5.5.25
FUSARIUM SPP. ASSOCIATED WITH DRY ROT OF POTATO IN IRELAND

JW CHOISEULl and E BANNON2

lScottish Agricultural Science Agency, East Craigs, Edinburgh, EH12 8NJ, Scotland; 2Department of Environmental Resource Management, Faculty of Agriculture, University College Dublin, Dublin 4, Ireland

Background and objectives
There have been no reports on the Fusarium spp. which cause dry rot of the potato in Ireland since the work of Pethybridge and Lafferty in 1917 [1] who attributed the disease to F.coeruleum. A study was therefore commenced to investigate the Fusarium spp. currently associated with dry rot symptoms. The objectives of this study were to identify the Fusarium spp. causing the disease and to assess their resistance to thiabendazole (TBZ), which up to 1997 was the only fungicide approved for the control of dry rot in Ireland.

Materials and methods
Tuber samples were collected in 1993 and 1994 from the main potato growing regions of Donegal, Cork, Wicklow and Dublin/Meath. A sample consisted of ten tubers of a cultivar with lesions typical of dry rot. Tissue from the leading edge of each rot was placed onto potato dextrose agar (PDA). Four pieces were taken from each tuber and single spore colonies were produced for each Fusarium isolate. The cultures were then grown on SNA, a minimum nutrient medium, and PDA and identified using characteristics of colony morphology, conidia and conidiogenous cells.
The sensitivity of the isolates to TBZ was assessed by taking plugs (5 mm diam.) from 10 day old colonies and placing them in petri dishes containing 10 ml of malt extract agar to which 0, 5 or 100 mg of TBZ per litre agar had been added. Three replicates of three plates were prepared for each isolate. The colonies were incubated for 10 d. at room temperature after which their diameters were measured.

Results and conclusions
In 1993, 105 samples were collected from which 170 isolates were obtained. The most commonly isolated species was F. coeruleum (47%) followed by F. avenaceum (23%), F. sambucinum (12%) and F. culmorum (11%). Although fewer samples were collected in 1994 (70) the occurrence of the common species was similar. Occasional isolates of F. aquaeductuum var. medium, F. compactum, F. equiseti, F. merismoides, F. oxysporum, F. sporotrichioides and F. tricinctum were found. The frequency of the various species within each region differed slightly but generally they reflected the national average. It seems that the regional climate within Ireland does not differ sufficiently to favour one species more than another. Although cultivars are known to vary in their susceptibility to different Fusarium spp., the occurrence of particular species was not related to particular cultivars. This is probably because the cultivars which show extreme susceptibility or resistance to the different Fusariumspp are not grown commercially. Those species which were most frequently isolated ie F. coeruleum and F. avenaceum were not influenced by cultivar. Less frequently isolated species varied much more between cultivars but as these trends were not consistent between years it is most likely that the variation was due to small sample size.
Resistance to TBZ in F. sambucinum was first reported by Meijers [2] in The Netherlands. In this study, 84 and 71 Fusarium isolates, collected in 1993 and 1994 respectively, were assessed for their sensitivity to TBZ. All isolates of F. coeruleum, F. avenaceum and F. culmorum were sensitive to TBZ. In 1993, 15 of the 33 F. sambucinum isolates tested were resistant to the fungicide and 23 out of 33 in 1994. The resistant isolates were not totally insensitive to the fungicide and in both years, growth of the isolates at 5 mg TBZ was 66% of the unamended medium. The percentage of F. sambucinum isolates resistant to TBZ in the population varied from between the two years but an average of 65% of isolates were resistant. Resistance to TBZ in the Irish F. sambucinum population occurred despite the relative rarity of this species. Since this survey was completed the use of TBZ for dry rot control in Ireland has been discontinued. It would be interesting to re-examine the current importance of F. sambucinum as a cause of dry rot in Ireland.

References
1. Pethybridge GH, Lafferty HA, 1917. Economic Proceedings of the Royal Dublin Society 1, 547-558.
2. Meijers CP, 1986. Aardappelwereld, July, 13-15.