Revitalizing maize downy mildew management: harnessing new-generation fungicides and host plant resistance

Maize Downy Mildew (MDM) is a devastating disease in the humid sub-tropical/tropical regions of Asia. In this study, the prevalence of MDM during the rainy Kharif seasons of south Karnataka state (India) ranged between 6.8% (2018) and 19.1% (2022). The research evaluated new fungicidal treatments and assessed the genetic tolerance of maize lines to develop robust management strategies that enhance maize productivity and stability.

During the Kharif seasons of 2021 and 2022, we conducted field trials to evaluate the effectiveness of six different fungicides, both individually and in combination. The most effective approach combined seed treatment with Metalaxyl (4%) and Mancozeb (64%) WP, followed by a foliar spray of Azoxystrobin (18.2%) and Difenoconazole (11.4%) SC. This treatment reduced MDM incidence by 97.6% and increased maize yield up to 85.6 quintals per hectare, with a benefit-cost ratio of 2.2. Additionally, screening of 317 maize inbred lines in Kharif 2019 identified 22 lines with stable MDM resistance over nine consecutive Rabi and Kharif seasons, indicating their potential for sustained resistance. Liquid Chromatography-Mass Spectrometry (LC-MS) analysis revealed significant increases in eighteen phenolic compounds and fifteen flavonoid compounds in resistant maize genotypes. Specifically, resistant genotypes exhibited elevated levels of salicylic acid (4.2 to 9.2-fold), p-Coumaric acid (3.7 to 4.8-fold), o-Coumaric acid (4.5 to 7.4-fold), Caffeic acid (2.4 to 3.1-fold), and Ferulic acid (2.3 to 2.8-fold). Flavonoid levels also increased, with Naringenin ranging from 34.4 µg/g in African Tall to 130 µg/g in MAI 224, Catechin from 22.9 µg/g in African Tall to 124.4 µg/g in MAI 10, and Epicatechin from 1.3 µg/g in African Tall to 8.2 µg/g in MAI 10. These heightened levels contribute to a robust chemical defence mechanism against Peronosclerospora sorghi.

This study provides crucial insights into managing MDM through host plant resistance and fungicidal treatments. We identified 22 resistant inbred lines as valuable genetic resources for breeding MDM-resistant maize hybrids. Enhanced levels of specific phenolic and flavonoid compounds in these resistant genotypes suggest a robust chemical defence mechanism, essential for developing resilient crops. Our findings offer practical recommendations for improving maize production and ensuring crop security in MDM-affected regions. Integrating these resistant maize lines and effective fungicidal treatments can significantly advance sustainable agricultural practices, contributing to crop resilience and food security in areas prone to MDM.