Disease of the Month - Rust diseases (Horticulture Crops)

  Hort Snacks - June 2017
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 Causal Organisms: Gymnosporangium nelsonii (Saskatoon Berry/Juniper Rust); Gymnosporangium juniperii-virginianae (Cedar/Apple Rust); Gymnosporangium globosum (Cedar/Hawthorn Rust); Puccinia asparagi (Asparagus Rust); Puccinia sorghi (Common rust – Corn); Uromyces appendiculatus (Bean Rust); Uromyces fabae (Pea Rust); other pathogenic species also exist

Crops Affected: Saskatoon berry, juniper species (alternate host); also causes rust on pears and some other Rosaceous species; asparagus, corn, beans, peas, lettuce, etc.

Disease Cycle:

  • Typically 2 hosts required to complete entire sexual cycle (alternate host is not always required – e.g. Asparagus rust)
    • Cycles between species
    • May also increase as specific spore types on each host
    • Symptoms may vary somewhat between spore types and/or on different hosts
  • Tends to be a defoliating pathogen, reducing photosynthetic capability of the host and weakening the plant
  • Depending on when the pathogen arrives on the crop (e.g. corn), it can have a varying degree of impact
  • In SB/J or C/A Rust, galls on junipers or cedars (or related species) produce jelly-like orange-brown “horns” in wet springs, releasing spores
    • Typically in May-June
    • Spores can travel several kilometres to infect Saskatoon berry/Apple/Hawthorn plants
  • Leaves and fruit may be infected, causing characteristic swellings and growths
  • Spores produced on one host travel to complete the life cycle on the alternate host
  • Regardless of host species, rusts all produce some type of rusty/orange-coloured pustules and spores, although spores of other colours may be produced at different stages
  • Globular woody galls which produce jelly-like spore bodies (horns) after rain
Saskatoon berries/Apples/Hawthorn
  • Early symptoms include yellowish spots and swellings on leaves and fruit
  • Swellings grow to become firm spiky outgrowths from leaves and fruit
  • Twigs and branches may swell and be distorted
  • Orangey rusty powder evident on and around outgrowths
  • Pale green, stretched oval lesions form, typically near the base of the shoot, eventually turning cream to orange in colour
  • Black spores produced at later stages (for overwintering)
  • Orange to yellow-brown blisters appear on leaves, bursting to release rust-coloured spores
Beans / Peas (similar symptoms)
  • Initially, visible as very small, white raised spots surrounded by a yellow halo
    • Red-brown circular pustules are produced on leaves, as well as petioles and pods – may be surrounded by yellow halo
    • Black spores produced for later stages
Conditions Favouring Disease Development
  • Infection and spread favoured by temperatures between 10-24C with wet plant surfaces
  • Moist or rainy conditions can increase spore production and spread of infection
  • Tends to be less prevalent in drier and more arid climates
  • Avoid planting near native stands of evergreen hosts (Junipers/Cedars, etc.)
    • Remove junipers or prune out galls from infected evergreens within approximately 1-2 km of orchards – fairly impractical in areas where rust is prevalent
    • Removing volunteer or wild hosts near cultivated crops can reduce inoculum (e.g. asparagus)
  • Remove infected leaves and debris, to try and reduce inoculum
  • Ensure that there is good air circulation in crops
  • Irrigate early in the day, to encourage drying prior to nightfall
  • Tolerant cultivars may be available for some crops (e.g. asparagus, bean)
  • Plant early to time crop development to ensure crop is beyond economic impact stage when infection occurs (e.g. corn)
  • Rotations to non-host crops can be effective for the management of some rust diseases (e.g. bean) but not others (corn)
  • Apply registered protective controls during late May – mid June
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on May 29, 2017.