Characterization Of Metalaxyl Resistant Pythium Ultimum From Chickpea Fields In The Pacific Northwest

Chickpea is an important component of the crop systems in the in the US Pacific Northwest. Pythium seed rot and damping-off is a serious disease of chickpea, but has been successfully managed with fungicide metalaxyl for more than 30 years. In recent years, however, seed rot and damping-off has occurred widely in metalaxyl-treated chickpeas in the US Pacific Northwest. This thesis study was aimed at identifying the cause of the emerging chickpea seed rot and damping-off and finding potential solutions to the problem. Pythium isolates were obtained from soil samples collected from inside and outside disease foci, and were characterized as either metalaxyl-resistant (MR) or metalaxyl-sensitive (MS) using a discriminatory dose (50 ppm) of metalaxyl. Most (67%) of the isolates obtained from inside disease foci were MR, whereas only a few (3%) of the isolates from outside of the disease foci were MR. Seven Pythium species were identified based on morphology and rDNA ITS sequences. The most common species are P. ultimum (115 isolates), P. irregulare (39) and P. abappressorium (28). Notably, all the identified 109 MR isolates belonged to Pythium ultimum. Metalaxyl resistance as measured by ED50 values is a stable trait. Comparative fitness studies showed that metalaxyl resistance does not carry any fitness cost (based on either colony diameter or mycelial dry weight). Pathogenicity tests further confirmed that metalaxyl cannot protect chickpea seeds from seed rot caused by MR isolates. However, the fungicide ethaboxam can protect chickpea seeds from seed rot caused by either MR or MS isolates of Pythium. This study showed that the emerging seed rot and damping-off of chickpea was caused by metalaxyl-resistant Pythium populations and the disease can be managed by including ethaboxam in seed treatments. Additionally, in efforts to provide empirical data for designing future studies on possible mechanisms of metalaxyl resistance, reactions of Pythium to metalaxyl were investigated in three aspects, hormesis, preconditioning and microsatellite haplotypes. Our results showed there was hormetic effect in MS P. ultimum. Preconditioning with metalaxyl always reduced the growth rate. Haplotypes based on SSR alleles were not correlated with metalaxyl resistance.