A technique of investigating apple scab infection periods using M M 109 rootstocks in the glasshouse is fully described. Inoculation by ascospores in aqueous suspension was less reliable than that by sedimenting the spores direct from source leaves on to the host plants, but fresh conidia in aqueous suspension consistently gave high levels of infection under optimum conditions.Ascospores required a shorter period of continuous wetness (6 hr.) than conidia (7-9 hr.) for infection at near-optimum temperatures. Maximum infection from both sources was reached after about 18 hr. continuous wetness; much longer periods were sometimes inimical. With discontinuous wetness, most ascospores tolerated a dry interval of 24 hr. on the host leaves, although infection was somewhat reduced if the dry period began when the spores were starting to germinate. Conidia were more inhibited than ascospores by 24 hr. drying during minimal infection periods, but many survived and produced lesions.Some ascospores survived dry periods of at least 96 hr., but mature leaves had acquired resistance during the interval and thus infection was reduced. The reduction was partly offset by greater infection of the youngest leaves, which meantime had expanded and were thus easier to wet. No infection resulted, however, when the dry interval was extended to 10 days.T h e implications of the results are discussed in relation to the interpretation of infection periods in the field.
Experiments are described in which the efficiency of undiluted lime‐sulphur applied as a spray mist of small droplets, formed by and carried in a stream of compressed air, was compared on small Cox's Orange Pippin apple trees with that of diluted lime‐sulphur applied either in the same way or by orthodox hydraulic spraying. The control without spray damage of apple scab (Venturia inaequalis (Cooke) Wint.) and apple mildew (Podosphaera leucotricha (Ell. & Everh.) Salm.) was the criterion.The undiluted concentrate proved at least as effective and safe as the hydraulically applied dilute spray when a satisfactory method of application, by a paint spray gun, had been established. A 50% dilution applied in this way at increased volume proved equally effective, but a 10% dilution similarly applied failed to give satis‐factory control of scab, probably because of the reduced dosage.
On the sandy loam at East Mailing, moderate annual dressings of inorganic sources of nitrogen and potash exerted no consistent influence on the incidence of apple scab, even when the dressings were continued over an eleven‐year period on established trees of four different varieties on M.IX rootstock. Likewise, no influence of such dressings continued over five seasons was shown on the incidence of apple canker on the rootstock M.VIII, which was grown under conditions favourable for severe infection.There was evidence from early observations that grassing‐down to excess can reduce the incidence of scab, but only at the expense of nitrogen starvation in the trees. Short‐term mixed leys had no such effect.The incidence of branch blister, a functional disease associated with adverse soil conditions and retarded vigour, was greatly reduced in the variety Cox's Orange Pippin by potash manuring, and was further influenced by the rootstock.
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