At the right you see a much-enlarged picture of a fungal rust looking like orange-yellow... well, rust... on the undersides of some Oxalis leaflets. About 2000 species of rust are known and all are parasitic on flowering plants or ferns. No other group of fungi is as dangerous to agricultural and horticultural crops. The rust in the photo is Puccinia oxalidis. The spore-producing "fruiting bodies" of this rust are tiny black specks you can barely see scattered here and there across the leaflets.
The life histories of rusts often are mind-bogglingly complex, often with the fungus spending part of its life on one kind of plant, then another part on a completely different kind of plant. For example, let's look at the life cycle of the Cedar-apple Rust, Gymnosporangium juniperi-virginianae.
Near my trailer there's an Eastern Redcedar tree, Juniperus virginiana. All winter some brown, bumpy, woody, tumor-like galls about the size of golf balls were visible growing on the branches. In early spring the bumps on the thing developed the slender, sharp "horns" you see emerging at the left. When rains came, the horns enlarged to form the orange, gelatinous masses seen below and to the right.
When those gelatinous masses dry out they will disintegrate into a huge number of spores. Wind will carry the spores to an apple or crabapple tree where they will infect newly emerging growth. Inside growing apple and crabapple leaves two strains of the fungus mate and produce a new kind of spore. This process causes a disease on the apple or crabapple tree; spots appear on the leaves. Eventually many leaves may fall off too early weakening the tree and fruit may be damaged.Spores produced in the leafspots are then blown back to redcedars in a time period from mid summer to fall, galls on the redcedar trees slowly develop over a period of about 17 months, and then the whole cycle begins again when spring rain results in the orange "horns" seen above. Wind-borne spread of spores between redcedars and apple and crabapples of several hundred yards is not unusual, and can take place over miles.
Right behind my trailer there's a young Loblolly Pine about 20 feet tall. The image at the right shows the swollen, blistered item appearing on the trunk at about the 10-foot level. It's about 2.5 inches thick (6 cm). This is Fusiform Rust, Cronartium quercuum f. sp. fusiforme. When you knock against the trunk, a cloud of orange powder is released from the blisters. This powder is composed of tiny aeciospores. These aeciospores land on oak leaves and later in spring pustules known as uredia will appear on the oak leaves' undersurfaces. During late spring or early summer, brown, hairlike structures called telia will form on the oak leaves. These telia will produce teliospores which will germinate into basidiospores, which will infect a pine and cause the infection shown at the left. This is a serious disease for Loblolly and Slash Pines in the US Southeast. Infections that occur on the main stem within the first 5 years of a tree's life normally cause tree death. My Loblolly Pine is about five years old, so it will probably die. Infections that occur on older trees weaken stems and trunks, resulting in wind breakage at the canker.