Effects of powdery mildew on cucumber yields, and its chemical control

SUMMARY Effects of Sphaerotheca fuliginea were deduced from experiments in which cucumber foliage, exposed to naturally occurring inocula, was fungicidally sprayed. Yields increased as the incidence of mildew decreased but the relation was sometimes affected by differing amounts of phytotoxicity. The fungitoxicity and phytotoxicity of a range of chemicals applied as high‐volume sprays, fumigants or soil drenches, were tested. Non‐phytotoxic concentrations of some fungicides adequately protected cucumber foliage from subsequent attack but were often insufficient to eradicate established infections, the increased amounts needed for this being phytotoxic. Powdery mildew was controlled more effectively when drazoxolon and quinomethionate were applied as sprays with 0·1 and 0·02% a.i. respectively, than when used as fumigants at 2–4 and 1·3 g/28·3 m3(= 1000 ft3). Sprays of drazoxolon (0·1% a.i.) increased yields from 7·0 to 11·2 kg/ plant during 8 weeks picking and in another experiment weights of fruit were increased by applying quinomethionate (0·02% a.i.) from 10·8 to 52·2 kg/plot of four plants. In the former experiment appreciable amounts of phytotoxicity and infection were tolerated before yields decreased but, in the latter, yields were inversely proportional to numbers of dead leaves which were directly related to the incidence of mildew. Spraying with quinomethionate, or drazoxolon plus tetradifon (0·012% a.i.) increased numbers of female flowers from 59·5 in the mildewed controls to 93·0 and 142·8 per plant respectively, and of these 17·6, 32·8 and 16·2 % subsequently produced marketable cucumbers. In addition to decreasing yields, severe S. fuliginea infestations were associated with increased numbers of misshapen cucumbers. Increasing sulphur concentrations from 0·5 to 1·0g/28·3m3during nightly fumigations significantly decreased mildew incidence and increased (a) yields from 35·8 to 52·9 kg during 5 weeks picking, and (b) the proportion of high‐quality cucumbers from 49 to 63 %. Similar trends occurred in another trial where sulphur concentrations were increased from 0·75 to 1·0 g/28·3 m3, but comparisons with fortnightly sprays of 0·1% drazoxolon suggest that these sulphur concentrations caused some damage.