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.

Multiple species of Colletotrichum can cause bitter rot disease of apple, but the identities and relative representation of the species causing the disease in Kentucky are unknown. In total, 475 Colletotrichum isolates were collected from diseased apple fruit in 25 counties and characterized both morphologically and by using various molecular approaches. Multigene sequence analyses revealed that sample isolates belonged to several newly erected species within the Colletotrichum acutatum and C. gloeosporioides species complexes. The isolates were identified as C. fioriniae and C. nymphaeae, which reside within the C. acutatum species complex, and C. siamense, C. theobromicola, and C. fructicola, which are placed within the C. gloeosporioides species complex. C. fioriniae was the most common species causing bitter rot in Kentucky, comprising more than 70% of the isolates. Infectivity tests on detached fruit showed that C. gloeosporioides species-complex isolates were more aggressive than isolates in the C. acutatum species complex. However, isolates within the C. acutatum species complex produced more spores on lesions compared with isolates within the C. gloeosporioides species complex. Aggressiveness varied among individual species within a species complex. C. siamense was the most aggressive species identified in this study. Within the C. acutatum species complex, C. fioriniae was more aggressive than C. nymphaeae, causing larger, deeper lesions. Apple cultivar did not have a significant effect on lesion development. However, Colletotrichum spp. produced more spores on ‘Red Stayman Winesap’ than on ‘Golden Delicious’. Fungicide sensitivity tests revealed that the C. acutatum species complex was more tolerant to thiophanate-methyl, myclobutanil, trifloxystrobin, and captan compared with the C. gloeosporioides species complex. The study also revealed that mycelial growth of C. siamense was more sensitive to tested fungicides compared with C. fructicola and C. theobromicola. These research findings emphasize the importance of accurate identification of Colletotrichum spp. within each species complex, because they exhibit differences in pathogenicity and fungicide sensitivity.

In August 2000, azoxystrobin was ineffective in controlling gray leaf spot of perennial ryegrass at a golf course in Lexington, KY and at two golf courses in Illinois. Isolates suspected of being fungicide-resistant (“suspect isolates”) were compared to “baseline” isolates obtained from sites with no known use of quinol-oxidizing inhibitor (QoI) fungicides. Conidial germination of Pyricularia grisea was tested in vitro with 100 μg of salicylhydroxamic acid per ml. For baseline isolates, 50% effective concentration (EC50) values for azoxystrobin and trifloxystrobin were 0.015 to 0.064 and 0.013 to 0.078 μg/ml, respectively; EC50 values for suspect isolates were 2.39 to 44.8 and 0.31 to 111, respectively. All suspect isolates exhibited significantly (P = 0.05) lower sensitivity to QoI fungicides than all baseline isolates. The mean EC50 values for suspect isolates for azoxystrobin and trifloxystrobin were 690 and 827 times higher, respectively, than the means for baseline isolates. In the laboratory, azoxystrobin and trifloxystrobin provided essentially complete control of disease induced by nine baseline isolates in vivo. Azoxystrobin and trifloxystrobin provided poor to no control of disease induced by six of eight suspect isolates; control of disease induced by the remaining two isolates was partial for azoxystrobin and complete for trifloxystrobin. We conclude that one or more biotypes of perennial ryegrass-infecting strains of P. grisea with resistance to QoI fungicides have emerged. This is the first report of resistance to QoI fungicides in P. grisea. Furthermore, this is one of two QoIresistant fungal pathogens collected in the United States during the 2000 growing season, the first instances reported for North America.

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Chlorothalonil and myclobutanil were applied separately to creeping bentgrass using two sets of nozzles: two low‐drift nozzles that provided incomplete coverage [Teejet Turbo Turfjet nozzle (1/4TTJ04‐VS) and the Raindrop RA‐4 nozzle (35654‐2)], and two nozzles that provided nearly complete coverage [XR Teejet flat fan nozzle (XR11004‐VS) and the Teejet Air Induction nozzle (AI1004‐VS)], as assessed using water‐sensitive paper. Spray volume and pressure were equivalent with all nozzles. For both fungicides tested, in 48% of our assessment dates over four trials, the complete‐coverage nozzles controlled dollar spot better (P < 0.1) than one or both nozzles that provided incomplete coverage. We saw no instance when nozzles providing incomplete coverage controlled dollar spot better than nozzles providing complete coverage. These results support the hypothesis that achieving complete spray coverage with fungicide provides more consistent control of dollar spot than incomplete spray coverage.