheidt, 2008.
Foliar rust on Crataegus Crimson Cloud."
Eric Smith, 2010.
Flowers and fruit as well as leaves are susceptible to rust infection.
Jay W. Pscheidt, 2019.
Hair-like (tubular) spore-bearing structures (aecia) of quince rust.
Cause Tomato mosaic virus or tobacco mosaic virus, which also affects pepper, eggplant, petunia, and many other plants. Poor sanitation and handling infected plants are the primary means of spread.
Cause Physiological. Oedema (edema) is more prominent when ambient temperatures are lower than soil temperatures and soil moisture and relative humidity are high. Low transpiration rates along with a rise in water absorption increase cell pressure, erupting epidermal cells so that inner cells enlarge and protrude. That causes the inner cells to die and discolor (yellow, brown, or rust), giving the impression that a parasitic organism is involved.
Note what look like strange growths on the underside of the leaf.
OSU Extension Plant Pathology Slide Collection, 19
Cause Passalora fulva (syn. Fulvia fulva, formerly Cladosporium fulvum), a fungus.Spores resist drying and may survive in the greenhouse up to one year after plants are removed.This pathogen can also survive in soil as sclerotia and conidia.
