Rust fungus belongs to the Basidiomycota group of fungi. (The rusts are a distinctive group of fungi which may have very complex life cycles involving five spore-forms within the same fungal species. When these different spore-forms occur on more than one host, e.g. blackcurrant rust (Cronartium ribicola), which attacks both blackcurrants and five-needle pines, the close planting of the two crops may give rise to high rust levels. For this reason, in some North American forested regions, blackcurrants may not be planted. Most horticultural rust species are found on only one host species.)
Damage. Carnation rust first appears as an indistinct yellowing of the leaf and stem, soon turning into an elongated raised brown spot which yields brown dust (spores) when rubbed (see Figure 15.9).
Life cycle and spread. The more common thin-walled spores (uredospores) are spread by wind currents and infect the leaf by way of the stomata in damp conditions. The less common thick-walled black spores (teleutospores) may survive and overwinter in the soil. The resistance of carnation cultivars varies, while related species, e.g. pinks or sweet williams, are rarely affected.
Control. Preventative control includes the use of rust-free cuttings, sterilization of border soils and careful maintenance of greenhouse ventilators to prevent damp patches occurring in the crop. Amateur and professional horticulturist are able to spray products containing a systemic ingredient, myclobutanil, and products containing the protectant mancozeb.
The occurrence of white rust (Puccinia horiana) on chrysanthemums has created serious problems for the horticultural industry and for gardeners, because of the ease with which the disease is carried in cuttings and its speed of increase and spread. Other common rusts are found on antirrhinum, hollyhock, rose and leek, and more recently European pear rust, with an alternate host of juniper, has become more common in the UK.
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