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ARS Home » Plains Area » Fargo, North Dakota » Edward T. Schafer Agricultural Research Center » Sugarbeet and Potato Research » Research » Publications at this Location » Publication #323573
Title: Stability of tetraconazole-resistant isolates of Cercospora beticola after exposure to different temperature and time treatments

Author
item ARABIAT, SAHAR - North Dakota State University
item KHAN, MOHAMED - North Dakota State University
item Bolton, Melvin
item SECOR, GARY - North Dakota State University

Submitted to: Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/19/2016
Publication Date: 4/3/2017
Citation: Arabiat, S., Khan, M.F., Bolton, M.D., Secor, G. 2017. Stability of tetraconazole-resistant isolates of Cercospora beticola after exposure to different temperature and time treatments. Journal of Plant Pathology. 99(1):177-184.

Interpretive Summary: Cercospora leaf spot caused by Cercospora beticola is the major foliar disease effecting sugar beet (Beta vulgaris L.) production in North Dakota and Minnesota. The fungicide tetraconazole is widely-used to manage Cercospora leaf spot. However, there has been an increase in prevalence of tetraconzole-resistant strains in recent years. Knowledge on whether tetraconazole-resistant strains are less hardy when no tetraconazole has been applied is important for fungicide resistance management. To explore this, we inoculated sugar beet plants with two fungicide-sensitive and two fungicide-resistant isolates of C. beticola. Four weeks after inoculation, infested leaves were harvested and subsequently exposed to six different temperature/time regimes: -20ºC (4 weeks), 4ºC (4 weeks), 20ºC (4 weeks), -20ºC (2 weeks)/4ºC (2 weeks), -20ºC (1 week)/4ºC (1 week)/-20ºC (1 week)/4ºC (1 week), and -20ºC (1 week)/20ºC (1 week)/-20ºC (1 week)/20ºC (1 week). Subsequently, spore production, spore germination, radial mycelial growth, sensitivity to tetraconazole, and disease severity were evaluated for each isolate and compared to control treatments that received no temperature treatment. Resistant isolates had no fitness penalty as measured by spore production, spore germination, radial mycelial growth, and disease severity after exposure to all temperature/time regimes. However, the tetraconazole-resistant isolate 09-347 reverted to a moderatly resistant level after exposure to two different temperature/time regimes; -20ºC, and -20ºC/4ºC/-20ºC/4ºC with a factor of change of 38.6 and 32.8, respectively. Although one tetraconazole resistant isolates was adversely affected by cold temperatures, this study suggests resistant isolates. So it may be important to avoid applying triazole fungicides early in the disease season in areas known to have triazoles-resistant C. beticola population, and use other chemistries with the aim of significantly reducing the population of triazole-resistant isolates so as to prolong the usefulness of triazole fungicides for controlling C. beticola. Taken together, cold temperatures do not appear to impart a significant fitness penalty in C. beticola.

Technical Abstract: Cercospora leaf spot caused by Cercospora beticola is the major foliar disease effecting sugar beet (Beta vulgaris L.) production in North Dakota and Minnesota. The sterol demethylation inhibitor (DMI) fungicide tetraconazole is widely-used to manage Cercospora leaf spot. However, there has been an increase in prevalence of tetraconzole-resistant strains in recent years. Knowledge on whether tetraconazole-resistant strains exhibit a fitness penalty in the absence of the selection pressure imposed by tetraconazole application is important for fungicide resistance management. To explore this, we inoculated sugar beet plants with two DMI-sensitive and two DMI-resistant isolates of C. beticola. Four weeks after inoculation, infested leaves were harvested and subsequently exposed to six different temperature/time regimes: -20ºC (4 weeks), 4ºC (4 weeks), 20ºC (4 weeks), -20ºC (2 weeks)/4ºC (2 weeks), -20ºC (1 week)/4ºC (1 week)/-20ºC (1 week)/4ºC (1 week), and -20ºC (1 week)/20ºC (1 week)/-20ºC (1 week)/20ºC (1 week). Subsequently, spore production, spore germination, radial mycelial growth, sensitivity to tetraconazole, and disease severity were evaluated for each isolate and compared to control treatments that received no temperature treatment. Resistant isolates had no fitness penalty as measured by spore production, spore germination, radial mycelial growth, and disease severity after exposure to all temperature/time regimes. However, the tetraconazole-resistant isolate 09-347 reverted to a moderatly resistant level after exposure to two different temperature/time regimes; -20ºC, and -20ºC/4ºC/-20ºC/4ºC with a factor of change of 38.6 and 32.8, respectively. Although one tetraconazole resistant isolates was adversely affected by cold temperatures, this study suggests resistant isolates. So it may be important to avoid applying triazole fungicides early in the disease season in areas known to have triazoles-resistant C. beticola population, and use other chemistries with the aim of significantly reducing the population of triazole-resistant isolates so as to prolong the usefulness of triazole fungicides for controlling C. beticola. Taken together, cold temperatures do not appear to impart a significant fitness penalty in C. beticola.