Fungicides for the management of blackleg disease of canola caused by Leptosphaeria maculans in the Western Cape province of South Africa

Blackleg disease, caused by Leptosphaeria maculans , severely impacts canola production in the Western Cape. Fungicides are an important component of integrated management, but little information is available on fungicide efficacy under South African conditions. With decreased sensitivity reported for L . maculans to the demethylation inhibitor (DMI) class fungicides in Australia and Europe, it is important that fungicide sensitivity is also evaluated locally. This study screened 118 L. maculans isolates, collected between 2007 and 2022 from canola growing regions of the Western Cape, for in vitro sensitivity to two active ingredients frequently used in canola production, namely tebuconazole and prothioconazole. Field trials were conducted at four locations in 2022 and 2023 to assess the efficacy of six fungicide products on two canola cultivars. Tebuconazole 50% effective concentrations (EC 50 ) ranged from 144.9 ng/mL to 985.3 ng/mL, which was within the range of EC 50 values found for sensitive isolates in similar studies. Tebuconazole EC 50 values steadily increased from 2020 to 2022, suggesting a shift in sensitivity. Prothioconazole EC 50 values were between 270.3 ng/mL and 1620.4 ng/mL. In the field trials, all fungicide products, except the unregistered product Bumper, were effective in lowering blackleg severity. The cultivar Diamond showed a greater fungicide response than the more resistance cultivar 44Y94. Under higher disease pressure, however, fungicide applications also reduced blackleg severity in 44Y94. When sequences of the CYP51 promotor region were screened for inserts associated with reduced DMI sensitivity, no inserts were detected in local isolates. This study provides a foundation for the effective and sustainable use of fungicides in an integrated blackleg management strategy in South Africa. • No triazole resistance detected in South African Leptosphaeria maculans populations • Steady reduction in tebuconazole sensitivity between 2020 and 2022 • No inserts associated with reduced DMI sensitivity in CYP51 promoter of L. maculans • Blackleg control with fungicides improved with low host resistance and high disease