Distribution of QoI Resistance in Populations of Tangerine-Infecting Alternaria alternata in Florida

Abstract

Vega, B., and Dewdney, M. M. 2014. Distribution of Qol resistance in populations of tangerine-infecting Alternada alternata in Florida. Plant Dis. 98:67-76.Chemical control, based on copper and quinone outside inhibitor (Qol) fungicides, has been essential for the management of brown spot of citrus, caused by Alternarla alternata. However, Qol control failures were detected recently in Florida. From 2008 to 2012, 817 monoconidial isolates of A. alternata from 46 citrus orchards were examined for sensitivity to azoxystrobin (AZ) and pyraclostrobin (PYR). Of the isolates, 57.6% were resistant to both fungicides, with effective concentration to inhibit 50% growth (EC,,,) values greater than 5 |ag/ml for AZ and 1 |ig/ml for PYR. The mean EC5,, values for sensitive isolates were 0.139 and 0.020 pg/ml for AZ and PYR, respectively. The EC50 values of both fungicides were highly correlated [P < 0.0001), indicating cross resistance. The proportion of resistant isolates differed sig-nificantly (P < 0.0001) among cultivars and with Qol applicadon frequency (P < 0.0001). However, resistance was not significantly related (P = 0.364) to disease severity in the field (low, moderate, and high) or isolate virulence {P = 0.397). The molecular basis for Qol resistance was determined for a subset of 235 isolates using polymerase chain reaction restricdon fragment length polymorphism of the cytochrome b gene. All resistant isolates showed the point mutadon G143A. Based on the presence of one or two introns, isolates were classified as profile I and profile II, respectively. The resistance frequency was significantly higher (P < 0.0001) in isolate profile II, suggesdng a higher selecdon pressure for resistant population profile II.Alternaria brown spot (ABS), caused by Alternarla alternata (Fr.;Fr.) Keissl., is a serious disease of tangerine cultivars {Citrus reticulata Blanco) and their hybrids in commercial citrus orchards in humid subtropical and semiarid growing regions (58). This fungus produces the host-selective ACT toxin (32,33), which plays an important role in host specificity and isolate virulence (23,34). Florida has 7,000 ha of tangerine and tangerine hybrids for freshfruit production (21). More than two-thirds of the tangerine cultivars planted in Florida are susceptible to ABS, including 'Minneola', 'Nova', and 'Orlando' tángelo; 'Dancy' and 'Sunburst' tangerine; 'Murcott' tangor, and some Dancy hybrids such as 'Lee' (56). The fungus can affect leaves, twigs, and young fruit, cause considerable yield loss, and produce blemishes on fruit that reduce marketability (56,58). Management of ABS includes the use of cultural methods such as disease-free nursery stock, wide tree spacing, reduction of excessive vegetadve growth, the elimination of overhead irrigation, and fungicide applications (16,57). Although cultural practices reduce disease severity, ABS control is largely dependent on fungicide applications (8,16,56). Copper-based fungicides (Fungicide Resistance Action Committee [FRAC] code Ml), ferbam (FRAC code M3), and ...

Sensitivity of <i>Leptosphaeria maculans</i> to pyraclostrobin in Alberta, Canada

“…More frequent or heavier use of fungicides in an area may cause a greater shift in sensitivity relative to areas with less frequent use of the same product (Brent and Hollomon 2007). There are many sensitivity studies involving pyraclostrobin and other fungal pathogens in the Pleosporales (Gossen and Anderson 2004;Pasche et al 2004;Mondal et al 2005;Avenot et al 2008;Patel et al 2012;Vega et al 2012;Ahmed et al 2014;Vega and Dewdney 2014;Bowness et al 2016). However, due to natural variation in the fungicide sensitivity of species, different EC 50 values are to be expected (Vega and Dewdney 2014).…”

“…There are many sensitivity studies involving pyraclostrobin and other fungal pathogens in the Pleosporales (Gossen and Anderson 2004;Pasche et al 2004;Mondal et al 2005;Avenot et al 2008;Patel et al 2012;Vega et al 2012;Ahmed et al 2014;Vega and Dewdney 2014;Bowness et al 2016). However, due to natural variation in the fungicide sensitivity of species, different EC 50 values are to be expected (Vega and Dewdney 2014). For example, Ahmed et al (2014) determined the EC 50 of Didymella rabiei Kovatsch to pyraclostrobin to be 1.0 mg L −1 with a range of 0.3 to 3.8 mg L −1 , which is much greater than the values observed for L. maculans in both the current study and the abstract of Liu et al (2013).…”

“…In an assessment of the fungicide sensitivity of A. alternata isolates, Vega and Dewdney (2014) determined the EC 50 of insensitive populations to be 5.507 μg mL −1 , which represented a 239-fold decrease in sensitivity relative to the baseline isolates (0.023 μg mL −1 ). The results of the growth plate assay in the current study suggest that there were no isolates insensitive to pyraclostrobin in the collection from Alberta, since no isolates with a <50% reduction in colony growth were identified.…”

QoI-Resistance Stability in Relation to Pathogenic and Saprophytic Fitness Components of Alternaria alternata from Citrus

“…In all, 10 Qol-resistant and 10 Qol-sensitive single-conidium-derived isolates of A. altemata were used in this study ( Table 1). The isolates were collected from tangerine and tangerine hybrid orchards in Florida for a study conducted from 2008 to 2012 to determine the fungicide sensitivity of A. altemata isolates to azoxystrobin and pyraclostrobin (56). The effective concentrations needed to reduce the fungal growth by 50% (EC50 values) were determined using the resazurin-based microtiter as say, according to a previously described procedure (57).…”

“…Multiple cases of Qol resistance, including A. altemata from tangerine, have been reported (19). A high fre quency of Qol-resistant isolates, carrying the G143A mutation, was found in the most important tangerine-producing counties in Florida (56), suggesting that Qol resistance in A. altemata popula tions may not be associated with a fitness cost.…”