Reduced Sensitivity to Demethylation Inhibiting Fungicides in New Zealand Strains of Pyrenophora Teres Drechsler and Related Barley Pathogens

Abstract

Pyrenophora teres Drechsler (anamorph Drechslera tens (Sacc.) Shoemaker) is a pathogen of barley causing the disease "net blotch". There are two types of net blotch present in New Zealand. The "net" type (P. teres f. teres) and the recently introduced "spot" type (P. teres f. maculata) first identified in 1986. The "net" type was initially well controlled by the demethylation inhibitor (D.M.I.) fungicide triadimenol, the first D.M.I, fungicide to be used as a cereal seed treatment in New Zealand. The problem of reduced sensitivity to D.M.I.'s was highlighted by the failure of triadimenol to control net blotch in the 1981-82 season. Imazalil replaced triadimenol in 1983 and gave very good control of net blotch. The common "net" type is still (1988) controlled by imazalil, prochloraz, and propiconazole but not by triadimenol, flutriafol and nuarimol and some of the more recently introduced D.M.I.'s. Some isolates of the "spot" type of net blotch have shown in laboratory studies reduced sensitivity to imazalil, prochloraz, and propiconazole and to the recently introduced D.M.I.'s cyproconazole, myclobutanil, terbuconazole. (The former 3 are the currently registered fungicides for net blotch control). Field resistance to these fungicides has not yet been found. Hybridization between the sensitive "fit" readily transmitting "net" type and the "unfit" but less sensitive "spot" type may possibly occur but this has not been demonstrated. In this study available D.M.I.'s were used in poison plate screening of the "net" and "spot" type isolates of P. teres over a 2 year period and Ec50's calculated from the data are presented. Also included are isolates of P. graminea (Ito & Kurib.) Drechs. (anamorph Drechslera graminea (Rabenh. ex Schlecht) Shoemaker) cause of leaf stripe of barley and Cochliobolus sativus (Ito & Kurib.) Drechs. ex Dastur (anamorph Drechslera sorokiniana (Sacc.) Subram. & Jain) cause of spot blotch/foot rot.