SCANNING ELECTRON MICROSCOPY OF EARLY INFECTION OF DYERS WOAD, ISATIS TINCTORIA BY GERMLINGS OF PUCCINIA THLASPEOS

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Michael T. Binns and Dr. Gary R. Hooper, Agronomy and Horticulture

Dyers Woad (Isatis tinctoria) is an introduced noxious weed of crop and rangeland in northern Utah. A native rust fungus (tentatively identified as Puccinia thlaspeos) is being investigated as a possible biological control agent of the weed. In order to effectively use the rust in a biocontrol program, it is necessary to understand its epidemiology and characteristics.

Weeds were inoculated by suspending previously infected weed leaf fragments over test plants. Teliospores on these fragments produced basidiospores which dropped onto the weed seedling leaf surfaces. Inoculated leaves were prepared for SEM by fixation in 2% cacodylate buffered glutaraldehyde, post fixed in 2% buffered osmium tetroxide and dehydrated through ethyl alcohol and critical point dried. Mounted specimens were sputter-coated with gold. Some tissues were freeze fractured in 100% alcohol and fragments were critical point dried, coated and mounted for SEM. 1

Basidiospores are roughly ovoid, 9-10 Jim in diameter with a prominent hilum, or attachment scar. Spores are firmly adhered to the epidermal surface and germinate by a germ tube varying in length from 2 to 14 VLm. A slight swelling at the end of the tube demarks the appressorium-below which the fungus penetrates the cell. The entire rust germling (spore, germ tube and appressorium) adheres tightly to the epidermis and appears to either dissolve cuticular waxes or mechanically disrupts them. Shortly after spore germination, and movement of its contents into the germ tube, the original spore collapses.

Unlike many species of fungi, this rust does not enter the plant through stomata, nor penetrate the leaf between cells. All observed penetration was directly through cell walls ven when the germ tube was very near stomatal openings. Once inside the leaf, the fungal hyphae ramifies between cells throughout the leaf. Puccinia thlaspeos is similar to some other microcyclic rusts in this behavior. 2 Even though this fungus becomes systemic it does not penetrate leaf veins nor move through xylem vessles or tracheids as a pathway through the plant.

References

  1. O’Donnell, K.C. and G.R. Hooper. 1977. Micologia 69: 309-320.
  2. Gold, Randall E., and Kurt Mendgen. 1991. pp. 67-99. In: The Fungal Spore and Disease Initiation in Plants and Animals. Plenum 2
    Press, New York.