Field Resistance to Strobilurin (Q_oI) Fungicides in Pyricularia grisea Caused by Mutations in the Mitochondrial Cytochrome b Gene

Abstract

Gray leaf spot caused by Pyricularia grisea is a highly destructive disease of perennial ryegrass turf. Control of gray leaf spot is dependent on the use of preventative fungicide treatments. Strobilurin-based (Q(o)I) fungicides, which inhibit the cytochrome bc(1) respiratory complex, have proven to be very effective against gray leaf spot. However, in August 2000, disease was diagnosed in Q(o)I-treated perennial ryegrass turf on golf courses in Lexington, KY, Champaign, IL, and Bloomington, IL. To determine if resistance was due to a mutation in the fungicide target, the cytochrome b gene (CYTB) was amplified from baseline and resistant isolates. Nucleotide sequence analysis revealed an intronless coding region of 1,179 bp. Isolates that were resistant to Q(o)I fungicides possessed one of two different mutant alleles, each of which carried a single point mutation. The first mutant allele had a guanine-to-cytosine transition at nucleotide position +428, resulting in a replacement of glycine 143 by alanine (G143A). Mutant allele two exhibited a cytosine-to-adenine transversion at position +387, causing a phenylalanine-to-leucine change (F129L). Cleavable amplified polymorphic sequence analysis revealed that neither mutation was present in a collection of baseline isolates collected before Q(o)I fungicide use and indicated that suspected Q(o)I- resistant isolates found in 2001 in Indiana and Maryland possessed the F129L mutation. The Pyricularia grisea isolates possessing the G143A substitution were significantly more resistant to azoxystrobin and trifloxystrobin, in vitro, than those having F129L. DNA fingerprinting of resistant isolates revealed that the mutations occurred in just five genetic backgrounds, suggesting that field resistance to the Q(o)I fungicides in Pyricularia grisea is due to a small number of ancestral mutations.

Mechanism of action and selectivity of a novel fungicide, pyribencarb

“…1) binds with a hydrogen bond to the amide group of glutamic acid at position 272 in domain C; therefore, this region plays an important role in accepting fungicides. Since substitution of glycine for alanine at position 143 (G143A) in domain B significantly lowers the affinity of strobilurin fungicides to cytochrome b, 7,8,13,14,[19][20][21][22][23] interaction between domain B and strobilurin fungicides must be strong. On the other hand, substitution of phenylalanine for leucine at position 129 (F129L) in domain A has been shown not to significantly lower the affinity of strobilurin fungicides to cytochrome b.…”

“…On the other hand, substitution of phenylalanine for leucine at position 129 (F129L) in domain A has been shown not to significantly lower the affinity of strobilurin fungicides to cytochrome b. 14,21) This suggests that interaction between domain A and strobilurin fungicides might be rather weak. The result obtained here that strobilurin fungicides inhibited SCR activities of crops, rats and carp rather potently, which have different amino acid residues from those of fungi at position 130 in domain A (Fig.…”

Mechanisms and significance of fungicide resistance

“…Mutations mediating a G143A-or a F129L exchange lead to complete loss or drastic reduction of fungicide efficacies (26,31). Single spore isolates of the apple powdery mildew Podosphaera leucotricha collected from commercial orchards were all heteroplasmic with respect to their mitochondrial cytochrome b gene and differed in strobilurin sensitivity.…”

Phylogenetic diversity and sensitivity to MBI and QoI fungicides of <i>Magnaporthe oryzae</i> in Taiwan

“…33,34) The mutation at codon 129 (TTC→ TTA) is associated with moderate resistance, and mutation at codon 143 (GGT→ GCT) is associated with resistance. 34) In Taiwan, rice blast is endemic to the first rice cultivation (March to June), affecting over 20,000 ha each year. 35) Previous studies showed the presence of more than 20 races of M. oryzae by the standard lines of rice.…”