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Written by: Carl-Adam Wegenschimmel

One of the greatest adversaries to garden and wild plants is the great host of pathogens that regularly attack them. These organisms can belong to a variety of groups, including; fungi, bacteria, nematodes, and viruses. As human beings, we often consider the economic costs these organisms have on our economy, particularly in the agricultural, garden, and forest industries. However, pathogens also play natural roles in our ecosystems, killing sick plants and controlling the population growth of certain species that could otherwise dominate a community.

With the growing concern and insight into climate change, understanding how these understudied groups may affect plants and ecosystems is becoming increasingly important. One of the most noticeable of these groups is fungi!

Fungi attack their plant hosts in a variety of ways, some may first kill their hosts and feed on dead material (necrotrophs), which enter their hosts through wounds and natural openings. Other fungi feed on living tissue (biotrophs) which often enter their hosts in more specialized ways (Doehlemon et al. 2017).

Necrotrophs can sometimes be very destructive, especially when they are invasive species. A well-known example is Dutch Elm Disease (Ophiostoma novo-ulmi), which has severely reduced Elm tree abundance in North America. In this case, the fungus attacks trees with the aid of insects like the Native Elm Bark Beetle (Hylurgopinus rufipes) and the introduced European Elm Bark Beetle (Scolytus multistriatus). Dutch Elm Disease is believed to have originally been introduced from Asia, and so our native Elm trees have evolved little resistance to the fungus (Hubbes 1999). American Elm (Ulmus americana) and Rock Elm (Ulmus thomasii) have suffered the worst with Red Elm (Ulmus rubra) being slightly more resistant. The disease is spread to Elm trees when the beetles feed on twigs in spring time entering and slowly spreading into the trunk of the trees, blocking vascular tissues and eventually killing the host. The beetles are attracted to the diseased elms for breeding and subsequently bore holes into the infected Elms. Eggs are laid inside infected Elms where newly hatching beetles pick up spores and continue the cycle. 

Biotrophic fungi require living hosts in order to feed and have evolved specifically to interact with a living organism rather than a dead one. One of the most visible groups of these plant parasites are the rust fungi, which is one of the largest orders of fungi containing more than 8000 species worldwide (Lorrain et al. 2018). Some rusts cause little damage to their hosts whereas other species are better referred to as hemibiotrophs, which start off as seemingly benign biotrophs but eventually kill their host and act as necrotophic fungi (Koeck et al. 2011).

Some hemibiotrophic rusts are known to cause devastating damage to crops. Other species of rusts are rarely seen but have complex lifestyles like Chrysomyxa pyrolae seen here (right) on American Pyrola (Pyrola americana), which cycles between its Pyrola and Spruce (Picea spp.) hosts. Although this species does not necessarily kill its hosts, it has been observed to negatively affect seed crop in spruce trees (Sutherland et al, 2011).