Fungi Causing Dry Tuber Rots of Seed Potatoes in Storage in Scotland

The occurrence, pathogenicity (1997 isolates only) and fungicide sensitivity of pathogens causing dry tuber rots in Scottish seed potatoes was investigated over three storage seasons between 1997 and 2000 in relation to region of production, cultivar, class and generation of seed potatoes, type of temperature control in store and earthiness of tubers. A total of 156 samples, each comprising up to ten rotted tubers, was received over the three seasons. The relative importance of each pathogen in causing rots was summarised by calculating a mean rot index that combined the prevalence of a pathogen in the samples with the incidence of tubers affected by the pathogen within those samples. Phoma foveata (gangrene) had the highest rot index, which was five times greater than for P. exigua and ten times greater than for P. eupyrena. This relationship was mirrored by the relative pathogenicities of P. foveata and P. eupyrena, as measured by size of rots developing at inoculated wounds in test tubers. Fusarium avenaceum appeared to be the greatest cause of Fusarium dry rots, having a rot index at least twice as great as that for F. solani var. coeruleum. Infection by F. sulphureum was relatively uncommon. In the pathogenicity test, F. avenaceum, F. solani var. coeruleum and F. sulphureum produced rots of similar depths and widths and larger than those of F. culmorum. The mean rot index for Cylindrocarpon spp. was slightly more than that for F. avenaceum, whereas C. destructans produced smaller rots in the tuber pathogenicity test. Region of production affected the prevalence of P. foveata and F. avenaceum, but only the occurrence of P. foveata was affected by class and generation of seed potatoes. Isolate sensitivity to thiabendazole and imazalil was examined in vitro over 2 years. Six out of seven isolates of F. sulphureum were resistant to thiabendazole, with the remaining isolate being partially resistant. The growth of two out of 34 isolates of F. avenaceum was inhibited by more than 50% only at 100 mg thiabendazole l⁻¹. All isolates, except those of F. avenaceum,, were inhibited in their growth by more than 50% at either 1 or 10 mg imazalil l⁻¹. However, the growth of 40% of isolates of F. avenaceum was inhibited only at 100 mg l⁻¹. The various changes in pathogen prevalence and isolate sensitivity to fungicides recorded in this study highlight the need for regular monitoring programmes to be conducted in order that disease-control strategies can remain effective.