The Susceptibility Of Certain Potato Varieties To Leaf‐Roll And Mosaic Infection

Abstract

Summary.1. A trial planned to ascertain whether any differences in susceptibility to leaf‐roll and mosaic infection existed in seven varieties—two of which had been included in a revious trial—was laid down in 1929. It consisted of four replicated plots for each disease; the varieties being randomised in the plots to secure uniform chances of infection. Sources of infection were provided by drills of diseased plants alternating with varieties under test.2. Mosaic was generally transmitted but with such faint symptoms that percentage infection could not be determined. The apparent loss of vigour was very slight, but losses ascertained by weighing the crop, against that given by normal plants, ranged from 0·8 to 24·4 per cent.3. Leaf‐roll infection was very heavy; most varieties showing from 90 to 100 per cent. infection of the plants. Tuber infection on the infected plants was also heavier than in 1924, and ranged from 77·3 to 99·9 per cent. Both haulm (or plant) infection and tuber infection thus showed a marked dependence upon seasonal factors which affect the breeding of insect vectors.4. The loss in yield due to leaf‐roll was found to be the most reliable guide to the susceptibility of a variety. This loss cannot be estimated from the degree of stunting or the number of rolled leaves on a plant, but only by weighing and grading the crop. Some evidence is presented to show that loss in yield of an infected plant is less dependent upon seasonal factors than percentage haulm or tuber infection, and that, therefore, figures showing such losses have a more general applicability to other localities and seasons. On this assumption a table of susceptibility of fifteen varieties has been constructed.5. Although the variety Field Marshal suffered little from leaf‐roll infection, heavy losses occurred when mixed leaf‐roll and mosaic viruses entered the plant. In large tubers receiving mixed infection the effect was to produce neither leaf‐roll nor mosaic symptoms, but “crinkle.” The small tubers, however, gave apparently pure leaf‐roll.6. The value to be attached to field trials of this kind, and their applicability to other conditions, is discussed in detail. Suggestions are offered for improvement in the technique for such trials.