The effect and transmission of one isolate of the rust Puccinia minussensis on five clones of Lactuca sibirica.

In a greenhouse experiment, one isolate of the systemic rust fungus Puccinia minussensis was applied to the host clone from which it was collected and to four other clones of the host Lactuca sibirica. The plants were grown in fertilized potting compost (N+) to promote growth and in peat (N-) to hamper growth, for three growing periods during one year. The results show that the expression of host plant resistance could not be determined visually, but there were differences in effects on the clones. The rust isolate was found to produce a significantly higher percentage of diseased shoots on clone A (the clone it was taken from). Furthermore, the rust also had the strongest effect on both biomass and shoot production on clone A compared to the other four clones. The data suggest that the rust isolate is highly adapted to the clone from which it originated. We suggest that selection in this system has not favoured a benign pathogen and that similar patterns are likely to occur for plants that (i) rarely establish by seeds; (ii) have strong lateral growth; and (iii) may persist for long periods once established.