FIELD EXPERIMENTS ON POTATO SKIN‐SPOT DISEASE CAUSED BY OOSPORA PUSTULANS OWEN & WAKEF

In experiments on skin‐spot disease of potatoes, caused by Oospora pustulans Owen & Wakef., Kerr's Pink proved the most susceptible of twenty‐four commercial varieties; Arran Banner, Majestic and King Edward were highly susceptible; Home Guard and Golden Wonder were highly resistant. Thymol and tetrachlornitrobenzene, applied as dusts to pits of potatoes at lifting, did not give satisfactory control of the disease. The efficacy of an organo‐mercury dip treatment at lifting time was confirmed. The incidence of the disease was considerably decreased by storing tubers in boxes instead of in pits, by digging the crop about a month before normal harvest, or by cutting the haulms at this time and digging at the normal time. Ware tubers showed a significantly higher infection than seed tubers of the same crop, indicating that susceptibility of tubers increases towards maturity. The difference in eye infection between varieties may not be related to that of general superficial infection. It is suggested that, in future, the assessment of varietal susceptibility should be based upon the degree of eye infection, which is in practice the measure of the economic importance of the disease. Tubers having pustules at or near all the eyes gave markedly reduced sprout emergence and plant stand.