The objective was to evaluate the efficacy of a Phytophthora blight fungicide program for controlling powdery mildew as well as Phytophthora blight. The impetus was observing that the severity of powdery mildew on upper leaf surfaces was lower than expected through Aug in previous evaluations of biopesticides.
The following fungicides with targeted activity for Phytophthora blight were applied throughout the season to foliage in treated plots (except the first application which was a directed spray to soil): Omega 24 fl oz/A was applied on 10 July, Omega 16 fl oz/A on 15 July, Ranman 2.75 fl oz/A on 20 Aug and 3 and 25 Sep, Orondis Ultra 7 fl oz/A on 13 and 27 Aug and 9 Sep, Presidio 4 fl oz/A on 23 July and 6 Aug, and Revus 8 fl oz/A on 30 July. The 11 applications were made on a preventive 7-day schedule using a tractor-mounted boom sprayer. The last application was unintentionally applied 14 days after the prior one was done.
The program with targeted fungicides for Phytophthora blight provided some control of powdery mildew on the upper leaf surface most notably during August, which corresponds with previous observations. It provided 47% and 17% control on upper and lower leaf surfaces, respectively, based on AUDPC values. This degree of control is not adequate for commercial production, but it is sufficient to impact results of powdery mildew fungicide evaluations, in particular biopesticides. The surfactants in these fungicides may be responsible for the control.
The only Phytophthora blight symptom observed was fruit rot. First affected fruit were observed on 30 August, 1 week after favorable conditions were created by 1.3 in. of rain on 22-23 Aug, which was the first rainfall exceeding 0.3 in. since 18 July. At every assessment there was a lower percentage of fruit affected by Phytophthora blight in the treated than control plots. Observing some disease suppression in an experiment in which half the plots were not treated and were interspersed among the treated plots is noteworthy for a difficult to control disease caused by a pathogen that can be splash dispersed.
McGrath, M. T. and Downing, C. T. 2023. Efficacy of a Phytophthora blight fungicide program for powdery mildew and Phytophthora blight, 2022. Plant Disease Management Reports 17:V0.
Images below were taken on 1 September when control of powdery mildew by the Phytophthora fungicide program was not very evident. Photographs were not taken previously.
The following plot photos taken on 1 September, 5 days after the 8th application.
