Pustules on wood as sources of inoculum in apple scab and their response to chemical treatments

The green stems and leaves of potted Cox's Orange Pippin trees were susceptible to infection by Venturia inaequalis from June to August. Some isolates of the fungus showed conspicuous ability to attack the wood. In the orchard, many conidia were released from pustules in early March (2—3 wk before bud-burst) but in the 3 years examined (1967—9) most were liberated in April and May. In 1969 it was shown that a decline in numbers in June was accompanied by a decrease in viability of conidia. Infections near the tips of leading shoots were partially controlled by a single treatment with 5% urea (aqueous solution) in October 1966 or 2% urea or 0·01% phenylmercuric chloride (PMC) in March 1967. The subsequent reduction in inoculum was correlated with a reduction in the numbers of primary infections developing on leaves. Basal infections on leading shoots were not susceptible to treatment in October 1967 or March 1968, but in 1969 similar infections were controlled with PMC applied mid-April when the pustules had burst and were actively sporulating.