Greenhouse evaluation of chemicals for control of powdery mildews

SUMMARY A method is described for the artificial inoculation of large numbers of potted apple rootstocks or barley seedlings with conidia of the appropriate powdery mildew, using settling towers large enough to hold 100 pots, and blowing spores via a central pipe into the upper part of the tower and allowing them to settle on the foliage. Plants are put in the tower some 24 hr. before introducing the spores, left enclosed for a further 24 hr., and then placed on the bench in a greenhouse with forced ventilation until lesions are clear enough for recording, usually 9 days after inoculation for apple mildew and 7 for barley mildew. Rooted shoots from stoolbeds of M. III apple rootstock provided suitable material for apple mildew experiments. The largest numbers of lesions per leaf were usually found on the third and fourth leaves down the shoots; records were usually taken on leaves 3 to 5. For barley mildew, the variety Plumage Archer was found to be suitable. Conidia can be collected and weighed as required, facilitating control over the inoculum load applied in each test. Seedlings are preferred before the second leaf emerges, i.e. before bending or twisting interferes with inoculation. The method is successful for the study of both the protective and curative effects of chemicals applied by a dipping technique.