2012 Volume 78 Issue 1 Pages 3-9
Gray blight caused by Pestalotiopsis longiseta is one of the most important diseases of tea plant. Thiophanate-methyl (benzimidazole) and azoxystrobin (strobilurin, QoI) fungicides have been used to control gray blight in Kagoshima Prefecture. However, benzimidazole-resistant strains of P. longiseta have been found in tea fields in Kagoshima Prefecture since 1983 and strobilurin-resistant strains since 2008. Therefore, we examined samples of P. longiseta from several green-tea-cultivation regions in Kagoshima Prefecture in 2008 and 2009 for their sensitivities to strobilurin and benzimidazole. As a result, 31.7% of the strains in 2008 and 33.6% in 2009 were strobilurin-resistant. The rate was especially high in the Nansatsu region and low in the Aira and Hokusatsu regions. On the other hand, 80.9% of the strains were benzimidazole-resistant in 2009, found widely in separated fields. Strains that were highly resistant to benzimadazole were frequently found in the same fields as moderately resistant strains. Highly resistant strains to both fungicides occurred in as much as 20.0% of the tea fields. Azoxystrobin was not effective against QoI-resistant strains. In addition, the control effect of azoxystrobin on P. longiseta was decreased with the increase in the rate of QoI-resistant strains in the tea fields. Just a one-time application of QoI-fungicide remarkably increased the percentage of QoI-resistant strains in the tea field. This was the case for thiophanate-methyl.