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Publication Title | Phragmidium violaceaum has all spore forms, but Puccinia xanthii has only teliospores and basidiospores

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Text | Phragmidium violaceaum has all spore forms, but Puccinia xanthii has only teliospores and basidiospores | 001



Phragmidium violaceaum has all spore forms, but Puccinia xanthii has only teliospores and basidiospores.

While some rusts do have a full life-cycle, all stages may not be present in the field. Only the uredinial and telial stages of Puccinia abrupta var. partheniicola have been found in the field in Mexico. While the teliospores are functional, germination has only been observed in the laboratory after dormancy had been broken by chemical treatment. The rust appears to cycle in the field by the urediniospore stage only. These spores have the ability to remain dormant over winter while retaining their viability.

Pathogenicitytesting

Pathogenicity tests are aimed at selecting the most effective strain of the pathogen for the biotype(s) of the target weed and the environmental conditions under which the weed grows. Specimens of the target weed, grown from seed collected from the target area, are inoculated under the target environmental conditions with the strains of the candidate pathogen that have been collected from different parts of its native range. Pathogenicity is then assessed from symptoms exhibited by the target weed and microscopic examination as for host specificity testing.

Host specificity

As for insects (Heard this volume), preliminary inform- ation about the host specificity of a potential agent pathogen in its country of origin might be obtained by searching the literature, particularly crop protection literature, checking herbarium collections and records, and checking with workers in agriculture to determine whether the pathogen is a known pest of crops.

In the absence of other preliminary information, host range tests against a small selection of plants, usually closely related to the target weed, should be done in the country of origin. This will determine whether

further, more detailed, testing is warranted.

Supply of test plants

Since testing for Australian weeds is carried out overseas, it is necessary to provide the laboratory carrying out the work with test plants. Where possible test plant seed is sent to the country where the testing is to be carried out. This is the easiest and cheapest method as there are usually fewer problems with quarantine requirements. However, in some cases it is necessary to grow the plants before despatch to the testing agency. In this case, the plants must be able to withstand reasonably rough handling, including being turned upside down, and exposure to extremes of temperature. A packaging method for sending plants overseas which has proven to be reliable involves sealing each plant in its own container which is lined with absorbent paper to prevent sweating and breakdown of the foliage in transit, and tightly packing the individual containers into a polystyrene box which is protected by an outer layer of strawboard.

Methodology

Host specificity testing involves assessing the response to infection by the test plants at the cellular level. Plants are inoculated in batches of four or five species along with the target weed as a control. Inoculation of each test species should be replicated three or four times. Each replicate is valid only if infection of the control plant is normal. The plants should be inoculated and incubated under ideal conditions for the development of the particular pathogen, and maintained for twice the length of the latent period for the pathogen on its natural host to allow complete development, eg the production of urediniospores. Pathogen development should be observed both macroscopically and microscopically. In the latter case, sample sections of the leaves are examined using techniques such as whole leaf clearing and staining technique (Bruuese and Hasan 1983), and scanning

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Image | Phragmidium violaceaum has all spore forms, but Puccinia xanthii has only teliospores and basidiospores



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