Competitive Ability and Fitness of Alternaria alternata Isolates Resistant to QoI Fungicides

Karaoglanidis, G. S., Luo, Y., and Michailides, T. J. 2011. Competitive ability and fitness of Alternaria alternata isolates resistant to QoI fungicides. Plant Dis. 95:178-182. Fungicides that act as quinone outside inhibitors (QoIs) constitute a fungicide group extensively used against Alternaria late blight of pistachio caused by Alternaria spp. However, developement of resistance to this fungicide class constitutes an important threat for the succesful control of the disease. This study was conducted to determine whether development of resistance to QoIs is associated with a fitness cost, by measuring several biological and epidemiological parameters and estimating the competitive ability in four QoI-resistant and four QoI-sensitive Alternaria alternata isolates. Fitness parameters measured were mycelial growth and spore production in vitro, disease latent period, aggressiveness, and spore production on detached pistachio leaves. The competitive ability of resistant isolates was assessed in coinoculation experiments with sensitive isolates on detached pistachio leaves, using a real-time polymerase chain reaction assay technique. Fitness parameters between grouped QoI-resistant and QoI-sensitive isolates were not significantly different (P = 0.13, 0.21, 0.31, and 0.27 for sporulation in vitro, mycelial growth, incubation period, and sporulation in vivo, respectively), while resistant isolates, as a group, showed a higher aggressiveness (P = 0.01) compared with the sensitive isolates. The data indicate that the resistant strains did not account for a fitness cost compared with the sensitive ones under the conditions of testing. The outcome of the competition experiments was isolate dependent. In two pairs, the resistance frequencies increased whereas, in the remaining two pairs of isolates, resistance frequency decreased, suggesting that the resistant isolates were competitive similarly to the sensitive isolates. Alternaria late blight of pistachio (Pistacia vera L.), caused by Alternaria alternata (Fr.) Keissl., A. tenuissima (Kunze) Wiltshire, and A. arborescens E.G. Simmons, is one of the most important fungal diseases of pistachio in California. The disease is characterized by the development of large necrotic lesions on the foliage that progressively coalesce, leading to premature defoliation. On the fruit, the disease is characterized by the development of small necrotic spots on the hull of immature nuts, causing staining of the nutshells that lead to a reduction of the fruit quality (30). Control of the disease is achieved by a combination of cultural practices (such as irrigation management and appropriate pruning to increase air movement and reduce relative humidity within tree canopy) and the use of fungicide spray applications. Fungicides registered for use against Alternaria late blight of pistachio in California include sterol demethylation inhibitors, quinone outside inhibitors (QoIs), and succinate dehydrogenase inhibitors (SDHIs) (2). QoIs constitute a fungicide class that has been developed from natural fungicidal derivatives such as strobilurin A, oudemansin A, and myxothiazol A (5). The mechanism of action that this fungicide class possesses consists of inhibiting mitochondrial respiration by binding at the Qo site of cytochrome b. Inhibition of mitochondrial respiration is achieved by blocking the electron transport between cytochrome b and cytochrome c1, which leads to a disruption of the energy cycle (5). Azoxystrobin was the first QoI fungicide registered for use against Alternaria late blight disease in California pistachio early in 2000. Later, two additional QoIs, trifloxystrobin and pyraclostrobin, were included in the spray pro