AK LEES, JE BRADSHAW, M DE MAINE and HE STEWART

Scottish Crop Research Institute, Invergowne, Dundee DD2 5DA, UK

Background and objectives
Soft rot caused by Erwinia carotovora subsp. atroseptica (Eca) and dry rot caused by Fusarium coeruleum (F.c.) and Fusarium sulphureum (F.s.), result in significant post-harvest losses in stored potato tubers. No effective chemical control is available for soft rot, and Fusarium isolates are commonly insensitive to Thiabendazole, the fungicide used widely to control dry rot. The principal cultivated potato S. tuberosum subsp. tuberosum is derived from a narrow genetic base and as a consequence, lacks genes for adequate resistance to a number of pathogens. Two approaches were taken in an attempt to locate sources of resistance to soft rot and dry rot.

Resistance to soft rot has been found in wild and other cultivated Solanum species but, even where such species hybridise with subsp. tuberosum, poor adaptation is a problem with regard to their use in European conditions [1]. This study details the assessment of long-day adapted S. phureja for soft rot resistance in order to identify parents which will allow the rapid introduction of resistance into commercially acceptable tetraploid material. In the case of dry rot, long-day adapted Neotuberosum clones derived from andigena were examined. The assessment of a sample of SCRI elite Neotuberosum clones for new sources of resistance to F.c. and F.s., and of progenies derived from crossing selected clones to a range of Tuberosum cultivars and breeding lines is described.

Materials and methods
Two long-day adapted S. phureja clones, one with high and one with intermediate resistance to soft rot were hybridised. Soft rot resistance of 173 progeny was assessed in 1996. Tubers were inoculated under vacuum, with 4 x 10⁶ c.f.u./ml Eca and incubated at 24^oC, 95% relative humidity for 5 days before symptom assessment. The test was repeated 2 weeks later. Twelve clones with good resistance and other favourable traits were selected as potential parents for the introgression of soft rot resistance into S. tuberosum, their resistance was confirmed in 1997.

One hundred and thirteen clones (84 Neotuberosum and 29 hybrids) were assessed for dry-rot resistance. Inoculum of F.s. and F.c. was prepared in sand-cornmeal medium incubated for 4 weeks at room temperature. Tubers were inoculated with either F.c. or F.s., incubated at 85-90% RH at 10^oC for 45 days and the resulting lesion measured. A crossing programme was carried out in 1994/5 comprising eight Neotuberosum parents chosen for their resistance to F.c. and/or F.s. and for putative resistance to late blight. The other set of parents comprised eight new cultivars from SCRI and two clones with good blight and PCN resistance1 respectively. A dry rot progeny test was carried out on 2 replicates of each of the resulting 72 progenies using the above inoculation method.

Results and conclusions
Of 173 offspring of the cross between the two S. phureja clones, over fifty per cent were highly resistant to Eca. Twelve clones were selected and their resistance confirmed. This study illustrates the value of S. phureja as a potentially valuable source of resistance that is uncommon in tuberosum cultivars. Introgression of characters from non-tuberosum germplasm into commercially acceptable material should be more rapid, using long-day, rather than short-day adapted cultivated clones and wild species.

Neotuberosum clones with differing levels of resistance to F.c. and F.s. were selected and used in crosses with tuberosum clones. Assessment of the resulting progenies showed that there was little correlation between disease scores for the two Fusarium species, indicating that resistance to each species is distinct. It is concluded that there are good prospects for combining resistance to the two Fusarium species from different sources and also for achieving high levels of other desirable characteristics.

References
1. Rousselle-Bourgeois F, Priou S, 1995. Potato Research 38, 111-118.