Edema (masses of plant cells often produced on the undersides of leaves) is common on plants when they are overwatered under conditions of high humidity.
The ergot pathogen produces survival structures called sclerotia that can germinate to form mushroom-like structures. These structures produce spores that can infect important grain crops like wheat, barley and rye.
Yellowing between the veins of oak leaves (called chlorosis) is typically caused by a lack of iron in trees that are grown in sites where the soil pH is over high. This is a common occurrence in Wisconsin.
Heterosporium is a fungal leaf pathogen that can cause leaf spots on iris and (in the example in this photo) on ash.
One symptom of an infection by odontoglossum ringspot virus on orchids can be necrotic (i.e., dead) streaks in flowers.
Many rust pathogens (such as the one that causes the common corn rust shown in this photo) produce masses of orange to rusty-brown spores that can be easily wiped from the leaf surface.
Powdery mildew is a common disease the leaves of many indoor and outdoor plants. On African violet, the disease more commonly occurs on flowers.
Flourine gas is often produced as a by-product of the manufacture of glass. It can build up to high concentrations in leaves and lead to tissue death.
Growth distortions and greening of flowers associated with an aster yellows phytoplasma infection of gladiolus.
Brooke Weber prepares soil for an alfalfa Aphanaomyces euteiches race test.