Believe it or not, what you are seeing here is the same species of plant. The one on the left is the normal reproductive state of an Arabis mustard while the one on the right is the same species of mustard that has been infected by a rust fungus known as Puccinia monoica.
The interaction of these two species is interesting on so many levels. I spent an entire summer, along with my botanical colleagues, completely stumped as to what this strange orange-colored plant could be only to eventually find out that it was a mustard that has been hijacked! The fungus in question, Puccinia monoica, is part of a large complex of interrelated rust fungi who are quite fond of mustards. They utilize such an elaborate form of sexual reproduction.
The life cycle is as follows: Fungal spores land on a young mustard plant and begin to invade the host tissue. As they grow, they gain more and more nutrients from the mustard. Eventually the fungi effectively neuters its host and causes it to begin forming what are referred to as "pseudoflowers." The pseudoflowers are basically leaves that have been mutated by the fungus to look and smell a lot like other plants blooming in early summer. They produce a sticky, nectar-like substance that smells quite nice to pollinators. The mimicry even goes as far as to produce yellowish pigments that reflect UV light, making them an even more attractive target for passing insects. On each pseudoflower are hundreds of small cups known as spermatogonia. These house the sex cells of the fungus. The insect becomes covered in these sex cells, which it then transfers to other infected plants thus achieving sexual reproduction for Puccinia monoica.
Still with me?
At this point, the pseudoflowers stop producing color and nectar and instead, the fused sex cells germinate into hyphae that begin to form specialized structures called "aecia." The aceia house the spores that will be responsible for infecting their secondary host plants, which are grasses. The spores germinate and infect the grass. From there, structures called "uredia" are formed that go on to produce even more spores to infect even more grass. Eventually, structures called "telia" are formed on the grass and the cycle finally comes full circle. The telia produce the spores that will infect the original mustard host plants.
Whew! To have stumbled across an evolutionary drama such as this serves as a reminder of just how much in nature goes largely unnoticed every day.