First report in England of changes in the susceptibility of Puccinia horiana, the cause of chrysanthemum white rust, to triazole and strobilurin fungicides

This is a first report of Puccinia horiana strains from England tolerant either to triazole or to both triazole and strobilurin fungicides. Elsewhere in Europe there is tolerance to carboxamide fungicides (Dirkse et al, 1982). Increasing tolerance to fungicides can undermine phytosanitary control measures against this EC II/AII listed organism.

For two decades, propiconazole controlled chrysanthemum white rust (CWR) caused by Puccinia horiana on Dendranthemum morifolium (Dickens, 1991). However, from March 1999 reports began to accumulate of chrysanthemum growers in the Midlands failing to control the disease on various cultivars, using propiconazole and another triazole, myclobutanil.

In September 1999 plants of cv White Fresco bearing pustules suspected of being fungicide tolerant were collected from a Manchester grower. A method to assess fungicides against CWR (Dickens, 1991) was adapted to confirm tolerance. This involved placing the suspect plants above rooted cuttings of susceptible cv Margaret for 3 days in a damp chamber, minimum temperature 20° C, to allow natural infection via sporidia. An isolate of CWR from cv Wendy, collected from a nursery in Clwyd, Wales, not exposed to fungicides, was used as a sensitive control. Curative sprays of propiconazole, myclobutanil or the strobilurin azoxystrobin, at 0, 100, 200 or 500 mg a.i./l were applied to run off with a hand sprayer to the latently-infected plants (at least three plants per cv per treatment).

Both triazoles failed to prevent the suspect tolerant isolate from developing pustules even when applied at 500 mg a.i./l (five times maximum permitted concentration). This contrasted with azoxystrobin where suppression was partial at 100 mg a.i./l and total at 500 mg a.i./l (twice maximum permitted concentration). On the other hand, both triazoles and azoxystrobin were still effective against the Welsh sensitive strain. Here suppression was either partial or total at 100 mg a.i./l (depending on inoculum level) and total at 500 mg a.i./l. Tests repeated three times gave similar results. Control of CWR with azoxystrobin is documented (O'Neill & Pye, 1997), the effect being curative (Wojdyla & Orlikowski, 1999). Triazole tolerance was again detected in July 2000 on cv Bradford in West Sussex.

In August 2000, growers in Cambridgeshire and West Sussex, failed to control CWR with both triazole and azoxystrobin and the presence of a doubly tolerant strain was suspected. The affected cultivars included White Fresco, White Reagan and Dark Rosy Reagan. Tests of the suspect isolates on the susceptible cvs Margaret, Tattoo Time, White Fresco, White Reagan and Dark Rosy Reagan confirmed the isolates were tolerant to curative sprays of both fungicides.

Finally, the tests were redesigned to measure tolerance to the fungicides applied as protectant sprays. The tests were performed as before except that the fungicides were applied 24 h before, (rather than after), inoculation of susceptible plants with the doubly tolerant strains. In three tests, the strains were confirmed tolerant to protectant sprays of propiconazole at 100 mg a.i./l and strobilurin at 100 and 500 mg a.i./l. The Welsh isolate remained sensitive to these protectant sprays.

Further cases of tolerance to these fungicides are suspected in England and CWR control in this commercially important crop could become increasingly difficult (Ruth Finlay, David Slawson, pers. comm.).