Figure 1. Diagram of spore germ tube that has formed an appressorium. Abbreviations: gt, germ tube; a, appressorium; ip, infection peg; h, haustorium. Reprinted with permission from Schumann GL (1991) Plant Diseases: Their Biology and Social Impact. St Paul, MN: APS Press. Copyright © 1991 The American Phytopathological Society.

Figure 2. Scanning electron micrographs of bean leaves infected by the rust fungus Uromyces phaseoli. a, Stoma on the leaf surface surrounded by epidermal cells. b and c, spores (with spines), germ tubes and appressoria forming over stomata on the leaf surface. Penetration peg (not seen) grows from the underside of the appressorium through the stomatal opening to gain access to the interior of the leaf. Reprinted with permission from Schumann GL (1991) Plant Diseases: Their Biology and Social Impact. St Paul, MN: APS Press. Copyright © 1991 The American Phytopathological Society.

Figure 3. Disease cycle of the stem rust fungus, Puccina graminis f.sp. tritici. a. The haploid basidiospores produced from teliospores on wheat stems infect leaves of barberry bushes. b, Infection of barberry produces spermagonia that contain both receptive hyphae and spermatia. c, When spermatia and receptive hyphae of different mating types fuse, the nucleus from the spermatium migrates into the receptive hyphae forming a dikaryotic mycelium. d, The dikaryotic hyphae produce fruiting bodies termed aecial cups on the lower surfaces of barberry leaves. The aecial cups produce dikaryotic aeciospores that are only able to infect wheat plants (they cannot reinfect barberry plants). e, Infected wheat stems produce uredial pustules that are filled with uredospores, able to reinfect wheat. This repeating stage of the rust disease cycle enables the fungus to produce large numbers of uredospores in a short period of time, resulting in disease epidemics under optimum conditions. g, Later in the growing season, another fruiting body, the telium, is formed by the dikaryotic hyphae in which teliospores are produced. Within the teliospores, the two nuclei of opposite mating types fuse (karyogamy) producing a diploid nucleus. This is followed shortly by meiosis after which the four products of the meiotic division migrate into four newly formed monokaryotic basidiospores, starting the disease cycle again. Reprinted with permission from Schumann GL (1991) Plant Diseases: Their Biology and Social Impact. St Paul, MN: APS Press. Copyright © 1991 The American Phytopathological Society.