Chickpea seed rot and damping-off caused by metalaxyl-resistant Pythium ultimum and its management with ethaboxam

Authors: WANG, MOYIN - Washington State University; VAN VLEET, STEPHEN - Washington State University; McGee, Rebecca; Paulitz, Timothy; Porter, Lyndon; SCHROEDER, KURTIS - University Of Idaho; Vandemark, George; Chen, Weidong

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/24/2020
Publication Date: 4/29/2021
Citation: Wang, M., Van Vleet, S., McGee, R.J., Paulitz, T.C., Porter, L.D., Schroeder, K., Vandemark, G.J., Chen, W. 2021. Chickpea seed rot and damping-off caused by metalaxyl-resistant Pythium ultimum and its management with ethaboxam. Plant Disease. 105(6):1728-1737. https://doi.org/10.1094/PDIS-08-20-1659-RE.
DOI: https://doi.org/10.1094/PDIS-08-20-1659-RE

Interpretive Summary: Stand establishment is the first crucial step for successful chickpea production. Pythium seed rot and damping-off is often the obstacle in chickpea seed germination and stand establishment. The disease has been managed for more than 30 years using metalaxyl in seed treatments. However, germination failure has occurred in metalaxyl-treated chickpea seeds since 2014. Our research has documented that the germination failure was due to emergence of metalaxyl-resistant populations in Pythium. High proportions of Pythium isolates from within disease foci were resistant to metalaxyl. All metalaxyl-resistant isolates (>120 isolates identified to species) belonged to Pythium ultimum. Metalaxyl seed treatment could not protect chickpea seeds from seed rot and damping-off caused by metalaxyl-resistant isolates. Metalaxyl resistance does not carry any observable fitness costs because wetalaxyl-resistant isolates grow equally well as metalaxyl-sensitive isolates. The metalaxyl resistance is a stable trait because metalaxyl resistant isolates stayed resistant after ten generations in the absence of metalaxyl. The stability of metalaxyl resistance and lack of fitness costs associated with the metalaxyl resistance demand active management. Experiments in the laboratory, greenhouse and field have shown that the fungicide ethaboxam can inhibit growth of metalaxyl-resistant isolates and can increase seed emergence in metalaxyl-resistant isolates infested soil. Ethaboxam has proved to be effective in managing the metalaxyl-resistant populations of Pythium.

Technical Abstract: Metalaxyl and its isomer mefenoxam have been the primary fungicides used as seed treatment in managing Pythium seed rot and damping-off of chickpea. However, recent outbreaks of seed rot and damping-off of metalaxyl-treated chickpea seeds were found in the dryland agriculture region in southeastern Washington and northern Idaho. Isolates of Pythium obtained from rotten seeds and associated soil showed high levels of resistance to metalaxyl. Large proportions of Pythium isolates resistant to metalaxyl were found in Pythium populations obtained from areas where severe chickpea damping-off occurred and were observed in commercial chickpea fields over several years. All metalaxyl-resistant isolates were identified as P. ultimum var. ultimum. The metalaxyl resistance trait measured by EC50 values was stable over 10 generations in the absence of metalaxyl, and no observable fitness costs were associated with metalaxyl resistance. Under controlled conditions, metalaxyl treatments failed to protect chickpea seeds from seed rot and damping-off following inoculation with metalaxyl-resistant Pythium isolates. In culture, ethaboxam inhibited mycelial growth of metalaxyl-resistant isolates, as well as metalaxyl-sensitive isolates. Greenhouse and field tests showed that ethaboxam is effective in managing metalaxyl-resistant Pythium. Ethaboxam in combination with metalaxyl is now commonly applied as seed treatments in commercial chickpea production.