Rusts

Rusts are fungal diseases caused by rust fungi (order Pucciniales) that affect plants worldwide. They are obligate parasites, often forming rust-colored pustules on leaves, stems, or fruits and causing chlorosis, necrosis, and sometimes defoliation. With thousands of described species, rusts attack grasses, trees, fruit crops, and ornamentals.

Many rusts have complex life cycles. Most are macrocyclic and heteroecious, requiring two hosts to complete development, though some are autoecious or demicyclic. A typical cycle may involve up to five spore stages: pycniospores, aeciospores, urediniospores, teliospores, and basidiospores. Teliospores overwinter and give rise to basidiospores that infect the alternate host.

Common examples include Puccinia graminis f. sp. tritici (wheat stem rust), Gymnosporangium spp. (cedar-apple rust), and Hemileia vastatrix (coffee leaf rust). Some rusts are historically important due to yield losses and trade disruption. The importance of rusts varies by region and crop.

Management emphasizes resistant cultivars, surveillance, removal of alternate hosts where feasible, crop rotation, and fungicide programs. Environmental conditions such as humidity and leaf wetness strongly influence infection and spread. Research continues to monitor races and deploy diverse resistance genes to slow adaptation by pathogens.