PINK ROT OF THE POTATO

SUMMARY. 1. A survey of the occurrence and importance of pink rot of the potato is given. The disease is widespread in Britain, but is only occasionally of serious economic importance. 2. The nature of pink rot is explained and it is shown how the disease may be caused by more than one pathogenic fungus. Phytophthora erythroseptica Pethybr. is the normal pathogene, but two other species of Phytophthora have been recorded as causing the disease in the field, viz. P. drechsleri Tucker in the U.S.A. and P. megasperma Drechsler in Northern Ireland. 3. The disease may be systemic in the potato plant, causing “misses” and “pink rot wilt” in the crop; normally, however, it is of importance only as a disease of the tuber. 4. Under conditions of high humidity and poor aeration the disease may cause heavy loss in an affected crop during storage. 5. Under normal conditions in the field the tuber becomes infected via the mother stolon; if conditions in the field or store are sufficiently moist, however, the tuber becomes infected directly, generally through the “eyes.” 6. Varietal susceptibility of the potato has been considered on the three bases: ( a ) artificial infection of the tuber with P. erythroseptica , ( b ) natural infection of the tuber when stored in contaminated soil, ( c ) susceptibility in the field when planted in contaminated soil. Altogether fifty‐one varieties (including nine mentioned as susceptible by Pethybridge and which were not tested) have been considered, all of which proved to be more or less suceptible. No variety was found to be particularly resistant. 7. Large numbers of oospores are produced by P. erythroseptica in the haulms and stolons and also to some extent in the roots of the dead potato plant. These oospores become disseminated and greatly increase the prevalence of the fungus in the soil. 8. It has been shown that, in the absence of the host plant, the fungus may persist indefinitely in the soil. This persistence is apparently principally due to the resting spores, but the fact that the fungus may exist to some extent as a saprophyte in the soil cannot be ruled out. 9. The disease may be satisfactorily controlled in practice by paying attention to the general principles of crop hygiene. Satisfactory crops can be grown in contaminated soil, provided that strongly sprouted tubers are planted and that soil drainage is sufficiently good to prevent the land from becoming unduly wet in times of heavy rainfall. Early harvesting of the crop tends to secure a minimum percentage of affected tubers at the time of digging. Prevention of the spread of the disease during storage may be obtained by ensuring dry and well aerated conditions in the store. The writers wish to express their very sincere thanks to Mr A. Murray, Culnafay, Toomebridge, County Antrim, who has given every facility for the carrying out of field experiments on his farm, and to the Potato Inspectors in Northern Ireland who assisted in recording occurrences of the disease.