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| Authors: | W. Rademaker, J. Jong |
Abstract:
Four types of interaction between host and pathogen are described.
The inheritance of resistance is reported as well as the use of resistance in chrysanthemum breeding.
- Complete resistance without any macroscopic symptoms is controlled by a single dominant gene.
This type of resistance could be similar to early hypersensitivity in cereals and, analogous to what has happened with cereal rusts, may be broken down by new virulence genes in the pathogen.
Since 1975, however, when screening for resistance started, no evidence for break down of resistance has been documented in Holland.
- Incomplete resistance, whereby some small colonies develop under heavy infection pressure, is probably based on several genes.
Sporulation is delayed but not prevented.
Under 'commercial' conditions this type of resistance offers sufficient protection.
Again, analogous to cereals, this type of resistance is expected to be stable.
- Resistance with visible necrosis of the host (late hypersensitivity) is found in a number of cultivars.
Upon infection, necrotic spots develop on the leaves which hinder the growth of the pathogen.
Sporulation may not be completely prevented.
It is presently studied whether greenhouse environment affects the severity of the necrosis and the degree of sporulation.
The progeny of late hypersensitive cultivars is mainly susceptible.
- Susceptible cultivars may show large or small pustules with abundant sporulation.
The progeny is fully susceptible.
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