A field experiment on the retention of Captan and Delan on apple
1996
Heijne, B. | Anbergen, R.H.M. (Research Station for Fruit Growing, Wilhelminadorp (Netherlands))
Individual apple trees were artificially rained with mini-sprinkles in a normal orchard. This was done at five occasions. Additional raining was done only during natural rain fill. Hence no additional infections of apple scab (Venturia inaequalis) were created. The quantity of extra rain varied between 6 and 28 mm of rain. Weekly sprayings with either Captan or Delan were applied to control apple scab. There were no significant differences between the retention of Captan and Delan on leaves of shoots and fruits of apple. Both Captan and Delan provided good control on shoots. While 33 per cent of leaves had scab lesions in untreated, this was no more than 1.8 per cent of leaves in the treatments with Captan and Delan. Similar results were obtained on fruits. The scab infection index was 42 for untreated, while it varied between 1.9 to 8.6 for different treatments with Captan and Delan. Apple scab infection increased as a result of the additional rain from 0.6 to 1.7. The amounts of additional rain were not extreme and can occur naturally as well. There were only two occasions when scab infections occurred after additional rain. In all other occasions first a new spraying was applied before infections took place. This means that only these two occasions were responsible for the extra scab infection. At these occasions only 5.8 and 17.4 mm of additional rain was given. And at the time of the infection residues were one and two days old respectively. It is remarkable that so little extra rain gave more scab on fruits. However, the natural rain should be included in the total amount of rain between two sprayings. Together, the additional rain and the natural rain were over 30 mm in both cases. The results imply that after heavy rain fall, residues of Captan and Delan do no longer control apple scab sufficiently. A shorter interval between treatments or the use of curative fungicides should be advised in such cases.
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