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Rusts of Horticultural and Agricultural Plants

June 1, 2005

This article is intended to familiarize you with the rust diseases that do or may occur in Illinois on a wide range of horticultural and agricultural plants. Some of the rusts addressed are established in Illinois and quite common; others are rare, not established, or of regulatory concern. The occurrence of each rust disease listed in the table represents the collective experience of this author, Nancy Pataky (Director, University of Illinois Plant Clinic), Dean Malvick and Mohammad Babadoost (Extension Specialists, University of Illinois), and Ian Thompson (Curator, Purdue University Arthur [Rust] Herbarium). Rather than providing detailed symptom and biology descriptions for each rust disease mentioned, Table 1 provides links to specific Internet resources. This document is considered a work in progress, and we welcome any suggestions you may have for improvements.

To many, “the rusts” are among most interesting and complicated plant diseases. The following points summarize the main sources of intrigue and confusion:

  • Nearly all rust fungi are obligate parasites, which means they can not be grown apart from a living host. Thus, conducting research with rust pathogens is relatively difficult.
  • Rust fungi may produce as many as five different stages in their life cycles: — Stage 0: Spermogonia bearing spermatia (sexual recombination occurs here) — Stage I: Aecia bearing aeciospores — Stage II: Uredinia bearing urediniospores (summer, or repeating spores) — Stage III: Telia bearing teliospores — Stage IV: Basidia bearing basidiospores
  • For some rust fungi, all five stages of the life cycle have been observed (macrocyclic); however, other rust fungi lack one or more stages and may be referred to as microcyclic (which produce only teliospores and basidiospores), demicyclic (which do not produce urediniospores), or endocyclic (which produce only spermatia and aeciospores).
  • Some rust fungi (such as hollyhock rust and pine-pine gall rust) are autoecious, meaning they can survive and complete their life cycle on a single host species.
  • Some rust fungi (such as cedar-apple rust) are heteroecious, meaning they can only complete their life cycle and survive by alternating between two very different host species. In these cases, the signs and symptoms on the two plant species can be radically different, causing observers to wrongly conclude that there are two distinctly different pathogens at work.
  • Some heteroecious rust fungi (such as common rust of corn and wheat leaf rust) appear in Illinois each year even though the alternate host(s) is absent or rarely infected in our region. These rusts don’t overwinter in our northern climate but arrive as wind-blown (or via infected plants) urediniospores from southern regions of the United States.

In terms of managing rusts, it is important to know how and when the pathogen arrives and whether or not it produces a repeating/summer spore (urediniospore) stage. For example, the cedar-apple rust pathogen does not produce urediniospores. This means that the springtime infection of crabapple will not lead to later reinfections of the same plant. By the time you notice rust lesions on crabapple, it is too late to apply a fungicide to the crabapple because these lesions produce only aeciospores that infect juniper.

For a more detailed explanation of the rust fungi, technical terms used, and their history, consider visiting the following Web sites: www.cals.ncsu.edu/course/pp318/intros/rusts/rusts.htm, www.botany.hawaii.edu/faculty/wong/BOT135/Lect08.htm, and www.apsnet.org/education/IllustratedGlossary.

Author: Bruce Paulsrud


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